Diffusion of solvents and cations in porous sol-gel glass

Abstract

Diffusion coefficients of selected solvents (acetone, acetonitrile, chloroform, cyclohexane, toluene) and rare-earth cation (erbium and neodymium) solutions were determined in porous sol-gel glass. The traditional diaphragm technique was used to measure the diffusion coefficients of solvents, and new experimental techniques have been developed which employ spectroscopy to determine the diffusion coefficients of cations and anions. The concentrations of the solvents were ascertained using IR spectroscopy. The diffusion coefficient for acetone within the pores was studied over the temperature range 11-42$\sp\circ$C and the activation energy was determined. It was shown that molecular translational motion inside the pores is hindered by geometrical restrictions and surface interactions. The effect of chemical interactions was best illustrated when selected samples had their internal surface modified by replacing hydroxyl groups with trimethylsilane groups, causing the diffusion coefficient for acetone to decrease by 50%. To explain this unexpected result, acetone diffusion was compared with molecular dynamics computer simulations. Concentration profiles of cations and anions within impregnated monoliths were ascertained using UV-visible absorption and Raman spectroscopy, respectively. To study the effects of ligands on the diffusion, four different erbium salts were used: nitrate, chloride, bromide, and perchlorate. Diffusion coefficients for the anions nitrate and perchlorate were found to diffuse with the same rate as the cations. Nitrate anions are found to hinder diffusion, while perchlorate anions do not seem to impede and possibly facilitate diffusion of erbium cations. Absorption spectra were recorded for Nd$\sp{3+}$ in bulk liquid, liquid inside pores, and for heat treated porous glass. Spectra indicate that cations are surrounded by similar species in bulk liquids and liquid within pores, and increasingly interact with the irregular structure of the porous matrix as the temperature increases. The transport of metal into porous glass is of great interest due to potential applications in the manufacture of gradient index (GRIN) lenses and doped optical fibers. By radially diffusing metal into a porous glass cylinder, it has been demonstrated that GRIN lenses can be produced. The transport of rare-earth cations in porous glass is also significant because of the utilization of doped silica fibers in optical communications devices.