Entropies of Adsorbed Molecules Exceed Expectations

The ability to accurately predict the rates of chemical reactions at surfaces is essential to improving technological applications that rely on molecule-surface interactions, such as designing new catalysts for use in chemical synthesis and power-generation applications. Such accurate predictions depend on knowledge of both the entropies and enthalpies of the reacting species. Although advances in molecular modeling have increased the accuracy for the enthalpies of adsorbed molecules, reliable methods for calculating the entropies of adsorbed molecules have been lacking; even estimating their magnitudes has remained elusive. Campbell and Sellers (1) now show that the entropies of adsorbed molecules on crystalline surfaces are surprisingly large and linearly correlate with the entropies of gaseous molecules. Their finding provides a simple yet reliable way to calculate the entropies and affords new insights into the motions of adsorbed molecules. An immediate impact of the Campbell-Sellers correlation is that it will greatly improve the accuracy of calculated desorption rates of adsorbed molecules.