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Ciprofloxacin and Azithromycin Antibiotics Interactions with Bilayer Ionic Surfactants: A Molecular Dynamics Study
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Cite this: ACS Omega 2024, 9, 30, 33174–33182
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https://doi.org/10.1021/acsomega.4c04673
Published July 17, 2024

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Abstract

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The introduction of pharmaceuticals into aquatic ecosystems can lead to the generation of antibiotic-resistant bacteria. This paper employed molecular dynamics simulations to examine the interactions between cationic/anionic surfactants and two antibiotics or drugs, namely, ciprofloxacin and azithromycin. The analysis focused on many factors to elucidate the mechanism by which the surfactant bilayer molecular structure affects the selected antibiotics. These factors include the tilt angle, rotational angle of the surfactants, electrostatic potential, and charge density along the bilayers. Our molecular-level investigation of the adsorption mechanisms of hydrophobic (azithromycin) and hydrophilic (ciprofloxacin) drugs on the cationic/anionic surfactant bilayer offers a crucial understanding for comprehending the optimal selection of surfactants for effectively separating antibiotics.

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Published as part of ACS Omega virtual special issue “Celebrating the 25th Anniversary of the Chemical Research Society of India”.

1. Introduction

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Antibiotics have revolutionized the field of medicine by providing effective treatment for bacterial infections. However, the overuse and improper disposal of antibiotics have made them find their way into our water resources, thereby causing detrimental effects on aquatic ecosystems and human health. (1) Furthermore, the discharge of antibiotics into the aquatic environment, resulting from the processes of the pharmaceutical industry and sewage waste, has instigated a worldwide surge in antimicrobial resistance, (2−4) making it more difficult to treat infections in both humans and animals. Additionally, antibiotics can disrupt the balance of microorganisms in aquatic environments, influencing the overall health of these ecosystems. (5,6) It is thus important to monitor and find possible ways to isolate these drugs from water sources using secondary techniques such as foam fractionation. (7) Numerous works have been performed on removing or minimizing the antibiotic concentrations in water with the help of various techniques such as adsorption, (8,9) advanced oxidation processes, (10,11) membrane filtration, (12,13) biological treatment methods, (14,15) and using various surfactants. (16−19)
Surfactants form an essential component in the drug industry for their ability to improve the therapeutic concentration of drugs by enhancing their bioavailability. (20) These amphipathic molecules form micelles that reduce surface tension in various systems, enhancing solubility, dissolution profile, permeation, and stability. Surfactants could be anionic, cationic, nonionic, or amphoteric, each with their own unique properties and applications. (21) They are commonly used in soaps, detergents, and industrial processes, leading to excessive concentrations in water bodies that pose environmental and toxicology issues due to their persistent nature. (22) Anionic surfactants, such as sodium dodecyl sulfate (SDS), are commonly used in cleaning products due to their ability to remove dirt and grease. Cationic surfactants, such as cetyltrimethylammonium bromide (CTAB), are often used in fabric softeners and hair conditioners for their ability to reduce static electricity. Nonionic surfactants, such as poly(ethylene glycol), are gentle on the skin and are commonly found in personal care products. Amphoteric surfactants, like cocamidopropyl betaine, have both positive and negative charges, making them versatile in a variety of applications. (23) The combined presence of surfactants and antibiotics in water bodies has increased manifold in recent decades due to their broad usage. Their synergistic coaction can increase the bioavailability of antibiotics, leading to higher levels of these drugs in aquatic systems. This can further exacerbate the development of antibiotic-resistant bacteria and disrupt the natural equilibrium of microbial communities. Therefore, understanding the interactions between surfactants and antibiotics is essential in developing effective strategies to mitigate their impact on water quality and ecosystem health, thereby protecting the environment and preventing potential health risks for both aquatic organisms and humans.
Numerous experimental investigations have been performed to probe the interactions between antibiotics and surfactants. Rahman et al. (24) experimentally investigated the interaction of ceftriaxone sodium trihydrate (CFT) drug with ionic and nonionic surfactants. The micellization of tetradecyltrimethylammonium bromide (TTAB) with CFT was studied based on conductivity measurements, whereas Triton X-100 (TX-100) and Tween 80 with CFT were based on cloud point measurement. The conductivity was measured by varying the concentration of the surfactants. A breakdown in the linearly increased conductivity is observed after a certain concentration, indicating micelles formation. Based on the conductivity results, on incorporating the CFT drug (at a concentration of 0.5 mmol/kg), the critical micellar concentrations (CMC) of the TTAB system were increased to 4.56 from 3.7 mmol/kg of pure TTAB system at 310 K. The nature of hydration (i.e., hydrophilic or hydrophobic) was hypothesized based on increased or decreased CMC values at different temperatures. Similarly, the interaction of CFT with TX-100 was explored based on the cloud point conditions (TCP-temperature at which cloud point is observed) at different surfactant and CFT concentrations. The TCP was found in the range of 332.43–339.75 K at the concentrations range of 1–10% (w/w) in the aqueous phase. A marginal increase in TCP of TX100 in the presence of CFT indicated the formation of micelle aggregation carrying the drug. The interaction of CFT with Tween 80 was investigated using the UV spectrophotometric method. It was found that the maximum wavelength of pure CFT increases in the presence of Tween 80, indicating the existence of interaction between them. Similar work (25) was also conducted by the same group on the interaction of the same drug (CFT) with the CTAB. In this study, the intermolecular interaction was investigated based on the micellization of surfactant in the aqueous phase in the presence of the drug at different CTAB concentrations. In another analogous work by Ahsan et al., (26) the interaction between moxifloxacin hydrochloride (MFH) with SDS was studied. The interaction was elucidated by measuring the CMC based on the conductivity at different surfactant concentrations and temperatures in the aqueous phase. It was observed that on increasing the surfactant concentration, the conductivity was initially increased and then decreased due to the formation of the surfactant assembly. The conductivity results showed that with the incorporation of MFH, the CMC of SDS was found to be almost 1.5 times more than that of pure SDS in the aqueous phase. Moreover, the CMC was also measured as a function of temperature in the range of 298–318 K for the SDS/MFH system, and it was found that the CMC decreases with an increase in temperature due to the reduction of the hydration effects. Pathania et al. (27) experimentally investigated the interaction between cefepime drug and CTAB and DTAB surfactants using densimetry and sound speedometry. The CMC of surfactants was determined based on the speed of sound data at different temperatures. The encapsulation of drugs in surfactants and their intermolecular interaction were evident in the Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR) results. (27) Rub et al. (17) reported the micellization behavior of amitriptyline hydrochloride (AMT) and a surfactant triton X-45 (TX-45). The CMC was measured based on the effect on surface tension at different concentrations of surfactant in drug-surfactant mixtures. The amount of surfactant added at which no further reduction in surface tension was observed was considered as a CMC of the system. It was observed that the CMC of the mixed drug-surfactant system in the aqueous phase was found to be lesser than the pure surfactant system. (17) On increasing the concentration of the drug, the CMC was significantly reduced, which indicated the micellization of the mixed process via attractive interactions. The micellization of surfactant in the system was also confirmed based on the obtained activity coefficients. The surfactant activity coefficients were less than the AMT activity coefficients, indicating the presence of a high amount of surfactant in the mixed micellar system. Their findings suggested that the TX-45 surfactant can facilitate the delivery of the AMT drug and enhance its bioavailability. Similar observations were also made in the study reported by Kumar et al. (28) for a system of ibuprofen with surfactants, hexadecyltrimethylammonium bromide (HTAB), and a gemini surfactant. The interaction of ibuprofen with both surfactants was investigated and compared based on the surface and thermodynamic properties of the systems. Based on the parametric investigations, it was found that the hydrophobicity of the ibuprofen-gemini surfactant mixture was more than the ibuprofen-HTAB mixture, indicating the favorable micellization and more interaction of the ibuprofen-gemini surfactant.
Several experimental and theoretical investigations have been performed employing surfactant bilayers as well. (29) Zhang et al. (30) performed molecular dynamics (MD) simulations of a bilayer consisting of SDS molecules in conjunction with a solid surface. Their work proposed a potential mechanism for the creation of curved SDS bilayers. The aggregates undergo a transition from curved bilayers to planar bilayers, to perforated bilayers, and finally to micelles as the cross-sectional area increases. Tang et al. (31) performed MD simulations of preassembled SDS micelles utilizing several force fields, namely, GROMOS, CHARMM36, OPLS-AA, and OPLS-UA. These simulations were performed for varying aggregation numbers and box sizes. The variation in force fields was found to have a minimal impact on the overall micelle structure of small aggregates with sizes of 60 or 100. However, for micelles with an aggregation number of 300 or greater, bicelle structures with organized tails were observed instead of the more realistic rodlike micelles with disordered tails.
Reverse micelles have garnered significant interest in recent decades as an innovative approach to the separation and purification of antibiotics. This is due to their ability to create a unique microenvironment within the organic medium. (32) Zhang et al. (33) conducted a study on the separation of bovine serum albumin (BSA) using a cationic surfactant, i.e., CTAB. At a pH of 7.4, which is higher than the isoelectric point of BSA, a maximum recovery ratio of 80.5% was achieved. Xu et al. (34) conducted a study on the adsorption of BSA in a foam fractionation process utilizing cationic, anionic, and nonionic surfactants. Their research showed that in the presence of both cationic and anionic surfactants, BSA migrated to the gas–liquid interface owing to a strong attraction.
Boukhelkhal et al. (32) worked on the experimental investigations to remove amoxicillin from aqueous solution using the anionic surfactant, i.e., SDS. The study unravels the optimal adsorption kinetic parameters, such as contact time, pH, temperature, and initial concentration of SDS. A removal efficiency of 87.7% of amoxicillin was observed by the authors. Huang et al. (35) developed a two-stage batch foam fractionator to recover creatine from wastewater using the SDS. The authors observed optimum operating conditions with an enrichment ratio of 3:1, leading to a recovery percentage of 70.6. Ghosh et al. (7) worked on the adsorption of fluoroquinolone antibiotics by employing a semibatch foam fractionation process. In their work, two surfactants were considered, i.e., cationic CTAB and anionic SDS. It was observed that ciprofloxacin partitions to the gas–liquid interface more readily in the presence of SDS compared to CTAB. The authors found higher removal efficiency with SDS (96.3%) in comparison to CTAB (52%) for the ciprofloxacin.
To briefly summarize the above discussion, the potential synergistic or antagonistic effects between surfactants and antibiotics must be thoroughly investigated to predict the effects caused by the interaction of these components in our water bodies. Understanding how different surfactants interact with specific antibiotics can lead to the development of more effective methods to isolate antibiotics efficiently. To handle this issue, we probed the SDS/CTAB bilayer interactions with the ciprofloxacin/azithromycin drugs. We have thoroughly analyzed the tilt angle, rotational angle of the surfactants, hydrogen bonds between drug and surfactant, density profile of drugs, electrostatic potential, and charge density along the bilayers. Our results explain the atomistic interaction mechanisms between the surfactants and antibiotics, which is lacking in the existing literature.

