Molecular Hydrophobicity Signature in Charged Bidimensional\nClay Materials

The\nunraveling of the hydrophobicity/hydrophilicity molecular signature\nof nanometric bidimensional confined systems represents a challenging\ntask with repercussions in environmental transport processes. Swelling\nclay minerals represent an ideal model system, as hydrophobicity can\nbe modified during material synthesis by substituting hydroxyls by\nfluorine in the structure, without additional surface treatment. This\nfollowing work presents a combined approach, integrating experimental\ninelastic neutron scattering spectroscopy and ab initio molecular\ndynamics simulations, with the objective of advancing our understanding\nof the role of surface hydroxylation/fluorination and the extent of\nconfinement on water properties. From computed structures, the analysis\nof molecular hydrophobicity/hydrophilicity signature was investigated\nin detail through water–cation–surface interactions.\nThe results elucidate the influence of fluorination on interlayer\nspecies, thereby tracing the impact of the surface on the diminished\nnumber of water molecules in such a sample. It is notable that the\nstrong cation–water interaction can overcome the disruptive\ninfluence of fluorine, thereby maintaining comparable water hydration\nshells around cations and resulting in an almost identical bidimensional\nconfinement geometry for both hydroxylated and fluorinated specimens.\nThe analysis of the hydrogen-bond network revealed a significant reorganization\nof the water molecules due to fluorination. Our results suggest that\na quantitative molecular signature of hydrophobicity/hydrophilicity\ncan be derived from the analysis of the formation of cavities in the\nconfined fluid. This new finding represents a robust approach for\ngeneralizing the hydrophobicity/hydrophilicity character for a wide\nvariety of bidimensional systems while proposing a framework for the\ndesign of new materials with controlled water properties.