Temperature-Programmed Desorption Studies of n-Alkane Derivatives on Graphite: Desorption Energetics and the Influence of Functional Groups on Adsorbate Self-Assembly

Abstract

Temperature programmed desorption (TPD) studies have been performed to determine the influence of functionalization on the desorption of n-alkanes from inert substrates. Monolayer and multilayer desorption was investigated for a set of six 1-bromoalkanes, six alkanoic acids, and four 2-bromoalkanoic acids adsorbed on the basal plane of highly-oriented pyrolytic graphite (HOPG). Full activation of the substrate surface for monolayer adsorption required annealing temperatures of approximately 700 K. Molecular desorption from physisorbed mono- and multilayers was found to exhibit first- and zeroth-order kinetics, respectively, with no measurable dependence of the activation energy on coverage. A Redhead analysis of monolayer desorption signals uncovered a profound influence of alkane functional groups on the desorption barrier. As compared with the case of unfunctionalized n-alkanes, the desorption energies of all derivative species studied here exhibit a reduced chain length dependence in conjunction with an increased adsorption energy in the limit of zero chain length. Both effects are weakest for 1-bromoalkanes, where the adsorption energy also displays a pronounced nonlinearity as a function of chain length. These effects reach their maximum for 2-bromoalkanoic acids, where the adsorption energy is nearly independent of the number of methylene units, at least up to 2-bromooctanoic acid. The trends observed here can be understood in the context of molecular self-assembly information provided by ambient scanning tunneling microscopy (STM) studies and molecular dynamics (MD) simulations. Additional interactions and steric considerations in these adsorbates arise from the presence of functional groups, which introduce added configurational constraints and cause a competition with alkyl chain interactions in determining self-assembly patterns.