Interaction of anionic compounds with gelatin. II: Effect on some physicochemical properties of gelatin

The interaction of gelatin with four monosulfonated or monocarboxylated azo dyes was investigated by measuring the surface tension and intrinsic viscosity of gelatin solutions containing the dyes at different concentrations, the rigidity and melting point of their gels, and the moisture regain of their films. The dyes, which were used as models for anionic drugs, differed in the size of their aromatic hydrocarbon moieties. Surface tension measurements showed that the gelatin did not affect the critical micelle concentration of the free dyes and that the bound dyes increased the surface activity of the gelatin. The dyes reduced the intrinsic viscosity of gelatin by as much as 2/3. They also lowered the rigidity and the melting point of dilute gelatin gels and reduced the moisture regain of dry gelatin films. These changes became more pronounced with increasing dye concentrations. The effectiveness of the dyes in producing these changes increased with the size of their hydrocarbon moieties because, as had been shown in a previous study, increasingly larger hydrocarbon moieties increased the binding of the dyes to gelatin. At the pH of the measurements, which was 1.9 units below the isoelectric point of the gelatin, the gelatin was a cationic polyelectrolyte. Binding of the dye anions by ion pairing, hydrogen bonds, and other secondary valence forces rendered the gelatin less ionic and less hydrophilic, which accounts for the present observations.