Liquid Crystallinity versus Gelation of κ-Carrageenan in Mixed Salts: Effects of Molecular Weight, Salt Composition, and Ionic Strength

The recently discovered isotropic/nematic phase transition in κ-carrageenan was further examined by macroscopic observations and by NMR. A state diagram, which is the equivalent of a phase diagram but including also nonequilibrium states (in our case a gel), was established in the mixed salt solutions of NaI/CsI where the competition between phase separation and gelation could be studied. The phase boundaries of the nematic phase depended on molecular weight and ionic strength qualitatively as expected for a charged rigid polymer. From these data the persistence length of the κ-carrageenan helix was estimated as 60−90 nm. The volume fraction of the nematic phase depended sensitively on the overall helical content. In coexisting phases, the helical content was larger in the nematic than in the isotropic phase.