Polysaccharide gelation in the presence of high concentrations of competing polymer: evidence for counter ion entropy effects

Solutions of the gelling polysaccharides agarose, kappa-carrageenan and gellan gum were prepared with gum arabic, gelatin, maltodextrin and low molecular weight grades of sodium alginate, carboxymethyl cellulose (CMC) and hydroxypropylmethyl cellulose (HPMC). The concentration where the cosolute prevented gelation of the poly saccharide was measured. It was found that carrageenan and gellan gum could form gels (continuous networks throughout the solution) at the highest level (14%) of HPMC whereas this cosolute inhibited agarose gelation. In contrast agarose gelled in the presence of 14% alginate or CMC, whereas they inhibited carrageenan and gellan gum gelation. These results are consistent with the hypothesis that phase separation in a mixture of a polyelectrolyte and a non polyelectrolyte is less likely because of the unfavourable entropy change due to an uneven distribution of counter ions. Agarose and carrageenan gelled in the presence of 14% gum arabic whereas gellan gum did not. The latter result observed can hardly be explained using the counter ions entropy concept. All three polysaccharides gelled in the presence of 14% maltodextrin.