Self-diffusion and mutual diffusion of small molecules in high-set curdlan hydrogels studied by 31P NMR

Self-diffusion and mutual diffusion are two different transport mechanisms experimentally characterized on different length and time scales. NMR spectroscopy is a highly suitable technique to characterize these two phenomena as both mechanisms can be studied on the same system and in the same experimental conditions. Pulsed field gradient (PFG) NMR was used to measure the self-diffusion whereas (31)P NMR profiling provided an approach to determine the mutual diffusion coefficients. We have characterized the diffusion of phosphate, trimetaphosphate, alendronate, and d-glucose-6-phosphate in hydrogels prepared with 10% (w/v) curdlan, a bacterial polysaccharide built of linear (1-->3)-beta-d-glucose repeating units. These solutes are small compared to the average pore size of the hydrogel, as inferred from environmental scanning electron microscopy (eSEM). Our results show that the self- and mutual-diffusion coefficients of small molecules in curdlan hydrogels are similar and are reduced by 30% compared to those measured in aqueous solutions. These observations are validated for the complete series of investigated analytes. It is therefore concluded that, for this system, the analyte diffusion in the gel is essentially reduced because of interactions at the molecular level and that the open structure of this gel has a very limited influence at the mesoscopic length scale. A literature survey indicates that these conditions prevail for the large majority of the systems that have been investigated up to now.