PROTON DIFFUSION IN THE PORES OF SILICATE SOL-GEL GLASSES

The rate of the recombination of protons with pyranine, or 8-hydroxy 1,3,6-trisulfonated pyrene, in the pores of sol-gel matrices prepared from tetraethoxysilane is measured by using a pH jump method. Irradiation of pyranine with a 10-ns laser pulse generates a proton concentration of about 10(-4) M within the time of the pulse. The rate of the ground-state back-protonation reaction is measured in sols, gels, and xerogels by monitoring the time dependence of the absorbance of the deprotonated form of pyranine after the excitation pulse. The rate constants in the gel and aged gel differ only slightly from those in aqueous solutions. In air-dried xerogels the rate constant is 4.6 x 10(10) M(-1) s(-1) The activation energy of the back-protonation reaction is calculated from the temperature dependence of the rate constant. It is 8.4 kcal/mol, about 2.4 times larger than that in aqueous solution.