Viscosity of stable sols

Solutions are stable at pH 3-11. Increasing temperature reduces the viscosity of solutions. Hypromellose undergoes a reversible sol-gel transformation upon heating and cooling, respectively. The gel point is 50-90°C, depending upon the grade and concentration of material. 

The particle diameters of some dilute Raffo sols have been determined as 0.1-0.5 fim [31, 32]. The densities of Raffo sols increase linearly with the sulfur content and reach a value of 1.24 g cm at 16 °C for a sulfur content of 450 g [30]. After evaporation of some of the water sulfur contents of up to 600 g have been obtained Such sols are of oily or honey-like viscosity [30]. More dilute sols are clear yellow liquids stable for several weeks and undecomposed even when heated to 100 °C, provided all salts have been carefully removed. No phase transition occurs on heating up to 150 °C [30] which indicates that the sulfur and the polythionates of the sol are in a liq-uid-like state. 

The pH of the double deionized sol was 3.4 and was then adjusted to 9.1 by adding enough 3 N sodium hydroxide solution while stirring vigorously in a blender. The sol was concentrated to 30 wt% silica by evaporation. A small sample was saved for characterization and the rest of the sol was further concentrated to 40 wt% silica. Both the 30% wt silica sol sample and the 40 wt% silica sol were filtered through Celite filter aid. Even the 40% silica sol was perfectly stable under normal storage conditions. The 30% sol had a specific surface area (S.A.) of 230 m /g and when diluted to 9 wt% SiOa had a viscosity of 5.1 cps at 25°C. 

The pH-value and Si02 content of this hydrosol are determined by the concentration of the raw materials and their mixing ratio. Typically acid excess is preferred, as under these conditions the intermediate sol is more stable and the process is less sensitive to feed fluctuations. During the sol-forming step an unstable intermediate-monomeric orthosilicic acid — is formed which then rapidly undergoes an acid-catalysed condensation reaction to form oligomers. When the molecular weight reaches ca. 6000, a sudden increase of both the viscosity and the modulus of elasticity is observed. This increase marks the transformation of the sol to a gel that will then further develop its internal structure. 

For stable sols such as colloidal silica, forced circulation evaporators have been generally used. Special precautions must be taken not to permit the sol to become too concentrated or to reach dryness at any point on the equipment walls and especially on the heat exchange surfaces. If this occurs, a layer of hard adherent silica is built up. The problem becomes acute as the silica approaches the final high concentration with increasing sol viscosity. 

Water molecules of hydrophilic sols, in the vicinity of dispersed molecules of high molecular weight substances (proteins and polysaccharides), are relatively tightly associated by non-bonding interactions. They are usually in the state of thermodynamic equi-Hbrium and are therefore relatively stable. Their viscosity and surface tension are significantly different (higher) than the viscosity and surface tension of the pure dispersion medium (water). 

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