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Propyliodone oily injection B.P. exhibits thixotropy under high shear rates and rheopexy under low shear rates. The rate and the degree of rheopectic build-up of structure are influenced by the particle size, the effect increasing as the median particle size is reduced.

The viscosity of dispersions of ascorbic acid in solutions of polysorbate 20 has been determined and found to be Newtonian at all concentrations studied. The course of the oxidation in polysorbate 20 appeared to be by the normal chain reaction, but the rate at which it occurred was modified by a number of factors. Incorporation within the micelle appeared to be responsible for an increase in oxidation rate, but the high viscosity of concentrated polysorbate 20 solutions, which would affect the diffusion of oxygen to the reaction site caused a much larger reduction in oxidation rate.

Changes induced by temperature in the relative position and extent of the isotropic and liquid crystal phases present in the ascorbic acid-water-polysorbate 80 system have been recorded. Increases in temperature reduce the size of the liquid crystal phase. This phase is pseudoplastic, but a small central region also shows thixotropic hysteresis loops. On recycling, most systems show Newtonian behaviour. In the isotropic regions, an increase in temperature results in a fall in log of viscosity proportionate to 1/Tabs, but in the liquid crystal region an increase in temperature may cause a similar fall or, because of a change in the relative position of the system within the liquid crystal region, an increase.

The solubility of ascorbic acid in dispersions of polysorbate 80 has been examined. Over a large region of the solubility range liquid crystal exists, bounded by isotropic liquid phases. Excess solubilizate separates out as crystalline material. The rheological properties are complex. Within the isotropic phases the viscosity is Newtonian, but as soon as liquid crystal is produced pseudoplastic or thixotropic flow occurs. In the centre of the liquid crystal region, there were concentrations exhibiting a “yield” value followed by relaxation before the usual thixotropic loop.