US EPA

1695898 

Journal Article 

Thermodynamic and structural properties of 1M, 2M and 3M mixtures of primitive model electrolytes at 25 degrees C with a common ion, one counterion of the same size, the other larger 

Sorensen, TS; Compan, V 

1996 

1 

Molecular Simulation
ISSN: 0892-7022 

18 

4 

225-275 

WOS:A1996VR76100002 

Using canonical ensemble Monte Carlo simulations in combination with the Widom test particle technique corrected for deviations from electroneutrality we have calculated excess free energies, excess heat capacities at constant volume, single ion activity coefficients, excess osmotic coefficients and radial distribution functions at constant total salt concentration for a mixture of primitive model electrolytes with a common ion. The monovalent, common ion (for brevity called K+) and the monovalent counterion no. 1 (called Cl-) have the same diameter, whereas the diameter for the monovalent counterion no. 2 (called F-) is larger (strongly hydrated ion). The conditions of the simulation correspond to a 3 mol/L total salt concentration in a dielectric continuum as water at 25 degrees C, if the diameters d(K+) and d(Cl-) are set to 0.29 nm and the diameter d(F-) to 0.37 nm. Linear Harned laws of variation with the salt fraction is found for the logarithms of the activity coefficients (In y) as well as for the other thermodynamic parameters. The variation of In y,, with salt fraction is much greater than the variations of In y(Cl-) and in y(F-). From the radial distribution functions we calculate the potential of mean force between the ions and the mean electric potential around each ion. Near to contact, the Debye-Huckel potential is a fair approximation to the potentials of mean force between all ions, but the electric potentials have to be fitted by Debye-Huckel expressions using individual screening lengths for each ion which are shorter than the Debye length. A comparison is made between the values obtained in the present study and the values obtained in earlier studies at 1 mol/L and 2 mol/L total concentrations and with values obtained by the MSA theory. A general theory for the variation of the Harned coefficients with the total concentration in terms of the concentration dependence of the activity coefficients of the pure salt solutions is proposed. It is shown that the latter dependence is an Akerlof-Thomas relation and that the Harned coefficients are almost independent of the total concentration which implies, that the trace activity coefficient of the two salts are almost identical at the same total concentration. 

canonical ensemble Monte Carlo; primitive model KCl-KF mixtures at 1,2 and 3M; excess energy; heat capacity; single ion activity coefficients; osmotic pressure; Harned rules; potentials of mean forces; electric potentials; MSA theory