Kinetics and mechanism of oligomer desorption from surfaces:: n-alkanes on graphite

A previous study of the desorption of straight-chain alkanes, [H(CH2)(N)H, N = 5-60] from the surface of graphite has revealed that the measured desorption barriers, DeltaE(des)(double dagger), have a nonlinear dependence on the chain length (Paserba, K. R.; Gellman, A. J. Phys. Rev. Lett. 2001, 86 (19), 4338). A model is proposed for the mechanism of oligomer desorption that accounts for the chain length dependence of the DeltaE(des)(double dagger) through consideration of both the energy and entropy of oligomers interacting with a surface. The segments of the oligomer are identified with individual backbone bonds such that an alkane with N carbon atoms has I = N - 1 segments. The oligomer segments are rapidly attaching to and detaching from the surface such that the various partially detached oligomers are in equilibrium with one another and can be classified by the number of detached segments i. Equilibrium among the partially detached oligomers is dictated by their relative free energies DeltaA(i), which are dependent on the numbers of detached segments. The energy E-i of an oligomer is simply linear in the number of detached segments. The entropy, S-i, is given by the statistics for detachment of i of I segments from the surface and by the number of trans-gauche conformations about each detached C-C bond. Finally, desorption is irreversible and occurs via a transition state with all segments detached from the surface. This model has been used to derive an expression for the measured DeltaE(des)(double dagger) which accounts for the contributions of both segment-surface detachment and trans-gauche conformational isomerism. The theory accurately reproduces the nonlinear dependence of the DeltaE(des)(double dagger) on chain length for alkane desorption from graphite.