A preliminary study of the interaction between gelatin and sodium dodecyl sulfate in water and water-ethanol solutions

Abstract

The objective of this work was to study the interaction between the protein gelatin and an anionic surfactant, Sodium Dodecyl Sulfate using various techniques. This research investigates the variation in several parameters, including temperature, gelatin type and ethanol concentration on the nature and the extent of the interaction taking place in solution. Conductivity studies were carried out on a considerable number of different systems. They have shown the presence of two surfactant critical aggregation concentrations (c. a. c 1 and c. a. c2) when water was used, but only the second aggregation concentration was detected in the presence of ethanolic solutions. The c. a. c 1 refers to the onset of micellisation in the presence of gelatin. The c.a.c2 represents the c.m.c. of the surfactant in the presence of gelatin. These studies have shown that altering the temperature, gelatin type and ethanol concentration affects the interaction because the critical aggregation concentrations were modified. An increase in temperature was shown to cause an increase in both the critical aggregation constants. Fish gelatin systems were found to have lower critical aggregation concentrations then systems containing bovine gelatin. Upon increasing the ethanol concentration the c.a.c1 became undetectable using conductivity, while the c.a.c2 was found to decrease. Solubilisation studies were only carried out on bovine gelatin at a fixed temperature, and similar trends were observed upon increasing the ethanol concentration although the aggregation concentrations were found to be significantly higher than the aggregation concentrations obtained using conductivity studies. Binding isotherms were constructed usmg equilibrium dialysis and cooperative binding seemed to be taking place. Binding studies showed that upon increasing the ethanol concentration, binding started at higher SDS concentrations when compared to water. The maximum amount of bound surfactant was also found to decrease. When an ethanol concentration of 20% was used cooperative binding became hindered, and a maximum value for bound surfactant was not obtained.