Analysis of 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 studied in detail. Tetra-methyl-ortho-silicate (TMOS) is often used as a precursor in sol-gel chemistry to produce silica gels in aqueous systems. TMOS can also be mixed with many hydrocarbons without chemical reaction, which allows for various applications in multiphase systems. In this study, TMOS was mixed with n-hexadecane and placed together with water in small cylinders. Upon contact of the mixture with the water, TMOS transfers completely to the aqueous phase where it forms a gel through a heterogeneous reaction. Nuclear magnetic resonance imaging and relaxation time measurements were employed to monitor the mass transfer of TMOS from the oleic to the aqueous phase. The longitudinal relaxation time (T-1) was calibrated and used to determine the concentration of TMOS in n-hexadecane during the transfer. The mass transfer rate was obtained at various temperatures (25-45 degrees C) and for several initial concentrations of TMOS. In the aqueous phase a sharp decrease in the transversal relaxation time (T-2) is observed which is attributed to the gel reaction, in particular the formation of methanol in the initial stage. The minimum in T-2 indicates the gelation point, and was found to be strongly dependent on temperature and concentration. (c) 2006 American Institute of Physics.