Resumen
A universal calculation model for theoretical description of structural and physicochemical properties of aqueous solutions of modified silicates with the involvement of complexing agents is offered. The model takes into consideration three types of equilibrium: acidic-basic, metal-complex and, for silicate oxygen anions (SOA), as well as polycondensation. The developed mathematical apparatus makes it possible to apply the model to virtually any type of water glass and its mixtures. For a particular case of the sodium-zinc-ammonium liquid glass, the model provides a numerical solution of the system with seventeen linear and nonlinear equations by using the Newton method.The new model was used to explain the experimentally observable effect of the rheological anomaly in aqueous solutions of liquid glass modified by zinc and ammonia. The effect is typical, most of all, for the solutions of amino silicates, where the rheological anomaly is associated with the shift of the molecular-mass distribution (MMD) of the SOA towards the process of polycondensation at heating. The calculation results show that such shift also takes place in this system and is explained by the transformation of amino complexes of zinc in hydroxo complexes. The total process can be expressed by equation [Zn(NH3)4]2++3OH-®[Zn(OH)3]-+4NH3, which is explained by the essential difference of magnitudes of enthalpy of formation of these complexes. This leads to a decrease in pH and to the shift of the MMD of SOA toward an increase in the degree of polymerization (a decrease in average basicity of SOA). The presence of other complex particles, such as [Zn(NH3)3]2+, [Zn(OH)4]2-, [Zn(OH)2], etc., does not play a significant role.The use of the proposed model allowed the calculation of dependences of parameters of the MMD of SOA, pH magnitudes and concentrations of complex and polycondensation structures on the composition of the solution and temperature. The evolution of relative content of different zinc and silicon containing structures in the course of a change in temperature and total concentration of zinc was traced in detail. It was concluded that the rheological anomaly in the studied system exists due to a special combination of thermodynamic parameters and is unlikely to be widespread among metal-complex silicates