Laser diagnostics of self-organization of amphiphiles in aqueous solutions on the example of sodium octanoate

In this study, a method of non-contact laser diagnostics of self-organization processes of surfactants in aqueous solutions is proposed. The study of solutions of sodium octanoate using laser Raman and near-IR spectroscopy has shown that spectral bands of stretching vibrations of CC-, CH- and OH-groups (including second overtone of the OH-vibrations) of the surfactant molecules and water, as well as bands of intermolecular vibrations of hydrogen bonds O-H center dot center dot center dot O are sensitive to the change the conformation of the hydrocarbon radical of the surfactant and the dynamics of hydrogen bonds during self-organization of surfactants in solution. It was found that the dependence of the spectral characteristics of these bands on concentration of surfactants has peculiarities in the region of critical premicelle concentration (CPMC), first critical micelle concentration (CMC1) and second critical micelle concentration (CMC2). A method of determining the critical concentrations of surfactants in water based on the revealed peculiarities is proposed. A comparative analysis of the obtained CMC values using laser Raman and IR spectroscopy and those measured by independent contact methods - pH-metry, conductivity, surface tension measurement - has shown a good match of the determined parameters. Advantages of the method of laser diagnostics of surfactant self-organization in water were discussed. (C) 2020 Elsevier B.V. All rights reserved.