Canonical Monte Carlo results are presented for mixtures of primitive model electrolytes with a common ion. Both symmetric mixtures, where the ions differ only in size, and charge asymmetric mixtures were studied for ionic strengths ranging from 0.003 to 1.0 M. The hypernetted chain (HNC) equation theory and the simple lsquo;lsquo;exponentialrsquo;rsquo; (EXP) approximation are both applied to the same electrolyte models. The electrolyte mixing coefficientsw0andw1were calculated from osmotic coefficients. Comparison with Monte Carlo data indicates that the HNC equation yields accurate predictions for the zeroth mixing coefficientw0, while the simple EXP approximation yields qualitatively correct results. The Monte Carlo results for the first mixing coefficient,w1, are not precise enough to allow a quantitative comparison with other theories. However, a strong concentration dependence ofw1for dilute solutions, found previously for the nonprimitive models in the HNC approximation, is confirmed by the Monte Carlo results.
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