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Abstract
A rheological anomaly, consisting in the reversible increase in solution viscosity with increasing temperature, is a typical property of aqueous solutions of aminosilicates (polysilicates of aliphatic amines, amino alcohols and other amino compounds). To explain this anomaly, a simple mathematical model of interrelated acid-base and polycondesation equilibria in such solutions is proposed. The proposed calculation model assumes a direct dependence of the solution viscosity on the polymerization degree of silicate anions, as well as the independence of the dissociation constant of silanols and the polycondensation equilibrium constant on the degree of anions polymerization. Based on the assumptions made, a system of eight linear and nonlinear equations was constructed and solved. The results of calculations performed for concentrated solutions of aminosilicates (methylamine and piperidine silicates), as well as sodium and tetramethylammonium silicates, confirm the presence of a positive temperature dependence of the polymerization degree of silicate anions in the aminosilicate solutions, in contrast to the solutions of silicates of the strong bases. Thus, the proposed mathematical model of polymerization-hydrolysis equilibria in aqueous silicate solutions qualitatively explains the existence of the rheological anomaly in aqueous solutions of aminosilicates and its absence in solutions of alkali silicates and silicates of quaternary ammonium bases. In addition, this model correctly reproduces the displacements of the molecular mass distribution of silicate anions with changes in chemical composition and concentration of solutions. The proposed approach makes it possible to clarify the mechanism of the rheological anomaly, to understand the roles of various thermodynamic factors in its occurrence, and, as a consequence, to conduct a more meaningful search for new silicate and non-silicate systems with the same anomaly.
Highlights
Aminosilicate (AS) solutions are aqueous solutions of polysilicates of aliphatic, alicyclic or heterocyclic amines as well as amino alcohols and other amino compounds
Resulting pH values and parameters of the molecular mass distribution (MMD) of silicate anions are determined by the amine type, amine and silica concentrations and the temperature
The most interesting property of aqueous solutions of AS is that all solutions, sufficiently concentrated in silica, exhibit the effect of a rheological anomaly, or thermogelation
Summary
Aminosilicate (AS) solutions are aqueous solutions of polysilicates of aliphatic, alicyclic or heterocyclic amines as well as amino alcohols and other amino compounds They may be obtained by ion exchange from alkali silicates or by direct dissolution of solid silicic acid in aqueous solutions of compounds with a sufficiently high basicity constant of amino or imino groups (Kb>10-4.5). The properties of such solutions are largely determined by two interrelated equilibria, namely, the amine ionization equilibrium and the polycondensation equilibrium of the silicon-oxygen anions (SOA). More than 20 years have passed since the discovery of the rheological anomaly in AS solutions, the physicochemical mechanism of the phenomenon is still not completely established
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