Symmetrical Poisson–Boltzmann and modified Poisson–Boltzmann theories

Symmetric pair and triplet ionic correlation functions in the electrolyte bulk are derived using Kirkwood's charging process. Four closures at the Poisson–Boltzmann level of approximation are considered for the pair distribution function and comparisons made with simulation thermodynamic properties for the following primitive model electrolytes: (i) 1 : 1 and 2 : 2 valencies with equal and unequal ion radii, (ii) 1 : 3 valency with equal ion radii. The modified Poisson–Boltzmann theory is extended to treat electrolytes with both unequal radii and/or unsymmetrical valencies.