Structure of glycerol-Mg2+-smectites/vermiculites complex based on molecular dynamics and implementation of the model for X-ray diffraction modeling

Smectites and vermiculites can be distinguished from each other after Mg2+-saturation and solvation with glycerol (GLY). Smectites generally expand to c.a. 18 ?, forming a two-layer complex, while vermiculites to c.a. 14.5 ?, forming a one-layer complex. Additionally, the distribution of layer charges within one population of crystallites can lead to mixed-layered one-/two-GLY layer complexes. Thus far, the model of the GLY intercalate has not been implemented in programs used for XRD modeling of clay minerals. The aim of this study was to obtain a model of the structure of Mg2+-GLY-smectite/vermiculite with the help of molecular dynamic (MD) simulations. A wide range of GLY and H2O contents, in the simulated structures of montmorillonites with charges: 0.3, 0.5, and 0.75 per half unit cell (phuc), and beidellite with a charge of 0.75 phuc, were considered. The average atomic density profiles along the z-direction were calculated for all of the simulations. Subsets corresponding to certain basal spacings (plateaus) were chosen as representative for one- and two-layer intercalates. The electronic density profiles of GLY, H2O, and Mg2+ ions were fitted with Gaussian functions. The purpose of the work was to minimize the number of functions in order to achieve a maximally simple, yet flexible model of the interlayer structures. It was possible to determine relationships between positions of the atomic distributions and the basal spacings. All the relationships were implemented in Sybilla and BGMN computer programs. The new model of the complex was used for fitting of diffractograms of Mg2+-montmorillonites and Mg2+-vermiculite with different layer charges.

Automated summary

This study aimed to obtain a model of the structure of Mg2+-GLY-smectite/vermiculite through molecular dynamic simulations, considering a wide range of charges and GLY/H2O contents. Relationships between atomic distribution positions and basal spacings were determined to create a simple and flexible model with minimal Gaussian functions. The new model was successfully used to fit diffraction patterns of Mg2+-montmorillonites and Mg2+-vermiculites with different layer charges.