Maximum in density of electrolyte solutions: Learning about ion–water interactions and testing the Madrid-2019 force field

Abstract

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 these solutions. We also performed molecular dynamics simulations to estimate the TMD at 1 and 2 m with the Madrid-2019 force field, which uses the TIP4P/2005 water model and scaled charges for the ions, finding an excellent agreement between experiment and simulation. All the salts studied in this work shift the TMD of the solution to lower temperatures and flatten the density vs temperature curves (when compared to pure water) with increasing salt concentration. The shift in the TMD depends strongly on the nature of the electrolyte. In order to explore this dependence, we have evaluated the contribution of each ion to the shift in the TMD concluding that Na+, Ca2+, and [Formula: see text] seem to induce the largest changes among the studied ions. The volume of the system has been analyzed for salts with the same anion and different cations. These curves provide insight into the effect of different ions upon the structure of water. We claim that the TMD of electrolyte solutions entails interesting physics regarding ion–water and water–water interactions and should, therefore, 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.

Phase Diagram of Aqueous Solutions of LiCl: a Study of Concentration Effects on the Anomalies of Water

“…For the latter case, in fact, the LLCP does not disappear upon increasing concentration and moves toward the limit of mechanical stability (lower pressures) and toward higher temperature as the salt concentration is increased, as shown in ref . The different behavior of LiCl can be connected to the results of ref , where the authors compare the TMD data of this potential for various anions and cations to the experimental data. They found out that the effect on the TMD of NaCl is much more than that of LiCl, and this decreased effect for the case of LiCl is attributed to the smaller volume occupied by the Li molecule that leaves more free volume for water to behave similarly to bulk.…”

“…Besides, it reproduces very well the experimental densities at different concentrations for ambient conditions, as it can be seen in Figure of ref . As further confirmation of the goodness of this potential and also the location of the TMD at ambient pressure and the corresponding value of the density maximum that it gives, agree very well with the experimental values as it can be seen in Figure and Table 3 of ref .…”

Water’s Unusual Thermodynamics in the Realm of Physical Chemistry

“…Thus, the evolution of the density maximum upon the addition of solutes has acquired a renewed interest. 23,24 This is also the case for the structural effects around solutes, as envisioned by H. S. Frank and M. W. Evans in 1945 and recently correlated with water's ice-like order with the aid of advanced spectroscopic techniques. 25 Furthermore, careful simulations yielding reliable values for the osmotic second virial coefficient and exact thermodynamic relations involving such property 26 suggest that water's unusual thermodynamics may affect the forces between nonpolar solute molecules mediated by water, which are predominantly attractive at high temperatures and repulsive at supercooling conditions.…”

Molecular Insights into the Influence of Major Marine Ions on Carbon Dioxide Hydrate Growth

“…The most important basis for our selection of the model is its accurate description of the hydrate phase boundary and the interaction between ions and hydrate. Therefore, as with our previous studies, ,, the TIP4P/Ice and TraPPE force fields and the optimized parameters for water–CO 2 interactions were used to model water and CO 2 molecules, which can well predict the phase diagram of pure CO 2 hydrate. , The ion–water interactions were modeled by the well-optimized Madrid-2019 force field parameters. , The remaining unlike interactions were calculated with the Lorentz–Berthelot combining rules.…”