NMR self-diffusion of molecular and macromolecular species in dextran solutions and gels

To provide insights into the diffusion properties of micelles in hydrogels and, in general, of solutes in dextran gels, self-diffusion coefficients of small organic solutes and large micelles were measured in dextran solutions and gels, using pulsed-field gradient NMR. The self-diffusion of the solutes was shown to be slower in dextran solutions than in D2O and even slower in dextran gels. The extent of the diffusion reduction was more pronounced for higher dextran concentrations. For a series of molecules with a molecular weight between 46 and 78, the self-diffusion coefficient in dextran gels (20% w/w) corresponded, on average, to about 0.39 of the values measured in bulk water. This hindered diffusion was mainly associated with the obstacles created by the polysaccharide segments. There was no evidence in the diffusion measurements of interaction between dextran and solutes capable of hydrogen bonding. The reduction of diffusion in dextran solutions and gels was very drastic in the case of micelles. For Triton X-100, the self-diffusion coefficient in dextran gel was about 7% of that observed in water. The values were, in fact, on the same order of the dextran chain diffusion coefficient. This finding suggests that these large macroassemblies can hardly move in dextran gels.