The influence of the solvent on the formation of micelles in colloidal electrolytes. I. Electrical conductivities of sodium dodecyl sulphate in ethyl alcohol-water mixtures

Abstract

In very dilute solutions in water sodium dodecyl sulphate behaves as a completely dissociated electrolyte. Above a concentration c = 0.00722 N the equivalent conductance ( Λ ) falls sharply and micelles are formed. To throw light on the mechanism of micelle formation, the variation of Λ with c has been measured in a series of mixtures of water and ethyl alcohol. The curves for Λ against √c show a gradual transition from the type associated with micelle formation to a uniform curve characteristic of strong electrolytes. Addition of alcohol decreases the tendency to form micelles. When there is 40% or more by weight of alcohol, micelles are no longer formed. The critical concentration first falls on addition of alcohol and then rises again. The effects of alcohol addition can be interpreted satisfactorily by considering the radius of the micelle to be equal to the length of the paraffin chain and thus to be independent of the concentration and nature of the solvent. The energy changes involved in micelle formation are calculated. Aggregation can occur if the interfacial energy available from the destruction of the paraffin-solvent interfaces of the ions is greater than the work to be done against electrical repulsion. The interfacial energy is no longer the greater beyond 40% of alcohol. It is shown that alcohol molecules are not in solution in the paraffin interior of the micelles in the mixed solvents, but are strongly adsorbed on the micelle surfaces. The influence of regions of lowered dielectric constant around the micelles is considered. This is more important at high concentrations. The final fall in Λ with highly supersaturated solutions is attributed to the increased viscosity of the solution.