Preferential interaction of Na+ over K+ with carboxylate-functionalized silver nanoparticles

Abstract

Elucidating mechanistic interactions between monovalent cations (Na+/K+) and engineered nanoparticle surfaces to alter particle stability in polar media have received little attention. We investigated relative preferential interaction of Na+ and K+ with carboxylate-functionalized silver nanoparticles (carboxylate–AgNPs) to determine if interaction preference followed the Hofmeister series (Na+>K+). We hypothesized that Na+ will show greater affinity than K+ to pair with carboxylates on AgNP surfaces, thereby destabilizing the colloidal system. Destabilization upon Na+ or K+ interacting with carboxylate–AgNPs was evaluated probing changes in multiple physicochemical characteristics: surface plasmon resonance/optical absorbance, electrical conductivity, pH, hydrodynamic diameter, electrophoretic mobility, surface charge, amount of Na+/K+ directly associated with AgNPs, and Ag+ dissociation kinetics. We show that Na+ and K+ react differently, indicating local Na+ pairing with carboxylates on AgNP surfaces is kinetically faster and remarkably favored over K+, thus supporting Hofmeister ordering. Our results suggest that AgNPs may transform into micron-size aggregates upon release into aqueous environments and that the fate of such aggregates may need consideration when assessing environmental risk.