Aqueous surfactant-alcohol systems: A review

Effect of N-glycinylmaleamic acid on microstructural characteristics and solubilization properties of sodium dodecyl sulfate micellar assemblies

The cosurfactant activity of N-glycinylmaleamic acid (NGMA) in sodium dodecyl sulfate (SDS) micelles has been demonstrated. The complementary techniques of electron spin resonance (ESR) and fluorescence spectroscopy have been used to draw information on hydration index (H), microviscosity (eta), and aggregation number (N) of micellar assemblies. The estimate of the critical micelle concentration of SDS in the presence of NGMA suggests a synergistic effect of NGMA. The enhanced solubilization of butyl propionate in the presence of NGMA in SDS micelles is explained on the basis of availability of larger interfacial area calculated from a simple spherical geometric model, combined with a low hydrophilicity index as estimated from ESR. Thus, addition of NGMA contributes to an increase of about 50% in ratio of area of polar shell (AP)/volume of hydration (Vh) ratio. The decrease in H accompanied by a decrease in eta with the incorporation of butyl propionate probably arises from solubilization of a butyl component inside the core with the adsorption of propionate ester on the interface.

Structural micellar transition for fluorinated and hydrogenated sodium carboxylates induced by solubilization of benzyl alcohol

The solubility of benzyl alcohol in micellar solutions of sodium octanoate and sodium perfluorooctanoate was studied. From the isotherms of specific conductivity versus molality at different alcohol concentrations, the critical micelle concentration and the degree of ionization of the micelles were determined. The cmc linearly decreases upon increasing the amount of benzyl alcohol present in aqueous solutions with two distinct slopes. This phenomenon was interpreted as a clustering of alcohol molecules above a critical point, around 0.1 mol kg(-1). Attending to the equivalent conductivity versus square root of molality, the presence of a second micellar structure for the fluorinated compound was assumed. The thermodynamic parameters associated with the process of micellization were estimated by applying Motomura's model for binary surfactant mixtures, modified by Pérez-Villar et al. (Colloid Polym. Sci 1990, 268, 965) for the case of alcohol-surfactant solutions. A comparison of the hydrogenated and fluorinated compounds was carried out and discussed.

Effect of ethanol on the interaction between poly(vinylpyrrolidone) and sodium dodecyl sulfate

The effect of ethanol on the interaction between the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic polymer poly(vinylpyrrolidone) (PVP) has been investigated using a range of techniques including surface tension, fluorescence, electron paramagnetic resonance (EPR), small-angle neutron scattering (SANS), and viscosity. Surface tension and fluorescence studies show that the critical micelle concentration (cmc) of the surfactant decreases to a minimum value around 15 wt % ethanol; that is, it follows the cosurfactant effect. However, in the presence of PVP, the onset of the interaction, denoted cmc(1), between the surfactant and the polymer is considerably less dependent on ethanol concentration. The saturation point, cmc(2), however, reflects the behavior of the cmc in that it decreases upon addition of ethanol. This results in a decrease in the amount of surfactant bound to the polymer [C(bound) = cmc(2) - cmc] at saturation. The viscosity of simple PVP solutions depends on ethanol concentration, but since SANS studies show that ethanol has no effect on the polymer conformation, the changes observed in the viscosity reflect the viscosity of the background solvent. There are significant increases in bulk viscosity when the surfactant is added, and these have been correlated with the polymer conformation extracted from an analysis of the SANS data and with the amount of polymer adsorbed at the micelle surface. Competition between ethanol and PVP to occupy the surfactant headgroup region exists; at low ethanol concentration, the PVP displaces the ethanol and the PVP/SDS complex resembles that formed in the absence of the ethanol. At higher ethanol contents, the polymer does not bind to the ethanol-rich micelle surface.

10-Methylacridinium ion as a fluorimetric probe measuring the activity of halide anions in aqueous solutions of cationic surfactants

The quenching of fluorescence of 10-methylacridinium ion by inorganic anions in aerated aqueous solutions was studied at room temperature. In the case of cationic surfactants, with chloride and bromide anions as counterions, characteristic breaks on the Stern-Volmer plots could be observed at concentrations corresponding to the critical micelle concentration of the surfactants. It is shown that the ratio of the slopes of the two linear fragments of the plots, in the micellar and premicellar concentration ranges, gives an estimate of the value of the ionization degree, alpha, of the micelles. This approach is applicable also in aqueous-alcohol systems.

