Interphase interactions of hydrophobic powders based on methilsilica in the water environment
Using modern physicochemical research methods and quantum chemical modeling, the surface structure, morphological and adsorption characteristics, phase transitions in heterogeneous systems based on methylsilica and its mixtures with hydrophilic silica were studied. It is established that at certain concentrations of interfacial water, hydrophobic silica or their composites with hydrophilic silica form thermodynamically unstable systems in which energy dissipation can be carried out under the influence of external factors: increasing water concentration, mechanical loads and adsorption of air by hydrophobic component. When comparing the binding energies of water in wet powders of wettind-drying samples A-300 and AM-1, which had close values of bulk density (1 g/cm3) and humidity (1 g/g), close to 8 J/g. However, the hydration process of hydrophobic silica is accompanied by a decrease in entropy and the transition of the adsorbent-water system to a thermodynamically nonequilibrium state, which is easily fixed on the dependences of interfacial energy (S) on the amount of water in the system (h). It turned out that for pure AM-1 the interfacial energy of water increases in proportion to its amount in the interparticle gaps only in the case when h < 1 g/g. With more water, the binding energy decreases abruptly, indicating the transition of the system to a more stable state, which is characterized by the consolidation of clusters of adsorbed water and even the formation of a bulk phase of water. Probably there is a partial "collapse" of the interparticle gaps of hydrophobic particles AM-1 and the release of thermodynamically excess water. For mixtures of hydrophobic and hydrophilic silica, the maximum binding of water is shifted towards greater hydration. At AM1/A-300 = 1/1 the maximum is observed at h = 3g/g, and in the case of AM1/A-300 = 1/2 it is not reached even at h = 4 g/g. The study of the rheological properties of composite systems has shown that under the action of mechanical loads, the viscosity of systems decreases by almost an order of magnitude. However, after withstanding the load and then reducing the load to zero, the viscosity of the system increases again and becomes significantly higher than at the beginning of the study. That is, the obtained materials have high thixotropic properties. Thus, a wet powder that has all the characteristics of a solid after a slight mechanical impact is easily converted into a concentrated suspension with obvious signs of liquid.
