Sticking Probabilities in Adsorption of Alkanethiols from Liquid Ethanol Solution onto Gold

The sticking probability (i.e., the rate of adsorption per molecular collision with the surface) is the most fundamental and powerful way to express adsorption rate constants, yet its accurate use has been restricted almost exclusively to adsorption from the gas phase. Here, we extend this concept to transient rate measurements of adsorption from liquid solutions, and apply it to clarify the dynamics of alkanethiol adsorption on gold. A numerical solution to Fick's law of diffusion, using the measured adsorption rate versus time as a boundary condition, provides the adsorbate concentration versus distance from the surface and time. The resulting concentration nearest the surface gives the collision frequency with the surface, used to calculate the sticking probability. Quantitative measurements of adsorption kinetics of a series alkanethiols onto gold from ethanol solutions by surface plasmon resonance (SPR) spectroscopy reveal first-order Langmuir kinetics up to a coverage of ∼4 × 10 14 molecules/cm 2, with an initial sticking probability that increases from ∼10 -8 to ∼10 -6 as the alkyl chain length increases from 3 to 19 C atoms. This implies that the free energy of the transition state is stabilized by ∼0.7 kJ/mol per CH 2 group, about half the stabilization of the adsorbed product. The solvent strongly increases the activation barrier for adsorption, since these same thiols stick onto clean gold in ultrahigh vacuum with a probability of ∼1.0. Addition of headgroups such as oligo(ethylene oxide) and −COOH decreases the sticking probability relative to a simple alkanethiol with the same total number of carbon atoms.