A vibrational spectroscopic study of some surfactant systems

Abstract

Infra red and Raman spectroscopy has been used to study: (i) the aggregation process of some polyoxyethylene surfactants in apolar media,(ii) the microenvironment of water at and near the micellar surface in seme cationic surfactants, and(iii) the nature of the hydrocarbon core within cationic surfactant micelles. We have used the V(_s)(O-H) stretching vibration of some polyoxyethylene surfactants of the type C(_12)H(_25) (OCH(_2)CH(_2)) (_n)-OH [where n = 0 to 8] to quantify the aggregation process in a variety of solvents. These have included heptane, decane, benzene, carbon tetrachloride, deutero-chloroform and decalin. In doing this we have gone some way in understanding the nature and mechanism for aggregation in such systene. Mid and far ir spectrcscopy has enabled us to study the nature of water at and near the mioellar surface in dodecyl and hexadecyl trimethyl ammonium chloride mesophase systems. We have studied the decoupled v(_s)(O-D) band of HDO and the v(_A)(H(_2)0), the combination mode of water in the mid ir. In addition v(_T), the intemolecular translational mode of water, in the far ir of these systems, has also been investigated. The results have been treated collectively, and a generalized model for the state of water has been proposed. Spectra for dodecyltrimethyl ammonium bromide were also obtained and compared with the corresponding chloride data. Raman spectroscopy of the hydrocarbon core in cationic surfactant micelles has shown that no abrupt changes occur in the gauche/trans equilibrium on crossing a (iiase boundary, with the exception of the gel/lamellar phase (where this is expected). However, examination of the C-ll stretching region has shown that the anisotropic phases (i.e. H(_1), L(_a)) display an increased lateral interaction. The isotropic phases (i.e. L(_1), I(_1), V_1)), show less interaction while the first intermediate phase in C(_16)TACl shows behaviour between the two.