Sticking probabilities in adsorption of alkanethiols from liquid ethanol solution onto gold

The sticking probability (i.e., the rate of adsorption per molecular collis ion with the surface) is the most fundamental and powerful way to express a dsorption rate constants, yet its accurate use has been restricted almost e xclusively to adsorption from the gas phase. Here, we extend this concept t o transient rate measurements of adsorption from liquid solutions, and appl y it to clarify the dynamics of alkanethiol adsorption on gold. A numerical solution to Fick's law of diffusion, using the measured adsorption rate ve rsus time as a boundary condition, provides the adsorbate concentration ver sus distance from the surface and time. The resulting concentration nearest the surface gives the collision frequency with the surface, used to calcul ate the sticking probability. Quantitative measurements of adsorption kinet ics of a series alkanethiols onto gold from ethanol solutions by surface pl asmon resonance (SPR) spectroscopy reveal first-order Langmuir kinetics up to a coverage of similar to4 x 10(14) molecules/cm(2), with an initial stic king probability that increases from similar to 10(-8) to similar to 10(-6) as the alkyl chain length increases from 3 to 19 C atoms. This implies tha t the free energy of the transition state is stabilized by similar to0.7 kJ /mol per CH2 group, about half the stabilization of the adsorbed product. T he solvent strongly increases the activation barrier for adsorption, since these same thiols stick onto clean gold in ultrahigh vacuum with a probabil ity of similar to1.0. Addition of headgroups such as oligo(ethylene oxide) and -COOH decreases the sticking probability relative to a simple alkanethi ol with the same total number of carbon atoms.