Amphiphiles: Molecular Assembly and Applications

Amphiphilic surfactants and polymers display characteristic molecular self-assembly behavior in solutions, at interfaces and in bulk, generating nanoscale structures of different shapes. These nanoscale features determine many characteristics of these amphiphiles, relevant for their practical applications in materials, pharmaceutical and biomedical technologies. The ability to generate desired nanoscale morphologies by synthesizing novel amphiphiles would allow the amphiphilic systems to be tailored for specific applications. Critical to achieving this goal is an understanding of the link between the molecular structure of the amphiphiles and their self-assembly behavior. In this chapter, we review the principles of self-assembly for both conventional low molecular weight surfactants and amphiphilic block copolymers. The main emphasis is on demonstrating how general principles of thermodynamics and considerations of molecular packing together help predict the self-assembled morphologies given the amphiphile molecular structure. The similarity between the behavior of classical surfactants and amphiphilic block copolymers are identified. The self-assembly behavior of novel amphiphiles can be extrapolated from these fundamental principles.

The success of gene therapy as a revolutionary method to treat diseases relies on finding delivery systems that can efficiently transfer and express (transfect) DNA into target cells while producing reduced cytotoxic effects. Recently, pyridinium amphiphiles were shown to generate promising transfection systems that can fulfill these requirements. The impact of various structural characteristics (hydrophobic chain type, linkage type, counterion) on the transfection activity and cytotoxicity among various classes of pyridinium amphiphiles (detergents, lipids, gemini surfactants, lipophilic polycations) is discussed, emphasizing the most important structural parameters, efficient designs and formulations.

The filamentous bacteriophage m13 and related viruses encapsulate DNA with protein, forming an organic nanowire about 1 micrometer long and less then 10 nanometers wide. The length of the wire is formed from many copies of a single protein, which is a single alpha helix formed from about 50 amino acids. It can be viewed as a very sophisticated surfactant, with hydrophilic regions that interact with the DNA and form the outer surface, and hydrophobic regions that pack against each other. We have implemented these design principles in peptoids (sequence-specific N-functional glycine oligomers) and have found that they form aggregates with DNA that have adjustable properties. This approach may complement phage display methods, providing new approaches to gene transfection and nanofabrication that do not require expression in bacteria and that provide a wider range of chemical stability and functionality.

Colloidal systems are bubbles, droplets, or/and particles dispersed in a continuous media, which are encountered in nature, biological system, and manufactured products. As site of interfaces between at least two bulk phases, their formation and stability need the presence of amphiphilic molecules. Carbohydrate-based surfactants (CBS) take part of these groups of compound having numerous attractive features: an abundance of precursor raw material sources, an excellent environmental compatibility, and a possible quasi-unlimited design of their molecular structure and geometry by different preparation routes. Numerous properties and activities including the aptitude to form and stabilize colloidal systems are therefore expected from a wide range of CBS compounds. Screening of their interfacial behaviors, which govern their functionalities in colloidal system properties, are then relevant in fundamental and practical point of view. In this chapter, interfacial properties of uronic acid-based surfactants are presented as examples of screening approaches. Experimental measurements of both dynamic and equilibrium properties at air-liquid and liquid-liquid interfaces, as well as a computational approach are reported.

Surfactants that bear chiral headgroups form a variety of supra self-assembled nanostructures and can be used for different applications. In particular, ascorbic acid possesses two chiral centers, a rigid planar ring, a powerful redox active moiety, two acidic –OH residues, and a primary –OH group, and therefore is one of the most versatile polar headgroups for its peculiar properties.

Here we report our studies on the nanoassemblies produced by L-(+)-ascorbyl-alkanoates (L-ASCn) and D-(-)-isoascorbyl-alkanoates (D-ASCn), carried out through DSC, SAXS, XRD, and surface tension experiments. The results suggest that the different configuration of the headgroups induces relevant changes in the structural and physico-chemical properties of the aggregates, due to the different hydration of the two epimers L-ascorbic and D-isoascorbic acid. The mixtures of D-ASC12 and L-ASC12 surfactants produce interesting phase diagrams, revealing the existence of a 1:1 compound, and of two eutectic points.

In order to obtain stable biphasic solvent mixtures such as emulsions and miniemulsions, distinct emulsifiers are required for the liquid/liquid interface. Until recently, nanoscale polymer particles have been mainly gained from waterborne heterophase techniques in radical processes, herein nonaqueous emulsions are presented. For these biphasic systems consisting of two aprotic organic nonmiscible solvents, high molecular weight amphipolar block copolymers were developed which show selective solubility in both phases and give additional stability by sterical shielding. Different emulsifiers, ranging from low molecular weight surfactats up to long chain block copolymers, were designed for the stabilization of hydrocarbon/perfluorocarbon mixtures and the metallocene-catalyzed synthesis of polyolefins therein. Furthermore, PI-b-PMMA emulsifiers were applied for mixtures of DMF dispersed in n-hexane. By these emulsions high molecular weight polyester and polyurethane particles were recieved, while further developments led to more sophisticated morphologies like core-shell structures.

