Gelatin–alpha olefin sulfonate interactions studied by dynamic light scattering

Abstract

Dynamic light scattering (DLS) measurements were performed to study the binding of anionic surfactant alpha olefin sulfonate (AOS) to gelatin chains at various NaCl concentrations at 30 °C in aqueous sodium phosphate buffer (pH=6.8) solutions. The surfactant concentration was varied from 0 to 80 mM and the NaCl concentrations chosen were 0.025, 0.05, and 0.1 M. AOS exhibited electrostatic binding to the positively charged sites of the polypeptide chain resulting in considerable reduction in its hydrodynamic radius up to critical micellar concentration (cmc=8 mM for no salt, 0.01 and 0.025 M, and 5 mM for 0.05 M and 2 mM for 0.1 M solutions). The correlation function revealed the presence of two types of structures above cmc; namely the micelles of AOS and gelatin–AOS micelle complexes. The micellar radii ( R m), the effective gelatin–surfactant complex radii ( R c), have been determined as a function of salt concentration. No critical aggregation concentration (cac) was observed. The inter-gelatin–surfactant complex ( k D 1 ) and inter-micellar interactions ( k D 2 ), were determined by fitting the concentration dependence of R m and R c to a virial expansion in reduced concentration ( c−cmc), which are compared. While k D 1 showed strong ionic strength dependence, k D 2 remained invariant of the same. The protein to surfactant binding ratio was found to be smaller than normal. Results have been discussed within the framework of the necklace-bead model of polymer–surfactant interactions.