Impact of Cations on the Hydration Properties of Carboxymethyl-Phytoglycogen Nanoparticles

We present a study of the hydration properties of a carboxymethylated (CM) version of phytoglycogen (PG), a glucose-based nanoparticle with an underlying dendritic architecture. We measure the water uptake and water ordering in the presence of different cations (Na+, Mg2+, and Ca2+). We find that introducing Ca2+ results in a large reduction in water uptake relative to that of CM-PG in the presence of Na+. The presence of Ca2+ also decreases the frequency of the carboxylate asymmetric stretch peak in infrared spectroscopy experiments, which is consistent with bidentate or bidentate bridging between the carboxylate groups facilitated by Ca2+. This form of bridging, which has been shown to collapse polyelectrolyte brushes, could explain the large reduction in the level of water uptake for the CM-PG-Ca2+ particles. We also find that the degree of water ordering in the CM-PG particles increases in the sequence Na+ → Ca2+ → Mg2+, consistent with increasing charge density on the cations.