The Raman Spectra of Surfactants and The Concentration Dependence of Their Molecular Conformations in Aqueous Solutions

Abstract

The laser Raman scattering by potassium n-alkyl carboxylates and sodium n-alkyl sulfates was measured in the solid state and aqueous solutions, and the concentration dependence of molecular conformations of these molecules was studied. For aqueous solutions of potassium n-hexanoate and potassium n-pentanoate, the Raman intensities of the accordion vibrations of the all-trans form relative to the skeletal deformation vibrations of the gauche isomers were found to increase with an increase in the concentration. This intensity change is remarkable at the critical micelle concentration. A concentration dependence of the intensity of the Raman lines was also observed in the frequency region of 1600–600 cm−1. These observations revealed that the percentage of the all-trans form of the surfactant molecules increases with an increase in the concentration above the critical micelle concentration.