High Pressure Raman Study of the Hydrolysis Reaction of Tmos and Teos
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HIGH PRESSURE RAMAN STUDY OF THE HYDROLYSIS REACTION OF TMOS AND TEOS G. HOANG, J. WATSON and T. W. ZERDA Physics Department, Texas Christian University, Fort Worth, TX 76129 ABSTRACT High pressure Raman spectroscopy is used to monitor the hydrolysis reaction of TMOS in solutions with methanol, acetonitrile, acetone, dioxane and formamide, and of TEOS as a function of pH and the catalyst used. The rate constants for various solvents, temperatures and pressures are experimentally determined from the time dependence of Raman band intensities. It is shown that the reaction is slow in dioxane and fast in methanol or formamide. The volume of activation is found from the pressure dependence of the rate constant. The volume of activation, the dielectric constant, dipole moments and hydrogen bonding properties and their role in the hydrolysis reaction are discussed. It is shown that solvents which can form hydrogen bonding with Si-OR groups can increase the rate of the reaction. INTRODUCTION The hydrolysis reaction, the first step of the sol-gel processing, has been studied both theoretically [1,2] and experimentally. Different experimental techniques, such as NMR [3-5], Raman [3,6,7], UV absorption [8], and chromatography [9] have been used. It is now generally accepted that hydrolysis proceeds by bimolecular nucleophilic displacement reactions (SN2 reactions) involving pentacoordinate intermediates [10]. However, there are still unanswered questions relating the details of the role of the solvents. Previously, we studied the effects of pH of water, temperature, pressure, and solvent on the hydrolysis reaction of TMOS [6,7]. Using high pressure Raman spectroscopy we determined the volume of activation, and we provided evidence for the pentavalent intermediate. In this report, we examine the effect of the dielectric constant, dipole moment and hydrogen bonding on the reaction rate. The overall hydrolysis reaction of alkoxide can be written as: Si(OR)4 + nH20
-ý
Si(OR)4-n(OH)n + nROH
(1)
where n varies from 1 to 4, and OR represents an alkoxide group. We studied the band due to Si-OR vibrations located at about 645 cm- 1 for TMOS and at about 656 cm- 1 for TEOS. The intensity of the Raman band is proportional to the concentration of the molecules in the system. The decay in time of the Si-OR vibrational band, due to the consumption of alkoxides by water molecules, can be used to characterize the reaction rate. For a system with an excess of water, one may assume that reaction (1) is a quasi-first order reaction and the rate, k, can be found from In I(t) = - kt + const.
(2)
where I is the intensity of the Si-OR band, and t is time elapsed. When only a small amount of water is used, alcohol producing condensation may decrease the the number of silicon alkoxides in the system [5]. In order to find the reaction rate, the concentrations of water have to be known at a given time, then it can be found from the intensity of the C-O band of alcohol. The rate is then found from the second order kinetics. The rate constant incre
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