Direct visualization of the hydrolysis kinetics of titanium(IV) alkoxides on functionalized gold surfaces

The hydrolysis kinetics of titanium(IV) alkoxides coordinated on functionalized gold surfaces was monitored with video-based contact angle measurements, and the experimental data were further modeled to evaluate pseudo-first-order reaction rate constants. In particular, titanium(IV) isopropoxide Molecules were attached to dithiothreitol monolayers on gold from dilute 2-propanol solution to form titanium-coordinated surfaces. The side view of a sessile drop of aqueous solution on the titanium-coordinated surface changed significantly as a function of time (corresponding to an apparent decrease in the contact angles), indicating that the surface became more hydrophilic as a result of its reaction with aqueous solution (i.e., the hydrolysis and polycondensation). The kinetics of this surface reaction was modeled, for the first time, based on the observed changes in the time-dependent contact angles. Pseudo-first-order kinetic parameters obtained showed an interesting two-step reaction feature when water was used (k(1) = 0.91 X 10(-3) s(-1) and k(2) = 2.1 x 10(-3) s(-1)), while the experiments using acid and base solutions displayed a clear enhancement effect and were better modeled with only one rate constant (k(overall) = 2.3 x 10(-3) s(-1) for 0.10 M HCl and 5.0 x 10(-3) s(-1) for 0.10 M NaOH).