Structure/Performance Relationships in Surfactants

Alkyl ether sulfates are, after alkyl benzene sulfonates(LAS), the group of technically important anionic surfactants with the largest production volume and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed, for example, in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application.

Data on the relationship of chemical structure to fundamental properties of polyoxyethylene(POE) nonionics in aqueous solution are reviewed. These include: 1) the adsorption, micelle formation and thermodynamics of a series of highly purified POE n-alkyl monoethers, varying systematically in chain length of both alkyl and POE groups, 2) the effects on the aqueous properties of multi-chain and alicyclic structure in hydrophobe, 3) the evaluation of hydrophilicity and surface properties for a new class of nonionics, alkyl crown ethers, 4) the adsorption and dissolution behavior of long N-acyl α-amino acid POE monoesters having a short chain of homogeneous POE, R¹CONR²CHR³COO(C₂H₄O)_mH, with special reference to the structural effects of the α-amino acid residue.

Zwitterionic surfactants of structure RN⁺(CH₂C₆H₅)-(CH₃)CH₂COO^-, where R is an alkyl chain of 10 or 12 carbon atoms, and RN⁺(CH₂C₆H₅)(CH₃)CH₂CH₂SO₃^-, where R is 8, 10, or 12 carbon atoms, have been synthesized. From surface tension-concentration curves in aqueous solution at 10°, 25°, and 40°C, surface excess concentrations and areas/molecule at surface saturation, critical micelle concentrations, efficiency and effectiveness of surface tension reduction, and thermodynamic parameters of adsorption and micellization have been calculated. The areas/molecule indicate that the entire ionic head group in each series is lying flat in the aqueous solution/air interface. For the glycines, the standard free energies of micellization and of adsorption per methylene group at the aqueous solution/air interface are -2.80 kJ and -3.05 kJ, respectively; for the taurines, the standard free energy of adsorption per methylene is -3.15 kJ, all at 25°C.

The effect of pH and electrolyte on the surface properties of a betaine surfactant, C₁₂H₂₅N⁺(CH₂C₆H₅)(CH₃)CH₂COO^-, were studied. The ΔG°_ad of both the zwitterionic form of the betaine and of the cationic protonated betaine were calculated. Surface activity decreases slightly with decrease in pH. As expected, the cationic form has somewhat lower surface activity than the zwitterionic form. Although the pK_a of the protonated betaine is 2.8, the properties of both the zwitterionic betaine and the cationic protonated betaine determine the surface properties, even in distilled water (pH = 5.85). At pH = 5.85, the betaine interacts more strongly with Na₂SO₄ than with NaCl or CaCl₂; anionic surfactants also show stronger interaction with the betaine than do cationic surfactants. These effects cannot be attributed to the zwitterion, but to the presence of the protonated cationic form in equilibrium with the zwitterion.

The slope of the linear relationship between log cmc and number of carbon atoms in the chain, nc, is -0.5 for nonionic and zwitterionic surfactants, whereas the slopes are -0.3 and -0.25 for univalent and bivalent ionic surfactants, respectively. When the activity of the surfactants at cmc, cma, is used instead of cmc, the slopes are -0.5—0.58 irrespective of head group. This results in a value of -680—777 cal/mol for the free energy of micelle formation per CH₂ group. Furthermore, the linear relationship between log cmc and log[counter ion] shows a slope of -0.6 for potassium dodecanoate, -0.67 for sodium dodecyl sulfate, and -0.95 for disodium dodecyl phosphate. When the critical micelle activity, cma, is used instead of cmc, the slopes are -0.9 for the univalent surfactants and -1.8 for the bivalent surfactants. These results indicate that the cma is substantially constant regardless of the counter ion concentration.

This paper presents a new finding that the oil solubilization rate is a function of surfactant aggregate size. Light scattering and conductance measurements were used in the experiments. Alcohol ethoxylates and SDS were used as surfactants. The aggregate size was changed by changing the surfactant structure or by adding chemicals. The solubilization rate shows a maximum at a certain aggregate size for a given surfactant and a given oil. Thus, we have found a measurable and controllable factor (size) in the process of oil solubilization. A theory was proposed to relate solubilization rate with micellar properties and surfactant structure. By using this theory, we can explain the performance of petroleum sulfonate in enhanced oil recovery and improve current formulation in achieving ultra-low interfacial tension. We can also explain the nonionic detergent performance as a function of surfactant structure.

The hydrotropic action of a dicarboxylic acid is discussed against the general features of hydrotropic action; the liquid crystal/isotropic solution equilibrium. It is shown that the hydrotropic action of the dicarboxylic acid in question, 8-[5(6)-carboxy-4-hexyl-cyclohex-2-enyl] octanoic acid, depends on its conformation at an interface.

