Phase separation, aggregation and gelation of a charged helixforming polysaccharide in aqueous salt solution

Abstract: The phase behaviour and gelation of the anionic polysaccharide kappa-carrageenan (KC) in different salt solutions was studied. The specific site binding of certain anions and certain cations to the KC helix was used as a means of controlling the interactions. In the presence of a binding anion (I-) the charge density of the helix increases and the aggregation of helices, and thereby the gelation, is prevented. By gradually introducing a site binding cation (Cs+) the effective charge density of the helix is reduced, as is the electrostatic repulsion between the helices. Most experiments were performed in 0.1 M salt solution of the mixed salts NaI/CsI. The molecular weight of the KC was varied. For a low molecular weight in 0.1 M NaI, a chiral nematic phase was observed above 5% KC. The volume of the chiral nematic phase was found to depend sensitively on the helical content which demonstrates a coupling between the coil-helix transition and the isotropic-nematic phase transition. From the liquid crystalline phase behaviour, the persistence length of the kappa-carrageenan helix was estimated to be in the range 90-120 nm. The development of the nematic phase was very slow for the high molecular weight KC, due to the high viscosity and slow diffusion for these large molecules. KC of a high molecular weight in 0.1 M NaI produced weak gels above 1% KC with - for KC - unusually low storage moduli. These “gels” were found to be caused by helix entanglements. The effect of mixing different proportions of specific and non-specific ions on the transition enthalpy was investigated experimentally by DSC for a wide range of salt compositions. Under conditions where no aggregation occurred (and thus no gelation), the variations in the observed enthalpies could be reproduced by Poisson-Boltzmann model calculations, taking into account the specific binding of ions to the helix. A change in the macroscopic properties, from isotropic/nematic phase separation to gelation, was found to occur above a certain cesium content in the mixed NaI/CsI salt solution. Studies on dilute solutions by cryo-TEM, viscometry, optical rotation and light scattering showed that at and above this molar ratio of cesium, the kappa carrageenan helices aggregate into rigid super-helical rods. These rods are not soluble, but self-aggregate in turn, giving rise to the gel. Thus, the properties of the gel seem to depend on the properties of this higher order kappa-carrageenan aggregate (the super-helical rod), rather than on the properties of the individual kappa-carrageenan helix.