Entropy of a restricted primitive model electrolyte using a mean electrostatic potential approach

The excess entropy of restricted primitive model electrolytes is calculated using a potential based approach through the symmetric Poisson-Boltzmann and the modified Poisson-Boltzmann theories. The theories are utilized in conjunction with a statistical thermodynamics equation that is shown to be equivalent to thermodynamic integration. Electrolyte systems having ionic valencies 1:1 and 2:2 with diameters 3 × 10−10 m and 4.25 ×10−10 m are treated over a wide range of concentrations. The exact radial distribution functions for the model electrolytes obtained from Monte Carlo simulations in the canonical ensemble are compared with the corresponding theoretical predictions. Furthermore, the radial distribution functions from the theories and the simulations are used in the Laird-Haymet entropy expansion equations [ J. Chem. Phys., 1994, 100, 3775] to estimate the excess entropy of the solutions. These equations take into account multi-particle distribution functions, which are approximated using a “ring” term. In general, the modified Poisson-Boltzmann theory gives results that are more consistent with the simulation data than those from the symmetric Poisson-Boltzmann theory. The results show that the excess entropy is negative with its absolute value increasing for 1:1 electrolytes with increasing concentration. The symmetric Poisson-Boltzmann values are slightly overestimated, while the modified Poisson-Boltzmann values are slightly underestimated relative to the simulations. The curves for 1:1 electrolytes including that from the Laird-Haymet equations are consistent with each other, while only the MPB curves for 2:2 electrolytes at 4.25 × 10−10 m are qualitative relative to the simulations up to about 1 mol/dm3. The 2:2 electrolyte curves reveal a characteristic inflection and plateau. The results obtained in the low concentration range (< 0.01 mol/dm3) are consistent with the predictions of the Debye-Hückel limiting law.