DIRECT DIFFUSION MEASUREMENTS OF NAPHTHACENE ON SILICA AS A FUNCTION OF SILANOL DENSITY

Fluorescence photobleaching is employed to obtain directly, and for the first time, diffusion coefficients and mobile fractions for naphthacene adsorbed on silica gel. The diffusion is slower than that estimated from bimolecular rate constants in comparable systems, and only about one-third to one-half of the molecules are mobile. The mobility parameters are studied as a function of the level of hydroxylation (chemisorbed water) and hydration (physisorbed water) of the silica gel controlled by appropriate heat treatments. The diffusion coefficient is independent of the preparation while the mobile fraction increases when the level of hydroxylation decreases and that of hydration is increased. These trends are rationalized in terms of the surface heterogeneity and the number of hydrogen bonds formed between naphthacene and surface silanol groups. © 1990, American Chemical Society. All rights reserved.