2. Computational Details

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Within this study, the DESMOND (36) simulation engine was used within Schrödinger simulation software. (37) The bonded and nonbonded interactions were described using the Optimized Parameters for Liquid Simulations (OPLS4) (38) force fields. Temperature control was accomplished using a Nose-Hoover Chain thermostat. Pressure control was applied using the Martyna-Tobias-Klein semi-isotropically at 300 K and 1.0 atm, and the relaxation times for the thermostat and barostat were 1 and 2.0 ps, respectively. A time step of 2 fs was used for the OPLS4 force field simulations. We performed 100 ps of Brownian minimization followed by 0.1 ns of NVT MD run (T = 10 K and time step = 1 fs) to slowly introduce the dynamics. Later, 20 ns of the NPγT run was performed (γ is surface tension) at 300 K and 1 atm. We have considered γ = 33.0 mN m–1 (39,40) for SDS and γ = 36.5 mN m–1 (41) for the CTAB bilayer.
Surfactant molecules were placed in a way that avoids atomic overlap, ensuring an initial configuration as shown in Figure 1. Atomic overlap was defined based on the van der Waals radii of the atoms, a theoretical hard sphere that represents the distance of closest approach between two atoms. The packing efficiency factor was used to uniformly scale the van der Waals radii of the atoms. This quantity directly impacts the density of the output structure. We used the packing efficiency factor of 0.6. We considered two drugs, mainly hydrophilic and hydrophobic drugs, i.e., ciprofloxacin and azithromycin. 4–8 wt % of the drug in aqueous solution was considered. Ciprofloxacin and azithromycin drugs were added to the surfactant bilayer, as shown in Figures 2a and 3a. Table 1 lists the number of molecules and box sizes considered for each simulation performed in this study.

Figure 1

Figure 1. (a) shows the snapshot of the MD run of SDS bilayer in an aqueous environment (b) shows the snapshot of the MD run of CTAB bilayer in an aqueous environment. Sulfur of SDS surfactant is shown in yellow color visualized in CPK model. Nitrogen atoms of CTAB surfactants are shown in blue color visualized in the CPK model. Water molecules are visualized in the ball and stick model. Gray color denotes the hydrophobic tail of surfactant.

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Figure 2

Figure 2. (a) Snapshot of the MD run of SDS Bilayer in an aqueous environment along with ciprofloxacin before the NPT run. (b) Snapshot of the MD run of SDS bilayer in an aqueous environment along with ciprofloxacin after the NPT run. Sulfur of the SDS surfactant is shown in yellow color visualized in the CPK model. Ciprofloxacin is visualized in the CPK model. Water molecules are visualized in the ball and stick model.

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Figure 3

Figure 3. (a) Snapshot of the MD run of CTAB Bilayer in an aqueous environment along with azithromycin before the NPT run. (b) Snapshot of the MD run of CTAB Bilayer in an aqueous environment along with azithromycin after the NPT run. Nitrogen atom of the CTAB surfactant is shown in blue color visualized in CPK model. Azithromycin is visualized in the CPK model. Water molecules are visualized in ball and stick model.

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Table 1. Number of Molecules and Box Sizes Considered for Each Simulation Run Performed in This Study
no. of surfactants moleculesno. of ciprofloxacinno. of azithromycinno. of water moleculesbox size before NPT run (Å3)box size after NPT run (Å3)
250 (SDS)  496252.05 × 52.16 × 102.0095.54 × 95.74 × 27.95
250 (SDS)10 496252.05 × 52.16 × 127.1597.16 × 97.36 × 27.67
250 (SDS) 5496252.05 × 52.16 × 135.1498.33 × 98.53 × 26.99
228 (CTAB)  239452.08 × 52.17 × 82.0085.75 × 85.90 × 28.85
228 (CTAB)10 239452.08 × 52.17 × 107.1587.48 × 87.64 × 28.33
228 (CTAB) 5239452.08 × 52.17 × 116.1886.37 × 86.52 × 29.27

3. Results and Discussion

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3.1. Surfactant Tilt and Rotational Angles

The distribution of the tilt and rotational angles provides information about the alignment or randomness of surfactants. The surfactant tilt angle and rotational angle are shown in Figure 4a,b. The tilt angle (θ) of a surfactant is the angle between the normal vector of a plane representing the surface and the vector extending from the surface to the hydrophobic end of the surfactant, indicating the orientation of the surfactant. The surface plane can be defined using a variety of approaches. In this work, the best fit for specific atoms is considered as shown in the yellow highlighted region in Figure 4 in the tilt angle schematic. In order to determine the rotational angle (φ) of a surfactant, the orientation vector is projected onto the surface plane. The angle between the projected vector and the x-axis represents the rotational angle of the surfactant.

Figure 4

Figure 4. Schematic representation of the calculated (a) Surfactant tilt angle and (b) Surfactant rotation angle. θ represents tilt angle and φ represents rotational angle.

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The time series surfactant tilt angle and rotational angle of the pure SDS and CTAB are shown in Figure 5a,b. Frequency distribution versus tilt/rotational angle is analyzed over 20 ns simulation time. The time-averaged tilt angle and rotational angle are listed in Table 2. The surfactant tilt angle of pure SDS surfactant in aqueous solution is compared with the presence of SDS and antibiotics in water. The tilt angle of the pure cationic SDS is found to be 47.416°, whereas a slight change is observed in the presence of both drugs as presented in Table 2. On the other hand, the tilt angle of the anionic CTAB is found to be independent of both the selected drugs, i.e., ca. 41°. The ciprofloxacin is a hydrophilic drug and it interacts with the hydrophilic head of the surfactant. This is the reason the tilt angle changes are minimum in the absence and presence of ciprofloxacin. It is interesting to note that the pronounced changes in the rotational angle are visible in the presence of both the drugs. We observed an interaction between the azithromycin hydrophobic drug and the hydrophobic tail of the SDS surfactant, due to which the rotational angle is changed from 89.114 to 90.353° corresponding to the change from pure SDS to SDS+Azi, whereas in the presence of ciprofloxacin, the rotational angle is found to be 90.463°. Furthermore, we noticed a decrease in rotational angle for the CTAB surfactant, where the rotational angle is changed from 92.443 to 87.453° corresponding to the change from pure CTAB to CTAB+Azi, whereas the rotational angle is found to be 92.885° in the presence of ciprofloxacin.

Figure 5

Figure 5. (a) Surfactant tilt and rotational angles of pure SDS versus time in aqueous solution. (b) Surfactant tilt and rotational angles of pure CTAB versus time in aqueous solution. Black line represents the tilt angle and the red line represents the rotational angle.

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Table 2. Calculated Surfactant Tilt and Rotational Angle for the Considered Systems along with the Standard Deviation of Time Series for Both the Tilt Angle and Rotational Angle Are Shown in the Table
systemtilt angle (°)tilt angle standard deviation (°)rotational angle (°)rotational angle standard deviation (°)
Pure SDS47.4166.7389.1143.56
SDS+Cipro45.7716.9890.4633.36
SDS+Azi46.2446.2290.3533.30
Pure CTAB40.3003.1992.4433.50
CTAB+Cipro40.9664.0892.8853.25
CTAB+Azi41.9017.7487.4534.65

