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Ethylene glycol intercalation in smectites. Molecular dynamics simulation studies.
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.