Solution Behavior of Surfactants in Ethylene Glycol: Probing the Existence of a CMC and of Micellar Aggregates

Abstract

We explore the solution behavior of surfactants in ethylene glycol from a theoretical point of view utilizing the thermodynamic approach developed earlier for aqueous solutions. In this approach, the standard free energy change on aggregation has three solvent-dependent contributions, namely, (a) the surfactant tail transfer free energy, (b) the aggregate core–solvent interfacial free energy, and (c) the free energy of ionic headgroup interactions at the aggregate surface. We develop estimates for these free energy contributions corresponding to ethylene glycol as the solvent and use them to compute the size distribution of aggregates for alkyl (C12, C14, C16) pyridinium bromides and alkyl (C10, C12, C14, C16) trimethyl ammonium bromides. For all surfactants considered, the theory predicts that aggregates form only at very large surfactant concentrations in contrast to the behavior exhibited by these surfactants in water. Further, the predicted aggregation numbers are quite small. The aggregates seldom exceed in size small oligomers when the surfactant tail length is not too large (fornc= 12 or below). As a result of the small aggregation numbers, the size-dependent solution properties are shown to change only gradually as a function of the total surfactant concentration rather than exhibiting any sharp transitions, thus contributing to the ambiguity in the precise determination of the critical micelle concentration (CMC). Finally, we show that the weak cooperativity in the association process, the resulting small sizes for the aggregates and the corresponding uncertainty in the determination of the CMC are all linked mainly to the surfactant tail transfer free energy and the aggregate core–solvent interfacial free energy contributions and not to the ionic headgroup repulsions.