Calculation of Entropy of Adsorption for Small Molecules on Mineral Surfaces

We performed density functional theory calculations for small organic molecules adsorbed on calcite, quartz, and kaolinite mineral surfaces and calculated the entropy change as a result of adsorption. Our results demonstrate that the entropy contribution to the free energy is mainly independent of the mechanism of binding, but it is affected by surface coverage, molecule size, character, and degrees of freedom. Compared with experimental results for weakly bonded molecules, density functional theory calculations predict a higher loss of entropy as a result of adsorption, which can be used as the upper limit for estimating the entropy contribution for adsorption to a mineral surface. The study has demonstrated a linear relationship between the entropy of the adsorbed molecule and the gas phase entropy, which can be used to estimate the entropy contribution to the free energy of adsorption on minerals without explicit density functional theory calculations, which is an important step forward in predicting the behavior of contaminants and other organic compounds in porous media such as soils, aquifers, and oil reservoirs.