Intrinsic hydrophobicity of smectite basal surfaces quantitatively probed by molecular dynamics simulations
Résumé
The siloxane surface of uncharged clays is known to be hydrophobic, which is supported by strong experimental
and theoretical evidence. For the siloxane surface of charged clays, like smectites, the picture is not as clear. We
are aiming to clarify this issue by molecular simulations in which smectite surface hydrophobicity is quantified
through the separate contribution of the surface itself, and the contribution due to the presence of chargebalancing cations on the surface. In order to explore systematically the effects of the total smectite charge and its
distribution in the structure, a series of molecular dynamics (MD) simulations was performed for several models
of dioctahedral smectites and compared with the results for uncharged pyrophyllite.
The largest difference between the simulation results for smectite models with naturally present surface
counterions and the models where these ions were artificially removed from the surface, while maintaining the
same total charge balance of the model, is in the shape of the water coverage. In the former case, full surface
wetting is observed and a relatively flat water film is forming on the surface. Its irregularity and thickness is
connected with number of ions on the surface. However, in all cases of smectite surfaces artificially devoid of
ions, a water droplet is always formed and the wetting is incomplete. The contact angles of the water droplets on
charged montmorillonites are very similar to that on uncharged pyrophyllite surface and range roughly between
110o and 90o
. These angles are also affected by the distribution of the octahedral and tetrahedral substitutions in
the structure and by their ratio. In the case of purely tetrahedral substitutions the contact angle on the bare
smectite surface can be as low as ~60o
, but still far from complete wetting.
The angular distributions of the H2O dipole vectors as a function of distance from the smectite surface show
two preferred surface-oriented types of water molecules when counterions are present, and the total surface is
highly hydrophilic. However, for surfaces devoid of ions, a population with dipole angles close to ~90o is
dominating, and the smectite surfaces can be considered hydrophobic. It can be thus concluded that, independent of the structural charge, bare smectite surfaces by themselves are either hydrophobic or only moderately hydrophilic. Their experimentally observed highly hydrophilic character is almost entirely due to the
charge balancing cations present on the surface.
Domaines
Physique [physics]Origine | Fichiers produits par l'(les) auteur(s) |
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