Self-assembly in linker-modified microemulsions

Linker molecules are added to microemulsion systems to enhance the interaction between the surfactant and oil (lipophilic linkers) or water (hydrophilic linkers) phases. Previous results suggest that when lipophilic and hydrophilic linkers are combined they behave as a self-assembled surfactant at the oil/water interface. In this work we investigate this self-assembly phenomenon as a function of surfactant, linker and electrolyte concentration. We find that middle phase microemulsion appears at a specific concentration higher than the critical micelle concentration (CMC), which we denote as the critical middle phase microemulsion concentration (CmicroC). When the lipophilic linker dodecanol is added in equimolar ratio to the hydrophilic linker sodium mono- and dimethyl naphthalene sulfonate (SMDNS), the middle phase microemulsion did not appear until the surfactant sodium dihexyl sulfosuccinate (SDHS) concentration was larger than the CmicroC of the SDHS-alone system. Dodecanol is shown to segregate near the surfactant tails following a Langmuir-type adsorption process. This segregation is not affected by the electrolyte concentration but is significantly reduced when the surfactant (SDHS) concentration approaches the CmicroC. The data suggest that the self-assembly between hydrophilic and lipophilic linkers to form middle phase microemulsions is only possible if a minimum amount of surfactant is present.

Influence of the addition of alcohol on the reaction methyl-4-nitrobenzenesulfonate + Br(-) in tetradecyltrimethylammonium bromide aqueous micellar solutions

The reaction methyl-4-nitrobenzenesulfonate + Br(-) was studied in tetradecyltrimethylammonium bromide (TTAB) aqueous micellar solutions in the absence and in the presence of various amounts of n-hexanol, n-pentanol, and n-butanol. Kinetic micellar effects provoked by the addition of the linear alcohols can be rationalized by using simple pseudophase kinetic models. The equilibrium binding constants of the methyl-4-nitrobenzenesulfonate molecules to the cationic micelles decreases when [alcohol] increases. The (k(2)(m)/V(m)) values found are practically the same for the different TTAB-alcohol micellar solutions studied, independent of the nature and concentration of the alcohol present in the reaction medium. This has been explained by considering the balance of two factors operating on reactivity in opposite ways: (1). an increase in the volume of the micellar interfacial region upon increasing alcohol concentration, and (2). a decrease in the polarity of the interfacial region as the amount of alcohol present in the micellar solutions increases.

Micellar behavior of tetradecyldimethylbenzylammonium chloride in water–alcohol mixtures

The effect of butanol and benzyl alcohol on the critical micelle concentration and the degree of ionization of micelles of tetradecyldimethylbenzylammonium chloride has been studied conductometrically in the temperature range of 5 to 40 degrees C at 5 degrees C intervals. The results indicate that some self-association process of benzyl alcohol in the aqueous phase commences when its concentration amounts to ca 0.05 m. By applying the theoretical treatment suggested by Motomura for binary surfactant systems, the molar fraction of alcohol in the micelle and the standard free energy of solubilization were estimated from the dependence of cmc on temperature and alcohol concentration. For comparative purposes the micellar properties of tetradecyltrimethylammonium bromide in water-alcohol systems at 25 degrees C have also been studied.

Micellar Solubilization: Structural and Conformational Changes Investigated by 1H and 13C Liquid-State NMR

A combination of (1)H self-diffusion measurements and (13)C chemical shift analysis has been used for the study of the solubilization of amphiphilic additives C(n)H(2n+1)X (n=4, 6; X=OH, NH(2)) in cetyltrimethylammonium bromide (CTAB) micelles. This approach, which could be extended to other mixed systems, allows complementary data on structures of micelles and conformations of alkyl chains to be obtained. Previous results on these systems are confirmed and new features emerge. All the additives studied behave as cosurfactants. Their degree of solubilization is determined solely by their alkyl chain length. In the case of cosurfactants with n=6, polar head group differences have been shown to modify micellar structures. This effect has been linked to differences in cosurfactant penetration into micelles. In parallel, the area of CTAB head groups and the length of cetyl chains gradually decrease on average when solubilizing more cosurfactants. Cetyl chain compression is strong in the case of cosurfactants with n=4, whereas it is slightly compensated by an extension below the micellar surface in the case of longer cosurfactants. These conformational changes are related respectively to the formation of smaller spherical micelles and of anisotropic or swollen micelles.

