Electrostatic-hydration trade-off in alkali-ion binding at sulfated cellulose nanocrystal interfaces.

HYPOTHESIS: Counterion specificity at sulfated cellulose nanocrystal (CNC) interfaces is not governed by ion charge, size, hydration strength, Hofmeister character, or residence time alone. Instead, alkali-ion binding is expected to emerge from a competition between access to localized sulfate electrostatic wells and the energetic cost of hydration-shell/interfacial-water reorganization. Simulations: Atomistic molecular dynamics simulations were used to investigate Li+, Na+, K+, and Cs+ at hydrated sulfated CNC interfaces. Realistic 18-chain CNC models were combined with flat cellulose slab models to analyze ion residence, diffusion, binding free energies, ion-surface interaction energies, nonbonded energy landscapes, preferred approach heights, interfacial water structure, and ion-water/surface coordination. FINDINGS: The simulations show that the ions do not follow a simple size, hydration, Hofmeister, or residence-time trend. Instead, their adsorption is governed by an electrostatic-hydration trade-off: small strongly hydrated ions access shorter ion-surface distances and localized electrostatic wells, but incur larger hydration and short-range repulsive penalties, whereas larger weakly hydrated ions interact more diffusely at larger distances. Na+ provides the most favorable binding free energy because it balances localized electrostatic stabilization with hydration retention. In contrast, Cs+ shows high interfacial residence but a more diffuse binding mode. These results distinguish dynamic surface association from thermodynamic adsorption affinity and provide molecular design principles for controlling ion-mediated behavior in charged nanocellulose colloids.