Effective Interaction Potentials for Alkali and Alkaline Earth Metal Ions in SPC/E Water and Prediction of Mean Ion Activity Coefficients

Abstract

The potential of mean force (PMF) acting between two simple ions surrounded by SPC/E water have been determined by molecular dynamics (MD) simulations using a spherical cavity approach. Such effective ion-ion potentials were obtained for Me-Me, Me-Cl-, and Cl(-)-Cl- pairs, where Me is a Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ cation. The ionic sizes estimated from the effective potentials are not pairwise additive, a feature in the frequently used primitive model for electrolytes. The effective potentials were used in Monte Carlo (MC) simulations with implicit water to calculate mean ion activity coefficients of LiCl, NaCl, KCl, MgCl2, CaCl2, SrCl2, and BaCl2. Predicted activities were compared with experimental ones in the electrolyte concentration range 0.1-1 M. A qualitative agreement for LiCl and a satisfactory agreement for NaCl were found, whereas the predictions for KCl by two K+ models were less coherent. In the case of alkaline earth metal ions, all experimental activities were successfully reproduced at c = 0.1 M. However, at higher concentrations, similar deviations occurred for all divalent cations, suggesting that the dependence of the permittivity on the salt concentration and the polarization deficiency arising from the ordering of water molecules in the ion hydration shells are important in such systems.

Monte Carlo simulations of H2O–CaCl2 and H2O–CaCl2–CO2 mixtures

“…Three potential models were considered for CaCl 2 : a model byÅqvist [18] (in the following referred to as the "A" model), a model by Deublein et al [19] ("DRVH" model), and a model by Smith and Dang [20] ("SD" model). The Ca 2+ parameters of the A model were obtained from the hydration free energy, while the Cl − parameters of the A model were taken from the SD model, as did Gavryushov and Linse [21]; the parameters of the DRVH model were adjusted to reproduce the liquid density at 293.15 K and 1 bar; and the parameters of the SD model were derived to reproduce binding enthalpies of ion-water clusters in the gas phase, solvation enthalpies, and solution structures. Three CO 2 models were also tested: a model by Harris and Yung [22] (in the following referred to as the EPM2 model), a model by Murthy et al [23] ("MSM" model), and a model by Potoff and Siepmann (TraPPE model).…”

Computing accurate potentials of mean force in electrolyte solutions with the generalized gradient-augmented harmonic Fourier beads method

“…[1][2][3][4][5][6][7][8][9] The parameters of the structural features to some extent depend on the ionic composition and concentration of the electrolytes in solution [10][11][12][13][14][15][16][17] and also on the size of the ions involved. 4,15,18 Importantly, in certain cases, the effective interactions between two ions of the same charge may become weakly attractive, seemingly defying the Coulomb repulsion. 5,[19][20][21][22][23][24][25][26][27][28] Incorporating these features of electrolyte solutions into simulations of macromolecules is challenging.…”

“…Because explicit solvent molecules often account for more than half the atoms in the system, it is important to develop accurate solvent models that incorporate the nontrivial properties of electrolyte solutions implicitly. An approximate method has been proposed 4,18,[32][33][34][35] that may substantially reduce the cost of simulations in electrolyte solutions by employing effective ion-pair potentials, and hence eliminating explicit water altogether. This procedure employs ion-pair RDFs from dilute electrolyte solutions to derive effective ion-pair potentials.…”

“…4,18 Therefore, it is important to compute RDFs or directly related PMFs and their derivatives with high accuracy. 4,32 The accurate ion-pair PMFs are also essential in theoretical studies of reaction rates, McMillan-Mayer solution theory, 36 calculations of mean activities and osmotic pressures, 4,13 and asymptotic calculations of dielectric constants.…”

Spontaneous Formation of KCl Aggregates in Biomolecular Simulations: A Force Field Issue?

“…A recent study devoted to the calculation of ion-ion potential of mean forces also addressed the respective qualities of the Åqvist (AMBER) and Dang models. 54 According to this author, "the Na + Lennard-Jones (LJ) parameters of Dang and Åqvist differ considerably with respect to each other. Thus, if only one experimental property is used to determine the LJ parameters, the determined LJ parameters become not necessarily unique.…”