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

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 (Corradini, D.; Rovere, M.; Gallo, P. J. Chem, Phys. 2010, 132, 134508) 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.