Perturbation of liquid-water structure by ionic substances

• Published in: Journal of molecular structure 1990-09, Vol.237, p.411-419
• Main Authors: Luu, D.V., Cambon, L., Mathlouthi, M.
• Format: Article
• Language: English
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/0022-2860(90)80155-D

The effect of ionic substances with different solubilities on liquid-water structure is studied by analyzing the laser-Raman spectra in the region of OH stretching. A model of the water structure is presented. This model is derived from the decomposition of the experimental Raman band of water in the OH region using a semi-empirical method for the calculation of the components of the observed band. Four components are found: a quasi-crystalline phase composed of dimers and tetramers of H 2O, a solid-like amorphous phase composed of flickering pentamer clusters, a liquid amorphous species formed with trimer balls, and a small amount of unassociated H 2O molecules. Analysis of the spectra of soluble salts (LiCl, CaCl 2) in the OH region shows a decrease in the total radiant energy and important shifts in the frequency and intensity of the component assigned to quasi-crystalline water. The decrease in intensity of this component is compensated by an increase of the intensity of the band assigned to the solid-like amorphous phase. The two other components are almost unchanged. The effect of less soluble salts (Li 2CO 3, ZnF 2) on the structure of water is not very marked. An increase in the total radiant energy is observed as well as slight shifts in the frequency and intensity of the four components. Analysis of the results reveals that the perturbation of liquid-water structure by ionic substances is more pronounced the more soluble the substance. Modifications of the hydrogen bonding in the clusters leads to modification of their relative weights and of their energy of vibration.