3.2. Electrostatic Potential and Charge Density

Ciprofloxacin being zwitterionic, it is important to probe the electrostatic potential. By utilizing the charge density, one can examine the configuration of charges within the system. Electrostatic potential measures the quantity of work required to transfer a unit charge from one reference point to another in the electric field generated by the surrounding environment. It is calculated directly from the charge density. In bilayer systems, the electrostatic potential ϕ is determined by calculating its value at different radial distances from a reference point which serves as the center of the trajectory. The reference point (r = 0) is established as the central location of the bilayer, and the electrostatic potential is then determined by integrating the electric field radially.
ϕ(r)=∫r∞dr′E(r′)
(1)
The radial electric field E(r) is defined as follows:
E(r)=14πr2ϵsϵ0∫0rdr′4πr′2ρ(r′)
(2)
The symbol ϵs represents the relative permittivity of the solvent, ϵ0 represents the vacuum permittivity, and ρ represents the charge density. The calculation of the charge density relies on the formal charge assigned to each individual atom. Atoms that have a formal charge of 0 are ignored. The electrostatic potential is nullified at radial distances far from the center of the bilayer.
For interfaces that are flat and perpendicular to the z-direction, the trajectory is segmented into bins of a thickness along the xy-plane. The charge density is obtained by averaging the ion distribution from each bin and then integrating it along the z-direction to calculate the electrostatic potential. The reference is the xy-plane located within the central region of the solvent. The expression for the electrostatic potential ϕ is formulated as
ϕ(z)=−12ϵsϵ0∫zzmdz′(zm−z′)ρ(z′)
(3)
where zm is the z value of the reference plane. The electrostatic potential is defined as zero in the reference plane.
Figure 6a displays the charge density as a function of the distance from the center of the solvent in the z dimension, which is perpendicular to the SDS bilayer. The value of z = 0 represents the centrality of the bulk solution, determined by the aqueous solvent in the calculations. This central point is occupied by positively charged sodium ions. The profile exhibits approximate symmetry with respect to z = 0 due to the symmetric nature of the bilayer geometry. As we move away from z = 0 in both directions (See Figure 6a), the average charge density in the region corresponding to the negatively charged sulfate head groups becomes negative. This charge density becomes less negative in the presence of the ciprofloxacin. This is attributed to stronger interactions between the hydrophilic head of the surfactant and ciprofloxacin drug. Continuing deeper inside the bilayer, the charge becomes somewhat positive as a result of a smaller number of sodium ions that enter the surface of the bilayer as shown in Figure 6a. Ultimately, the charge density reaches zero within the uncharged alkane tails. Figure 6b illustrates the relationship between the charge density and distance from the center of the solvent in the z dimension, which is perpendicular to the CTAB bilayer. The central point of the bulk solution is occupied by bromide ions, which have a negative charge. As we move away from z = 0 in both directions, the average charge density in the region corresponding to the positively charged nitrate head groups becomes positive. As one moves further into the bilayer, the charge becomes increasingly negative, because fewer bromide ions are able to enter the surface of the bilayer.

Figure 6

Figure 6. Charge density profile across the bilayer surfactant as a function of z-coordinate, which measures the distance from the midplane of the bilayer along the normal direction. (a) pure SDS, SDS+Cipro and SDS+Azi (b) pure CTAB, CTAB+Cipro and CTAB+Azi. Black line represents charge density of pure surfactant in aqueous solution, red line represents charge density of surfactant + ciprofloxacin in aqueous solution, and blue line represents charge density of surfactant + azithromycin in aqueous solution.

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Figure 7 shows the electrostatic potential along the z dimension, relative to the reference position at z = 0. As mentioned before, the electrostatic potential is the amount of work needed to move a unit charge from a reference point to a second point within an electric field. Therefore, the electrostatic potential at the reference point z = 0 is, by definition, 0. Moving within the first 5 Å on either side of the reference point, the electrostatic potential is greater than 0, indicating that work must be performed in order to move a charge from the reference position. The electrostatic potential reaches a minimum in the region corresponding to the negatively charged sulfate groups as shown in Figure 7a. A single positive charge would be more favorable in this position. Finally, the electrostatic potential levels off at a negative value in the region of the alkane tails. Both electrostatic and steric barriers along the way hinder charge diffusion from the highly charged region around the reference point for SDS. Figure 7b shows the electrostatic potential along the z dimension, relative to the reference position at z = 0. Moving within the first 5 Å on either side of the reference point, the electrostatic potential is less than 0, indicating that less work must be done in order to move a charge from the reference position. The electrostatic potential reaches a maximum in the region corresponding to the positively charged nitrate groups. A single negative charge would be more favorable in this position. Finally, the electrostatic potential levels off at a positive value in the region of the alkane tails for CTAB. Due to the strong interaction of the alkane tails of CTAB with azithromycin, we observe a lesser value than the maximum as shown in the blue line in Figure 7b. The hydrophobic attraction between the large aromatic ring of the quinoline and the extended alkyl chain of the surfactant also plays a role in the binding of the surfactant and drugs. In the SDS-ciprofloxacin system, the carboxyl acid group and carbonyl group created electrostatic repulsion, which shielded the SDS monomers from the quinoline.

Figure 7

Figure 7. Electrostatic potential profile across the bilayer surfactant as a function of z-coordinate, which measures the distance from the midplane of the bilayer along the normal direction. (a) pure SDS, SDS+Cipro and SDS+Azi (b) pure CTAB, CTAB+Cipro and CTAB+Azi. Black line represents electrostatic potential of pure surfactant in aqueous solution, red line represents electrostatic potential of surfactant + ciprofloxacin in aqueous solution, and blue line represents electrostatic potential of surfactant + azithromycin in aqueous solution.

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3.3. H-Bond Interactions

The H-bond analysis is crucial to understand the nonbonded interactions between surfactants and drugs. The hydrogen bonds can be formed between the hydrogen of the hydroxyl group of the drug and with oxygen of surfactants. The hydrogen bond mechanism between ciprofloxacin-SDS and azithromycin-SDS is shown in Figure 8a, b. Based on the experimental evidence, it was observed that the SDS is more efficient in removing ciprofloxacin than CTAB. (7) This can be attributed to the formation of the hydrogen bonds between ciprofloxacin and SDS as observed in our study. No hydrogen bonds were observed between CTAB and ciprofloxacin/azithromycin in our study. This can be attributed to the fact that CTAB is known to have neither hydrogen donors nor acceptors. (42) However, head groups of CTAB can be hydroxylated (43) to form hydrogen bonds with substrate molecules. The hydrogen bond time series for ciprofloxacin-SDS and azithromycin-SDS are shown in Figure 9 (top) & (bottom). It is evident from the figure that the number of hydrogen bonds between SDS and ciprofloxacin is in the range of 2 to 10, whereas it is in the range of 1 to 8 between SDS and azithromycin.

Figure 8

Figure 8. (a) H-bond formation between SDS and ciprofloxacin (b) shows the H-bond formation between SDS and azithromycin. Yellow dashed line represents the hydrogen bond. Drug and SDS surfactant molecules are visualized in a ball and stick model.

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Figure 9

Figure 9. (Top) Number of the H-bonds between SDS and ciprofloxacin pair versus simulation time. (Bottom) Number of the H-bonds between SDS and azithromycin pair versus simulation time.

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3.4. Drug Density Profiles

Density profiles show the drug distribution within the surfactant bilayer, and it is crucial to understand the localization of the drug at the head/tail of the surfactant. The density profile is determined by dividing the object into layers of a set thickness that are perpendicular to a z-axis. In order to determine the density profile at a specific point along the axis (defined as the coordinate at the bottom of the layer), the fraction of the van der Waals volume of each atom that intersects with the layer is multiplied by its atomic mass. The resulting values are then added together for all specified atoms and divided by the volume of the layer.
Figure 10a shows the ciprofloxacin density profile in SDS and CTAB bilayers, and Figure 10b shows the azithromycin density profile in SDS and CTAB bilayers. It is evident from Figure 10a that the ciprofloxacin is localized within 40 Å of the SDS bilayer along the z-axis, whereas the ciprofloxacin is delocalized within 50 Å in the z-axis for the case of the Cipro+CTAB system. This can be attributed to the presence of hydrogen bond interactions between ciprofloxacin and the hydrophilic sulfate group. The delocalization can be attributed to the absence of hydrogen bond interaction between ciprofloxacin and CTAB. Figure 10b shows that the density distribution of the azithromycin in SDS is in the range of 60 Å which can be attributed to the large molecule size of the azithromycin. Azithromycin in CTAB is observed to be localized within 45 Å along the z-axis due to the dominant interaction between the CTAB hydrophobic tail and hydrophobic azithromycin.

Figure 10

Figure 10. (a) Shows the ciprofloxacin density profile as a function of z-axis in SDS and CTAB in an aqueous environment. (b) Shows the azithromycin density profile as a function of z-axis in SDS and CTAB in an aqueous environment.

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The synergistic interaction of two amphiphiles is facilitated by various types of molecular interactions. These include electrostatic repulsion between two charged groups, steric repulsion between bulky hydrophilic and hydrophobic groups, ion-dipole interaction between ionic hydrophilic groups, and hydrogen bonding. However, experiments suggest that the electrostatic contacts between the negatively charged surface of SDS micelles and the localized positive charge on the nitrogen atom of moxifloxacin are stronger compared to the interactions between the positively charged surface of dodecyl-trimethylammonium bromide micelles. (44) It is important to understand the steric effects, as well. We have analyzed how diffusion rates (45) are kinetically hindered due to the steric effects of the hydrophilic head of the surfactant in the Supporting Information.
The calculated diffusion rates are summarized in Table S1. It can be clearly seen that the mobility of the azithromycin, i.e., 1.425 × 10–10 m2/s is twice higher than the ciprofloxacin, i.e., 0.786 × 10–10 m2/s. This can be attributed to the higher number of average H-bonds between SDS-ciprofloxacin than SDS-azithromycin. Enhanced H-bonds interactions along with the steric effect of the hydrophilic head of the surfactant slow down the mobility of the ciprofloxacin. On the other hand, both drug molecules diffuse at almost the same rate along the z-axis of the CTAB bilayer, which can be attributed to the steric effects. Overall, the drug diffusion rate is lower in CTAB in comparison to the SDS bilayer.

