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 f rom tetraethoxysilane is measured by using a pH jump method. Irradiati on of pyranine with a 10-ns laser pulse generates a proton concentrati on of about 10(-4) M within the time of the pulse. The rate of the gro und-state back-protonation reaction is measured in sols, gels, and xer ogels by monitoring the time dependence of the absorbance of the depro tonated form of pyranine after the excitation pulse. The rate constant s in the gel and aged gel differ only slightly from those in aqueous s olutions. 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 calc ulated from the temperature dependence of the rate constant. It is 8.4 kcal/mol, about 2.4 times larger than that in aqueous solution.