Experimental Studies on Diffusion of Liquids in Porous Glass
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STUDIES ON
DIFFUSION GLASS
OF LIQUIDS IN POROUS
YONG SHAO, NEIL KOONE, AND T. W. ZERDA
Texas Christian University, Physics Department, P. 0. Box 32915, Fort Worth, TX 76129 ABSTRACT Diffusion coefficients of water and cyclohexane in porous sol-gel glass of average pore diameter 2.9 nm were obtained using a radioactive tracer technique. This information was applied to"calibrate the membrane in the diaphragm cell which was subsequently used to measure the diffusion coefficients for cyclohexane, acetone, toluene, acetonitrile, and chloroform. Results for cyclohexane were compared with computer simulation of molecular motion of cyclohexane in a model cylindrical pore of diameter 2.9 nm. Translational motion of polar liquids inside the pores was found to be faster than that of neutral, not wetting solvents. INTRODUCTION Since recent developments in production of large monolithic porous sol-gel glass of controlled porosities numerous experimental studies have been published on motion in restricted geometries [1-4]. Investigations showed that molecular transport mechanism is greatly affected by surface interactions and pore geometry [5]. Various experimental techniques, such as diaphragm cell [1,6], NMR [2,3], laser light scattering [4], and fluorescence [7], have been used to measure the self-diffusion coefficients in small pores. The diaphragm method is inexpensive and very accurate but it requires the membranes to be calibrated before use [8]. In order to calibrate the diaphragm cell one needs to know diffusion coefficients for solvents that can be used for calibration runs. Experimental data on the diffusion coefficients of water and cyclohexane were obtained using a radioactive tracer diffusion and sectioning technique. We chose water because this liquid
is both a precursor and a byproduct of the sol-gel reaction, and the data on translational motion may be useful for improving the drying stage of the sol-gel process. Also, in many applications doped sol-gel glass is obtained by diffusing a dopant rich aqueous solution into the pores. Cyclohexane was selected because it is an inert solvent, and its motion through porous glass has been studied both experimentally and theoretically [2,9-11]. In addition, interpretation of the data is simpler than in the case of water because no specific interactions between the solute and silica need to be considered. Other solvents, such as acetone, toluene and chloroform were selected because they are very useful in impregnating porous glass with metal salts, semiconductor clusters, and large organic molecules. A solvent possessing a large diffusion coefficient in a porous medium is a good candidate for a carrier of metal cations to be encapsulated within the glass. It is especially important to use a solvent of large D value if the goal is to produce a porous glass with a given concentration profile across the sample, such as Gradient Index (GRIN) lens. As shown
elsewhere [12], it is relatively easy to produce a sample where a concentration of a dopant material changes as a pa
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