LIGHT-SCATTERING STUDY OF PHASE-TRANSITIONS IN AQUEOUS-SOLUTIONS OF NONIONIC AMPHIPHILES

The dynamical behavior of water in aqueous solutions of the nonionic p olyoxyethylene surfactant C10H21(OCH2CH2)(5)OH has been studied using Raman and depolarized Rayleigh-wing scattering, with particular attent ion to changes due to the structural phase transitions apparent at rel atively large amphiphile concentrations. The frequencies and intensiti es of the two principal components evident in Raman spectra of the wat er O-H stretching band change at the hexagonal to isotropic phase tran sition. The width and intensity of the single resolvable line in the d epolarized Rayleigh spectra similarly change at this transition: the r elaxation time of the rotational water mode increases in the hexagonal phase. The changes are all consistent with an increase in the hexagon al phase of the proportion of water bound to the oxyethylene head grou ps: the environment of water in the system differs significantly from that in bulk water. At the lamellar to isotropic phase transition very much smaller changes were apparent, but only in the Raman scattering data. This lesser sensitivity suggests that in the lamellar phase almo st all of the water is bound to the amphiphile head groups, as in the isotropic phase at such volume fractions. The changes in water dynamic s reflect the structural changes in the solutions at the phase transit ions, in particular the differing packing constraints in the various p hases.