Aggregation of amphiphiles in nonaqueous media

Abstract

The physical factors controlling aggregation of surfactants in nonaqueous solutions are identified and their contribution to the standard free energy of formation of the aggregates are calculated. In contrast to aqueous systems, there is no strong tendency (like that due to the hydrophobic bonding) favoring the formation of large aggregates to the formation of small aggregates. The resulting size distribution is such that under all conditions larger amounts of surfactant molecules are contained within the small aggregates than within the larger ones. For these reasons, the size dependent properties, such as the apparent average aggregation numbers, change only gradually with increasing surfactant concentration. Hence, in contrast to the aqueous systems, the nonaqueous systems do not exhibit a critical micelle concentration (CMC). The unique structure of water (which gives rise to the hydrophobic bonding of the amphiphiles) is responsible for the preferential formation of large aggregates at sufficiently large surfactant concentrations and, hence, for the occurrence of the cmc in aqueous systems. 23 references, 12 figures.