Recent progress in molecular simulation of aqueous electrolytes: force fields, chemical potentials and solubility (original) (raw)
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The Journal of Chemical Physics, 2013
We extend the osmotic ensemble Monte Carlo (OEMC) molecular simulation method (Moucǩa et al. J. Phys Chem. B 2011, 115, 7849−7861) for directly calculating the aqueous solubility of electrolytes and for calculating their chemical potentials as functions of concentration to cases involving electrolyte hydrates and mixed electrolytes, including invariant points involving simultaneous precipitation of several solutes. The method utilizes a particular semigrand canonical ensemble, which performs simulations of the solution at a fixed number of solvent molecules, pressure, temperature, and specified overall electrolyte chemical potential. It avoids calculations for the solid phase, incorporating available solid chemical potential data from thermochemical tables, which are based on well-defined reference states, or from other sources. We apply the method to a range of alkali halides in water and to selected examples involving LiCl monohydrate, mixed electrolyte solutions involving water and hydrochloric acid, and invariant points in these solvents. The method uses several existing force-field models from the literature, and the results are compared with experiment. The calculated results agree qualitatively well with the experimental trends and are of reasonable accuracy. The accuracy of the calculated solubility is highly dependent on the solid chemical potential value and also on the force-field model used. Our results indicate that pairwise additive effective force-field models developed for the solution phase are unlikely to also be good models for the corresponding crystalline solid. We find that, in our OEMC simulations, each ionic force-field model is characterized by a limiting value of the total solution chemical potential and a corresponding aqueous concentration. For higher values of the imposed chemical potential, the solid phase in the simulation grows in size without limit. t of the remaining species are fixed. An example is an aqueous solution of s ions with a fixed
It is known that none of the available simple molecular interaction models of aqueous electrolytes based on SPC/E water and their associated force fields are able to reproduce the concentration dependence of important thermodynamic properties of even the simplest electrolyte, NaCl, at ambient conditions over the entire experimentally accessible concentration range [Moucka, F.; Nezbeda, I.; Smith, W. R. J. Chem. Phys. 2013, 138, 154102]. This paper explores the possibility of improving their performance by incorporating concentration-dependent experimental data for the total ionic chemical potential and the density into the fitting procedure, in addition to experimental values of solubility and solid chemical potential. We describe a general parameter estimation methodology for a studied class of models that incorporates the aforementioned experimental data. When the entire concentration range is considered, although the resulting force field is a slight improvement over others currently available in the literature, overall quantitative agreement with the experimental data over this range remains unsatisfactory. This indicates an inherent limitation of such simple molecular interaction models and strongly suggests that more complex mathematical forms of such models are required to quantitatively predict the properties of aqueous electrolyte solutions when the entire concentration range is of interest. Our parameter estimation methodology is also applicable to such cases.
Fluid Phase Equilibria, 2018
We review progress in the development and application of molecular simulation methodology to predict the thermodynamic properties of aqueous electrolytes, focussing on work published since our previous review along similar lines [I. Nezbeda, et al., Mol. Phys. 114 (2016) 1665]. We consider such developments in the context of the use of Monte Carlo (MC) or Molecular Dynamics (MD) simulation methodologies using classical force fields. Special attention is paid to the incorporation of charge scaling approaches in the force fields, as well as to the simulation methodology used to compute solubility and osmotic pressure, and the use of the latter quantity to calculate the water activity and osmotic coefficient, and the electrolyte activity coefficient. We emphasize the importance of the statistical analysis of thermodynamic properties obtained from simulation data, and illustrate it with an example analyzing simulation osmotic pressure and electrolyte chemical potential data.
Journal of Physical Chemistry B, 2005
We have developed a molecular-level simulation technique called the expanded-ensemble osmotic molecular dynamics (EEOMD) method, for studying electrolyte solution systems. The EEOMD method performs simulations at a fixed number of solvent molecules, pressure, temperature, and overall electrolyte chemical potential. The method combines elements of constant pressure-constant temperature molecular dynamics and expanded-ensemble grand canonical Monte Carlo. The simulated electrolyte solution systems contain, in addition to solvent molecules, full and fractional ions and undissociated electrolyte molecular units. The fractional particles are coupled to the system via a coupling parameter that varies between 0 (no interaction between the fractional particle and the other particles in the system) and 1 (full interaction between the fractional particle and the other particles in the system). The time evolution of the system is governed by the constant pressure-constant temperature equations of motion and accompanied by random changes in the coupling parameter. The coupling-parameter changes are accepted with a probability derived from the expanded-ensemble osmotic partition function corresponding to the prescribed electrolyte chemical potential. The couplingparameter changes mimic insertion/deletion of particles as in a crude grand canonical Monte Carlo simulation; if the coupling parameter becomes 0, the fractional particles disappear from the system, and as the coupling parameter reaches unity, the fractional particles become full particles. The method is demonstrated for a model of NaCl in water at ambient conditions. To test our approach, we first determine the chemical potential of NaCl in water by the thermodynamic integration technique and by the expanded-ensemble method. Then, we carry out EEOMD simulations for different specified values of the overall NaCl chemical potential and measure the concentration of ions resulting from the simulations. Both computations give consistent results, validating the EEOMD methodology.
