Models based on free energy minimisation have been made for surface and interfacial systems, where the surface, interfacial or adsorbed atomic or molecular layers are modelled as separate phases. Gibbs-Duhem compatible activity coefficients for the surface and interfacial layers are estimated based on known bulk thermodynamics and geometric and other considerations. The models allow the estimation of the surface energies and compositions [1] of mixtures based on pure component and bulk thermodynamic data and interfacial energies based on bulk thermodynamic values.

In an analogous fashion the partitioning effect of a semi-permeable membrane in a two-compartment aqueous system can be modelled. In such system the large ions, not permeable through the membrane, cause an uneven charge distribution of ionic species between the two compartments. In this case, the electrochemical potential difference between the two aqueous phases becomes calculated for the multi-component system. The calculated results are consistent with the Donnan equilibrium theory; however the multi-phase system may also include the gas phase and several precipitating phases, which extends the applicability of the new method [2].

The interfacial models are solvable in a single equilibrium calculation by using a traditional Gibbs energy minimising program. Various example cases and applications in industrial practice will be presented.

References:

[1] R. Pajarre, P. Koukkari, T. Tanaka, Y. Lee: Calphad, 30, (2006) 196-200.

[2] R. Pajarre, P. Koukkari, E. Räsänen: Journal of Molecular Liquids, 125 (2006), 58-61.