A dielectric study of systems exhibiting molecular association.

Time domain spectroscopic methods have been developed and in combination with bridge methods have provided interpretable data on selected alcohol-water systems and micellar solutions of nonionic surfactants. Dielectric relaxation studies have shown that addition of water to an alcohol causes a reduction in the relaxation time of the principal dispersion which for all compositions is Debye-type. The relaxation behaviour has been interpreted using a model based on hydrogen-bond rupture and subsequent dipole reorientation. The possibility of relaxation times being modified by the formation of associated species has been examined. Static permittivites of the polyoxyethylene dodecyl ether surfactants C[12]E (n = 4, 6 and 8) and Brij 30 in binary mixtures with water are interpreted as originating from normal micelles at all compositions. In C12E4 and Brij 30 mixtures, the behaviour at higher surfactant concentrations is most successfully described by heterogeneous mixture equations for spherical dispersions; deviations at low surfactant concentration are attributed to either oblate spheroidal micelle shape or secondary aggregation of micelles. Axial ratios for oblate spheroids have been calculated using equations derived in this work. In mixtures containing higher n-ethylene oxides, the heterogeneous mixture approach becomes progressively less applicable because of smaller micelle size and an increasing contribution of molecular association between surfactant headgroups and water. Static permittivities of C[12]E[4] and Brij 30 in ternary mixtures with water and an n-alkane are attributed to normal micelles at some compositions and inverted micelles at others. Results for microemulsions of C[12]E[4] have been interpreted as indicating micellar inversion in which normal spherical micelles change to oblate spheroidal with increasing temperature and then invert to form prolate spheroidal and finally spherical inverted micelles. Axial ratios for both types of spheroidal shape have been calculated.