Strand-like phase separation in mixtures of xanthan gum with anionic polyelectrolytes

Binary aqueous mixtures of xanthan gum with anionic polyelectrolytes (alginate, polyacrylate, carboxymethylcellulose, λ-carrageenan, and low methoxyl pectin) exhibited phase separation above threshold concentrations of added polyelectrolyte. Dynamic oscillatory rheology showed that separation was associated with a large decrease in storage modulus, and when viewed under plane-polarised light, the dispersed phase exhibited a birefringent, strand-like morphology indicative of a liquid crystalline phase. At constant xanthan concentration, the strand thickness was inversely related to the viscosity of the added polyelectrolyte solution. Phase diagrams obtained for xanthan:alginate, and xanthan:carboxymethylcellulose mixtures showed that whilst the strands contained both xanthan and polyelectrolyte, the continuous phase was xanthan-free. Mixtures of xanthan gum with neutral polysaccharides (maltodextrin, methylcellulose) did not show phase separation, and separation was not observed when xanthan was replaced by scleroglucan a neutral polysaccharide which, like xanthan, adopts a stiff, helical structure in solution. The alginate concentration required to initiate phase separation was inversely related to alginate molecular weight, and was independent of the alginate mannuronate/guluronate ratio. Higher concentrations of alginate were required to elicit phase separation when the salt content of the mixture was increased. The results suggest that segregative phase separation, driven by repulsive interactions between polyelectrolytes, promotes the formation of liquid crystalline structures in the xanthan-rich domain. The consequence is that the decreased viscosity reported when xanthan solutions undergo liquid crystal formation, occurs at much lower xanthan concentrations, when in the presence of an anionic polyelectrolyte.