Competition Between Electrostatics and Hydrophobicity in the Rheology of Modified Phytoglycogen Nanoparticles

Phytoglycogen (PG) is a highly branched polysaccharide that occurs naturally as soft, highly hydrated, compact nanoparticles with a dendritic architecture. Collectively, these properties make PG desirable as sustainable additives for personal care, nutrition, and biomedical applications. The functionality of PG can be modified through covalent attachment of chemical groups such as octenyl succinic anhydride (OSA), a negatively charged, hydrophobic molecule. High degrees of substitution (DS) of OSA result in the hydrophobic collapse of OSA-modified chains on the surface of the PG particles [1]. In the present study, we vary the DS value and use rheology to measure the divergence of the zero-shear viscosity of aqueous dispersions of OSA-PG with increasing OSA-PG concentration 𝐶. We find that the glass transition concentration 𝐶_g, at which the relative viscosity is 10⁵, changes nonmonotonically with DS: it decreases rapidly with DS for low DS values and then increases more gradually with DS for high DS values. We attribute this nonmonotonic dependence of 𝐶_g on DS to the competing effects of electrostatics, which dominate at small DS, and hydrophobicity, which dominates at large DS.