Molecular dynamics simulations study of supercooled aqueous solutions of sodium perchlorate: the effect of Martian solutes on thermodynamics and structure of water*

I will show recent results of molecular dynamics simulations on the influence of solutes on the phase diagram of water. I will focus into the supercooled region where the anomalies that water shows with respect to other simple liquids, are enhanced. After revieweng previous results with alkaly halides solutes, I will concentrate on the thermodynamic behavior of sodium perchlorate solutions in supercooled water. These solutions are of special interest because of the recent experimental results that led to hypothesize the presence of liquid water in perchlorate solutions beneath the Martian soil. Water is modeled using the TIP4P/2005 potential. The phase diagrams obtained for solutions with concentrations 1.63 and 15.4 wt% show the temperature of maximum and minimum density lines and a liquid–liquid phase transition where the liquid–liquid critical point shifts to slightly higher temperatures and lower pressures with respect to the bulk water value. The structure of the systems is also analyzed, and it is shown that even at the highest concentration considered, the solutions still retain water anomalous behavior. These anomalies might play an important role in the existence of Martian liquid water since they are controlled by the local structure of water that also controls ice nucleation.