Adsorption of short-chain thiols and disulfides onto gold under defined mass transport conditions: coverage, kinetics, and mechanism.

The adsorption of water-soluble alkane thiols and their corresponding disulfides onto gold was followed in real time using highly sensitive surface conductivity measurements. Particular attention was paid to producing clean surfaces and to the purity of the adsorbates. The rate of mass transport to the surface was constant, controlled, and measured, over the whole time course of the experiment (1-10(4) s), by convective diffusion. An adsorption rate equation derived for coupled steady state convective-diffusion mass transport and Langmuir kinetics shows that systems limited by mass transport must also be slowed by Langmuir kinetics. Thiols and disulfides adsorbed at the same rate, limited mainly by mass transport. The distinct slowdown in adsorption rate for longer alkanethiols, attributed to conformational transitions (lying down → standing up), was less evident for the neutral thiols/disulfides. The slower rate of charged thiol adsorption is thought to stem from steric interactions of large, hydrated tail groups, although calcium as a counterion accelerated monolayer formation. The adsorption kinetics of a charged thiol were almost the same under screened (by extra added salt) or unscreened conditions. Therefore, long-range electrostatic interactions appear to be less important than short-range steric ones in limiting adsorption rates at surfaces.