High Precision Canonical Ensemble Monte Carlo Simulations of Very Dilute, Primitive Z:Z and 2:1 Electrolytes and of Moderately Concentrated 1:1 Electrolyte Mixtures

Abstract

Abstract Long-run Canonical Ensemble Monte Carlo Simulations of highly dilute, primitive model z:z and 2:1 electrolytes and of moderately concentrated (1 mol/L) mixtures of 1:1 electrolytes have been performed for a wide range of the number of ions (N) in the simulation cell (up to N = 1728). The excess energy, the excess Helmholtz' free energy, the excess heat capacity and the single ion activity coefficients have been simulated directly. The proper way of extrapolating the data to the thermodynamic limit is emphasized. In particular, the importance of the previously found analytical correction of the Widom method for the deviation from electroneutrality is stressed. The methodology presented opens up for the possible use of ≫ fast Monte Carlo ≪ simulations of e.g. single ion activity coefficients using systems with only a small number of ions. The Poirier formula—base on the exponential integral—for the mean ionic activity coefficients for the restricted primitive case (same ionic radii) seems to be confirmed for z:z and 2:1 electrolytes in highly dilute as well as moderately concentrated solutions. The ≫ negative deviations ≪ from the limiting law predicted at high dilution by the Poirier formula are found also by MC simulations. In mixtures of moderately concentrated 1:1 electrolytes, the Harned rule of linearity in the salt fraction was found valid for the mean ionic excess chemical potentials as well as for the single ion excess chemical potentials, the excess energy and the excess heat capacity at constant volume. The MSA theory gives a quite good approximation of the values. The radial distribution functions are also found by direct simulations and closely examined. In all cases investigated—except for high valency z:z electrolytes—the electric contributions to the potentials of mean force between the ions are very closely given by the electric potential of the linear Debye-Hückel theory. This is the content of the DHX model. Small deviations from the DHX potentials of mean force occur mainly between ions of the same sign and at large separations between the ions. At high Bjerrum parameters, formation of linear triplets perturbs the RDF's, however.