The newly discovered polyelectrolyte (PE) self-assembly by fluorinated hydrogen bonding (FHB) is harnessed to assemble FHB films and hydrogels. PE couples of poly(vinyl alcohol)/ perfluorosulfonic acid PTFE copolymer (PVA/Nafion®) or poly(allyl amine) PAH/Nafion® would self assemble into layer-by-layer films or FHB hydrogels under high centrifuge using controlled pH and ion strength conditions. After centrifuge, FHB hydrogels can be isolated from their liquor, washed, and sliced. Selected physical properties such as stability, solubility, molecular partitioning, and ion conductivity are discussed therein aided by ATR-FTIR and potentiostatic/ac impedance analysis. Stability to a wide pH range and relatively high ionic conductivity (i.e. > 0.1 Scm^-1) makes these FHB gels potential candidates for electrolyte applications in solid-state electrochemical systems.

The solid-liquid interface behaviour and the structure of adsorption layers of ionic-amphiphilic acrylic acid (AA)/ isobornyl acrylate (iBA) based copolymers on hydrophilic titanium dioxide (TiO₂) and hydrophobic copper phthalocyanine (CuPc) has been studied by the electrokinetic sonic amplitude (ESA) method. It was shown that the polymer gel layer theory can be applied to polyelectrolytes, giving detailed information not only about the polymer-particle interaction but also the thickness and structure of the coating layer around the particles.

CuPc was found to be covered by a relatively thin copolymer layer irrespective of the copolymer architecture, i.e., for both PAA-b-PiBA block copolymers with sharp block boundary and PAA-b-P(AA-co-iBA) block-like copolymers with tapered block-transition and isolated AA units in the hydrophobic block. In terms of the gel layer theory, PiBA blocks form the inner dense layer, and the PAA blocks represent the outer, permeable region of the adsorbed layer. Contrarily, different adsorption mechanisms of block and block-like copolymers were observed for the TiO₂. In case of block copolymers, a multilayer coating consisting of PAA anchor blocks, an interphases layer of PiBA blocks, and an outer PAA shell is obtained, whereas a solloid monolayer is formed for block-like copolymers.

Swelling states of poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl) ammonatopropanesulfonate) (MAPS) brush in aqueous solutions were investigated by atomic force microscopy (AFM) and neutron reflectivity measurement. The thickness of swollen poly(MAPS) brush evaluated by AFM was increased from 61 to 87 nm with an increase in NaCl concentration from 0 to 0.05 M, and was dramatically increased up to 153 nm at 0.5 M NaCl aqueous solution. At low salt concentration, poly(MAPS) brush chains formed shrunk structure due to the inter- and intrachain attractive electrostatic interactions, while the brush chains stretched up with increasing salt concentration due to the screening effect by hydrated salt ions.

Rheology provides a powerful way to access molecular scale data from a macroscopic measurement. Ultrasonic rheology, performed with a quartz-crystal microbalance (QCM) instrument, provides a straightforward route to sensitive (< 0.1 mPa·s STD) rheology of either bulk or surface phases. Presented is the necessary theory (Johannsmann and Voinova models) to perform such measurements, and some applications of that theory - including rheology on polymer brush layers, lipid vesicle adsorption, proteins in the bulk phase, and microemulsions. In each of these applications, an advantage of QCM-based rheology is described, such as small sample size, precision and the ability to perform either bulk or surface measurements. Additionally, corrections to the standard Sauerbrey approximation, which often results in an underestimation of adsorbed mass, are presented. In conclusion, the QCM is shown to provide a facile route to rheological measurements in the MHz regime.

The effect of hydrophobization of gold quartz crystal microbalance with dissipation (QCM-D) crystals is investigated in terms of its effect on surfactant adsorption at the solid-liquid interface. Hydrophilic gold crystals were modified with octadecanethiol to produce a hydrophobic surface. The gold surface was characterized prior to and after coating. Water contact angles were obtained to confirm hydrophobicity and atomic force microscopy (AFM) images were acquired to confirm surface modification and alkylthiol monolayer morphology. Modified crystals were tested for QCM-D response uniformity and then exposed to surfactant solutions. The effect of hydrophobization on the adsorption of surfactants is investigated for sodium dodecyl sulfate (SDS), ‘dioctyl’ sulfosuccinate sodium salt (AOT), hexadecyltrimethylammonium bromide (CTAB), and dihexadecyldimethylammonium bromide (DHDAB). Surfactant solutions were studied above their respective critical micelle concentration (CMC) in the absence of added electrolyte. Adsorption for SDS and AOT on hydrophilic gold was minimal and was below the detection limit for QCM-D at concentrations just above the CMC. For CTAB and DHDAB solutions at concentrations well above the CMC, adsorption on hydrophilic gold was substantial, forming more than one bilayer of coverage. On hydrophobed gold, SDS, AOT, CTAB, and DHDAB all adsorbed to form a surfactant monolayer.

