Effects of conformational isomerism on the desorption kinetics of n-alkanes from graphite

Abstract

The dynamics of oligomer desorption from surfaces have been studied by measuring the desorption kinetics of a set of n-alkanes from the surface of single crystalline graphite. Desorption rates were measured using a set of 21 monodispersed n-alkanes (CNH2N+2,5⩽N⩽60) each adsorbed at coverages in the range <0.1 to >1 monolayers. Desorption is observed to be a first-order process with a desorption barrier (ΔEdes‡) that is independent of coverage. The pre-exponential of the desorption rate constant is independent of the oligomer chain length and has a value of ν=1019.6±0.5 s−1. We also find that ΔEdes‡ has a nonlinear dependence on chain length and takes the empirical form ΔEdes‡=a+bNγ, with the exponent having a value of γ=0.50±0.01. More interestingly, we have proposed a mechanism for the desorption process and a model for the energetics and the entropy of the oligomers on the surface that provide an extremely good quantitative fit to the observed chain length dependence of ΔEdes‡. ΔEdes‡ is given by the difference in energy between the gas phase n-alkane and the conformation of the adsorbed n-alkane with the minimum free energy at the desorption temperature. These results reveal that conformational isomerism plays a significant role in determining the desorption kinetics of oligomers from surfaces.