Free-energy analysis of physisorption on solid-liquid interface with the solution theory in the energy representation

Physisorption of urea on its crystal in contact with water was subject to energetics analysis with all-atom molecular dynamics simulation. The transfer free energy of urea to an adsorption site was treated in the framework of the energy-representation theory of solutions, which allows a fast computation of the free energy in an inhomogeneous environment with solid-liquid interface. The preference of adsorption was then compared between the (001) and (110) faces, and it was found that the physisorption is more favorable on (001) than on (110) in correspondence to the hydrogen bonding between the adsorbed urea and the crystal urea. Among the terrace configurations of adsorption, the attractive interaction governs the preferable site with a minor role of the repulsive interaction. The effect of an edge was also treated by examining the terrace and step and was shown to be strongly operative on the (110) face when the CO group of the adsorbed urea points toward the edge. The present work demonstrates that the solution theory can be a framework for analyzing the energetics of physisorption and addressing the roles of the crystal and liquid at the interface through the systematic decomposition of free energy. Published by AIP Publishing.