Light scattering of sodium dodecyl sulphate in methanol-water mixtures

Modification of adsorption, aggregation and wetting properties of surfactants by short chain alcohols

The adsorption of methanol, ethanol and propan-1-ol at the solution-air and solid-solution interfaces, their aggregation in the aqueous media as well as wetting properties regarding their applications as additives or co-surfactants in the surfactants aqueous solution were discussed based on the literature data. Mutual influence of alcohols and surfactants on the solution-air and solid-solution interface tension was considered. For this purpose there were used different methods allowing to describe or predict changes of water surface tension as a function of alcohols concentration. These, in turn, as a function of alcohol and/or surfactant concentration were also analyzed by means of the methods applied for prediction of surface tension of aqueous solution of the classical surfactants mixture. The same considerations related to the behaviour of alcohol and surfactant at the solid-solution and solution-air interfaces were made. To explain the behaviour of alcohols and surfactants mixture at the solution-air and solid-solution interfaces the components and parameters of water, alcohols, surfactants and solids surface tension as well as the Gibbs free energy changes during the adsorption process were taken into account. It was proved that wettability of some solids can be predicted based on alcohol and surfactants adsorption as well as surface tension components and parameters. As follows the mutual influence of alcohol and surfactant on their adsorption at the solution-air and solid-solution interfaces as well as on the wetting properties at the alcohol concentration from zero to its critical aggregation concentration (CAC) is different from that at its concentration higher than CAC.

Thermodynamic modeling of the duality of linear 1-alcohols as cosurfactants and cosolvents in self-assembly of surfactant molecules

The effect of adding an alcohol to surfactant systems depends much on the alcohol chain length. Investigations on the effect of alcohols in micellar systems point out that medium-chain alcohols are appreciably incorporated in the micellar phase whereas short-chain alcohols are localized mainly in the aqueous phase. Nonetheless, penetration of the hydrocarbon chain of alcohols in the micellar shell has been experimentally observed for the entire homologous series of linear 1-alcohols. We present a thermodynamic model in which the alcohol molecules play two roles: cosurfactant and cosolvent. The cosurfactant effect of the alcohols is included by assuming that the alcohol molecules are nonionic surfactants. The cosolvent effect is modeled by accounting for the changes in the free energy to relocate the surfactant tail from the solvent to the aggregate core. The effects of short-chain alcohols in the macroscopic interfacial tension and dielectric constant of the solvent medium are also taken into account. For short-chain alcohols the partition coefficient of the alcohols between water and liquid hydrocarbons provides knowledge of the fraction of the molecules that participate in each function. Our proposed thermodynamic model improves the modeling of the effect of short- and medium-chain alcohols in self-assembly of molecules that are of increasing importance in modern scientific research and technological processes.

Study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Sodium dodecyl sulfate (SDS)-poly(propylene oxide) methacrylate (PPOMA) (of molecular weight M(w) = 434 g x mol(-1)) mixtures have been studied using conductimetry, static light scattering, fluorescence spectroscopy, and 1H NMR. It has been shown that SDS and PPOMA form mixed micelles, and SDS and PPOMA aggregation numbers, N(ag SDS) and N(ag PPOMA), have been determined. Total aggregation numbers of the micelles (N(ag SDS) + N(ag PPOMA)) and those of SDS decrease upon increasing the weight ratio R = PPOMA/SDS. Localization of PPOMA inside the mixed micelles is considered (i) using 1H NMR to localize the methacrylate function at the hydrophobic core-water interface and (ii) by studying the SDS-PPO micellar system (whose M(w) = 400 g x mol(-1)). Both methods have indicated that the PPO chain of the macromonomer is localized at the SDS micelle surface. Models based on the theorical prediction of the critical micellar concentration of mixed micelles and structural model of swollen micelles are used to confirm the particular structure proposed for the SDS-PPOMA system, i.e., the micelle hydrophobic core is primarily composed of the C12 chains of the sodium dodecyl sulfate, the hydrophobic core-water interface is made up of the SDS polar heads as well as methacrylate functions of the PPOMA, the PPO chains of the macromonomer are adsorbed preferentially on the surface, i.e., on the polar heads of the SDS.

Light Scattering Study of the Effect of n-Butanol on the Micellar Properties of Undecylammonium Chloride in Aqueous Electrolyte Solutions

Light scattering measurements have been performed on aqueous solutions of undecylammonium chloride in the presence of 0 to 0.2 mol dm(-3) NaCl and 0 to 0.5 mol dm(-3) n-butanol at 25 degrees C. The critical micelle concentration (CMC), aggregation number, and degree of dissociation of the micelles have been determined. The observed decrease of the CMC with the increase of the n-butanol concentration was explained by the effect of the n-butanol on water structure and by the selective solvation of the micelles with n-butanol, which counteract the decrease of the polar character of the solvent caused by n-butanol addition. An observed increase in the degree of dissociation of the micelles and a decrease in the aggregation number following alcohol addition have been explained by considering the effect of this additive on the electrostatic and other interactions involved in free energy of micellization. Our results support the concept of opposing effects between n-butanol and NaCl on the cooperativity in the micellization process of this surfactant, with the n-butanol disfavoring micellar growth. Copyright 2000 Academic Press.

Spin label study of detergents in the region of critical micelle concentration

A series of spin probes was employed to examine the behavior of the detergent sodium dodecyl sulfate (SDS) at concentrations above and below the critical micelle concentration (cmc). The existence of detergent aggregates below the cmc was evidenced by the appearance of composite electron spin resonance (ESR) spectra for probes that have measurable solubility in water. The spectra were indicative of two probe populations: one in an aqueous environment and another in detergent aggregates. The ESR spectra of probes which are highly insoluble in water exhibited line broadening due to intermolecular spin exchange interactions, indicating that the probes were concentrated in detergent aggregates present below the experimental cmc. The results are discussed in terms of their significance for the study of the mechanisms of micelle formation and for the detection of detergent aggregates at very low concentrations.