Charge-dependent free-energy landscapes of ciprofloxacin adsorption on curved graphitic interfaces

Understanding how adsorption emerges from coupled configurational degrees of freedom remains a central challenge in the statistical-mechanical description of interfacial systems. Here, molecular dynamics free-energy calculations were employed to investigate the charge-dependent adsorption of ciprofloxacin at curved graphitic interfaces. Umbrella-sampling simulations combined with rigorous free-energy reconstruction were performed for the cationic and anionic forms of ciprofloxacin on a systematic series of armchair carbon nanotubes (CNT4–CNT14). Adsorption thermodynamics were quantified along a surface-distance coordinate using explicit statistical-mechanical definitions of the bound and bulk states. Both charge states exhibit favourable adsorption but distinct curvature-dependent free-energy signatures. Two-dimensional free-energy surfaces reveal pronounced coupling between adsorption distance and molecular orientation, leading to distinct configurational ensembles for the two charge states. The cationic form exhibits stronger orientational confinement and a more localised adsorption basin, whereas the anionic form retains broader orientational distributions and greater configurational flexibility near the surface. These results demonstrate that adsorption at curved graphitic interfaces is governed by a multidimensional free-energy landscape in which molecular charge, surface curvature, and solvent-mediated configurational organisation jointly determine adsorption thermodynamics.