Structural Properties of High and Low Density Water in a Supercooled Aqueous Solution of Salt

TL;DR

This study compares the structural properties of supercooled bulk water and saltwater, revealing how ions influence water's HDL and LDL phases, especially in the LDL structure, through chloride substitution in oxygen coordination shells.

Contribution

It demonstrates how sodium chloride ions modify the LDL structure of supercooled water, making it more similar to HDL, and elucidates the microscopic mechanism behind this change.

Findings

Ions alter the LDL structure of supercooled water.

Chloride ions replace oxygen in coordination shells, modifying water structure.

Water-water structure in saltwater becomes more HDL-like in LDL regions.

Abstract

We consider and compare the structural properties of bulk TIP4P water and of a sodium chloride aqueous solution in TIP4P water with concentration c = 0.67 mol/kg, in the metastable supercooled region. In a previous paper [D. Corradini, M. Rovere and P. Gallo, J. Chem. Phys. 132, 134508 (2010)] we found in both systems the presence of a liquid-liquid critical point (LLCP). The LLCP is believed to be the end point of the coexistence line between a high density liquid (HDL) and a low density liquid (LDL) phase of water. In the present paper we study the different features of water-water structure in HDL and LDL both in bulk water and in the solution. We find that the ions are able to modify the bulk LDL structure, rendering water-water structure more similar to the bulk HDL case. By the study of the hydration structure in HDL and LDL, a possible mechanism for the modification of the bulk LDL structure in the solution is identified in the substitution of the oxygen by the chloride ion in oxygen coordination shells.