Individual Ion Activity Coefficients in Aqueous Electrolytes from Explicit-Water Molecular Dynamics Simulations

Abstract

We compute individual ion activity coefficients (IIACs) in aqueous NaCl, KCl, NaF, and KF solutions from explicit-water molecular dynamics simulations. Free energy changes are obtained from insertion of single ionsaccompanied by uniform neutralizing backgroundsinto solution by gradually turning on first Lennard-Jones interactions, followed by Coulombic interactions using Ewald electrostatics. Simulations are performed at multiple system sizes, and all results are extrapolated to the thermodynamic limit, thus eliminating any possible artifacts from the neutralizing backgrounds. Because of controversies associated with measurements of IIACs from electrochemical cells with ion-selective electrodes, the reported experimental data are not widely accepted; thus there remains a knowledge gap with respect to the contributions of individual ions to solution nonidealities. Our results are in good qualitative agreement with these reported measurements, though significantly larger in magnitude. In particular, the relative positioning for the activity coefficients of anions and cations matches the experimental ordering for all four systems. This work establishes a robust thermodynamic framework, without a need to invoke extra hypotheses, that sheds light on the behavior of individual ions and their contributions to nonidealities of aqueous electrolyte solutions.

Computational and experimental study of different brines in temperature swing solvent extraction desalination with amine solvents

“…To evaluate the solvation behavior of the individual ions (Na + , K + , Cl − , and Br − ) in the solvent phase, the excess chemical potentials μ ion ex of the ions in DIPA were calculated based on the multistate Bennett acceptance ratio estimator (MBAR) [48][49][50]. The coupling parameters for the van der Waals and Coulombic interactions λ vdw/coul = {0.00, 0.05, 0.10, …, 1.0} provided a reasonable overlap matrix [51]; using the convergence test proposed by Klimovich et al [51], 1 ns of equilibration followed by 10 ns of production proved sufficient to obtain accurate estimates of the free energies and reproduce the results of Saravi et al [52] The Alchemical Analysis package [51] was used to obtain the free energy estimates using the MBAR method. Then, in order to maintain consistency with previous work [52][53][54], the total chemical potential was obtained by adding the ideal gas contribution:…”

“…where P 0 is the standard pressure (1 bar), V is the average volume, k B is the Boltzmann constant, μ i 0 is the standard chemical potential obtained using the NIST-JANAF tables [55], and N is the number of ion pairs, estimated following the approach of Saravi and Panagiotopoulos [52] (i. e., the midpoint value before and after the individual ion insertion).…”

“…Here, μ i 0 at 343 K was obtained by using the standard enthalpy and μ i 0 values at 350 K. In the free energy calculations, a box with 800 molecules of DIPA was used, and a single ion was gradually inserted (N=0.5). We emphasize that although the gradual insertion of an individual ion results in a non-neutral box, it has been previously shown that the box background neutralization performed by the PME method under periodic conditions does not introduce changes in the ensemble distribution [52,56,57]. Finally, the transfer free energy of water from the 3 M brine phase to the pure DIPA phase (i.e., infinite dilution condition of water in DIPA) was calculated using:…”

Influence of Ion Specificity and Concentration on the Conformational Transition of Intrinsically Disordered Sheep Prion Peptide

“…In Figure S12, we have shown the radial distribution function and preferential binding graph for proteinions, ions-water, and protein-water by taking the center of mass as a function of interatomic distance for systems. Further, we calculated the water-ion, water-water, and ion-ion interaction energy to show the efficiency of the chosen force-field [71,72] to capture the interactions at a high salt-concentration regime as depicted in Figure S13. At higher salt concentrations the increment in water-ion and ion-ion interaction restricts their interaction with a protein molecules.…”

Computational and Experimental Study of Different Brines in Temperature Swing Solvent Extraction Desalination with Amine Solvents

“…where P 0 is the standard pressure (1 bar), V is the average volume, k B is the Boltzmann constant, μ i 0 is the standard chemical potential obtained using the NIST-JANAF tables [55], and N is the number of ion pairs, estimated following the approach of Saravi and Panagiotopoulos [52] (i. e., the midpoint value before and after the individual ion insertion).…”