Current Issues in Sol-Gel Reaction Kinetics
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CURRENT ISSUES IN SOL-GEL REACTION KINETICS Roger A. Assink and Bruce D. Kay Sandia National Laboratories Albuquerque, NM 87185
ABSTRACT This paper surveys a few of the current issues in sol-gel reaction kinetics. Many times seemingly modest changes in reactants or reaction conditions can lead to substantial differences in the overall reaction rates and pathways. For example, qualitative features of the reaction kinetics can depend on catalyst concentration. At very high acid-catalyst concentrations, reverse reactions are significant for TMOS solgels, while for moderate acid-catalyst concentrations, reverse reactions are substantially reduced. The reaction kinetics of two similar tetraalkoxysilanes: tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), can be markedly different under identical reaction conditions. Under acid-catalyzed reaction conditions, a TMOS sol-gel undergoes both water- and alcoholproducing condensation reactions while a TEOS sol-gel undergoes only water-producing condensation. The early time hydrolysis and condensation reactions of a TMOS sol-gel are statistical in nature and can be quantitatively described by a few simple reaction rate constants while the reaction behavior of a TEOS sol-gel is markedly nonstatistical. A comprehensive theory of sol-gel kinetics must address these diverse experimental findings. INTRODUCTION There have been many studies of the reaction kinetics of acid-catalyzed tetraalkoxysilanes.
The experimental techniques
used to study the chemical reaction kinetics include
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H and
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Si
NMR spectroscopy [1,2], capillary gas chromatography (3]and infrared and Raman spectroscopy [4]. Most of these studies have investigated a single tetraalkoxysilane under very specific conditions. These conditions include acid-catalyst concentration, silane concentration, water to silane ratio and even a specific time regime. Very often one wishes to extend the conclusions reached for a single system under specific conditions to sol-gel systems in general. This paper will attempt to show that one must be careful in making such extrapolations. We will examine three areas which have been the subject of This work performed at Sandia National Laboratories, supported by the U. S. Department of Energy under contract number DE-AC0476DP00789. Mat. Res. Soc. Symp. Proc. Vol. 180. ©1990 Materials Research Society
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recent study. These areas are: (1) the relative importance of thermodynamic versus kinetic factors in governing the product distribution of sol-gels, (2) the existence of both water- and alcohol-producing condensation pathways, and (3) the statistical (or nonstatistical) behavior of the hydrolysis and condensation reactions.
RESULTS AND DISCUSSION Thermodynamic vs Kinetic Control Recently Klemperer and Ramamurthi demonstrated the importance of thermodynamic factors in determining the final product distribution in an acid-catalyzed TMOS sol-gel [3]. Using capillary gas chromatography they were able to show that in three days the species distribution of an acid-catalyzed 3 M TM
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