The temperature of maximum density of diluted aqueous solutions of non-polar solutes: A molecular simulation study using TIP4P/2005 water and LJ point solutes

Abstract

The effect of non polar solutes on the temperature of maximum density of water, TMD, is analyzed in detail by means of molecular simulations over an ample pressure interval ranging from negative to very high pressures. Non polar solutes are represented by single Lennard-Jones (LJ) sites, whose non-polar character is tuned with the width and depth of the Lennard-Jones water-solute potential parameters. Our analysis is focused on the behavior of the partial molar volume of the solute at infinite solution, whose thermal dependence is correlated with changes in the TMD. We find that for pressures below −200 bar all possible LJ water-solute potential parameters decrease the TMD, i.e. the solute acts as a “structure breaker”. At higher pressures, it is always possible to find a range of LJ parameters which induce an increase of the TMD, whose magnitude rises gradually with pressure. Our results are consistent with previous simulations of non polar gases in solution and with theoretical studies in the case of small LJ centers. It will be shown that the partial molar volume of an ideal gas at infinite dilution can provide information concerning the existence and size of the LJ parameter space that induces TMD increases for a given water model. Finally, we analyze the changes in the TMD in terms of the competition between low density and high density water states. We find that the temperature dependence of the variation of the low density water mole fraction with the addition of LJ solutes seems to be at the source of the changes in the TMD.