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 modifi ed silica gel address the influence of pore diameter, particle size, a nd chemical modification on the heating kinetics of the silica gel. Th e results reveal that surface modification and pore diameter affect th e connectivity of the intraparticle pore network. Heating of packed be ds 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 so lvents. A decrease in the silica pore diameter, an increase in the len gth of surface ligands, or an increase in bonding density of the ligan ds causes a reduction in the rate of the energy dissipation in the tem perature-jump cell. The temperature equilibration time or minimum heat ing 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 25 0 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-expone ntial response of the fluorescence thermometer, while a biexponential response for larger particle silica shows evidence of nonuniform heati ng. The rate and uniformity of heating depend upon the particle size a nd length of the alkyl ligands attached to the surface.