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(cyanophenyl)-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 Møller–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