Nonlinear Ion-Specific Control of Underwater Adhesion by Bottlebrush Polymer Interfaces.

Programming interactions at aqueous interfaces remains challenging because structured hydration layers resist molecular contact and obscure the role of dissolved ions. As a result, electrolytes are often treated as passive screening agents rather than active design parameters. Here, we show that electrolyte identity can deterministically regulate the mechanical state of soft interfaces through ion-specific hydration effects. Using quaternized bottlebrush polymers on mica as a force-resolved model system, we identify a two-channel mechanism that separates polymer-substrate interactions from interpolymer cohesion. Cation hydration controls access to surface binding sites: strongly hydrated Na+ preserves polymer anchoring by remaining solvated near the surface, whereas weakly hydrated K+ directly occupies lattice sites and suppresses adhesion. In parallel, anion valency activates a distinct cohesion pathway, where multivalent anions bridge polymer chains across the confined interface. Coupling between these pathways produces non-additive behavior, enabling regimes in which substrate anchoring is suppressed while interpolymer bridging sustains adhesion. These findings establish electrolyte composition, not simply ionic strength, as a primary control parameter for interfacial mechanics, providing a framework for designing adaptive adhesion and hydration-mediated interactions in aqueous environments.