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

Abstract

We quantify the hydrodynamic screening length (ζ) in -carrageenan solutions by observing restrictions on the diffusion of several pullulan probes, each with a different molecular weight (M w). The diffusion coefficient of pullulan in a pure solvent (D pull,0) and a κ-carrageenan solution (D pull) were measured using a pulsed field gradient stimulated spin-echo 1H nuclear magnetic resonance. The hydrodynamic radii (R H) of the pullulan samples were determined using D pull,0 measured at 25°C and the Stoke-Einstein equation. The echo signal intensity of -carrageenan without a gradient, I kappa (0), decreased steeply at around the sol-to-gel transition temperature (T sg), suggesting that the κ-carrageenan chains formed helical aggregates at T sg diluting the solute -carrageenan concentration. The ratio, D pull /D pull,0, was less than 1, indicating that the mobility of pullulan was restricted by the κ-carrageenan chains. Over the entire temperature range, pullulan with larger molecular size yielded more restrictions on the molecular mobility, and D pull /D pull,0 decreased as the M w of pullulan increased. Below T sg, D pull /D pull,0 increased as the temperature decreased. This behavior suggests that a decrease in the restriction degree by the κ-carrageenan chains is concomitant with the formation of aggregates. To quantify the restriction degree, ζ was estimated at various temperatures using the relation D pull /D pull,0 exp(-R H/ζ). © 2012 The Society of Polymer Science, Japan (SPSJ) All rights reserved.