Silicate Gel Sample Clauses

Silicate Gel. UTCHEM was modified to simulate the gel reaction of the silicate gel. Polymer and chromium were replaced with silicate (SiO2) and hydroxyl ion (OH-), respectively. The gelation was limited to occur only for pH > 7 (Xxxxxxx et al., 1988; Xxxx, 1979) to eliminate complex behavior of gel reaction rate at pH < 7, and the aqueous-phase permeability-reduction factor was independent of silicate viscosity. Silicate gel is formed by polymerization when appropriate conditions are established. The exact mechanism of gelation is not clear yet; several authors (Xxxx, 1979; Xxxxxxx et al., 1989) explain the general mechanism of gelations of various types of gels. The general process of gelation is as follows (Xxxxxxx et al., 1989): • condensation of monomer and dimer silicate species to form higher-order oligomers, • intramolecular condensation of silanol groups within polymers leading to ring closure and eventual particle formation, and • aggregation of individual particles to form chains and microgel. The rate of gelation (Xxxxxxxxxx et al., 1993) is a function of • silicate concentration • pH • ionic strength • temperature The basic equations that govern polymerization of silicate (Xxxx, 1979) are as follows: SiO2 + 2 H2O = Si(OH)4 (6.10) - SiOH + HOSi - = - SiOSi - + H2O (6.11) In general form, (nx-z) n SiO2 + z OH- = XxxXx(OH)z- + (y-2) H2O , (6.12) where n = degree of polymerization x = ratio of OH:Si x = 4.85 n- 1/3 - 7.8 n- 2/3 + 4.2 n- 1 2 y = 4 n - n x + z z = number of charges on polymer Equation 6.12 can be written in simplified form as [SiO2] + m [OH-] = [silicate gel] , (6.13) where m is the stoichiometric ratio.
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