2 - Sol-Gel Kinetics

Abstract

This chapter discusses sol kinetics. The study of various chemicals, such as formamide or oxalic acid/ DCCAs (Drying Control Chemical Additives) added to the silicon alkoxide derived sol led to the discovery that large gels could be casted, and dried routinely without cracking. The chemical kinetics data are correlated with the x-ray small angle scattering (XSAS), Raman ultrastructure analysis of the wet gels, and pore size distributions of the dried gels using N2(nitrogen gas) adsorption isotherm analyses. The results revealed that sol compositions could be processed into easily dried monoliths. Control of the rate of hydrolysis of tetramethyl ortho silicate (TMOS) (kH), and the rate of condensation (kc) to form the gel network is the key to reliable control of the pore size distribution, and subsequent drying kinetics. Linkage of additional Si (OH)4 tetrahedra occurs as a poly condensation reaction, and results in a three dimensional SiO2 network. A typical sequence of condensation products is: monomer- dimer- linear trimer - cyclic trimer - cyclic tetramer, and a higher order generation of discrete colloidal particles that are commonly observed in aqueous systems. The effect of sol chemistry, and gel ultrastructure on the physical properties of the gel-silica monoliths is enormous.