Micellisation kinetics of ionic surfactants

Abstract

For solutions of ionic surfactants a recent equilibrium theory leads to the conclusion that previous treatments of micellisation kinetics require modification to allow for changes in rate coefficients with solution composition. These changes arise from micelle–counterion interactions and other factors responsible for non-ideal behaviour. For the fast process associated with micelle–monomer equilibria the appropriate modifications are obtained by (a) supposing that the specific reaction rates for molecules in micellar and monomeric states depend on a small number of composition variables all of which are independently variable at equilibrium and (b) using correct equilibrium conditions in deriving the kinetic equations. This method also applies to multi-component micelles. It allows for non-ideal mixing therein and for changes in micellar degree of dissociation with micelle composition. The treatment of the slower process associated with changes in the concentration of micelles is free from the approximations associated with (a). For single component micelles the new expressions for fast and slow relaxation times become identical to the previous expressions when applied to non-ionic surfactants. The additional terms for ionic surfactants have minor implications in the interpretation of experimental data. However, the inconsistency involved in applying previous treatment is removed. Methods similar to those developed for micelle–monomer exchange may also be of value in other kinds of non-ideal system. As an example, the kinetics of solute hydration in aqueous solutions is briefly considered.