Aqueous solution properties of the twenty-one carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C₂₁-DA) in salt form - alone and in the presence of a nonionic, anionic or cationic detergent - are reported. Membrane osmometry results indicate that C₂₁-DA alkali salt forms low molecular weight aggregates or micelles, its aggregation behavior appearing to resemble that of certain polyhydroxy bile salts. In the presence of detergent, small aggregates are also formed provided the weight fraction ofC ₂₁-DA salt in the micelles exceeds ca. 0.5. Phase equilibrium studies show that C₂₁-DA (as the dipotassium or full triethanolamine salt) acts as a hydrotrope above certain concentration levels in concentrated detergent solutions, retarding build-up of anisotropic aggregates responsible for mesophase formation, in accordance with previous investigations by Friberg and co-workers.

Lauryl, myristy, cetyl and steary-dimethylamine oxide (LDAO) aqueous solutions alone and in combination with sodium dodecyl sulfate (SDS) have been investigated by surface tension, viscosity and pH measurements. 1) LDAOsolutions: At any particular pH, the amine oxide is in equilibrium with its protonated form. The viscosity of the solution depends strongly on the degree of ionization (β) and maximum viscosity was observed when β = 0.5, due to the formation of elongated structure. 2) LDAO/SDSsolutions: Progressive addition of SDS to dodecyl or myristyl DAO results in pH and viscosity increases, both values reach their maximum values at a 3:1 LDAO/SDS molar ratio. However, the change in pH remains constant over a wide range of molar ratios, while the viscosity value depends strongly on that ratio. The viscous mixture is non-Newtonian in nature, and exhibits liquid crystalline behaviour. In the case of cetyl- and stearyl-DAO/SDS mixtures, solutions are turbid with low viscosity. This difference in behaviour is attributed, in part, to the compatibility in chain length of the LDAO and SDS.

Calorimetric measurements are used to examine interactions between different surfactant components in nonideal mixed micelles and assess the effects of surfactant structure on the thermodynamics of mixed micellization. Results for some anionic/nonionic surfactant mixtures show that variations in surfactant structure can have important effects on heats of mixing in the micelles and significantly influence the critical micelle concentration (cmc) of the mixed surfactant systems. Here, both the heats of mixing and deviations of the cmc from ideality are smaller for alkyl ethoxylate sulfates than alkyl sulfates when mixed with alkyl ethoxylate nonionics. The calorimetric results for these systems are also used to examine the appropriateness of the regular solution approximation used in pseudo-phase separation models for treating mixed micellization. The failure of the regular solution approximation to account for the observed heats of mixing in these systems suggests that the net interaction parameter of the nonideal mixed micelle models be interpreted as an excess free energy parameter in such cases.

The theories of microemulsion formation and stability have been reviewed. Three main approaches, namely mixed film, solubilisation and thermodynamic theories, have been briefly discussed. This is then followed by a section on factors determining w/o versus o/w microemulsion formation. The influence of surfactant and cosurfactant structure and chain length on the structure of microemulsions was described. In particular the cosurfactant chain length and structure has a considerable effect on the structure of the microemulsion. With short chain alcohols (<C₆) and/or surfactants, there is no marked separation into hydrophobic and hydrophilic domains and the structure is best described by a bicontinuous solution with easily deformable and flexible interfaces. With long chain alcohols (>C₆), well defined "cores" may be distinguished with a more pronounced separation into hydrophobic and hydrophilic regions. It was also concluded that microemulsions can be formed by a single surfactant provided this has the right geometry for packing at the interface and is capable of reducing the interfacial tension to low values. Addition of a cosurfactant is necessary in some cases to ensure packing of the surfactant molecule and to lower the interfacial tension.

The rate constants for the reaction of a pyridinium ion with cyanide have been measured in both a cationic and nonionic oil in water microemulsion as a function of water content. There is no effect of added salt on the reaction rate in the cationic system, but a substantial effect of ionic strength on the rate as observed in the nonionic system. Estimates of the ionic strength in the "Stern layer" of the cationic microemulsion have been employed to correct the rate constants in the nonionic system and calculate effective surface potentials. The ion-exchange (IE) model, which assumes that reaction occurs in the Stern layer and that the nucleophile concentration is determined by an ion-exchange equilibrium with the surfactant counterion, has been applied to the data. The results, although not definitive because of the ionic strength dependence, indicate that the IE model may not provide the best description of this reaction system.

