Influence of Solvation and the Structure of Adsorbates on the Kinetics and Mechanism of Dimerization-Induced Compositional Changes of Mixed Monolayers on TiO2

Mixed monolayers of thiol-terminated (T) and methyl-terminated (Me) carboxylic acids on nanocrystalline TiO2 Films underwent dimerization-induced compositional changes. At short reaction times, the compositions of mixed monolayers were kinetically controlled and mirrored the compositions of coadsorption solutions. On time scales up to several hours, well after the establishment of saturation surface coverages, the monolayers relaxed to thermodynamically Controlled compositions through the displacement of Me by T. Equilibration was driven by the formation of intermolecular disulfide bonds between thief groups of adsorbed T, which yielded polydentate dimeric adsorbates that were bound more strongly than monomeric adsorbates to TiO2. The rate of compositional changes increased with decreasing solvent viscosity and decreasing alkyl chain length of T, suggesting that the rate of adsorption of T to TiO2 strongly influenced the overall kinetics under certain conditions. Steric bulk within adsorbates and the strength of surface-attachment interactions also influenced the rate of compositional changes. A kinetic model, derived on the basis of Langmuir adsorption and desorption kinetics, accounts for key aspects or the mixed-monolayer compositional changes. The rate-determining step in the overall mechanism involved either the adsorption of T or the formation of disulfide bonds, depending on the conditions under which monolayers were prepared. Our findings illustrate that dimerization and other intermolecular interactions between adsorbates may dramatically influence the composition and terminal functionalization of mixed monolayers.