Effects of Solvents on Adsorption Energies: A General Bond-Additivity Model

While a vast body of knowledge exists about adsorption energies of catalytic reaction intermediates on solid surfaces in gas or vacuum conditions based on experimental studies and calculations using quantum mechanics, much less is known about adsorption energies in the presence of liquid solvents. We present here a method for estimating adsorption energies in the liquid phase based on the gas-phase adsorption energy, the solvent’s adhesion energy to the solid surface, and the gas-phase adsorbate’s solvation energy. A simple bond-additivity model was recently developed for approximating the change in adsorption energy (relative to gas phase) due to the additional presence of liquid solvents using the solvent’s adhesion energy and the gaseous adsorbate’s solvation energy, but that model was limited to adsorbates whose thickness is much smaller than its lateral dimension (parallel to the surface). Here we present a simple extension of that model to adsorbates of finite thickness and general shape. We propose a model to convert the experimental solvation energy of a gaseous molecule into a molecule–solvent adhesion energy by assuming isotropic interaction of the molecule with the solvent. This adhesion energy allows us to estimate the fraction of this solvation energy that is retained when the molecule is adsorbed, based on the molecule’s shape, size, and adsorption geometry. As in the earlier bond-additivity model, adsorption energies in solvent are lower in magnitude than in the gas phase by an amount approximately equal to the adhesion energy of the solvent to the surface times the surface area of the solvent molecules displaced upon adsorption. We also report the predicted effects of different solvents for molecules on metal surfaces where solvation energies, gas-phase adsorption energies, and solvent/surface adhesion energies are available in the literature.