DIRECT CALCULATION OF THE ELECTRIC-POTENTIAL DISTRIBUTIONS AROUND IONS FROM HIGH-PRECISION CANONICAL ENSEMBLE MONTE-CARLO SIMULATIONS OF SOME PRIMITIVE MODEL ELECTROLYTE SYSTEMS

The conditional electric potential around the ions are found by Poisson integration of the conditional charge densities calculated from the radial distribution functions found by the high precision canonical ensemble Monte Carlo simulations of some Z:Z,2:1 and 1:1:1 primitive model electrolyte systems described in earlier studies. In most cases, the correspondance with the solution given by solving the linear Poisson-Boltzmann equation is very nearly perfect - at least not too far from contact. This is so in spite of the fact, that the linearisation condition of Debye and Huckel is violated near to contact in dilute solutions and in spite of the anticipation that the Debye-Huckel approximation should be of little relevance in more concentrated systems, where the Debye length is of the same order of magnitude as the ionic diameters. The linear Poisson-Boltzmann equation even continues to be of relevance in zones around ions of different sizes, where only part of the ions appearing in the solution should be able to come. This parallels the earlier finding, that - not too far from contact - the electric contribution to the potentials of mean force do also very closely follow the Debye-Huckel expressions (the DHX model). The present findings do not mean, that the Debye-Huckel expressions for the thermodynamic quantities are correct (except for low Bjerrum parameters). The surface potentials on the ions calculated by the Debye-Huckel approach are quite correct, but the classical charging procedures (Debye or Guntelberg) are not. The relative success of the DHX + hard sphere model is explained by the numerical fact, that the linear DH potential is a quite accurate >>eigen-function<< when used as potentials of mean forces in the Poisson integral operator.