Hydration of Sodium Alginate in Aqueous Solution

Abstract

Alginates are naturally occurring biocompatible polysaccharides. They have a broad range of applications, mainly in connection to their ability to control the rheology of aqueous solutions. Specifically, addition of a small amount of alginate (10% wt) leads to a ∼100-fold increase in viscosity. Here we explore whether that pronounced retardation of the long-range correlations is accompanied by molecular-level changes of the water structure. We employ viscometry, dielectric spectroscopy (DS) and femtosecond infrared (fs-IR) pump–probe spectroscopy to study water dynamics in sodium alginate solutions. Remarkably, despite the large rheological effects of alginates in solution, the rotational dynamics of water are remarkably similar to those observed in bulk water. Only a small subensemble of water molecules is slowed down significantly, amounting to 6 ± 2 water molecules per saccharide unit. Furthermore, DS measurements reveal an additional ∼5 water molecules to be slowed down by the counterion (Na+). Our results reveal that the effect of alginate on the dynamics of water is restricted to the first hydration shell. This indicates that the large viscosity increase is determined by the polysaccharide network, with large water pools present between the polysaccharide chains.