Rheological study of the pH dependence of interactions between gelatin and anionic surfactants: Flow behaviour and gelation

The interactions between gelatin and two anionic surfactants (sodium di-sec-butylnaphthalene-sulfonate and sodium dodecyl-benzenesulfonate, respectively) were investigated using rheological methods, charge and surface tension measurements. Upon the addition of surfactants, the viscosity of aqueous gelatin solutions increases at pH values higher than the isoelectric point (IEP) of the gelatin, provided that a distinct surfactant concentration is exceeded. The increase in viscosity depends on the structure of the hydrophobic moiety of the surfactant. Surface tension measurements suggest the formation of gelatin-surfactant-complexes. If the surfactant is added in high concentration, the viscosity does not further increase, and free micelles are formed in the solution. Directly at the IEP and at pH values below, the addition of surfactants leads to flocculation because of electrostatic interactions. At all surfactant concentrations, the flow behaviour was strictly Newtonian. As a model of the structure of the complex, a modified 'bead and necklace'-model is suggested. This model proposes the nucleation of micelles at the hydrophobic gelatin regions ('micellar surfactant-gelatin complexes'). The number of micelle moieties per gelatin chain could be estimated to be about three. The complex stability is dependent on the extent of hydrophobic interaction. The gelatin behaviour of gelatin is strongly affected by the addition of the surfactant. The ratio between the attainable linear storage and loss moduli, G' and G '', decreases strongly. Gelation is generally hindered, but the effect is stronger at pH values below the IEP than above.