The effect of chain packing on surfactant aggregation in aqueous solution

Abstract

Hydrophobic interactions provide the main driving force for the aggregation of amphiphilic compounds in water. However, the morphology and dynamic properties of the aggregate represent the outcome of a compromise between a variety of partly opposing factors. It is obvious that efficient alkyl chain packing in the interior of the molecular assembly is a prerequisite for stabilisation. Unfortunately, the highly dynamic nature of micelles and smaller aggregates hampers a straightforward experimental approach. Small amphiphilic molecules like t-BuOH, 2-butoxyethanol (2-BE), and N-cyclohexylpyrrolidone (CHP) start to form highly dynamic, small clusters at the critical hydrophobic interaction concentration (chic). In addition to other techniques, the occurrence of a chic can be probed using kinetic studies of suitable model reactions. A detailed study of the aggregation behavior of a series of l-alkyl-4-(C12-alkyl)pyridiniumiodides (1-11) illustrates that the preferred morphology of the aqueous surfactant assemblies is primarily determined by the molecular architecture of the surfactant molecule. Depending on the branching and stiffness of the 4-(C12-alkyl) moiety, the length of the 1-alkyl substituent and the surfactant concentration, the surfactants form spherical micelles, rod-like micelles or vesicles. These differences in aggregation behavior are rationalized by considering the variation of the packing parameter of the surfactant monomer as a function of surfactant structure. The chain packing in the core of the spherical micelles was probed by an analysis of proton T1 relaxation times within the framework of Wennerström's two-step model. © 1990 De Gruyter