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Thermodynamics provides one of the scientific cornerstones of chemical engineering. This review considers how thermodynamics is and has been

In an aqueous electrolyte solution, the potential of mean force (PMF) for two macroions is affected not only by the size and charge of each electrolyte ion but also by the ion's polarizability. The Lifshitz theory provides a basis for calculating the van der Waals interaction between cation-colloid, anion-colloid, cation-cation, and anion-anion pairs. Monte Carlo simulations are used to determine how salt identity affects the PMF between colloidal particles or globular proteins in a saline solution, a phenomenon observed experimentally by Hofmeister for aqueous proteins more than 100 years ago. The calculations show that the PMF and, hence, solution phase behavior are sensitive to the van der Waals interaction between an ion and a macroion. The calculations described here may be useful for interpretation of experimental phase diagrams and for guiding design of separation processes where a salt is used to induce colloid or protein precipitation.

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Monte Carlo simulations have been performed for ion distributions outside a single globular macroion and for a pair of macroions, in different salt solutions. The model that we use includes both electrostatic and van der Waals interactions between ions and between ions and macroions. Simulation results are compared with the predictions of the Ornstein-Zernike equation with the hypernetted chain closure approximation and the nonlinear Poisson-Boltzmann equation, both augmented by pertinent van der Waals terms. Ion distributions from analytical approximations are generally very close to the simulation results. This demonstrates that properties that are related to ion distributions in the double layer outside a single interface can to a good approximation be obtained from the Poisson-Boltzmann equation. We also present simulation and integral equation results for the mean force between two globular macroions (with properties corresponding to those of hen-egg-white lysozyme protein at pH 4.3) in different salt solutions. The mean force and potential of mean force between the macroions become more attractive upon increasing the polarizability of the counterions (anions), in qualitative agreement with experiments. We finally show that the deduced second virial coefficients agree quite well with experimental results.

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