The interaction of sodium dodecyl sulfate with gelatin

The separation of adsorption complexes formed by the interaction of sodium dodecyl sulfate (SDS) with gelatin at pH values below the isoelectric point has been investigated. Two pH conditions were employed. In one, the pH of the reaction mixture was adjusted to 4.1 after the component solutions had been mixed. In the other, the pH of each of the component solutions was adjusted to 4.1 before the solutions were mixed and the pH of the reaction mixtures was allowed to vary with the reaction. Concentration regions of complex solubility, complex precipitation, maximum gelatin precipitation, and complex solubilization characterized these reactions. Analyses of the supernatant solutions for both gelatin and SDS indicate that the adsorption is not exclusively continuous. Under the first pH condition, the reaction mechanism appears to be all-or-none in the region of complex precipitation forming a complex in which only one-half of the cationic sites are occupied, and predominantly statistical in the region of maximum gelatin precipitation, terminating in a complex in which all the cationic sites are occupied. Solubilization of the precipitated complex is achieved by the physical adsorption of additional SDS with the hydrophilic groups oriented toward the water. Under the second pH condition, an exclusively all-or-none mechanism appears to prevail in the regions of complex precipitation and solubilizations. The reactions result in the formation of an insoluble complex in which one-half the cationic groups on the gelatin are involved and a soluble complex in which all the cationic groups are involved.