Desorption Energies of Linear and Cyclic Alkanes on Surfaces: Anomalous Scaling with Length

The desorption energies, ΔEdes‡, of linear and cyclic alkanes on the Cu(1 1 1) and Pt(1 1 1) surfaces have been measured and compared to the values of the ΔEdes‡ measured on a number of other surfaces. For sets of short linear and cyclic alkanes (n-CNH2N+2 and c-CNH2N, N=3–12) desorbing from single crystalline surfaces the ΔEdes‡ scale linearly in chain length with the form ΔEdes‡(N)=ΔE+ΔECH·N. The quantity ΔECH can be equated with a segmental contribution to the ΔEdes‡, however, the physical origin of the non-zero intercept, ΔE, is not clear. Comparison of the intercepts, for the linear and cyclic alkanes show that they are similar to one another on the Cu(1 1 1) surface and on the Pt(1 1 1) surface. This comparison rules out the possibility that ΔE arises from endgroup effects. Scaling of the ΔEdes‡ against the polarizabilities, α, of the alkanes also reveals that ΔE cannot arise from the scaling of the polarizability with chain length. We discuss other possible origins of ΔE including lattice commensurability effects, temperature, and desorption order.