We have presented the first experimental evidence that the molecular dipole and the electron density on the S-atom affect the adsorption process of thiols on gold. The adsorption kinetics of five rigid 4-mercaptobiphenyls onto a polycrystalline gold surface has been studied by the quartz crystal microbalance (QCM) technique. The kinetics data cannot be fitted to the Langmuir equation because it does not take interadsorbate interactions into consideration. A new lattice-gas adsorption model was developed that approximates the chemisorbed layer of interacting mercaptobiphenyls as lattice-gas particles with pair interactions between nearest-neighbor sites. The interacting lattice-gas model produces much better fits to experimental data and provides quantitative estimates of the strength of the chemisorption potential and the dipolar interactions, as well as of the rate constants. The formation of self-assembled monolayers (SAMs) from mercaptobiphenyls in toluene solutions is faster than that of n-alkanethiols, unless strong electron-attracting groups are substituted at the 4'-position. From the QCM measurements combined with ellipsometric data, we found that the initial adsorption rate constants at room temperature were directly related to the molecular dipole moment. The larger the dipole moment (|Δσ|) is, the more repulsive the intermolecular interactions (ε), and the slower the overall rate constant (Γ). The chemisorption potential (μ(s)) is bigger than for alkanethiolate, probably because the S-gold bond is stronger. The plot of μ(s) vs |Δσ| shows a maximum, suggesting that μ(s) includes the contributions of both the S-gold bond strength and the intermolecular repulsion.
ASJC Scopus subject areas
- Colloid and Surface Chemistry