JOULE-DISCHARGE HEATING STUDIES OF PORE CONNECTIVITY IN SILICA-GEL - INFLUENCE OF PORE DIAMETER, CHEMICAL MODIFICATION, AND PARTICLE-SIZE

Studies of Joule-discharge heating of packed beds of chemically modified silica gel address the influence of pore diameter, particle size, and chemical modification on the heating kinetics of the silica gel. The results reveal that surface modification and pore diameter affect the connectivity of the intraparticle pore network. Heating of packed beds consisting of 60-, 100-, and 147-angstrom pore-diameter silica gels modified with n-triacontyldimethylsilyl (C30), dimethyloctadecylsilyl (C18), or trimethylsilyl (C1) ligands are monitored by observing the temperature-dependent fluorescence of NBD-hexanoic acid in MeOH/H2O solvents. A decrease in the silica pore diameter, an increase in the length of surface ligands, or an increase in bonding density of the ligands causes a reduction in the rate of the energy dissipation in the temperature-jump cell. The temperature equilibration time or minimum heating time, tau(h,min), depends on particle size with 5-, 10-, and 40-63-mum octadecylsilica exhibiting tau(h,min) values of ca. 5, 10, and 250 mus, respectively. Joule heating at a discharge rate of 2 x 10(5) s-1 of 5- and 10-mum silicas is uniform, as indicated by a single-exponential response of the fluorescence thermometer, while a biexponential response for larger particle silica shows evidence of nonuniform heating. The rate and uniformity of heating depend upon the particle size and length of the alkyl ligands attached to the surface.