A dielectric method to investigate the interfacial composition of micellar aggregates

Abstract

The aggregation state of micellar solutions is mainly determined by the specific chemical and physical conditions within the interfacial region constituted by the polar head terminations and solvent molecules. In particular the mutual head group interactions and their interactions with solvent and cosolvent molecules strongly affect the overall shape, size and size distribution function of micellar solutions. It then becomes evident how important the determination of the composition and structural arrangement of the interfacial region is. Permittivity measurements of an heterogeneous system allow the evaluation of the permittivity of the suspended particles using one of the available mixture equations. If the suspended particles are constituted by separated regions with different dielectric properties it is possible to iterate the procedure to extract information on each of the regions. In the case of micellar aggregates there is the hydrocarbon core region, equivalent to an oil liquid phase, and an interfacial region, constituted by the polar head group terminations, solvent and cosolvent molecules. By comparing the interfacial permittivity with the permittivity of mixtures composed by the solvent and free head groups, it is possible to evaluate the composition of the micellar interface. We apply this methodology on two different surfactant mixtures: C 12E 6 in water and in water–urea (2, 4 and 6 M); octyl- β- d-glucopyranoside in water and in water–glyclne (0.3 and 0.6 M). The results obtained concerning the conformation and composition at the interface are consistent with the overall behaviour of the solutions studied by many other different techniques supporting the proposed procedure.