4. Conclusions

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This work provides atomistic insights into the interaction between ciprofloxacin and azithromycin antibiotics with the SDS/CTAB bilayer using molecular dynamics simulations. Ciprofloxacin being a zwitterionic compound exhibits a strong molecular interaction with SDS. Also, ciprofloxacin being a hydrophilic drug interacts with the hydrophilic head of the surfactant. On the contrary, the hydrophobic azithromycin interacts with the hydrophobic tail of the surfactant. Our results show that the ciprofloxacin can be separated more effectively in the presence of SDS than CTAB. This conclusion is drawn based on the density profiles along the bilayers, rotational angle of surfactants, and dominant hydrogen bond interaction between SDS and ciprofloxacin. The anionic surfactant SDS is observed to have a better influence on the adsorption of ciprofloxacin than CTAB, whereas the cationic surfactant CTAB exhibits a more pronounced effect on the adsorption of hydrophobic azithromycin compared to SDS. Furthermore, it is evident from our study that the presence of the drugs alters the SDS/CTAB surfactant tilt angle and rotational angle in comparison to pure SDS/CTAB bilayer. A pronounced localization of the ciprofloxacin drug is observed within the SDS bilayers based on the density profiles along the z-axis. To summarize, our approach can be generalized to understand the molecular interaction of emerging pharmaceutical pollutants with surfactants.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c04673.

  • Diffusion rates of ciprofloxacin and azithromycin along the z-direction of the bilayer (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Sriprasad Acharya - Department of Chemical Engineering, Manipal Institute of Technology (MIT), Manipal Academy of Higher Education (MAHE), Manipal, Karnataka 576104, India;  Orcidhttps://orcid.org/0000-0002-8897-9940
    • Jitendra Carpenter - Department of Chemical Engineering, Manipal Institute of Technology (MIT), Manipal Academy of Higher Education (MAHE), Manipal, Karnataka 576104, India;  Orcidhttps://orcid.org/0000-0002-7216-7227
    • Muddu Madakyaru - Department of Chemical Engineering, Manipal Institute of Technology (MIT), Manipal Academy of Higher Education (MAHE), Manipal, Karnataka 576104, India;  Orcidhttps://orcid.org/0000-0002-2518-5190
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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A.K.V. would like to thank Schrödinger Centre for Molecular Simulations, MAHE, Manipal for their support.

References

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This article references 45 other publications.

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    A Collection of Papers in Honour of Nikolay Churaev on the Occasion of his 80th Birthday.

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  • Abstract

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    Figure 1

    Figure 1. (a) shows the snapshot of the MD run of SDS bilayer in an aqueous environment (b) shows the snapshot of the MD run of CTAB bilayer in an aqueous environment. Sulfur of SDS surfactant is shown in yellow color visualized in CPK model. Nitrogen atoms of CTAB surfactants are shown in blue color visualized in the CPK model. Water molecules are visualized in the ball and stick model. Gray color denotes the hydrophobic tail of surfactant.

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    Figure 2

    Figure 2. (a) Snapshot of the MD run of SDS Bilayer in an aqueous environment along with ciprofloxacin before the NPT run. (b) Snapshot of the MD run of SDS bilayer in an aqueous environment along with ciprofloxacin after the NPT run. Sulfur of the SDS surfactant is shown in yellow color visualized in the CPK model. Ciprofloxacin is visualized in the CPK model. Water molecules are visualized in the ball and stick model.

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    Figure 3

    Figure 3. (a) Snapshot of the MD run of CTAB Bilayer in an aqueous environment along with azithromycin before the NPT run. (b) Snapshot of the MD run of CTAB Bilayer in an aqueous environment along with azithromycin after the NPT run. Nitrogen atom of the CTAB surfactant is shown in blue color visualized in CPK model. Azithromycin is visualized in the CPK model. Water molecules are visualized in ball and stick model.

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    Figure 4

    Figure 4. Schematic representation of the calculated (a) Surfactant tilt angle and (b) Surfactant rotation angle. θ represents tilt angle and φ represents rotational angle.

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    Figure 5

    Figure 5. (a) Surfactant tilt and rotational angles of pure SDS versus time in aqueous solution. (b) Surfactant tilt and rotational angles of pure CTAB versus time in aqueous solution. Black line represents the tilt angle and the red line represents the rotational angle.

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    Figure 6

    Figure 6. Charge density profile across the bilayer surfactant as a function of z-coordinate, which measures the distance from the midplane of the bilayer along the normal direction. (a) pure SDS, SDS+Cipro and SDS+Azi (b) pure CTAB, CTAB+Cipro and CTAB+Azi. Black line represents charge density of pure surfactant in aqueous solution, red line represents charge density of surfactant + ciprofloxacin in aqueous solution, and blue line represents charge density of surfactant + azithromycin in aqueous solution.

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    Figure 7

    Figure 7. Electrostatic potential profile across the bilayer surfactant as a function of z-coordinate, which measures the distance from the midplane of the bilayer along the normal direction. (a) pure SDS, SDS+Cipro and SDS+Azi (b) pure CTAB, CTAB+Cipro and CTAB+Azi. Black line represents electrostatic potential of pure surfactant in aqueous solution, red line represents electrostatic potential of surfactant + ciprofloxacin in aqueous solution, and blue line represents electrostatic potential of surfactant + azithromycin in aqueous solution.

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    Figure 8

    Figure 8. (a) H-bond formation between SDS and ciprofloxacin (b) shows the H-bond formation between SDS and azithromycin. Yellow dashed line represents the hydrogen bond. Drug and SDS surfactant molecules are visualized in a ball and stick model.

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    Figure 9

    Figure 9. (Top) Number of the H-bonds between SDS and ciprofloxacin pair versus simulation time. (Bottom) Number of the H-bonds between SDS and azithromycin pair versus simulation time.

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    Figure 10

    Figure 10. (a) Shows the ciprofloxacin density profile as a function of z-axis in SDS and CTAB in an aqueous environment. (b) Shows the azithromycin density profile as a function of z-axis in SDS and CTAB in an aqueous environment.