An octadecylphosphonic acid (ODPA) film was formed on the surface of stainless steel 316L and thermally treated at 100 – 120 °C for 30 – 60 minutes. AFM images, and water contact angles revealed that the film was stabilized enough after the thermal treatment to survive in water flush. DRIFT spectra showed that thermal treatment enhanced P-O stretches of the film. A model is suggested to explain the stability of ODPA film enhanced by thermal treatment. An ODPA molecule is physisorbed to the surface weakly by hydrogen bonding. Heating drives away water molecules leading to the formation of strong monodentate or mixed mono/bi-dentate bonds of ODPA molecules to the surface.

Amphiphilic invertible polymers (AIPs) are composed of the hydrophilic, poly(ethylene glycol), and hydrophobic, aliphatic dicarboxylic acid moieties, alternately distributed along the main backbone. They build micelles, self-assemble by increasing concentration and are capable of accommodating otherwise insoluble substances in aqueous medium. The resulting complex nanostructures can undergo inverse conformational changes and are functional in both polar and non-polar environments, giving potentially broad use in many applications, especially in the pharmaceutical industry, agriculture, and cosmetics.

Bio-inspired adhesive vesicles are designed by self-assembly of di-acetylenic fatty acids that were modified at the carboxylic acid head-group with either cell-adhesive or mussel-adhesive peptide sequences. The amphiphiles are stabilised by UV-light induced polymerisation of the hydrophobic tail into well-defined structures, and allow for controling the presentation of the adhesive groups at the vesicular surface. The vescicles can be immobilized on a surface-attached polymer hydrogel support in laterally confined structures (micro-arrays). The adhesive properties towards living cells and artificial oxidic substrates depend on the vesicle composition, concentration and presentation of the adhesive groups at the interface. This technique may serve as an interesting platform for creating functional adhesive interlayers.

Large-pore organosilicas with methylene and ethylene bridges in the framework were synthesized at low initial temperature (15 °C) using hexane as a swelling agent and Pluronic P123 (EO₂₀PO₇₀EO₂₀) block copolymers as template. Organosilica materials with the nominal (KJS) pore sizes of 18 and 10 nm were obtained in case of methylene and ethylene bridges, respectively. The variation of amount of precursor was found to have almost negligible effect on the interplanar spacing and pore size. The use of swelling agents with different chain length allowed us to tailor the pore size from 9 to 11 nm in case of ethylene-bridged organosilicas, when the first step of the synthesis was carried out at room temperature.

A single component, multifunctional resorcinarene thiol-ene monomer, resorcinarene tetra alkene tetra thiol (RTATT), has been used as a photopolymerizable building block, for the synthesis of hollow nanocapsules, and other morphologically distinct polymeric architectures. Photopolymerization of RTATT in chloroform resulted in the direct formation of nanocapsules, in the absence of any template or preorganization. We show that the polymerization media plays a crucial role in determining the polymer morphology, as a variety of architectures, like lattices, fibrous networks, and nanoparticles were produced by varying the solvent. Further, we have developed a one-pot, two-stage method for the synthesis and in situ functionalization of resorcinarene nanocapsules.

Studies of sub-micron-sized polymer-core/shell particles, or polymer nanocapsules, where the capsule core consists of a liquid hydrocarbon are discussed. The capsules are prepared via a one-step, miniemulsion polymerization procedure. The capsule stability and the effect of droplet size and the polymer/oil interfacial tension are investigated using differential scanning calorimetry. Changes in the integrity of the polymer shell can be observed by measuring the solidus temperature of the encapsulated n-hexadecane. The type of polymer and volume of the oil droplet also affect the solidus temperature. We observe a limited agreement with the Gibbs-Thomson equation.

Nanoparticles (NPs) and molecular surfactants are two classes of compounds which spontaneously localize at oil/water interfaces. Industrial and commercial applications of these systems often require precise two-dimensional organization of the localized NPs. An impediment to the realization of such systems is our under-developed understanding of the physics which governs the behavior of NPs in the presence of surfactants. Here we present a range of molecular dynamics simulation studies on non-ionic NP/surfactant systems. Analysis of the results allows us to relate the dispersive interactions of the NPs and surfactants to their physical behavior at oil/water interfaces.