Self-Assembly of n-Alkanethiols: A Kinetic Study by Second Harmonic Generation

Thiolate formation of n-alkanethiols (CH3(CH2)(m-1)SH) adsorbing on polycrystalline Sold was studied in situ and in real time under a variety of experimental conditions. The chain length of the thiols studied varied from m = 4 to m = 22. The influence of the thiol concentration in different solvents (ethanol, hexane, dodecane, and hexadecane) was examined in the range between 0.5 and 20.0 mu mol/L. Among the models proposed in the literature, only Langmuir kinetics can explain the data, irrespective of the experimental conditions. As will be shown, all other kinetic models fail to describe the measured formation kinetics of thiolate films on a polycrystalline gold surface. However, small but significant differences between the experimental data and the Langmuir model assuming statistical adsorption are identified. A modified Kisliuk model accounting fur different adsorption sites yields the best agreement with the experimental data. On the basis of these results we propose a kinetic model of thiolate formation which identifies the displacement of solvent molecules adsorbed on the substrate and/or incorporation of alkanethiols into thiolate islands as the rate-limiting steps.