Solubility, solution rheology and salt-induced gelation of welan polysaccharide in organic solvents

摘要

Aqueous solutions of welan show gel-like rheology, including a c~2-dependence of G′ (where c is concentration and G′ is elastic modulus). Network formation is attributed to weak interactions between ordered species in the double-helical conformation recently characterised by X-ray fibre diffraction in the solid state. High concentrations of salt cause only a slight increase in modulus. Solutions of welan in dimethylsulphoxide (DMSO), by contrast, have properties broadly similar to those of disordered polysaccharides in water. Loss of 'weak-gel' character in mixed solvents of DMSO and water occurs over a narrow range of composition (between ～86 and ～91% v/v DMSO for 1% w/v welan, increasing somewhat at higher concentrations of polymer). Addition of salt to disordered welan (in 91% DMSO) causes rapid re-ordering, with slower development of a 'true' gel network by cation-mediated helix-helix aggregation. On exposure to excess water, the gels swell (attributed to reversal of the aggregation process) but remain intact (attributed to direct crosslinking of chains through stable double helices). Gels formed by addition of salt to ordered welan (in 80% DMSO), by contrast, dissolve completely on immersion in water. At moderate salt concentration (100 mM NaCl), both types of network revert to 'weak gels' on heating to ～80℃, but those formed from the disordered state (in 91% DMSO) show a second melting process at higher temperature, attributed to loss of double-helix structure in this more destabilising solvent. Both processes are reversible on cooling, with the thermal hysteresis anticipated for an aggregating system, giving stronger networks than were obtained initially by direct addition of salt. The solubility of ordered welan in polar organic solvents (ethylene glycol and aqueous DMSO) decreases sharply with increasing concentration of salt. This behaviour, and the salt-induced gelation, are attributed to enhancement of electrostatic attraction between negatively charged helices and positive counterions in solvents with dielectric constants substantially lower than that of water.