Sticking Probabilities in Adsorption from Liquid Solutions: Alkylthiols on Gold

Abstract

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.

Importance of reactant mass transfer in the reproducible preparation of self-assembled monolayers

“…The work herein extends earlier investigations by other laboratories on the role of mass transfer with respect to adlayer deposition [49,58,59]. It takes, however, a slightly different tactic by examining how the geometry (i.e., size and shape) of the substrate and reaction vessel influence reactant mass transport and, therefore, the structure, wetting, and electrochemical properties of the resulting adlayer.…”

“…Our impingement model does not take into account the fate of a reactant after collision with the evolving surface structure [81]. It also ignores, for example, issues related to the role of the sulfhydryl hydrogen [82], the impact of charge buildup on the gold surface as the adlayer forms [82], how the sticking coefficient varies with adlayer surface concentration [49,58,59,83], and the possible formation of a bilayer-like structure during assembly [84]. Rather, our model examines only how the rate and diffusional mode of reactant delivery may affect the structure and, therefore, barrier properties of the resulting adlayer [5].…”

“…Karpovich and Blanchard [61] followed the concentration dependence of deposition kinetics via a goldcoated quartz crystal microbalance. Jung and Campbell [58,59] utilized Fick's Law to calculate concentration profiles during deposition to determine a sticking coefficient. Camillone [49] derived a linear diffusion model to ascertain the time required for impingement of one-half of a monolayer from solution.…”

Current understanding of the structure, phase transitions and dynamics of self-assembled monolayers on two- and three-dimensional surfaces

“…Several studies exist on the adsorption energetics of SAMs, but all are not completely consistent [104][105][106]. Electron diffraction and STM measurements reveal that the monolayers form a ð p 3 Â p 3Þ R308 overlayer structure, which is a balance between the S-Au chemisorption and the interaction between the chains.…”

Potential-assisted deposition of mixed alkanethiol self-assembled monolayers

“…Although the principles of the adsorption appear to be known, the kinetics data, including the time constants of SAM formation, in fact reveal discrepancies [22] of many orders of magnitude. The most widely used models, such as the simple first order Langmuir law [25,[27][28][29][30][31][32] show some deviation [28] especially for short and long times. Peterlinz and Georgiadis found that data were equally well described by a second or first order Langmuir kinetics [26].…”