Calculation of Gibbs free energies of aqueous electrolytes to 350.degree.C from an electrostatic model for ionic hydration (original) (raw)
Related papers
A New Gibbs Energy Model for Obtaining Thermophysical Properties of Aqueous Electrolyte Solutions
Journal of Solution Chemistry, 2008
In this paper, a new Gibbs energy model is proposed to study the thermophysical properties of aqueous electrolyte solutions at various temperatures. The proposed model assumes that the electrolytes completely dissociate in solution. The model also has two temperature-independent adjustable parameters that were regressed using experimental values of the mean ionic activity coefficients (MIAC) for 87 electrolyte solutions at 298.15 K. Results from the proposed model for the MIAC were compared with those obtained from the E-Wilson, E-NRTL, Pitzer and the E-UNIQUAC models, and the adjustable model parameters were used directly to predict the osmotic coefficients at this temperature. The results showed that the proposed model can accurately correlate the MIAC and predict the osmotic coefficients of the aqueous electrolyte solutions better on the average than the other models studied in this work at 298.15 K. Also, the proposed model was examined to study the osmotic coefficient and vapor pressure for a number of aqueous electrolyte solutions at high temperatures. It should be stated that in order to calculate the osmotic coefficients for the electrolyte solutions, the regressed values of parameters obtained for the vapor pressure at high temperatures were used directly. The results obtained for the osmotic coefficients and vapor pressures of electrolyte solutions indicate that good agreement is attained between the experimental data and the results of the proposed model. In order to unequivocally compare the results, the same experimental data and same minimization procedure were used for all of the studied models.
Thermodynamic Model for Aqueous Electrolyte Solutions with Partial Ionization
Industrial & Engineering Chemistry Research, 2013
An equation of state has been developed to describe the thermodynamic properties of single electrolytes in water within a wide range of temperatures from 25 °C to near the critical point of the solvent. The new equation of state was obtained from an analytical expression of the Helmholtz free energy containing three major contributions: (1) a discrete-solvent term to account for short-range interactions between uncharged particles based on the Peng-Robinson equation of state, (2) an ion charging term described by the continuum-solvent model of Born, and (3) a charge-charge interaction term given by the explicit mean-spherical-approximation (MSA) expression. The thermodynamic model proposed here incorporates chemical equilibrium for the dissolved electrolyte allowing the calculation of the corresponding degree of dissociation of the salt at different temperatures. The present equation of state was applied to the representation of mean ionic activity coefficients, osmotic coefficients, standard free energies of hydration of ions, and densities for NaCl, CaCl 2 , K 2 SO 4 , and MgSO 4 salts in water over a wide range of temperatures and salt molalities. The results indicated a good agreement between the experimental data and those calculated using the present equation of state.
Chemical Engineering Science, 1992
The concept of ionic hydration has been used earlier to get a new representation of the excess free energy of aqueous, single-electrolyte solutions, which leads to the prediction of y* and # values using only two parameters for each electrolyte at 25°C. Here this concept is extended to cover higher temperatures (up to 300°C) using temperature-dependent parameters. The resulting equations arc tested with experimental data for several electrolytes of different charge types, covering temperatures up to 300°C and concentrations up to an ionic strength of 15 mol kg-l. It is found that six or seven parameters are enough to get excellent predictive accuracy for y* and # over these concentration and temperature ranges. A detailed comparison with equations of earlier works clearly brings out the predictive superiority of the present method. In recent years, one of the earlier approaches, based on a virial equation for excess free energy, has been shown to give comparable predictive accuracy. However, it has been demqnstrated only for a few electrolytes, and involves 15 or more parameters. The genera1 applicability of the present method is therefore obvious. It has also been shown to be useful in the accurate calculation of the thermal properties, such as enthalpy and heat capacity, which involve successive differentiation of the excess free energy with temperature.
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.
Ion-specific thermodynamics of multicomponent electrolytes: A hybrid HNC/MD approach
Chemical Physics, 2009
Using effective infinite dilution ion-ion interaction potentials derived from explicit-water molecular dynamics (MD) computer simulations in the hypernetted-chain (HNC) integral equation theory we calculate the liquid structure and thermodynamic properties, namely the activity and osmotic coeffcients of various multicomponent aqueous electrolyte mixtures. The electrolyte structure expressed by the ion-ion radial distribution functions is for most ions in excellent agreement with MD and implicit solvent Monte-Carlo (MC) simulation results. Calculated thermodynamic properties are also represented consistently among these three methods. Our versatile HNC/MD hybrid method allows for a quick prediction of the thermodynamics of multicomponent electrolyte solutions for a wide range of concentrations and an efficient assessment of the validity of the employed MD force-fields with possible implications in the development of thermodynamically consistent parameter sets.
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...
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.
A thermodynamic model has been developed for calculating phase equilibria and other properties of multicomponent electrolyte systems. The model has been designed to reproduce the properties of both aqueous and mixed-solvent electrolyte systems ranging from infinite dilution to solid saturation or pure solute limit. The model incorporates formulations for the excess Gibbs energy and standard-state properties coupled with an algorithm for detailed speciation calculations. The excess Gibbs energy model consists of a long-range interaction contribution represented by the Pitzer-Debye-Hückel expression, a second virial coefficient-type term for specific ionic interactions and a short-range interaction term expressed by the UNIQUAC equation. The accuracy of the model has been demonstrated for common acids and bases and for multicomponent systems containing aluminium species in various environments.
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