Journal of Chemical Theory and Computation, 2015
We describe a computationally efficient molecular simulation methodology for calculating the concentration dependence of the chemical potentials of both solute and solvent in aqueous electrolyte solutions, based on simulations of the salt chemical potential alone. We use our approach to study the predictions for aqueous NaCl solutions at ambient conditions of these properties by the recently developed polarizable force fields (FFs) AH/BK3 of Kiss and Baranyai (
Revisiting electrolyte thermodynamic models: Insights from molecular simulations
AIChE Journal, 2018
Pitzer and electrolyte non-random two-liquid (eNRTL) models are the two most widely used electrolyte thermodynamic models. For aqueous sodium chloride (NaCl) solution, both models data satisfactorily up to salt saturation concentration, i.e., ionic strength around 6 molal. However, beyond 6 molal, the model extrapolations deviate significantly and diverge from each other. We examine this divergence by calculating the mean ionic activity coefficient over a wide range of concentration based on molecular simulations and Kirkwood-Buff (KB) theory. We show that the asymptotic behavior of the activity coefficient predicted by the eNRTL model is consistent with the molecular simulation results and supersaturation experimental data.
Molecular dynamics simulations of concentrated aqueous electrolyte solutions
Molecular Simulation, 2011
Transport properties of concentrated electrolytes have been analysed using classical molecular dynamics simulations with the algorithms and parameters typical of simulations describing complex electrokinetic phenomena. The electrical conductivity and transport numbers of electrolytes containing monovalent (KCl), divalent (MgCl2), a mixture of both (KCl+MgCl2) and trivalent (LaCl3) cations have been obtained from simulations of the electrolytes in electric fields of different magnitude. The results obtained for different simulation parameters have been discussed and compared with experimental measurements of our own and from the literature. The electroosmotic flow of water molecules induced by the ionic current in different cases has been calculated and interpreted with the help of the hydration properties extracted from the simulations.
Journal of Chemical & Engineering Data, 2016
Many industrial processes involve processing aqueous electrolyte solutions. There is thus a need for accurate theories to predict their thermophysical properties. Recent studies have shown that the size of the hydrated ion plays an important role in determining these properties. In this study, we first used molecular dynamics simulations to estimate the effective hydrated ionic size and the free energy of solvation, and then developed correlations allowing for the prediction of these quantities. The temperature dependence of these solution properties was also investigated. Our studies have shown that the effective (hydrated) size, the charge density, and the free energy of solvation of the ions are strongly interdependent. The effective hydrated ionic size also plays an important role in determining the selectivity of membranes to remove such hydrated ions from solutions, for example, in membrane based desalination processes, and related water purification technologies.
Solubility of NaCl in water by molecular simulation revisited
The Journal of Chemical Physics, 2012
In this paper, the solubility of NaCl in water is evaluated by using computer simulations for three different force fields. The condition of chemical equilibrium (i.e., equal chemical potential of the salt in the solid and in the solution) is obtained at room temperature and pressure to determine the solubility of the salt. We used the same methodology that was described in our previous work [E. Sanz and C. Vega, J. Chem. Phys. 126, 014507 ] although several modifications were introduced to improve the accuracy of the calculations. It is found that the predictions of the solubility are quite sensitive to the details of the force field used. Certain force fields underestimate the experimental solubility of NaCl in water by a factor of four, whereas the predictions of other force fields are within 20% of the experimental value. Direct coexistence molecular dynamic simulations were also performed to determine the solubility of the salt. Reasonable agreement was found between the solubility obtained from free energy calculations and that obtained from direct coexistence simulations. This work shows that the evaluation of the solubility of salts in water can now be performed in computer simulations. The solubility depends on the ion-ion, ion-water, and water-water interactions. For this reason, the prediction of the solubility can be quite useful in future work to develop force fields for ions in water.