A LASER-LIGHT SCATTERING STUDY OF GELLAN GELS

A study of gellan has been made using the technique of photon correlation spectroscopy. It has been confirmed that gellan gels are largely stationary at a molecular level like other polysaccharide gels and quite unlike the gels of flexible polymers such as polyacrylamide. Solution-gel transitions of deacetylated gellan in 0.025M NaCl have been studied both as a function of concentration and temperature, and the results compared with those of a parallel investigation of agarose. The interstitial spaces within gellan gels have also been studied by measuring the diffusion coefficients of dextran fractions within the gels. Since all gels are nonergodic systems, the theory of dynamic light scattering from such systems is discussed insofar as it affects the present work. It has been shown that the gellan and agarose aqueous systems are fundamentally different, in that agarose does not form a solution at very low concentrations, but splits up into macroscopic gel particles. At very low concentrations, gellan forms a solution in the presence of both gelling and nongelling ions, the molecules of which show little change in hydrodynamic diameter with temperature in the range 20-80 degrees C. At higher concentrations where gels are formed, both gellan and agarose exhibit hysteresis in their temperature transitions from gel to solution and solution to gel, the solutions being of large molecular aggregates. The transitions are sharp, but in both cases there is a continuous rearrangement in the structural morphology over the entire temperature range on heating, rendering the system more homogeneous prior to dissociation. In the case of gellan, however, there are two distinct phases in these structural changes-this is not true of agarose. The mean mass per unit length of the gellan fibre in the presence of 0.025M NaCl is 19 k daltons/nm at 0.7% concentration and varies with concentration to the power 0.15. The mass per unit length of the agarose fibre is much larger (ca. 110 k Daltons/nm), this difference being consistent with the difference in properties at very low concentrations. (C) 1994 John Wiley & Sons, Inc.