Dynamical structure of water in NaCl aqueous solution

Abstract

We report on the dynamical structure of water in NaCl aqueous solutions as functions of temperature and concentration by a low-frequency Raman spectroscopy. The spectral profiles in the frequency range from −50 to 250 cm−1 have been analyzed with a superposition of one Cole–Cole type relaxation mode and two damped harmonic oscillator modes. The distributions of the characteristic frequencies of the intermolecular vibrational modes observed around 50 and 180 cm−1 in NaCl aqueous solutions are always wider than those in pure water. In NaCl aqueous solutions with various concentrations, the spectral line width g1=1/(2πcτ) of a central component, which corresponds to the reciprocal relaxation time, linearly changes with temperature from about 255 K to 300 K, while the relaxation time above 300 K holds an Arrhenius-type behavior. The slope of the spectral linewidth against temperature below 300 K decreases with increasing concentration. The distribution parameter of the relaxation time in the Cole–Cole type relaxation formula decreases with decreasing temperature and it becomes smaller with increasing concentration.