Calculating the Entropy Loss on Adsorption of Organic Molecules at Insulating Surfaces

Although it is recognized that the dynamic behavior of adsorbing molecules strongly affects the entropic contribution to adsorption free energy, detailed studies of the adsorption entropy of large organic molecules at insulating surfaces are still rare. We compared adsorption of two different functionalized organic molecules, 1,3,5-tri(4-cyano-4,4-biphenyl)benzene (TCB) and 1,4-bis(cyanopheny1)-2,5-bis(decyloxy)benzene (CDB), on the KCl(001) surface using density functional theory (DFT) and molecular dynamics (MD) simulations. The accuracy of the van der Waals corrected DFT-D3 was benchmarked using Moller-Plesset perturbation theory calculations. Classical force fields were then parametrized for both the TCB and CDB molecules on the KCl(001) surface. These force fields were used to perform potential of mean force (PMF) calculations of adsorption of individual molecules and extract information on the entropic contributions to adsorption energy. The results demonstrate that entropy loss upon adsorption are significant for flexible molecules. Even at relatively low temperatures (e.g., 400 K), these effects can match the enthalpic contribution to adsorption energy