A study of the interaction between fish gelatin and sodium dodecyl sulfate

Abstract

The objective of this work was to study the interaction between gelatin obtained from cold water fish skin and SDS. This research work investigates the variation in several parameters, such as pH, temperature, ionic strength and the concentration of both fish gelatin and SDS on the nature and extent of the interaction taking place in solution. Turbidity studies were used to investigate the precipitation behaviour of the fish gelatin SDS system. Significant precipitation of the gelatin-SDS complex occurred at pH values below the gelatin isoionic point (pl), with the precipitation being heavier with decreased pH. The amount of turbidity decreased significantly at pH values above the gelatin pl. The proton abstraction studies consisted of mixing fish gelatin with SDS having the same pH, following which the final pH of the mixture was measured. These studies have shown that at pH 6.0, there are significant electrostatic interactions involved because the pH increased on mixing fish gelatin and SDS, and the increase in pH was more pronounced with increasing SDS concentration in the mixture. The increase in pH is attributed to the abstraction of protons from solution by negatively-charged sites on the gelatin which become exposed following the electrostatic interaction of SDS with gelatin. Conductivity and solubilisation results have shown the presence of two surfactant critical aggregation concentrations (cac1 and cac2). The cac1 refers to the onset of micellisation in the presence of fish gelatin. The cac2 represents the cmc of SDS in the presence of the gelatin. These studies have shown that altering ionic strength and gelatin concentration affects the interaction because both cac1 and cac2 are modified. In particular, the presence of salt lowers the value of cac2, while an increase in fish gelatin concentration seems to decrease the SOS concentration at which the interaction begins, while increasing the value of cac2. Additionally, increasing the solution pH results in an increase in cac1, confirming the importance of electrostatic interactions. Finally, binding isotherms constructed using equilibrium dialysis and surfactant-selective electrode data seem to indicate that the binding is not particularly cooperative, especially in the absence of salt. The maximum amount of bound SDS is lower in the presence of salt, which probably competes with SDS for attachment to binding sites on the fish gelatin molecules.