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      Levy, Stuart B.; Marshall, Bonnie
      Nature Medicine (New York, NY, United States) (2004), 10 (12, Suppl.), S122-S129CODEN: NAMEFI; ISSN:1078-8956. (Nature Publishing Group)
      A review. The optimism of the early period of antimicrobial discovery has been tempered by the emergence of bacterial strains with resistance to these therapeutics. Today, clin. important bacteria are characterized not only by single drug resistance but also by multiple antibiotic resistance: the legacy of past decades of antimicrobial use and misuse. Drug resistance presents an ever-increasing global public health threat that involves all major microbial pathogens and antimicrobial drugs.
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    3. 3
      Laxminarayan, R.; Chaudhury, R. R. Antibiotic resistance in India: drivers and opportunities for action. PLoS medicine 2016, 13, e1001974  DOI: 10.1371/journal.pmed.1001974
      There is no corresponding record for this reference.
    4. 4
      Mangla, D.; Annu; Sharma, A.; Ikram, S. Critical review on adsorptive removal of antibiotics: Present situation, challenges and future perspective. Journal of Hazardous Materials 2022, 425, 127946  DOI: 10.1016/j.jhazmat.2021.127946
      4
      Critical review on adsorptive removal of antibiotics: Present situation, challenges and future perspective
      Mangla, Divyanshi; Annu; Sharma, Atul; Ikram, Saiqa
      Journal of Hazardous Materials (2022), 425 (), 127946CODEN: JHMAD9; ISSN:0304-3894. (Elsevier B.V.)
      A review. This gives a proper dedicated understanding of the contamination level, sources, and biol. dangers related with different classes of antibiotics in consumable water. The literature on the adsorption of antibiotics is relatively uncommon and developments are still under progression, esp. for adsorbents other than activated carbon. Also, adsorption technique has already been applied vastly for water treatment. Notwithstanding significant progressions, designed natural wastewater treatment frameworks are just bearably effective (48-77%) in the expulsion of antibiotics. Hence, the compilation of available literature esp. for antibiotic adsorption was much needed. Moreover, the conventional adsorbents have some limitations of their own. In this study, the main focus was laid on unconventional adsorbents such as Biochar, Biopolymers, Carbon Nanotubes, Clays, Metal-Org. Frameworks, Microalgae and some misc. adsorbents. The mechanism of adsorption by the unconventional adsorbents includes electrostatic interactions, π-π bonding, weak Van der Waal forces, H-bonding and surface complexation, which was similar to that of conventional adsorbents and hence these unconventional adsorbents can easily replace the costlier conventional adsorbents with even better adsorption efficiency. This paper also briefly discussed the thermodn., adsorption equil.; isotherm and kinetics of adsorption. This paper seizes the crit. advances of adsorption phenomenon at various interfaces and lays the foundation for current scenario assocd. with further progress. Besides, this study would help in understanding the antibiotic adsorption, cost estn. and future goals that will attract the young the researchers of this field.
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    5. 5
      Schwartz, T.; Kohnen, W.; Jansen, B.; Obst, U. Detection of antibiotic-resistant bacteria and their resistance genes in wastewater, surface water, and drinking water biofilms. FEMS microbiology ecology 2003, 43, 325– 335,  DOI: 10.1111/j.1574-6941.2003.tb01073.x
      5
      Detection of antibiotic-resistant bacteria and their resistance genes in wastewater, surface water, and drinking water biofilms
      Schwartz, Thomas; Kohnen, Wolfgang; Jansen, Bernd; Obst, Ursula
      FEMS Microbiology Ecology (2003), 43 (3), 325-335CODEN: FMECEZ; ISSN:0168-6496. (Elsevier Science B.V.)
      In view of the increasing interest in the possible role played by hospital and municipal wastewater systems in the selection of antibiotic-resistant bacteria, biofilms were studied using enterococci, staphylococci, Enterobacteriaceae, and heterotrophic bacteria as indicator organisms. Biofilms were also studied in drinking water from river bank filtrate to est. the occurrence of resistant bacteria and their resistance genes, thus indicating possible transfer from wastewater and surface water to the drinking water distribution network. Vancomycin-resistant enterococci were characterized by antibiograms, and the vanA resistance gene was detected by mol. biol. methods, including PCR. The vanA gene was found not only in wastewater biofilms but also in drinking water biofilms in the absence of enterococci, indicating possible gene transfer to autochthonous drinking water bacteria. The mecA gene encoding methicillin resistance in staphylococci was detected in hospital wastewater biofilms but not in any other compartment. Enterobacterial ampC resistance genes encoding β-lactamase activities were amplified by PCR from wastewater, surface water and drinking water biofilms.
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    6. 6
      Bouki, C.; Venieri, D.; Diamadopoulos, E. Detection and fate of antibiotic resistant bacteria in wastewater treatment plants: a review. Ecotoxicology and environmental safety 2013, 91, 1– 9,  DOI: 10.1016/j.ecoenv.2013.01.016
      6
      Detection and fate of antibiotic resistant bacteria in wastewater treatment plants: A review
      Bouki, Chryssa; Venieri, Danae; Diamadopoulos, Evan
      Ecotoxicology and Environmental Safety (2013), 91 (), 1-9CODEN: EESADV; ISSN:0147-6513. (Elsevier B.V.)
      A review. Antibiotics are among the most successful group of pharmaceuticals used for human and veterinary therapy. However, large amts. of antibiotics are released into municipal wastewater due to incomplete metab. in humans or due to disposal of unused antibiotics, which finally find their ways into different natural environmental compartments. The emergence and rapid spread of antibiotic resistant bacteria (ARB) has led to an increasing concern about the potential environmental and public health risks. ARB and antibiotic resistant genes (ARGs) have been detected extensively in wastewater samples. Available data show significantly higher proportion of antibiotic resistant bacteria contained in raw and treated wastewater relative to surface water. According to these studies, the conditions in wastewater treatment plants (WWTPs) are favorable for the proliferation of ARB. Moreover, another concern with regards to the presence of ARB and ARGs is their effective removal from sewage. This review gives an overview of the available data on the occurrence of ARB and ARGs and their fate in WWTPs, on the biol. methods dealing with the detection of bacterial populations and their resistance genes, and highlights areas in need for further research studies.
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    7. 7
      Ghosh, R.; Hareendran, H.; Subramaniam, P. Adsorption of Fluoroquinolone Antibiotics at the Gasâ“Liquid Interface Using Ionic Surfactants. Langmuir 2019, 35, 12839– 12850,  DOI: 10.1021/acs.langmuir.9b02431
      There is no corresponding record for this reference.
    8. 8
      Antonelli, R.; Malpass, G. R. P.; da Silva, M. G. C.; Vieira, M. G. A. Adsorption of ciprofloxacin onto thermally modified bentonite clay: Experimental design, characterization, and adsorbent regeneration. Journal of Environmental Chemical Engineering 2020, 8, 104553  DOI: 10.1016/j.jece.2020.104553
      There is no corresponding record for this reference.
    9. 9
      Attallah, O. A.; Al-Ghobashy, M. A.; Nebsen, M.; Salem, M. Y. Adsorptive removal of fluoroquinolones from water by pectin-functionalized magnetic nanoparticles: process optimization using a spectrofluorimetric assay. ACS Sustainable Chem. Eng. 2017, 5, 133– 145,  DOI: 10.1021/acssuschemeng.6b01003
      9
      Adsorptive Removal of Fluoroquinolones from Water by Pectin-Functionalized Magnetic Nanoparticles: Process Optimization Using a Spectrofluorimetric Assay
      Attallah, Olivia A.; Al-Ghobashy, Medhat A.; Nebsen, Marianne; Salem, Maissa Y.
      ACS Sustainable Chemistry & Engineering (2017), 5 (1), 133-145CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      The efficiency of adsorption of two photosensitive fluoroquinolones; Ciprofloxacin (CIP) and Moxifloxacin (MOX) on the surface of synthesized magnetite/pectin nanoparticles (MPNPs) and magnetite/silica/pectin nanoparticles (MSPNPs) was studied from aq. soln. under varying exptl. conditions. A validated spectrofluorimetric assay was developed for monitoring of CIP and MOX intact drugs and their photodegraded mols. concns. To optimize the working conditions which influence the drugs sorption, a 24 full factorial exptl. design was implemented. The max. percentage of removal was attained as 89% (Type of sorbent = MSPNPs, pH = 7.0, initial drug concn. =5 mg/L and contact time =30 min). The studied factors; pH, NPs loading, initial drug concn. and contact time were significant for both types of sorbents. The most significant variable was pH and the highest CIP and MOX adsorption occurred at pH = 7.0. Equil. isotherm data were fitted to Langmuir, Freundlich and Sips equations and the Sips model showed the best fit with equil. isotherm data. Furthermore, pseudo-first and pseudo-second order kinetic models were used to analyze sorption kinetics and it was found that adsorption of the investigated fluoroquinolones followed pseudo-second order kinetics. We believe that our synthesized NPs can be used as effective adsorbents for fluoroquinolones and their photodegraded mols. removal from aq. solns.
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    10. 10
      Cuerda-Correa, E. M.; Alexandre-Franco, M. F.; Fernández-González, C. Advanced oxidation processes for the removal of antibiotics from water. An overview. Water 2020, 12, 102,  DOI: 10.3390/w12010102
      There is no corresponding record for this reference.
    11. 11
      Cuprys, A.; Thomson, P.; Ouarda, Y.; Suresh, G.; Rouissi, T.; Kaur Brar, S.; Drogui, P.; Surampalli, R. Y. Ciprofloxacin removal via sequential electro-oxidation and enzymatic oxidation. Journal of Hazardous Materials 2020, 389, 121890  DOI: 10.1016/j.jhazmat.2019.121890
      There is no corresponding record for this reference.
    12. 12
      Khajavian, M.; Shahsavarifar, S.; Salehi, E.; Vatanpour, V.; Masteri-Farahani, M.; Ghaffari, F.; Tabatabaei, S. A. Ethylenediamine-functionalized ZIF-8 for modification of chitosan-based membrane adsorbents: Batch adsorption and molecular dynamic simulation. Chem. Eng. Res. Des. 2021, 175, 131– 145,  DOI: 10.1016/j.cherd.2021.08.033
