The effect of pH on coupled mass transfer and sol-gel reaction in a two-phase system

The coupled mass transfer and chemical reactions of a gel-forming compound in a two-phase system were recently analyzed in detail [Castelijns et al. J. Appl. Phys. 2006, 100, 024916]. In this successive work, the gel-forming chemical tetramethylorthosilicate (TMOS) was dissolved in a mineral oil and placed together with heavy water (D2O) in small cylinders. The transfer of TMOS from the oleic phase to the aqueous phase was monitored through nuclear magnetic resonance (NMR) relaxation time measurements of hydrogen in the oleic phase. The rate of gelation was measured through NMR relaxation time measurements of deuterium in the aqueous phase. The temperature, the initial concentration of TMOS, and the type of buffer in the aqueous phase were varied in the experiments. The mass transfer is driven by the rate of hydrolysis, which increases with temperature. The hydrolysis rate is the lowest at a neutral pH and is the highest at a low pH. In the aqueous phase, a sharp decrease in the transverse relaxation time (T2) of 2H is observed, which is attributed to the gel reaction. The plateau in T2 indicates the gel transition point. The gel rates increase with increasing temperature and increasing concentration, and are the highest at a neutral pH.