Mapping Structural Perturbations of Water in Ionic Solutions

Abstract

The structure of water in sodium halide aqueous solutions at different concentrations is studied through molecular dynamics. Emphasis is placed on the extent of ionic-induced changes in the water structure, and the concept of kosmotropes/chaotropes is probed, in terms of perturbations to the tetrahedral H-bond network of water. The results show that at low salt concentrations, the halide anions slightly increase the tetrahedrality of the H-bond network of water in the anionic second hydration shell and I(-) is found to be the strongest kosmotrope, contrary to its structure breaker reputation. The sodium cation in turn induces a significant loss of tetrahedrality in the second cationic hydration shell. At higher concentrations, the dominant disruptive effect of Na(+) cancels the anionic effects, even in the anionic second hydration shell. According to a kosmotropes/chaotropes classification of ions, based on the tetrahedrality of the H-bond network of water, halide anions are therefore weak kosmotropes, while Na(+) is a strong chaotrope. However, if this classification is applied to the salts, rather than to the ions, all of the sodium halides are classified as structure breakers even at low concentrations. Further, the effect of pressure on the tetrahedrality of the H-bond network of water is found to be similar to the average effect of the dissolved salts. The present results indicate that the classification of ions in kosmotropes/chaotropes in terms of long-range perturbations to the tetrahedral H-bond network of water is not correlated to the position of the ions in the respective Hofmeister series.