Prediction of mechanical properties of grafted kaolinite - A DFT study

Mechanical properties of kaolinite modified by grafting and/or intercalation were studied theoretically by means of the first principle calculations based on the density functional theory (DFT) method. Elastic constants, (C-ij), and bulk (B-0), shear (G) and Young's (E) moduli were predicted for structural models of pure kaolinite (Kaol), kaolinite intercalated with methanol (K-INT), kaolinite grafted with methoxy group (K-MTX), mixed grafted/intercalated kaolinite (K-MIX), and K-MIX model with residual water content (K-MIXW). Generally, all calculated values of the mechanical properties of the modified structures were lower than the reference values of the pure kaolinite. For example, bulk modulus decreased in a following order: 67.8 GPa (Kaol) > 41.1 GPa (K-MTX) > 34.3 GPa (K-MIXW) > 31.1 GPa (K-MIX) > 29.7 GPa (K-INT). The modification of the interlayer space by the grafting or intercalation reduced the strength of interactions between layers what was also evidenced by the calculated cohesion energies. Our calculations showed that the grafted/intercalated kaolinites represent softer materials and can be delaminated/exfoliated easier than pure kaolinite.