Raman Spectroscopic Study of Water Structure in Aqueous Nonionic Surfactant Solutions

Water structure in aqueous solutions of two poly(oxyethylene) surfactants, C12E5 and C12E6, were studied by Raman spectroscopy in the whole range of concentrations. A method based on recording Raman spectra with polarization rotated to 60° with respect to the excitation light was used, allowing one to follow isothermal paths that intersect phases with different polarization properties, viz., the micellar solution phase (isotropic), lamellar phase (anisotropic), and reverse micellar solution phase (isotropic). Besides the O−H stretching band, the conformational bands of the oxyethylene groups were measured to evaluate the dehydration process of the hydrophilic headgroups of the surfactant. The results showed that the behavior of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes of the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant headgroups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at very low content in the system.