Kinetics and Mechanism of Oligomer Desorption from Surfaces:  n-Alkanes on Graphite

Abstract

A previous study of the desorption of straight-chain alkanes [H(CH 2 ) N H, N ) 5-60] from the surface of graphite has revealed that the measured desorption barriers, ∆E des ‡ , 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 ∆E des ‡ 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 ∆A 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 ∆E des ‡ which accounts for the contributions of both segment-surface detachment and trans-gauche conformational isomerism. The theory accurately reproduces the nonlinear dependence of the ∆E des ‡ on chain length for alkane desorption from graphite.

Desorption energies of linear and cyclic alkanes on surfaces: anomalous scaling with length

“…These configurations represent a distribution of states from which desorption occurs and thus yield values of DE z des that are lower than the values of the adsorption energies, DE ads . This is the same conclusion as was reached in the analysis of alkane desorption from graphite surfaces although the model used for the alkane-graphite interaction [12] was much more primitive than the united atom model used to simulate the alkane-Au(1 1 1) interaction [9].…”

“…As was the case for n-C 7 H 16 desorption, the monolayer desorption temperature for c-C 7 H 14 increased as the heating rate was increased. The inset shows a plot of lnðb=T [7,11,12]. We found that the Values of DE z des for alkanes on Cu(1 1 1) and Pt(1 1 1) surfaces can be calculated using the Redhead equation and the peak desorption temperatures [18].…”

“…The exception is the study of alkane desorption from the Ru(0 0 1) surface which showed that the pre-exponential factor decreased with increasing chain length [4]. In contrast, models for alkane desorption from both Au (1 1 1) and graphite indicate that the pre-exponent should increase with increasing chain length at least for short chain lengths (N < 20) [9,12]. Thus, although errors in the desorption pre-exponential factor could serve to explain the origin of DE there is no experimental evidence from data obtained on metal surfaces to suggest that the desorption preexponential factor increases with chain length.…”

“…The fact that the desorption measurements have been made at different temperatures for the different alkanes might, in principle, introduce an artifact into the measured values of DE z des that could be manifested in the form a non-zero intercept, DE. This issue has been addressed in a prior study of linear alkane desorption from the surface of graphite [7,11,12]. That study included linear alkanes ranging in length from N ¼ 5 to 60 which desorbed from the surface at temperatures ranging from T p ¼ 180 to 730 K. Those measurements revealed that DE z des is non-linear in the chain length.…”

“…A puzzling aspect of the experimental results is that most indicate that the desorption pre-exponential factor is independent of chain length [3,7,11,12]. As mentioned, Wetterer et al [1] report that the intercepts of their Arrhenius plots indicate a value of v % 10 13 s À1 that is independent of chain length.…”