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

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.

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.