Excess entropy of water in a supercooled solution of salt

Abstract

We consider the relationship between the excess entropy and anomalies of water. We investigate by molecular dynamics simulations the thermodynamic region of supercooled water and a supercooled aqueous solution with NaCl at a salt concentration of 0.67 mol kg⁻¹. The TIP4P potential model displays, as already established, in pure water and in solution a phase diagram with a liquid–liquid critical point. We explore how the two-body excess entropy calculated from the radial distribution functions is an indicator of density and structural anomalies of supercooled liquid water, both in the pure system and in the NaCl(aq). The two-body excess entropy shows a peculiar behaviour associated with the density anomaly and structural changes in water as revealed by the radial distribution functions. The signature of a change in the structural relaxation of water from fragile to strong is also found by examining the behaviour of the excess entropy at decreasing temperature.

Keywords:

• ionic aqueous solutions
• excess entropy
• supercooled water
• anomalies of water
• structure of supercooled water

Acknowledgements

We gratefully acknowledge the computational support of CASPUR, CINECA, and the Roma Tre INFN-GRID.