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 (CNH2N12,5<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 (DEdes ‡ ) 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 n 510 s. We also find that DEdes ‡ has a nonlinear dependence on chain length and takes the empirical form DEdes ‡ 5a1bN, with the exponent having a value of g50.5060.01. 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 DEdes ‡ . DEdes ‡ 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. © 2001 American Institute of Physics. @DOI: 10.1063/1.1398574#