Estimation of the hydrodynamic screening length in -carrageenan solutions using NMR diffusion measurements

We quantify the hydrodynamic screening length (n) in j-carrageenan solutions by observing restrictions on the diffusion of several pullulan probes, each with a different molecular weight (Mw). The diffusion coefficient of pullulan in a pure solvent (Dpull,0) and a j-carrageenan solution (Dpull) were measured using a pulsed field gradient stimulated spin-echo 1H nuclear magnetic resonance. The hydrodynamic radii (RH) of the pullulan samples were determined using Dpull,0 measured at 25 1C and the Stoke–Einstein equation. The echo signal intensity of j-carrageenan without a gradient, Ikappa(0), decreased steeply at around the sol-to-gel transition temperature (Tsg), suggesting that the j-carrageenan chains formed helical aggregates at Tsg diluting the solute j-carrageenan concentration. The ratio, Dpull/Dpull,0, was less than 1, indicating that the mobility of pullulan was restricted by the j-carrageenan chains. Over the entire temperature range, pullulan with larger molecular size yielded more restrictions on the molecular mobility, and Dpull/Dpull,0 decreased as the Mw of pullulan increased. Below Tsg, Dpull/Dpull,0 increased as the temperature decreased. This behavior suggests that a decrease in the restriction degree by the j-carrageenan chains is concomitant with the formation of aggregates. To quantify the restriction degree, n was estimated at various temperatures using the relation Dpull/Dpull,01⁄4 exp( RH/n). Polymer Journal (2012) 44, 901–906 doi:10.1038/pj.2012.107; published online 13 June 2012