Kinetic Micellar Effects in Tetradecyltrimethylammonium Bromide–Pentanol Micellar Solutions

The reactions 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane + OH(-) and 2-(p-nitrophenyl)ethyl bromide + OH(-) were studied in tetradecyltrimethylamonium bromide, TTAB, and TTAB-pentanol micellar solutions. The influence of changes in the surfactant concentration as well as changes in the hydroxide ion concentration on the observed rate constant was investigated. If changes in the cmc and ionization degree provoked by the presence of the different amounts of n-pentanol in the micellar solutions are taken into account, the experimental kinetic data can be rationalized quantitatively by using the PIE model. Assuming that the ion-exchange equilibrium constant, K(OH(-)/Br(-)), for the competition between the bromide and the hydroxide ions in all TTAB and in TTAB-pentanol micellar solutions studied is the same, a good agreement between the theoretical and the experimental kinetic data was found in all the micellar media for the two processes studied. This assumption was checked by experimentally determining the ion-exchange equilibrium constant K(OH(-)/Br(-)) in TTAB and TTAB-pentanol micellar solutions through a spectroscopic method, results showing that the presence of n-pentanol does not affect substantially the value of K(OH(-)/Br(-)). The second-order rate constants obtained from the fittings decrease slightly when the amount of pentanol increases, being greater than that in aqueous solution. This acceleration can be explained considering that micelles accelerate the reactions in which the charge is delocalized in the transition state.

Factorial design of electrolyte systems for the separation of fatty acids by capillary electrophoresis

In this work, a capillary zone electrophoretic methodology using UV indirect detection (224 nm) for the analysis of fatty acids (FAs) in saponified oils is proposed. The electrolyte consisted of a 5 mmol l(-1) phosphate buffer, pH 7. containing 4 mmol l(-1) sodium dodecylbenzenesulfonate (SDBS) as chromophore, 4 mmol l(-1) dimethyl-beta-cyclodextrin and 45% acetonitrile (ACN). The composition of the electrolyte was optimized by a 2(3) factorial design with triplicate at the central point. The design established practical concentration boundaries for SDBS and ACN. In a defined concentration range of 2-4 l(-1), SDBS can certainly be used as a chromophore for indirect detection without imparting excessive baseline noise. For ACN, a suitable interval of 45-55% was found to enhance FAs solubilization without overflowing the system with bubble formation and current interruption. Additionally, the design revealed the importance of dimethyl-beta-cyclodextrin in the resolution of difficult pairs and its function as a solubilizing agent for long chain FAs. At the optimized conditions, nine FAs from C10 to C20, including mono- di- and tri-unsaturated C18 fatty acids were baseline separated in less than 10 min. The proposed method was applied to the separation of FAs in edible oils and polyunsaturated fatty acid enriched margarine. Additionally, spectral monitoring at 206 nm was used to confirm peak identity in the samples.

Shape Transition in Micelles of Undecylammonium Chloride in the Presence of Sodium Chloride and n-Butanol: A Study by Dynamic Light Scattering

Dynamic light scattering measurements have been performed on micellar solutions of undecylammonium chloride in the presence of 0 to 0.5 mol dm(-3) n-butanol and 0 to 0.2 mol dm(-3) sodium chloride. The results support opposing effects of these additives on micellar growth, with the n-butanol causing a reduction of micelle size. The dependence of the aggregation number on the hydrodynamic radius suggests a transition from sphere to prolate ellipsoid when the aggregation number approximates that predicted for the maximum aggregation number of a spherical micelle. Micelle dimensions calculated from the aggregation number, conformational considerations, and packing criteria of the monomers in the micelle are compared with those from dynamic light scattering data. Copyright 2001 Academic Press.

