A DIELECTRIC METHOD TO INVESTIGATE THE INTERFACIAL COMPOSITION OF MICELLAR AGGREGATES

The aggregation state of micellar solutions is mainly determined by th e specific chemical and physical conditions within the interfacial reg ion constituted by the polar head terminations and solvent molecules. In particular the mutual head group interactions and their interaction s with solvent and cosolvent molecules strongly affect the overall sha pe, size and size distribution function of micellar solutions. It then becomes evident how important the determination of the composition an d structural arrangement of the interfacial region is. Permittivity me asurements of an heterogeneous system allow the evaluation of the perm ittivity of the suspended particles using one of the available mixture equations. If the suspended particles are constituted by separated re gions with different dielectric properties it is possible to iterate t he procedure to extract information on each of the regions. In the cas e of micellar aggregates there is the hydrocarbon core region, equival ent 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 mixtu res composed by the solvent and free head groups, it is possible to ev aluate the composition of the micellar interface. We apply this method ology on two different surfactant mixtures: C12E6 in water and in wate r-urea (2, 4 and 6 M); octyl-beta-D-glucopyranoside in water and in wa ter-glyclne (0.3 and 0.6 M). The results obtained concerning the confo rmation and composition at the interface are consistent with the overa ll behaviour of the solutions studied by many other different techniqu es supporting the proposed procedure. (C) 1998 Elsevier Science B.V. A ll rights reserved.