Attempt at a unified interpretation of the self-association of 1-1 ionic surfactants in solvents of low dielectric constant

Abstract

On the basis of electrostatic energies alone, 1-1 ionic surfactants in nonpolar solvents are expected to form cyclic dimers and compact oligomers. However, approximate model calculations show that when internal entropy contributions are included, strongly polar acyclic dimers and oligomers may become the species of lowest free energy. This implies that two distinct patterns of aggregation are available for these solutes, as seems to be required by available experimental results. The calculations suggest that when the solvent has a relatively high dielectric constant, ϵ, or the sum of the radii of the ionic headgroups, d, is large, association is best represented by stepwise sequential formation of open-chain oligomers with approximately equal equilibrium constants for the binding of additional monomers. When ϵ and d are both small, compact clusters are preferred. Their electrostatic binding energy increases with increasing aggregation number, n, but their growth is eventually limited at n = n max by the steric demand of the hydrocarbon chains. When n max ≥ 16 most of the aggregated surfactant should be present in the form of clusters with n ⋟ n max , and then the association process is much more nearly analogous to micellization in aqueous solutions.