Aggregation of amphiphiles as micelles or vesicles in aqueous media

Abstract

The physical factors responsible for the aggregation of amphiphiles in aqueous media are examined and expressions for their contribution to the attractive or repulsive components of the free energy change of aggregation are established. Whereas in previous treatments, an arbitrary repulsive force was necessary to explain the behavior of nonionic systems, no such ad hoc assumption is made here. Rather the free energy changes due to interfacial tension at the hydrocarbon core (of the aggregates)-water interface and to the loss of a part of translational and rotational degrees of freedom of the amphiphiles when they aggregate are the two main repulsive contributions. On the other hand, the factors favoring aggregation are: (i) the van der Waals interactions between the hydrocarbon tails of the amphiphiles, and (ii) the structural changes in water and the changes in the interactions between amphiphiles and water resulting from aggregation. For ionic and zwitterionic amphiphilar systems additional free energy contributions are included to account for the repulsive electrostatic interactions between the head groups. For vesicles, the repulsion caused by the overlapping electrical double layers inside the vesicles is also considered. The expressions established for the various free energy changes associated with aggregation are used to examine the formation of micelles and vesicles, from single and double chain amphiphiles with nonionic, ionic, or zwitterionic head groups. In general, single chain amphiphiles aggregate as micelles, rather than as vesicles, for all types of polar head groups. Depending upon the nature of the head groups small and/or large micelles can form. Nonionic amphiphiles which have head groups of small cross-sectional areas form large micelles, whereas those with large cross-section aggregate as small micelles. This happens because the repulsion caused by the loss of translational degrees of freedom is larger in the latter of the two cases. Ionic or zwitterionic amphiphiles form small micelles even though they have small head groups because of the electrostatic repulsion between the head groups. At large ionic strengths, large micelles can form because the repulsive interactions between the head groups are small. Nonionic double chain amphiphiles aggregate predominantly as vesicles. Ionic or zwitterionic double chain amphiphiles aggregate as micelles when the electrostatic repulsion between the head groups is large and as vesicles when this repulsion is small. However for intermediate values of these interactions both micelles and vesicles form depending upon the length of the hydrocarbon tail. Most biologically significant double chain amphiphiles have long, complex polar head groups and they aggregate as micelles when the hydrocarbon tail length is short, even if the electrostatic repulsion between the head groups is weak; but they aggregate as vesicles when the hydrocarbon tail length is long. The size distributions calculated for different types of amphiphiles can be unimodal, representing a single population of aggregates, bimodal, or trimodal representing the coexistence of two or three distinct populations of aggregates. The three possible populations are small micelles, large micelles, and vesicles.