Mechanical and Bonding Behaviors Behind the Bending Mechanism of Kaolinite Clay Layers.

Daniel Tunega,Ali Zaoui

doi:10.1021/acs.jpcc.9b11274

Abstract

The density functional theory-based calculations were performed on stripe models of the single kaolinite layer. The calculations helped to explain why halloysite mineral, a member of the kaolinite group existing in a tubular form, has rolled tubes only in one way. In that form, aluminol octahedral sheet, terminated by surface hydroxyl groups, represents the inner surface of the nanotubes. The bending models with the inner surface formed by the SiO tetrahedral sheet showed significant structural instability with monotonically increasing strain energy as a function of the curvature. In contrast, for the bending models with the octahedral sheet as the inner surface, stabilization energetic minima were found at curvatures of about 10 nm. The calculations were also performed on the individual sheets (tetrahedral and octahedral) of the kaolinite layer to show their contribution to the bending strain. We found that the decrease of the bending energy and the layer stabilization with respect to the planar configuration for curvatures with radii RC > ∼5 nm can be attributed mainly to three factors—(i) better match between octahedral and tetrahedral sheets, (ii) local structural changes of the SiO and AlOH polyhedral units, and (iii) increasing effectivity of hydrogen bonding of the outer surface OH groups.

Highlights

Clay minerals belong to important components of soils and sediments having significant impact on soil properties
density functional theory (DFT) calculations were performed on several models of stripes derived from the individual kaolinite layer, and the stability of the bending models was measured as a function of the radius of the bending
The DFT study was performed on several stripe models of the single kaolinite layer in order to explain how bending deformation can affect the structural stability of kaolinite minerals, tubular forms of halloysite

Summary

■ INTRODUCTION

Clay minerals belong to important components of soils and sediments having significant impact on soil properties. There is no tubular analog with the inner surface formed from basal surface atoms of the tetrahedral sheet For this purpose, DFT calculations were performed on several models of stripes derived from the individual kaolinite layer, and the stability of the bending models was measured as a function of the radius of the bending. Using fully cylindrical model even with a single layer wall in periodic DFT calculations is practically impossible with the current computational sources of the highest performance because computational cells would be extremely large (e.g., model with a diameter of 10 nm would contain ∼2000 atoms) As the octahedral (AlOH) sheet is symmetric, the bending deformation was performed only in one direction

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES