Grand-canonical Monte-Carlo simulation of solutions of salt mixtures: theory and implementation (original) (raw)
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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.
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
Fluid Phase Equilibria, 2005
In this work the Ghotbi-Vera mean spherical approximation (GV-MSA) model, coupled with two different expressions for the cationhydrated diameters, was used in predicting the mean ionic activity coefficients (MIAC) of electrolytes for a number of the mixed-solvent and mixed-salt electrolyte solutions at 25 • C. In all cases the cation diameters in solutions changed with concentration of electrolyte while the anion diameters were considered to be constant and equal to the corresponding Pauling diameters. In application of the GV-MSA model to the electrolyte systems, two different expressions were used for concentration dependency of cation-hydrated diameters, i.e., the GV-MSA1 and GV-MSA2 models. In case of the electrolyte solutions containing the mixed-solvent of water and alcohol, the dielectric constants of the mixed solvents were obtained by simple regression of polynomial equations in terms of weight fraction of alcohol to the pertinent experimental data available in the literature. For the mixed-salt and mixed-solvent electrolyte solutions, in order to directly calculate the MIAC of electrolytes without introducing any new adjustable parameter, the values obtained in this work for the cation-hydrated diameters in the single aqueous electrolyte solutions were used. The results obtained in this work showed that the GV-MSA2 could more accurately correlate the MIAC of electrolytes in the single aqueous electrolyte solutions in comparison to those of the GV-MSA1 and Pitzer models. Also, the results showed that the GV-MSA-based models could accurately predict the MIAC of electrolytes in the mixed-solvent electrolyte solutions in comparison to those obtained from the model of Pitzer. In case of the mixed-salt electrolyte solutions the results of the two GV-MSA-based models studied in this work reasonably predict the MIAC of electrolytes in the mixed-salt electrolyte solutions without introducing any additional adjustable parameters compared to those obtained from the model of Pitzer with two adjustable parameters.
Simulation of Osmotic Pressure in Concentrated Aqueous Salt Solution
Accurate force fields are critical for meaningful simulation studies of highly concentrated electrolytes. The ion models that are widely used in biomolecular simulations do not necessarily reproduce the correct behavior at finite concentrations. In principle, the osmotic pressure is a key thermodynamic property that could be used to test and refine force field parameters for concentrated solutions. Here we describe a novel, simple, and practical method to compute the osmotic pressure directly from molecular dynamics (MD) simulation of concentrated aqueous solutions by introducing an idealized semipermeable membrane. Simple models for Na þ , K þ , and Clare tested and calibrated to accurately reproduce the experimental osmotic pressure at high salt concentration, up to the solubility limit of 4-5 M. The methodology is general and can be extended to any type of solute as well as nonadditive polarizable force fields.
Using Monte Carlo simulation to compute osmotic coefficients of aqueous solutions of ionic liquids
Chemical Physics, 2010
a b s t r a c t We perform, for the first time to our knowledge, Monte Carlo simulation to compute osmotic coefficient of ionic liquid aqueous solutions. The ionic liquids chosen are 1-ethyl-3-methylimidazolium bromide [Emim][Br], 1-methyl-3-methylimidazolium chloride [Mmim][Cl], 1-methyl-3-methylimidazolium bromide [Mmim][Br], 1-methyl-3-methylimidazolium iodide [Mmim][I] and 1-methyl-3-methylimidazolium hexafluorophosphate [Mmim][PF 6 ]. Simulations are carried out in the NVT ensemble at 298.15 K. The Unrestricted Primitive Model (UPM) of electrolyte is used as microscopic model in simulation process. Accuracy of simulation to predict osmotic coefficients is verified by a direct comparison of simulation results with experimental data. Computed osmotic coefficients are in good agreement with available experimental values.
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.
Iranian Journal of Oil and Gas Science and Technology, 2017
In this work, the performance of four electrolyte models for prediction the osmotic and activity coefficients of different aqueous salt solutions at 298 K, atmospheric pressure and in a wide range of concentrations are evaluated. In two of these models, (electrolyte Non-Random Two-Liquid e-NRTL and Mean Spherical Approximation-Non-Random Two-Liquid MSA-NRTL), association between ions of opposite charges for simplification purposes is ignored and in the other two ones, (Associative Mean Spherical Approximation-Non-Random Two-Liquid AMSA-NRTL and Binding Mean Spherical Approximation BiMSA) association and solvation effects are considered. The predictions of these four models for the osmotic and activity coefficients of electrolyte solutions at 298 K and atmospheric pressure are compared with the experimental data reported in the literature. This comparison includes, 28 different aqueous salt solutions including thio-cyanates, perchlorates, nitrates, hydroxides, quaternary ammonium sal...