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 $\sim {10}^{-8\mathrm{}}$ to $\sim {10}^{-6\mathrm{}}$ with alkyl chain length, implying a stabilization of the transition state by $\sim 0.65\mathrm{kJ}/\mathrm{mol}$ per ${\mathrm{CH}}_2$. Since their sticking probabilities in the gas phase are $\sim 1.0$, the solvent increases the activation free energy by $\sim 40\mathrm{kJ}/\mathrm{mol}$.