Strong polyelectrolyte – Induced mixing in concentrated biopolymer aqueous emulsions

Using a rheo-small angle light scattering methodology, optical microscopy, phase analysis, static light scattering, rheology, and isoelectric focusing (IEF), we studied the effect of a polyelectrolyte (sodium salt of dextran sulfate/DSS/) on phase equilibrium, intermacromolecular interactions and structure of water-inwater biopolymer emulsions. These emulsions resulted from the mixture of a conformationally variable protein chain (alkaline gelatin type B), and a highly branched non-ionic high molecular weight coil (dextran). Addition of DSS to the emulsion leads to its compatibilization and subsequent homogenization in both liquid (at +45 °C) and gel (at +18 °C) states, and to substantial increase in viscosity and moduli (G′, G″). Homogenization is observed at DSS/gelatin weight ratio (q) = 0.04 and 0.08 respectively. Increasing the ionic strength in the resulting single-phase system induces phase separation, showing that DSS-induced compatibilization is reversible. The zeta potential and IEF measurements showed that interaction between DSS and gelatin is weak and results mainly in an increase of the negative charge of gelatin and in a slight shift of its isoelectric point to the acid range. Our results suggest that the change of the total charge of gelatin molecules in the presence of sulfate polysaccharide is the main driving force for compatibilization and mixing. This is in agreement with a modified Flory–Huggins model, in which dissociated counter ions are included as a separate component contributing to the entropy of mixing. The proposed mechanism of strong polyelectrolyte-induced compatibilization could be generalized to other biphasic concentrated protein–polysaccharide mixtures, provided that the capacity of the protein to form large polymer complexes is weak and provided that the polysaccharide does not contain charged functional groups.