Sticking probabilities in adsorption from liquid solutions: alkylthiols on gold

The sticking probability, defined as the rate of adsorption per molecular collision with the surface, directly expresses the difficulty encountered by a molecule in scaling the barrier to adsorption. Its prior use has been restricted to adsorption of gases. A method extending this concept to adsorption from liquid solutions is applied to transient measurements of alkylthiol adsorption onto gold from ethanol solutions. The initial sticking probability increases from approximately 10(-8) to approximately 10(-6) with alkyl chain length, implying a stabilization of the transition state by approximately 0.65 kJ/mol per CH2. Since their sticking probabilities in the gas phase are approximately 1.0, the solvent increases the activation free energy by approximately 40 kJ/mol.