THE EFFECT OF CHAIN PACKING ON SURFACTANT AGGREGATION IN AQUEOUS-SOLUTION

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