New model of ethylene glycol intercalate in smectites for XRD modelling

Several models of ethylene glycol (EG) intercalates were implemented in computer programs which are used to simulate XRD patterns of oriented clay minerals. So far the models assumed distributions of atomic positions perpendicular to the c* direction with similar or significantly higher temperature coefficients B for the EG molecules (B = 1 angstrom(2) or 11 angstrom(2)) than for atoms of crystal structure (approximately: B = 1 angstrom(2)). The temperature coefficients for the interlayer H2O molecules and Ca2+ cations (B = 1 angstrom(2) or 1.68 angstrom(2)) were close to these of clay minerals' atoms. This assumption is in disagreement with the distributions described recently with molecular dynamics (MD) simulations. An adjustable model of one and two EG planes in the interlayer is provided in this study. This new model is based on the results of MD simulations. A wide range of EG and H2O contents were considered in the simulated structures. Theoretical structures corresponding to certain basal spacings were chosen as representative for one- and two-plane intercalates. The electronic density profiles of EG molecules, H2O molecules, and Ca2+ ions were fitted with Gaussian functions. Their number was minimized to achieve the simplest but yet flexible model of the interlayer structures. The obtained new models show significant differences with regard to the former ones; especially in the atoms' positions. Based on the MD results, it was also possible to determine the relationships between positions of the atoms/molecules and basal spacing. For some atoms was also a relationship between basal spacing and intensity ratio of Gaussian distributions found. Temperature factors for interlayer species were larger than in the former models and Ca2+ ions can exist both as inner-sphere and/or outer sphere complexes, while its ratio can be optimized. All the relationships were implemented in the BGMN Rietveld refinement program. Using the new model it is possible to receive slightly better or similar matching between experimental and theoretical X-ray diffractograms, comparing to the former models. The new model shows limitations due to the fact that electronic distributions of water and EG molecules in the interlayer space are quite similar. Therefore, EG and water molecules in the interlayer space were not treated fully separately during the minimalization procedure.