Gel formation, effect

For the preparation of unbuilt, cleai—type, HDL formulations, the order of addition is of importance to minimize viscosity resistance to mixing and to avoid possible gel formation. Effective stirring should be maintained during addition of all ingredients, and each ingredient should be in solution before the next is added. A blending temperature somewhat above ambient (e.g., 80-90F) is recommended but not essential. 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Torkelson and coworkers [274,275] have developed kinetic models to describe the formation of gels in free-radical pol5nnerization. They have incorporated diffusion limitations into the kinetic coefficient for radical termination and have compared their simulations to experimental results on methyl methacrylate polymerization. A basic kinetic model with initiation, propagation, and termination steps, including the diffusion hmitations, was found to describe the gelation effect, or time for gel formation, of several samples sets of experimental data. 

Nakanishi, K. and Soga, N., Phase-separation in silica sol-gel system containing polyacryhc-acid. 1. Gel formation behavior and effect of solvent composition, Journal of Non-Crystalline Solids 139(1), 1-13, 1992. 

TABLE 2. Effects of Polymer Concentration, Buffer Solution pH, and Incubation Temperature on Gel Formation... 

This effect is observed at other low absorbed doses (Figure 4) and in each case a minimum of gel formation occurs at around 3% PS. However, it is noticeable that as the absorbed dose is increased the extent of this protective effect decreases. 

---