Ls. Jung et Ct. Campbell, Sticking probabilities in adsorption of alkanethiols from liquid ethanol solution onto gold, J PHYS CH B, 104(47), 2000, pp. 11168-11178
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.