      12
      Ethylenediamine-functionalized ZIF-8 for modification of chitosan-based membrane adsorbents: Batch adsorption and molecular dynamic simulation
      Khajavian, Mohammad; Shahsavarifar, Samaneh; Salehi, Ehsan; Vatanpour, Vahid; Masteri-Farahani, Majid; Ghaffari, Fahimeh; Tabatabaei, Seyed Ali
      Chemical Engineering Research and Design (2021), 175 (), 131-145CODEN: CERDEE; ISSN:1744-3563. (Elsevier B.V.)
      In the current investigation, the incorporation of ethylenediamine (EDA)-modified ZIF-8 (zeolitic imidazolate framework) nanostructures into the chitosan/polyvinyl alc. (CS/PVA) membrane adsorbents was explored for the elimination of reactive green 19 (RG) from aq. solns. Characterization of the synthesized EDA-modified ZIF-8 nanostructures and the membranes was carried out using XRD, SEM, BET and ATR-FTIR analyses. The CHN calcd. the deacetylation degree of chitosan as 93%. To attain the max. removal efficiency, three affecting operational parameters, including pH, adsorbent dosage, and initial concn. of dye, were optimized using the central composite design technique. The optimal conditions were resulted as; pH of 4, the adsorbent mass of 0.047 g, and the initial concn. of 70 mg L-1. Afterwards, the isothermal, kinetic, and thermodn. studies were done under the optimal conditions attained. The outcomes displayed that the equil. and rate of reactive green 19 adsorption on the CS/PVA/EDA-modified ZIF-8 membrane surface obeyed the Freundlich isotherm and the pseudo-second order kinetic model, resp. The adsorption capacity was found as 144.61 mg g-1. Furthermore, the entropy change (ΔS) and the enthalpy change (ΔH) for the adsorption were equal to 0.28 (kJ mol-1 K-1) and 4.83 (kJ mol-1), resp. Neg. values of ΔG° confirmed the spontaneity of the adsorption. Mol. dynamic simulation results showed that the interaction energies of CS/PVA/EDA-modified ZIF-8 membrane + RG system surpassed that of the CS/PVA + RG, which was in agreement with the adsorption performance results.
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    13. 13
      Nasrollahi, N.; Vatanpour, V.; Khataee, A. Removal of antibiotics from wastewaters by membrane technology: Limitations, successes, and future improvements. Science of The Total Environment 2022, 838, 156010  DOI: 10.1016/j.scitotenv.2022.156010
      13
      Removal of antibiotics from wastewaters by membrane technology: Limitations, successes, and future improvements
      Nasrollahi, Nazanin; Vatanpour, Vahid; Khataee, Alireza
      Science of the Total Environment (2022), 838 (Part_1), 156010CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
      A review. Antibiotics and related pharmaceuticals are applied to enhance public health and life quality. A major environmental concern is wastewaters from pharmaceutical industries, which contain significant amts. of antibiotics. Pharmaceutical industries apply conventional processes (biol., filtration, coagulation, flocculation, and sedimentation) for wastewater treatment, but these approaches cannot remove antibiotics completely. Moreover, unmetabolized antibiotics released by humans and animals are dangerous for municipal and effluent wastewater. Besides, antibiotic resistance is another challenge in treatment of wastewater for superbugs. This comprehensive study summarizes different techniques for antibiotic removal with an emphasis on membrane technol. in individual and hybrid systems such as chem., phys., biol., and conditional-based strategies. A combination of membrane processes with advanced oxidn. processes (AOPs), adsorption, and biol. treatments can be the right soln. for perfect removal. Furthermore, this review briefly compares different procedures for antibiotic removal, which can be helpful for further studies with their advantages and drawbacks.
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    14. 14
      Langbehn, R. K.; Michels, C.; Soares, H. M. Antibiotics in wastewater: From its occurrence to the biological removal by environmentally conscious technologies. Environ. Pollut. 2021, 275, 116603  DOI: 10.1016/j.envpol.2021.116603
      14
      Antibiotics in wastewater: From its occurrence to the biological removal by environmentally conscious technologies
      Langbehn, Rayane Kunert; Michels, Camila; Soares, Hugo Moreira
      Environmental Pollution (Oxford, United Kingdom) (2021), 275 (), 116603CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
      A review. In this crit. , we explored the most recent advances about the fate of antibiotics on biol. wastewater treatment plants (WWTP). Although the occurrence of these pollutants in wastewater and natural streams has been investigated previously, some recent publications still expose the need to improve the detection strategies and the lack of information about their transformation products. The role of the antibiotic properties and the process operating conditions were also analyzed. The pieces of evidence in the literature assoc. several mol. properties to the antibiotic removal pathway, like hydrophobicity, chem. structure, and electrostatic interactions. Nonetheless, the influence of operating conditions is still unclear, and solid retention time stands out as a key factor. Addnl., the efficiencies and pathways of antibiotic removals on conventional (activated sludge, membrane bioreactor, anaerobic digestion, and nitrogen removal) and emerging bioprocesses (bioelectrochem. systems, fungi, and enzymes) were assessed, and our concern about potential research gaps was raised. The combination of different bioprocess can efficiently mitigate the impacts generated by these pollutants. Thus, to plan and design a process to remove and mineralize antibiotics from wastewater, all aspects must be addressed, the pollutant and process characteristics and how it is the best way to operate it to reduce the impact of antibiotics in the environment.
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    15. 15
      Mishra, S.; Singh, A. K.; Cheng, L.; Hussain, A.; Maiti, A. Occurrence of antibiotics in wastewater: Potential ecological risk and removal through anaerobic-aerobic systems. Environmental Research 2023, 226, 115678  DOI: 10.1016/j.envres.2023.115678
      There is no corresponding record for this reference.
    16. 16
      Rasheed, T.; Shafi, S.; Bilal, M.; Hussain, T.; Sher, F.; Rizwan, K. Surfactants-based remediation as an effective approach for removal of environmental pollutantsâ“A review. J. Mol. Liq. 2020, 318, 113960  DOI: 10.1016/j.molliq.2020.113960
      There is no corresponding record for this reference.
    17. 17
      Rub, M. A.; Pulikkal, A. K.; Azum, N.; Asiri, A. M. The assembly of amitriptyline hydrochloride + triton X-45 (non-ionic surfactant) mixtures: Effects of simple salt and urea. J. Mol. Liq. 2022, 356, 118997  DOI: 10.1016/j.molliq.2022.118997
      There is no corresponding record for this reference.
    18. 18
      Lalthlengliani, J.; Gurung, J.; Pulikkal, A. K. Solubilization of aqueous-insoluble phenothiazine drug in TX-100 micellar solution and interactions of cationic/anionic surfactants with phenothiazine-TX-100 system. J. Mol. Liq. 2022, 354, 118823  DOI: 10.1016/j.molliq.2022.118823
      There is no corresponding record for this reference.
    19. 19
      Pokhrel, D. R.; Sah, M. K.; Gautam, B.; Basak, H. K.; Bhattarai, A.; Chatterjee, A. A recent overview of surfactantâ“drug interactions and their importance. RSC Adv. 2023, 13, 17685– 17704,  DOI: 10.1039/D3RA02883F
      There is no corresponding record for this reference.
    20. 20
      Schittny, A.; Philipp-Bauer, S.; Detampel, P.; Huwyler, J.; Puchkov, M. Mechanistic insights into effect of surfactants on oral bioavailability of amorphous solid dispersions. J. Control. Release 2020, 320, 214– 225,  DOI: 10.1016/j.jconrel.2020.01.031
      There is no corresponding record for this reference.
    21. 21
      Knepper, T. P.; Berna, J. L. Analysis and Fate of Surfactants and the Aquatic Environment; Comprehensive Analytical Chemistry. Elsevier 2003, 40, 1– 49,  DOI: 10.1016/S0166-526X(03)40004-4
      There is no corresponding record for this reference.
    22. 22
      Fainerman, V.; Lucassen-Reynders, E. Adsorption of single and mixed ionic surfactants at fluid interfaces. Adv. Colloid Interface Sci. 2002, 96, 295– 323,  DOI: 10.1016/S0001-8686(01)00086-0
      22
      Adsorption of single and mixed ionic surfactants at fluid interfaces
      Fainerman, V. B.; Lucassen-Reynders, E. H.
      Advances in Colloid and Interface Science (2002), 96 (1-3), 295-323CODEN: ACISB9; ISSN:0001-8686. (Elsevier Science B.V.)
      A review. Two different approaches were used in the literature to describe the effects of ionization of surfactants on the surface pressure, Π. One approach is based on a mol. model for a charged monolayer, in which the mutual repulsion of the long-chain surfactant ions results in an addnl. surface pressure, Πel, calcd. with the Gouy-Chapman theory for the formation of a diffuse elec. double layer, and with counterion binding in the Stern-Helmholtz layer adjacent to the surfactant monolayer. The other approach regards the surface as a two-dimensional soln. defined as a Gibbs dividing surface, which is electroneutral by definition. In this approach, the adsorption of any ion is the sum of its excesses in the monolayer and the elec. double layer; no assumptions are made about the spatial distribution of charges. Both models can produce a reasonable description of exptl. results obtained for solns. of a single ionic surfactant (RX) with or without inorg. electrolyte (XY). In many cases, measurements of Π vs. mean ionic activity at different salt concns. (cXY) coincide on a single curve, implying that at given mean ionic activity both adsorption and Π are independent of cXY, i.e. that double-layer contributions to the surface pressure are negligible. In addn., the electroneutral 2-dimensional soln. approach resulted in a simple explanation of several typical features of mixed ionic surfactant solns., in particular for mixts. of anionic and cationic surfactants. In mixed solns. too, double-layer effects appear to be negligible. The authors present arguments for such negligibility. One reason is a significant degree of binding between adsorbed surface active ions (R) and counterions (X); another is that for 1:1 electrolytes, the contribution of the diffuse double layer to the adsorption of the combination (RX) vanishes. As a result, it is possible to interpret the same exptl. data in terms of both models.
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      A Collection of Papers in Honour of Nikolay Churaev on the Occasion of his 80th Birthday.