Many nonionic surfactants of the poly(oxyethylene) alkyl and aryl ether class interact with biological membranes increasing their permeability and causing increased trans-membrane solute transport. The mechanisms of such effects are not fully understood. Studies of the interaction of homologous series of nonionic surfactants (varying either in hydrophobic or hydrophilic chain length) with a variety of biological substrates, reviewed here, indicate there to be an optimal lipophilicity for maximal membrane activity. Problems with the hydrophile-lipophile balance (HLB) as an index of lipophilicity result from the structural non-specificity of this value. The biological effects of surfactants are concentration dependent and structure dependent. In many cases C₁₂ hydrocarbon chain compounds appear to exert maximal effects and in all series a parabolic relationship between membrane activity and lipophilicity is observed. The effects are complex resulting from penetration of the membrane, its fluidization and, at high surfactant concentrations, solubilization of structural components.

This work describes the application of a previous study which dealt primarily with organic synthesis and physical properties of reaction products of pure fatty acids with DETA. Derivatives to amplify the previous study were prepared from various industrial fatty materials. The reaction product, from 1 mole diethylenetriamine (DETA) with 2 moles fatty acid, was thought to be the primary amine, RCON(CH₂CH₂NH₂)-CH₂CH₂NHCOR, rather than the secondary amine, as cited in the literature. The amine was readily dehydrated to the imidazoline, R_{x230a;C = NCH2CH2Nx230b;}CH₂CH₂NHCOR. The imidazolines in the presence of moisture hydrolyzed upon standing, to the open chain derivatives. These cationic surfactants were examined as water repellents for soil. Water repellency was evaluated by contact angle measurements and water infiltration through sand, sandy soil, and soil containing 30% clay. A large number of derivatives made clay soil hydrophobic, whereas only a few caused this effect on sandy soils. The following factors influenced soil water repellency. Open chain derivatives were more hydrophobic than the corresponding imidazolines. Hydrophobicity intensified with increasing molecular weight of the saturated fatty acids. Unsaturation, as in the oleic acid derivatives, enhances hydrophilicity. Hydrocarbon branching in the fatty acid also reduces water repellency. The soil hydrophobing agents in treated soils greatly restrict seed germination.

Solid state electrodes selective for alkylpyridinium cations are used to study the binding of these surfactants cations, with C₁₂, C₁₄ and C₁₆ alkyl chainlengths, to a number of anionic polyelectrolytes. The electrodes are shown to be effective from very low surfactant concentrations to the cmc, and can be used for accurate cmc determinations in solutions of high ionic strength. Binding isotherms of the alkylpyridinium cations with polyacrylate, alginate, pectate and pectinates are presented. All isotherms are highly cooperative. The surfactant chainlength dependence of the overall binding constant is identical to the case of micelle formation of the free surfactant, but for a given surfactant the overall binding constant depends strongly on the charge density of the polyion.

Pure homologs of linear alkylbenzene sulfonate sodium salts (LAS) were evalulated for detergency performance. The surface tensions of the wash liquors used for these detergencies were then measured and used in conjunction with a wetting model to calculate a theoretical detergency performance. The theoretical and experimental detergency results were compared. The molecular structure effects of the LAS homologs on detergency performance were calculated by incorporating into the detergency equation several molecular structure theories such as the cohesive energy ratio concept, molar-attraction constants, internal liquid pressure, and liquid thermal properties. The assets and deficiencies of these approaches are discussed.

Adsorption of a surfactant on solids is dependent, among other things, on the structure of both the hydrophobic and hydrophilic portions of it. There are a number of mechanisms proposed for surfactant adsorption and an understanding of the effects of the structure of the surfactant can help in elucidating the role of these mechanisms. In this study, the effect on adsorption on alumina of some structure variations of sulfonates (chain length and the branching and the presence of ethyoxyl, phenyl, disulfonate and dialkyl groups) is examined above and below CMC as a function of surfactant concentration, pH and salinity. Co-operative action between an ionic alkylsulfonate and a nonionic ethoxylated alcohol is also studied.

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system.Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity.

The mechanism of interaction of amino acids at solid/aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption.

The low interfacial tensions between two liquids have been measured for different systems by using the pendant drop method. In the case of the quaternary system: C₁₂H₂₅SO₄Na+H₂O+n-Butanol+Toluene, the interfacial data as measured by pendant drop method are compared with reported literature data, using other methods (with varying NaCl concentration). In order to understand the role of co-surfactant, ternary systems were also investigated. The pendant drop method was also used for measuring the interfacial tension between surfactant-H₂O/n-alcohol (with number of carbon atoms in alcohol varying from 4-10). The interfacial tension variation was dependent on both the surfactant and alcohol.