SOLUBILITY, SOLUTION RHEOLOGY AND SALT-INDUCED GELATION OF WELAN POLYSACCHARIDE IN ORGANIC-SOLVENTS

Aqueous solutions of welan show gel-like theology, including a c(2)-de pendence of G' (where c is concentration and G' is elastic modulus). N etwork formation is attributed to weak interactions between ordered sp ecies in the double-helical conformation recently characterised by X-r ay fibre diffraction in the solid state. High concentrations of salt c ause only a slight increase in modulus. Solutions of welan in dimethyl sulphoxide (DMSO), by contrast, have properties broadly similar to tho se of disordered polysaccharides in water. Loss of 'weak-gel' characte r in mixed solvents of DMSO and water occurs over a narrow range of co mposition (between similar to 86 and similar to 91% v/v DMSO for 1% w/ v welan, increasing somewhat at higher concentrations of polymer). Add ition of salt to disordered welan (in 91% DMSO) causes rapid re-orderi ng, 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 hel ices). Gels formed by addition of salt to ordered welan (in 80% DMSO), by contrast, dissolve completely on immersion in water. At moderate s alt concentration (100 mM NaCl), both types of network revert to 'weak gels' on heating to similar to 80 degrees C, but those formed from th e disordered state (in 91% DMSO) show a second melting process at high er temperature, attributed to loss of double-helix structure in this m ore destabilising solvent. Both processes are reversible on cooling, w ith the thermal hysteresis anticipated for an aggregating system, givi ng stronger networks than were obtained initially by direct addition o f salt. The solubility of ordered welan in polar organic solvents (eth ylene glycol and aqueous DMSO) decreases sharply with increasing conce ntration of salt. This behaviour, and the salt-induced gelation, are a ttributed to enhancement of electrostatic attraction between negativel y charged helices and positive counterions in solvents with dielectric constants substantially lower than that of water.