Phase diagram and structure of TIP4P/2005 water and magnesium perchlorate: A molecular dynamic study of Martian aqueous solutions

We investigate the phase diagram and the structural behaviour of magnesium perchlorate solutions in supercooled TIP4P/2005 water via molecular dynamics numerical simulations for solute concentrations of 2.95 wt% (mole fraction 2.45×10−3) and 24.4 wt% (mole fraction 25.4×10−3). These solutions hold particular significance due to recent experimental findings about the presence of liquid water in perchlorate solutions underneath the Martian surface. Upon increasing the concentration of the solutes, an interplay between the high-density liquid (HDL) and the low-density liquid (LDL) phases of water is found. Similar to the pure phase of water, at low concentration the system shows a liquid-liquid critical point. No liquid-liquid critical point was found for the higher concentration. Nonetheless, several water anomalies persist in both solutions. We observe that the LDL region shrinks as the concentration is increased. The structural analysis cofirms the interplay between HDL and LDL, albeit HDL local structure prevails in the solutions more than in pure water.