Influence of Sodium Salts on the Swelling and Rheology of Hydrophobically Crosslinked Hydrogels Determined by QCM-D

Hydrophobically modified copolymers provide a versatile platform of hydrogel materials for diverse applications, but the influence of salts on the swelling and material properties of this class of hydrogels has not been extensively studied. Here, we investigate model hydrogels with three different sodium salts with anions chosen from the classic Hofmeister series to determine how these counterions influence the swelling and mechanical properties of neutral hydrogels. Our measurements utilize a quartz crystal microbalance with dissipation (QCM-D) to quantify both swelling and rheological properties of these gels. Overall, the observed trends are broadly consistent with more kosmotropic ions causing diminished solubility (‘salting out’) and strongly chaotropic ions causing improved solubility (‘salting in’), a trend characteristic of the Hoffmeister series governing the solubility of many proteins and synthetic water soluble polymers, but trends in the shear stiffness with gel swelling are clearly different from those normally observed in chemically cross-linked gels.