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

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 less than or equal toN less than or equal to 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 (DeltaE(des)double dagger) 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 nu = 10(19.6+/-0.5) s(-1). We also find that DeltaE(des)double dagger has a nonlinear dependence on chain length and takes the empirical form DeltaE(des)double dagger =a+bN(gamma), with the exponent having a value of gamma =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 DeltaE(des)double dagger. DeltaE(des)double dagger 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. (C) 2001 American Institute of Physics.