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 $({\mathrm{C}}_N{\mathrm{H}}_{\text{2N+2}}\text{,5⩽N⩽60)}$ each adsorbed at coverages in the range 1 monolayers. Desorption is observed to be a first-order process with a desorption barrier ${\mathrm{(\Delta E}}_{\mathrm{des}}^{‡})$ 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 ${\text{ν=10}}^{\text{19.6±0.5}} {\mathrm{s}}^{\mathrm{-1}}.$ We also find that ${\mathrm{\Delta E}}_{\mathrm{des}}^{‡}$ has a nonlinear dependence on chain length and takes the empirical form ${\mathrm{\Delta E}}_{\mathrm{des}}^{‡}{\mathrm{=a+bN}}^{\gamma },$ with the exponent having a value of $\text{γ=0.50±0.01.}$ 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 ${\mathrm{\Delta E}}_{\mathrm{des}}^{‡}.$ ${\mathrm{\Delta E}}_{\mathrm{des}}^{‡}$ 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.