Ionic strength effects in aqueous solutions of gelatin and sodium dodecylsulphate

The interaction between the anionic surfactant sodium dodecylsulphate and the polyampholyte gelatin in aqueous solution has been studied by viscosity and fluorescence, Three different gelatin molecular weights, a polydisperse gelatin and two fractionated samples derived from it, have been examined at two ionic strengths. On addition of sodium dodecylsulphate SDS, the viscosity of a low molecular weight fractionated (alpha) gelatin increases smoothly with SDS concentration, unlike the polydisperse (standard) and high molecular weight (HMW) fractionated gelatins which show maxima. The viscosity increase at the maximum increases with gelatin molecular weight. On addition of 0.1 M salt, the viscosities of the gelatin-surfactant mixtures are lower than the equivalent no-salt cases. Fluorescence studies suggest that the SDS micelles adsorbed onto these various gelatins under two different ionic strengths are remarkably similar.