Probing the origins of linear free energy relationships with molecular theory and simulation

Linear free energy relationships (also known as entropy-energy compensation) are seen in a range of physical processes, including adsorption and diffusion. The most interesting compensation effects presumably arise from geometric and energetic effects at the molecular level, but their origins are not well understood. We utilize molecular modeling of simple adsorbate-adsorbent systems to discover these origins in a semi-quantitative way. Sets of calculations were constructed to represent certain experimental processes, e.g. adsorption of a homologous series of molecules, for which linear free energy relationships might be expected. We find that the general correlation between energy and entropy is often much more complex than a simple linear relationship, although a linear approximation might be sufficient across limited ranges.