where c = concentration in g per 100 cm3 and nrel is the viscosity relative to the solvent. K = 1000K0. Viscosity is essentially independent of pH over the range 0-10 and aqueous solutions exhibit a high tolerance for many inorganic salts. Its wide solubility in organic solvents is unusual. The viscosity of a range of aqueous solutions of PVP is shown in Fig. 8.20.

Figure 8.20 Viscosity of polyvinylpyrrolidone (PVP) solutions as a function of molecular weight (PVP K15 (mol. wt. 40 000) to PVP K90 (mol. wt. 700 000)) and concentration of the polymer in water.

Reproduced from J. L. Azorlosa and A. J. Martinelli, in Water Soluble Resins (ed. R. L. Davidson and M. Sittig), Reinhold, New York. 1962.

PVP forms molecular adducts with many substances. Insoluble complexes are formed when aqueous solutions of PVP are added to tannic acid, poly(acrylic acid) and methyl vinyl ether-maleic anhydride copolymer. Soluble complexes, called iodophors, are formed with iodine: the solubility of iodine is increased from 0.034% in water at 25°C to 0.58% by 1% PVP. The resulting iodophor retains the germicidal properties of iodine. It is thought that the iodine is held in a PVP helix in solution. The influence of two samples of PVP on the solubility of testosterone is shown in Fig. 8.21. The PVP correspondingly increases the rate of solution of the steroid from solid dispersions.

8.4.7 Polyoxyethylene glycols (Macrogols)

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