Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?

Abstract

In addition to isomorphic cation substitutions, smectite layers may present anionic substitutions with fluorine replacing the structural hydroxyl groups and inducing a reduced number of H2O molecules hydrating interlayer cations. The resulting additional versatility of smectite layers could be used to fine-tune the hydrophilicity of pure or intercalated smectite pending a detailed understanding of interlayer water organization. The present article thus reports on the hydration of (fluoro-)hectorite samples exhibiting similar charge density (structural formulae: [Na0.8·nH2O]inter[Mg5.2Li0.8]oct[Si8.0]tetO20(OH,F)4). Water sorption isotherms and PIGE/PIXE analyses allowed constraining the water content over the probed range of relative humidity and the bulk chemistry, respectively. Modeling of X-ray diffraction (XRD) profiles obtained along water vapor desorption isotherms allowed gaining additional insights in the distribution of interlayer H2O molecules. Compared to hydroxylated smectites of similar charge, fluorinated hectorites contain ∼30% less interlayer H2O molecules although transitions between discrete hydration states are observed for similar values of relative humidity. These molecules (3–4 and 6–8 H2O molecules per cation in the mono- and bi-hydrated states, respectively) predominantly belong to the hydration sphere of interlayer cations, although additional H2O molecules are present in the bi-hydrated state. As a consequence, the positional disorder of interlayer H2O molecules is much reduced in fluorinated samples, thus increasing the minimum distance from an interlayer H2O molecule to the smectite layer, most likely owing to the hydrophobicity of fluorinated layers. Finally, the distribution of layers with a given hydration state is more heterogeneous within smectite crystals for fluorinated smectites, compared to hydroxylated ones, possibly as the result of the improved crystallinity.