    23. 23
      Sarkar, R.; Pal, A.; Rakshit, A.; Saha, B. Properties and applications of amphoteric surfactant: A concise review. J. Surfactants Deterg. 2021, 24, 709– 730,  DOI: 10.1002/jsde.12542
      There is no corresponding record for this reference.
    24. 24
      Rahman, M.; Anwar, S. J.; Molla, M. R.; Rana, S.; Hoque, M. A.; Rub, M. A.; Khan, M. A.; Kumar, D. Influence of alcohols and varying temperatures on the interaction between drug ceftriaxone sodium trihydrate and surfactant: A multi-techniques study. J. Mol. Liq. 2019, 292, 111322  DOI: 10.1016/j.molliq.2019.111322
      24
      Influence of alcohols and varying temperatures on the interaction between drug ceftriaxone sodium trihydrate and surfactant: A multi-techniques study
      Rahman, Marzia; Anwar, Sk. Jahir; Molla, Mohammad Robel; Rana, Shahed; Hoque, Md. Anamul; Abdul Rub, Malik; Abdullah Khan, Mohammed; Kumar, Dileep
      Journal of Molecular Liquids (2019), 292 (), 111322CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      The interaction between ceftriaxone sodium trihydrate drug and surfactants such as tetradecyltrimethylammonium bromide, Triton X-100, and Tween-80 was studied under various conditions by using three different physico-chem. methods. Different parameters such as crit. micelle concn., various thermodn. parameters for drug-TTAB aq. mixts. in an ethanol solvent, etc., have been measured. The varying cmc value of TTAB with the addn. of CFT in the TTAB soln. signifies the existence of interaction between TTAB and CFT. This interaction and the cmc values of mixts. of drug and TTAB, depend upon the employed concn. of alcs. and the system temp. Parameters such as heat capacity (ΔC°p.m.) and various transfer energies were also evaluated and are discussed. In the case of the cloud point (CP) method the phase sepn. of TX-100 in an aq. system was intensified in the presence of CFT. The std. free energy of clouding (ΔGoc), for the (CFT-TX-100) system was evaluated and found to be pos.; this indicates the non-spontaneous characteristic of this clouding action. For the system of CFT-TW-80 mixts., the value of the binding const. (Kb) is dependent on temp., and the types of solvent of different compn. in both aq. and alc. mediums. Therefore, hydrogen bonding and electrostatic interactions are important due to the binding interaction between CFT and TW-80.
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    25. 25
      Rahman, M.; Khan, M. A.; Rub, M. A.; Hoque, M. A. Effect of temperature and salts on the interaction of cetyltrimethylammonium bromide with ceftriaxone sodium trihydrate drug. J. Mol. Liq. 2016, 223, 716– 724,  DOI: 10.1016/j.molliq.2016.08.049
      25
      Effect of temperature and salts on the interaction of cetyltrimethylammonium bromide with ceftriaxone sodium trihydrate drug
      Rahman, Marzia; Abdullah Khan, Mohammed; Abdul Rub, Malik; Hoque, Md. Anamul
      Journal of Molecular Liquids (2016), 223 (), 716-724CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      Interaction of the third-generation cephalosporin antibiotic drug ceftriaxone sodium trihydrate (CFT) with a cationic conventional surfactant cetyltrimethylammonium bromide (CTAB) has been carried out by conductometric technique in water and in the presence of various salts such as NaCl, Na2SO4 and Na3PO4 at varying temps. with gaps of T = 5 K. Two crit. micelle concns. (c*1 and c*2) were obtained for (CFT + CTAB) mixts. in all the cases. The change of c* values of CTAB due to the addn. of CFT is indicative of the interaction between CFT and CTAB. For the (CFT + CTAB) mixed system, the values of c* are higher in magnitude in contrast to that of pure CTAB in aq. soln. at a particular temp. Both the c* values for (CFT + CTAB) mixed system in the presence of different salts are lower in magnitude as compared to aq. medium which means addn. of salt favors the micellization of (CFT + CTAB) mixed system. The Δ G0m values are obtained to be neg. in all the cases. The heat capacity (ΔC0p.m.), transfer energy (Δ G0m.tr.), transfer enthalpy (ΔH0m.tr.) and transfer heat capacities (ΔC0p.m.tr.) were also detd. and discussed as an extension of the usual thermodn. quantities of micellization.
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    26. 26
      Ahsan, S. A.; Al-Shaalan, N. H.; Amin, M. R.; Molla, M. R.; Aktar, S.; Alam, M. M.; Rub, M. A.; Wabaidur, S. M.; Hoque, M. A.; Khan, M. A. Interaction of moxifloxacin hydrochloride with sodium dodecyl sulfate and tween 80: Conductivity & phase separation methods. J. Mol. Liq. 2020, 301, 112467  DOI: 10.1016/j.molliq.2020.112467
      There is no corresponding record for this reference.
    27. 27
      Pathania, L.; Chauhan, S. Aggregation and interactional behavior of cationic surfactants in the presence of cephalosporin drug: A thermo-acoustic and spectroscopic approach. J. Mol. Liq. 2020, 299, 112210  DOI: 10.1016/j.molliq.2019.112210
      There is no corresponding record for this reference.
    28. 28
      Kumar, D.; Azum, N.; Rub, M. A.; Asiri, A. M. Aggregation behavior of sodium salt of ibuprofen with conventional and gemini surfactant. J. Mol. Liq. 2018, 262, 86– 96,  DOI: 10.1016/j.molliq.2018.04.053
      28
      Aggregation behavior of sodium salt of ibuprofen with conventional and gemini surfactant
      Kumar, Dileep; Azum, Naved; Abdul Rub, Malik; Asiri, Abdullah M.
      Journal of Molecular Liquids (2018), 262 (), 86-96CODEN: JMLIDT; ISSN:0167-7322. (Elsevier B.V.)
      Aggregation behavior of (SIBU) and conventional surfactant-(HTAB) along with gemini surfactant - 1,6-Bis(N-hexadecyl-N,N-dimethylammonium)hexane dibromide (16-6-16) mixts. have been carried out at 298.15 K by employing tensiometric method. Gemini surfactant is more effective in decreasing the crit. micelle concn. (cmc) of the drug as compared to conventional surfactant. The mixed systems show lower cmc value in comparison to ideal cmc (cmcid) value representing the non-ideal behavior of present soln. mixt. of drug and surfactant. The nonideality of the mixed system also stated in terms of the interaction parameter (βm/βσ). Addnl. parameters concerning tensiometry methods, for instance, micellar mole fraction of mixed micelles/mixed interface (Xm1/Xσ1), area occupied per amphiphile mol. (Amin), surface excess concn. (Γmax), etc. were computed and discussed one by one. Packing parameters were also evaluated and discussed. The ΔGom and ΔGoads values for every studied soln. were achieved to be neg. i.e., micellization, as well as adsorption processes, are energetically favorable and the value of ΔGoads were more neg. than their corresponding ΔGom, showing that work is done in transferring the amphiphilic monomers from interface to the micellar stage across the aq. soln. to form micelles.
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    29. 29
      Shelley, J. C.; Shelley, M. Y. Computer simulation of surfactant solutions. Curr. Opin. Colloid Interface Sci. 2000, 5, 101– 110,  DOI: 10.1016/S1359-0294(00)00042-X
      29
      Computer simulation of surfactant solutions
      Shelley, John C.; Shelley, Mee Y.
      Current Opinion in Colloid & Interface Science (2000), 5 (1,2), 101-110CODEN: COCSFL; ISSN:1359-0294. (Elsevier Science Ltd.)
      A review with 98 refs. Major advances were made at several levels of computer simulation of surfactant solns. Atomistic level studies of preassembled surfactant structures have become fairly routine. The development of structure in surfactant solns. was studied using atomistic, coarse grain, and mesoscopic models. Coarse grain and mesoscopic simulations were used to det. phase diagrams. The challenges involved in treating complex surfactant solns. will continue to drive this field forward.
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    30. 30
      Zhang, H.; Yuan, S.; Sun, J.; Liu, J.; Li, H.; Du, N.; Hou, W. Molecular dynamics simulation of sodium dodecylsulfate (SDS) bilayers. J. Colloid Interface Sci. 2017, 506, 227– 235,  DOI: 10.1016/j.jcis.2017.07.042
      30
      Molecular dynamics simulation of sodium dodecylsulfate (SDS) bilayers
      Zhang, Hongshu; Yuan, Shiling; Sun, Jichao; Liu, Jianqiang; Li, Haiping; Du, Na; Hou, Wanguo
      Journal of Colloid and Interface Science (2017), 506 (), 227-235CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Sodium dodecylsulfate (SDS), a simple single-tailed surfactant, forms stable vesicles from its micellar soln. without additives under the effect of solid surfaces. To further understand the mechanism of this transition on a mol. level, mol. dynamics simulations systematically assessed SDS bilayer segments (as part of vesicles) in bulk soln. at the moment the lower leaflet of bilayers detached from solid surfaces. The SDS membrane would rather retain its bilayer structure than return to micelles when the initial interdigitated degree (δi) between alkyl chains was >8.0 ± 1.4%. The degree of interdigitation always approached 31.7 ± 2.0% when equil. was attained. The aggregates behaved as curved, planar, and perforated bilayers and micelles with an increase in lower leaflet cross-sectional area. Salt bridge and water bridge structures were formed between DS- and Na+ or water mols., which contributed to SDS bilayer stability. The salt bridge distribution difference in the direction of the S-O axis between the two leaflets led to bilayer asymmetry, which played supplementary role to formation of bilayer curvature. It is expected this work will help shed light on understanding interface phenomena and simple, single-tailed surfactant vesicle self-assembly on a mol. level.
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    31. 31
      Tang, X.; Koenig, P. H.; Larson, R. G. Molecular Dynamics Simulations of Sodium Dodecyl Sulfate Micelles in Waterâ“The Effect of the Force Field. J. Phys. Chem. B 2014, 118, 3864– 3880,  DOI: 10.1021/jp410689m
      31
      Molecular Dynamics Simulations of Sodium Dodecyl Sulfate Micelles in Water-The Effect of the Force Field
      Tang, Xueming; Koenig, Peter H.; Larson, Ronald G.
      Journal of Physical Chemistry B (2014), 118 (14), 3864-3880CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      Mol. dynamic (MD) simulations of preassembled sodium dodecyl sulfate (SDS) micelles are carried out using three versions of GROMOS, as well as CHARMM36, OPLS-AA, and OPLS-UA force fields at different aggregation nos. and box sizes. The differences among force fields have little effect on the overall micelle structure of small aggregates of size 60 or 100, but for micelles of an aggregation no. of 300 or higher, bicelle structures with ordered tails, rather than the more realistic rodlike or cylindrical micelles with disordered tails, occur when using versions of GROMOS45A3 or the OPLS-AA force fields that are adapted to model the sulfate head group atoms using methods given in the literature. We find that the Lennard-Jones (L-J) parameters for the sodium ions and the ionic oxygens of the SDS head group, as well as the water model, control the transition to bicelles, regardless of other L-J parameters. A closer binding of the sodium ions to the head group ionic oxygens screens the electrostatic repulsions more strongly, resulting in condensation of SDS head groups, leading to unphys. bicelles for GROMOS45A3 or the OPLS-AA force fields, when the aggregation no. is large. A telltale sign that the sodium-oxygen interaction is too strong shows up in high nearest neighbor peaks (height >8 and height >20 for micelles with 60 and 100 surfactants, resp.) in the radial distribution functions (RDFs) of sodium ions to ionic oxygens. In the 100-surfactant micelles, the high RDF peak is accompanied by "crystal-like" layering of sodium ions onto the surface of the micelle. The distance between the sodium ions and micelle also depends on the no. of waters binding to sodium ions in the presence of surfactant head groups, which depends on both the sodium ion and water models, and for the same sodium model increases as the water model is changed in the order: TIP4P, SPC/E, SPC, and TIP3P.