Effect of a variety of organic additives on retention and efficiency in micellar liquid chromatography

The effect of 21 organic additives (alkanols, alkane diols, dipolar aprotic solvents, alkanes) on the chromatographic behavior (retention, elution strength, efficiency) of probe solutes of widely differing hydrophobicity, such as benzene and 2-ethylanthraquinone, have been examined using a C18 stationary phase and sodium dodecyl sulfate (SDS) micellar mobile phases. The mobile-phase elution strength parallels the octanol-water partition coefficients of the additives or their ability to bind to the SDS micellar system, due to the increased solubility in the mobile phase and reduced affinity for the additive-modified surfactant-coated stationary phase. The comparison of the elution strength of micellar mobile phases with that of a reference acetonitrile-water system indicates that the elution strength is lower for micellar systems and depends on the nature of the eluted solute. The displacement of the solute-micelle and solute-stationary phase binding equilibria is quantified for several probe solutes eluted with micellar mobile phases in the presence of 1-propanol, 1-butanol, 1-pentanol, and acetonitrile. A correlation was also observed between the number of theoretical plates and the hydrophobicity of the alcohol additives: the efficiency initially increased steeply and reached a plateau. Compared to benzene, a more hydrophobic additive was needed to attain the maximum efficiency for the more hydrophobic 2-ethylanthraquinone analyte. Dipolar aprotic solvents appear to be somewhat more effective in enhancing the efficiency than alcohols. The results are rationalized in terms of the ability of the organic additives to alter the composition, structure, dynamics, and properties of the micelles and the surfactant-coated stationary phase.

Miscibility of Butanol and Cationic Surfactant in the Adsorbed Film and Micelle

Mixed adsorbed films and micelles were investigated by measuring surface tension of the aqueous solutions for the following four mixtures: the n-butanol (NBA)-dodecylammonium chloride (DAC), t-butanol (TBA)-DAC, NBA-dodecyltrimethylammonium chloride (DTAC), and TBA-DTAC. Experimental results were analyzed thermodynamically. Phase diagrams of adsorption and micelle formation and that exhibiting the composition relation of micelle and adsorbed film at CMC were compared among the four systems. Excess Gibbs energy in the adsorbed film gH,E and activity coefficient in the micelle fM2+/- were calculated from the phase diagrams to estimate the miscibility of the butanol and cationic surfactant quantitatively. By comparing the results of the pairs of NBA-DAC and TBA-DAC systems with those of the NBA-DTAC and TBA-DTAC systems, it was shown that the substitution of the polar head group from ammonium to trimethylammonium changes the gH,E value from positive to negative and increases fM2+/-. Furthermore, by comparing the results of the pairs of NBA-DAC and NBA-DTAC systems with those of the TBA-DAC and TBA-DTAC systems, it was shown that the bulky hydrophobic group weakens the molecular interaction between the butanol and the cationic surfactant in the adsorbed film. Copyright 1998 Academic Press.

Structure Effect on Miscibility of Butanol and Dodecyltrimethylammonium Chloride in Adsorbed Film

Miscibility of components in the adsorbed film for the n-butanol (NBA)-dodecyltrimethylammonium chloride (DTAC) and tert-butyl alcohol (TBA)-DTAC mixtures were investigated by measuring surface tension of the aqueous solutions. The experimental results were analyzed thermodynamically. Diagrams of the excess Gibbs energy in the adsorbed film gH,E vs the composition in the adsorbed film XH<INF POS="STACK">2 were obtained from the phase diagrams of adsorption. The gH,E values of the NBA-DTAC system are more negative than those of the TBA-DTAC system. It is concluded that the molecular interaction between NBA and DTAC is stronger than that between TBA and DTAC. With respect to the mean molar area in the adsorbed film A vs XH<INF POS="STACK">2 curve at a fixed surface tension gamma, the criterion of ideal mixing is demonstrated to be a straight line connecting the pure values. The A vs XH<INF POS="STACK">2 curves are convex downward in both systems and the deviation from the straight line is attributed to the dependence of gH,E on gamma. Copyright 1998 Academic Press.