Máximo en densidad de disoluciones de electrolitos

Abstract

Trabajo de Máster Universitario en Simulación Molecular (2021/22). Directores: Dra. Dª. Eva González Noya; Dr. D. Carlos Vega de las Heras. In this work we studied the effect of Li+, Na+, K+, Mg2+ and Ca2+ chlorides and sulfates on the temperature of maximum density (TMD) of aqueous solutions at room pressure. Experiments at 1 molal salt concentration were carried out to determine the TMD of the solutions. For density measurements below -15 ◦ C, the Sorensen methodology was adopted [1]. We also performed molecular dynamics simulations to estimate the TMD of these solutions at 1 m and 2 m with the Madrid-2019 force field [2], which uses the TIP4P/2005 water model [3] and scaled charges for the ions. Our experiments at room temperature are consistent with those from Laliberte [4] and the prediction of both, the maximum in density and the TMD from simulations is in excellent agreement with our experimental results. All the salts studied in this work shift the TMD of the solution to lower temperatures and flatten the density profile (when compared to pure water) with increasing salt concentration. The shift in the TMD (ΔTMD) depends strongly on the nature of the electrolyte. Despretz established a linear dependence of ΔTMD with the salt concentration (in molality) as early as 1840 [5, 6]. We hereby confirm that the Despretz law holds for all the solutions with monovalent cations considered here at least up to 2 m, whereas for those with divalent cations, small deviations are observed at this concentration. In order to further explore this shift in the TMD, we have evaluated the contribution of each ion to ΔTMD (from the experimental results), concluding that Na+, Ca2+ and SO2− 4 seem to induce the largest changes among the studied ions. We have proved that these group contributions are able to accurately predict the shift in TMD for 1 m and 1.5 m mixtures. Finally, the volume of the system has been analyzed for salts with the same anion and different cation. These curves provide an insight into the effect of different ions upon the structure of water. We conclude that, for those ions with a strong coordinated first layer, namely Li+ and Mg2+ (where water is not just coordinating the cation but forming a complex), the remaining water molecules accommodate in an expanded structure resulting from the configurations they adopt to enable hydrogen bonding with the “rigid" water molecules from the first solvation shell. We therefore claim that the TMD of electrolyte solutions entails interesting physics regarding ion-water and water-water interactions and should then be considered as a test property when developing force fields for electrolytes. This matter has been rather unnoticed for almost a century now and we believe it is time to revisit it.