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    32. 32
      Boukhelkhal, A.; Benkortbi, O.; Hamadeche, M.; Hanini, S.; Amrane, A. Removal of amoxicillin antibiotic from aqueous solution using an anionic surfactant. Water Air Soil Pollut. 2015, 226, 323,  DOI: 10.1007/s11270-015-2587-z
      There is no corresponding record for this reference.
    33. 33
      Zhang, F.; Wu, Z.; Yin, H.; Bai, J. Effect of ionic strength on the foam fractionation of BSA with existence of antifoaming agent. Chemical Engineering and Processing: Process Intensification 2010, 49, 1084– 1088,  DOI: 10.1016/j.cep.2010.07.016
      There is no corresponding record for this reference.
    34. 34
      Xu, Z.; Wu, Z.; Zhao, Y. Foam fractionation of protein with the presence of antifoam agent. Sep. Sci. Technol. 2010, 45, 2481– 2488,  DOI: 10.1080/01496391003674258
      34
      Foam Fractionation of Protein with the Presence of Antifoam Agent
      Xu, Zhong-He; Wu, Zhao-Liang; Zhao, Yan-Li
      Separation Science and Technology (2010), 45 (16), 2481-2488CODEN: SSTEDS; ISSN:0149-6395. (Taylor & Francis, Inc.)
      Foam fractionation is a promising technol. for protein concn. or purifn. However, the presence of an antifoam agent in fermn. broth restricted direct application of the technol. A preliminary approach of the surfactant-assisted foam process was conducted with a simulated system consisting of targeted protein bovine serum albumin (BSA), and a mixed antifoam agent (AF520, silicon oil/PGE mixt.). The effects of all three classes of surfactants (anionic SDBS, cationic CTAB, and non-ionic Tween-20) on BSA foam fractionation were examd. resp. Also, the influences of soln. pH, PGE, BSA, and NaCl were taken into account. The results revealed that all three classes of surfactants could stabilize foam film, so that the foam process could be operated, while the ionic surfactant exhibited excellent performance on condition that it was allowed to firmly interact with BSA to form a more hydrophobic complex, esp. for cationic CTAB. When soln. pH was adjusted to 7.5 and CTAB was 20 mg/mL-1, 90% of BSA could be extd. from a previous non-foaming system contg. 100 mg/mL-1 BSA and 4 mg/mL-1 AFA, and the enrichment reached 7.42. A higher enrichment of BSA could be obtained with increasing addn. of AFA but at the expense of the recovery. On the contrary, increasing BSA concn. gave rise to an opposite performance. The expts. also showed that the foam stability of the system was substantially enhanced by NaCl, significantly lowering the enrichment.
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    35. 35
      Huang, D.; Wu, Z. L.; Liu, W.; Hu, N.; Li, H. Z. A novel process intensification approach of recovering creatine from its wastewater by batch foam fractionation. Chemical Engineering and Processing: Process Intensification 2016, 104, 13– 21,  DOI: 10.1016/j.cep.2016.02.005
      There is no corresponding record for this reference.
    36. 36
      BlackBowers, K. J.; Chow, E.; Xu, H.; Dror, R. O.; Eastwood, M. P.; Gregersen, B. A.; Klepeis, J. L.; Kolossvary, I.; Moraes, M. A.; Sacerdoti, F. D.; Salmon, J. K.; Shan, Y.; Shaw, D. E. Proceedings of the 2006 ACM/IEEE Conference on Supercomputing; SC’06; ACM: New York, NY, USA, 2006.
      There is no corresponding record for this reference.
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      Release, S. Schrödinger Release 2023–2; Schrödinger, LLC: New York, NY, 2023.
      There is no corresponding record for this reference.
    38. 38
      BlackLu, C.; Wu, C.; Ghoreishi, D.; Chen, W.; Wang, L.; Damm, W.; Ross, G. A.; Dahlgren, M. K.; Russell, E.; Von Bargen, C. D.; Abel, R.; Friesner, R. A.; Harder, E. D. OPLS4: Improving Force Field Accuracy on Challenging Regimes of Chemical Space. J. Chem. Theory Comput. 2021, 17, 4291– 4300,  DOI: 10.1021/acs.jctc.1c00302
      There is no corresponding record for this reference.
    39. 39
      Gimel, J. C.; Brown, W. A light scattering investigation of the sodium dodecyl sulfateâ“lysozyme system. J. Chem. Phys. 1996, 104, 8112– 8117,  DOI: 10.1063/1.471496
      There is no corresponding record for this reference.
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      Hernainz, F.; Caro, A. Variation of surface tension in aqueous solutions of sodium dodecyl sulfate in the flotation bath. Colloids Surf., A 2002, 196, 19– 24,  DOI: 10.1016/S0927-7757(01)00575-1
      40
      Variation of surface tension in aqueous solutions of sodium dodecyl sulfate in the flotation bath
      Hernainz, F.; Caro, A.
      Colloids and Surfaces, A: Physicochemical and Engineering Aspects (2002), 196 (1), 19-24CODEN: CPEAEH; ISSN:0927-7757. (Elsevier Science B.V.)
      A study was carried out on the modification of surface tension in aq. solns. of the collector SDS, at temp. of 293 K and pH between 4-11. The surface tension varies very slightly when pH in the medium of flotation bath is modified. The crit. micelle concn. (cmc) was identified and certain thermodn. magnitudes assocd. with the adsorption of SDS in the soln.-air interface were also obtained. The values for the mol. limiting area range between 19-31 Å per mol. and std. adsorption energy between -19.82 and -23.52 kJ mol-1, at pH between 4-10.
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    41. 41
      Qazi, M. J.; Schlegel, S. J.; Backus, E. H.; Bonn, M.; Bonn, D.; Shahidzadeh, N. Dynamic Surface Tension of Surfactants in the Presence of High Salt Concentrations. Langmuir 2020, 36, 7956– 7964,  DOI: 10.1021/acs.langmuir.0c01211
      41
      Dynamic Surface Tension of Surfactants in the Presence of High Salt Concentrations
      Qazi, Mohsin J.; Schlegel, Simon J.; Backus, Ellen H. G.; Bonn, Mischa; Bonn, Daniel; Shahidzadeh, Noushine
      Langmuir (2020), 36 (27), 7956-7964CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      We study the influence of high NaCl concns. on the equil. and dynamic surface tensions of ionic (CTAB) and nonionic (Tween 80) surfactant solns. Equil. surface tension measurements show that NaCl significantly reduces the crit. micellar concn. (CMC) of CTAB but has no effect on the CMC of Tween 80. Dynamic surface tension measurements allow comparing the surface tension as a function of time for pure surfactant solns. and in the presence of NaCl. For the ionic surfactant, the dynamics agree with the usual diffusion-limited interfacial adsorption kinetics; however, the kinetics become orders of magnitude slower when NaCl is present. Sum-frequency generation spectroscopy expts. and the equil. adsorption measurements show that the presence of NaCl in CTAB soln. leads to the formation of ion pairs at the surface, thereby neutralizing the charge of the head group of CTAB. This change, however, is not able to account for the slowing down of adsorption dynamics; we find that it is rather the decreases in the monomer concn. (CMC) in the presence of salt which has the major influence. For the nonionic surfactant, the kinetics of interfacial tension is seen to be already very slow, and the addn. of salt does not influence it further. This also correlates very well to the very low CMC of Tween 80.
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    42. 42
      Liu, B.; Hoopes, M. I.; Karttunen, M. Molecular Dynamics Simulations of DPPC/CTAB Monolayers at the Air/Water Interface. J. Phys. Chem. B 2014, 118, 11723– 11737,  DOI: 10.1021/jp5050892
      42
      Molecular Dynamics Simulations of DPPC/CTAB Monolayers at the Air/Water Interface
      Liu, Bin; Hoopes, Matthew I.; Karttunen, Mikko
      Journal of Physical Chemistry B (2014), 118 (40), 11723-11737CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
      An atomistic-level understanding of cationic lipid monolayers is essential for development of gene delivery agents based on cationic micelle-like structures. The authors employ mol. dynamics (MD) simulations for a detailed atomistic study of lipid monolayers composed of both pure zwitterionic dipalmitoylphosphatidylcholine (DPPC) and a mixt. of DPPC and cationic cetyltrimethylammonium bromide (CTAB) at the air/H2O interface. The authors aim to study how the compn. of the DPPC/CTAB monolayers affects their structural and electrostatic properties in the liq.-expanded phase. By varying the molar fraction of CTAB, the authors found the cationic CTAB lipids have significant condensing effect on the DPPC/CTAB monolayers, i.e., at the same surface tension or surface pressure, monolayers with higher CTAB molar fraction have smaller area per lipid. The DPPC/CTAB monolayers are also able to achieve neg. surface tension without introducing buckling into the monolayer structure. The authors also found the condensing effect is caused by the interplay between the cationic CTAB headgroups and the zwitterionic phosphatidylcholine (PC) headgroups which has electrostatic origin. With CTAB in its vicinity, the P-N vector of PC headgroups reorients from being parallel to the monolayer plane to a more vertical orientation. Also, detailed anal. of the structural properties of the monolayers, such as the d. profile anal., H bonding anal., chain order parameter calcns., and radial distribution function calcns. were also performed for better understanding of cationic DPPC/CTAB monolayers.
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    43. 43
      Black Silva, M. G. A. D.; Meneghetti, M. R.; Denicourt-Nowicki, A.; Roucoux, A. New and tunable hydroxylated driving agents for the production of tailor-made gold nanorods. RSC Adv. 2013, 3, 18292– 18295,  DOI: 10.1039/C3RA42949K
      There is no corresponding record for this reference.
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      Black Sohail, M.; Rahman, H. M. A. U.; Asghar, M. N. Drug–ionic surfactant interactions: density, sound speed, spectroscopic, and electrochemical studies. Eur. Biophys. J. 2023, 52, 735– 747,  DOI: 10.1007/s00249-023-01689-2
      There is no corresponding record for this reference.
    45. 45
      Frenkel, D.; Smit, B., Eds. Understanding Molecular Simulation (Second ed.), second ed.; Academic Press: San Diego, 2002.
      There is no corresponding record for this reference.

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