Physical-chemical and biochemical factors in silica sol-gels

Abstract

Recent findings show that additions of certain organic chemicals, such as formamide, to TMOS and TEOS based sols make it possible to produce large monolithic dried gels routinely and rapidly. The role of the drying control chemical additives (DCCAs) in controlling the rates of hydrolysis and polycondensation reactions is described based upon 29Si NMR, laser Raman, and Mo solubility analysis of the sols. Effect of the DCCAs on gelation time and depolymerization rates is also discussed. Wide ranges of gel pore size distribution result depending upon the concentration and type of DCCA used. Results from automatic analysis pore size distributions are presented which show shifts from mean pore sizes of 10 Å to > 200 A ̊ . The effects of the DCCA on gel aging time and temperature is shown. Silica gels containing wide ranges of alkalis, alkaline earths, transition metals, and rare earth elements can also be made using similar processes. Thus, the optical absorption spectra, index of refraction, and dispersion can be varied greatly. Thermal densification of the gels results in even wider ranges of optical properties depending upon temperature, time, and pO 2 in the atmosphere. A modification of the above so-gel process is used to produce gels containing variable quantities of Al 2O 3, as well as SiO 2, Li 2O, TiO 2, etc. Organically fixed silicon has been shown by Schwartz and by Carlisle to be a critical constituent in the mineralization of tissues. Recent work has also implicated silicon as a critical element in arterial walls and resistance to hardening of the arteries and coronary heart disease. These and other biochemical observations are related to recent experiments on the biological behavior of silica gel forming implants which bond to living tissues. a potential relevance to differentiation of cells and use in genetic engineering is briefly discussed.