How Surface Functionalization Controls Confined Electrolyte Structure and Dynamics at Graphene Interfaces

Understanding how surface chemistry modulates confined electrolyte behavior is critical for advancing electrochemical, membrane, and nanofluidic technologies. Here, we present a comprehensive molecular dynamics study of aqueous NaCl solutions confined between graphene functionalized with -COOH, -OH, ═O, and -CH₃ groups across multiple surface coverages and electrolyte concentrations. We systematically disentangle how functional group identity and abundance independently shape interfacial layering, ion adsorption, and water dynamics. Polar, hydrogen-bonding groups (-COOH, -OH) strongly structure the interface and suppress water mobility, while weakly polar (═O) and nonpolar groups (-CH₃) lead to more diffuse, mobile profiles. Importantly, we show that functional group chemistry sets the morphology of interfacial structure, while coverage scales its intensity, a distinction that holds across electrolyte concentrations. These findings enable a quantitative framework for designing chemically heterogeneous surfaces that precisely modulate ion and solvent behavior in complex electrolyte environments.