Dynamic Light Scattering Study of Gelatin−Surfactant Interactions

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Abstract

Binding of anionic (sodium dodecyl sulfate, SDS), cationic (cetyl trimethyl ammonium bromide, CTAB), and nonionic (TX-100) surfactants to gelatin chains in aqueous buffer (pH = 7.0) medium has been studied by dynamic light-scattering technique performed at T = 30 °C. In the surfactant concentration range varying from 0 to 100 mM, SDS exhibited electrostatic binding to the charged groups of the polypeptide chain resulting in considerable reduction in the hydrodynamic radius (Rh) of gelatin up to the critical association concentration (CAC), and at higher concentrations both the SDS micelles and gelatin−SDS complexes were found to be coexisting in equilibrium. In the case of CTAB, almost the opposite was observed: the gelatin chains showed small increase in size up to the CAC. Beyond this, the gelatin−CTAB complexes were observed to grow significantly, and these were found to be in equilibrium with CTAB micelles. TX-100 exhibited little hydrophobic binding to gelatin, and no observable change in gelatin size was observed. The micellar shapes were found to be near-spherical for SDS and oblate ellipsoidal for CTAB micelles. Results have been explained through the necklace-bead model of polymer−surfactant interactions.

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