Paolo La Francesca — Università degli studi Roma Tre #
Liquid-Liquid Critical Point, Dynamics and Structure of  Aqueous Solutions of Perchlorates: a Molecular Dynamics Study #
We employ molecular dynamics (MD) simulations to determine how magnesium perchlorate
(Mg(ClO4)2) and calcium perchlorate (Ca(ClO4)2) alter the phase diagram, structure, and dy-
namics of supercooled TIP4P/2005 water. We focus on the interplay between its low-density
(LDL) and high-density liquid (HDL) phases and the glassy dynamics, simulating solutions at
concentrations from low to moderate. Thermodynamic analysis confirms the persistence of water anomalies, albeit shifted by the solutes. A second-order liquid-liquid critical point (LLCP) was located at low concentrations, but not at moderate concentrations within the simulated temperature range. Dynamically, similar to the bulk, water in these solutions follows the Mode Coupling Theory, exhibiting a fragile-to-strong crossover. Structurally, radial distribution functions demonstrate that increasing solute concentration enhances HDL-like behaviours and suppresses LDL ordering, the nucleation being favoured in LDL water. This correlates with a marked contraction of the LDL-like region in the (p, T) phase diagram, inferred from compressibility maxima shifts, which serve as a proxy for the maxima of the HDL-LDL correlation length. In light of recent potential radar evidence for aqueous solutions of perchlorates approximately 1.5 km underneath Mars’ south pole, it is of importance to assess their low-temperature stability and the possible crucial role of water anomalies in this context.