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Найдены 3 публикации
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Статья
Szczerba M., Klapyta Z., Kalinichev A. G. Applied Clay Science. 2014. Vol. 91. P. 87-97.

Molecular dynamics simulations were performed in order to study the interactions of ethylene glycol (EG) with smectite. The simulations have also taken into account that EG–smectite complex contains, as a rule, some adsorbed water molecules. The simulations results show that in the two-layer glycolate the content of water is about 1.0 H2O per half of the smectite unit cell. For a typical smectite a clear thermodynamic preference for one- or two-layer structure of the complexes was observed. The calculated radial distribution functions and running coordination numbers indicate that the H2O and EG molecules compete for the coordination sites near the calcium ions in the clay interlayer spaces. The EG and H2O packing in the interlayer space is controlled by the differences in the total smectite layer charge, charge distribution, and the type of the interlayer cation, strongly affecting the basal spacing and the structure of the complex. Varying amounts of EG and water and the ratio EG/H2O are, however, the most important factors influencing the extent of the smectite expansion. A comparison of the two-layer structure obtained from MD simulations with previous models leads to the conclusion that the arrangement of EG molecules in the interlayer spaces, typically used in simulations of clay mineral X-ray diffractograms, can be modified. In contrast to the earlier Reynolds model (1965), the main difference is that the interlayer ions tend to change their positions depending on the specific distribution of the clay mineral charge. In the case of montmorillonite, Ca(2+) ions are located in the middle of the interlayer space, while for beidellite they are located much closer to the clay mineral surface. Water molecules in this structure do not form distinct layers but are instead spread out with a tendency to be concentrated closer to the interlayer ions and to the smectite surface. One-layer structure of EG/water–smectite complex, characteristic of vermiculite is also proposed.

Добавлено: 30 апреля 2017
Статья
Kraevsky S. V., Tournassat C., Vayer M. et al. Applied Clay Science. 2020. Vol. 186. P. 105442.

Statistical information on the edge surface area and edge crystallographic orientation of clay nanoparticle surfaces is essential for proper accounting of the protonation-deprotonation reactions as a part of mechanistic surface complexation models. A combination of atomic-force microscopy (AFM) measurements and molecular dynamics computer simulations made it possible to quantify the relative contributions of the most frequently occurring montmorillonite edge surfaces to the total edge surface area. Edge surfaces normal to the [110] and [010] crystallographic directions are found to be the most abundant (~60% and ~20%, respectively), in agreement with previous estimations.

Добавлено: 26 февраля 2020
Статья
Szczerba M., Kalinichev A. G., Kowalik M. Applied Clay Science. 2020. Vol. 188. P. 105497.

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 charge-balancing 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.

Добавлено: 26 февраля 2020