How Grafting Density Shapes Analyte Adsorption in HILIC Chromatography: Nonmonotonic Trend Revealed by Umbrella...

Grafting density plays a significant role in governing separation performance in hydrophilic interaction chromatography (HILIC) by modulating the interfacial solvent organization and analyte transport behavior. This study employs all-atom molecular dynamics simulations combined with umbrella sampling free energy analysis to systematically investigate the effects of grafting density on solvent-mediated adsorption and diffusion anisotropy at cyanopropyl-grafted surfaces in methanol-water systems with naphthol as the representative analyte. Simulation results reveal a pronounced nonmonotonic dependence of adsorption free energy on grafting density, with the strongest analyte stabilization occurring at intermediate grafting density. Such stabilization corresponds to the molecular configuration parallel to the surface, resulting from the joint effect of penetration of the weaker adsorption layer and accessibility of the apolar environment. In contrast, both low and high grafting densities incur elevated penalties due to structured hydration layers and high-density solvent peaks, respectively. Furthermore, diffusion dynamics display nonisotropic behavior: steric constraints at high grafting density suppress parallel diffusion near the interface, while perpendicular diffusion retains significant mobility even at adsorption minima due to solvent displacement and confinement effects. These results demonstrate how grafting density balances the retention strength against interfacial mobility limitations, providing molecular design principles for HILIC stationary phases.