Molecular Dynamics Simulations of Supercooled Aqueous Solutions of Calcium Perchlorate: Thermodynamics and Structure of Martian Solutes in TIP4P/2005 Water

We employ molecular dynamics simulations to determine how calcium perchlorate modifies the phase diagram and structure of supercooled TIP4P/2005 water. These solutions are particularly relevant in light of recent experimental evidence of liquid water in perchlorate solutions underneath Martian soil. We focus on the interplay between its low-density liquid (LDL) and high-density liquid (HDL) phases, simulating solutions at concentrations of 3.15 and 25.7 wt%. Thermodynamic analysis confirms the persistence of several water anomalies, albeit shifted by the solutes. A second-order liquid-liquid critical point (LLCP) is observed at a concentration of 3.15 wt%. Structurally, radial distribution functions demonstrate that upon increasing the solute concentration, the HDL-like behavior is enhanced, as the thermodynamic LDL-like region appears to be shrinking. Calcium ions with perchlorate ions in water appear to be particularly effective at favoring supercooling.