Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Research Paper Prediction of mechanical properties of grafted kaolinite – A DFT study Eva Scholtzová a, ⁎ , Daniel Tunega b,c, ⁎⁎ a Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 36 Bratislava, Slovak Republic b Institut für Bodenforschung, Universität für Bodenkultur, Peter-Jordan-Strasse 82b, A-1190 Wien, Austria c School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, PR China ARTICLE INFO Keywords: Grafted kaolinite Mechanical properties Density functional theory Dispersion corrections Cohesion energy ABSTRACT 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 calcu- lated 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 evi- denced 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. 1. Introduction Kaolinite (Al 2 Si 2 O 5 (OH) 4 ) is a layered 1:1 dioctahedral aluminum silicate with layers formed by linked tetrahedral and octahedral sheets. The octahedral side of the layer is terminated by surface hydroxyl groups of μ-OH type (O atom is bound to two Al atoms). The tetrahedral side is formed by the plane of the basal surface atoms. In the kaolinite structure, layers keep together through hydrogen bonds formed among the basal oxygen atoms of the tetrahedral surface with the surface hy- droxyl groups of the adjacent layer. Kaolinite is one of the most abundant clay minerals found in the Earth's crust and has found a lot of industrial applications. This mineral can be also used as a starting material for the synthesis of new organo- mineral nanomaterials specifically designed for particular applications, for example, in environmental and remediation processes. Although the layers are held together via hydrogen bonds, kaolinite structure can expand relatively easily by intercalating various small polar molecules into the interlayer space such as potassium acetate (Wada, 1961), di- methylsulfoxide (Thompson and Cuff, 1985), N-methylformamide (Komori et al., 1998), methanol (Mako et al., 2015), and many others. The intercalation process opens access to basal surfaces and inter- calated molecules interact usually with basal surface atoms through physical forces, mostly having hydrogen bond origin. Kaolinite intercalates can be modified by further treatment (e.g., by varying temperature and pressure) during which also chemical changes can appear (Johnston and Stone, 1990; Franco and Ruiz Cruz, 2004; Li et al., 2009). An increasing demand exists for the preparation of new hybrid materials on the kaolinite base that can be used in many in- dustrial and environmental applications such as adsorbents, catalysts, rheological control agents, or fillers used for polymer nanocomposites, synthesis and preparation of mesoporous materials. Further, dehy- droxylated kaolinite is an important part of the aluminosilicate in- organic polymers generally called geopolymers (Alzeer and MacKenzie, 2013). The surface OH groups of the aluminol side are suitable for so-called grafting process with suitable organic agents (Tunney and Detellier, 1993; Itagaki and Kuroda, 2003; Murakami et al., 2004; Gardolinski and Lagaly, 2005; Letaief and Detellier, 2007; Avila et al., 2010; Hirsemann et al., 2011). Grafting, in general, can lead to modification of physicochemical properties of the parent material. Grafting with methanol, for example, can lead to modification of hydrophilicity/hy- drophobicity of clay surfaces. Therefore, kaolinite-methanol (KM) de- rived structures are perspective materials applicable as precursors for obtaining kaolinite-amine derivatives and kaolinite-polymer nano- composites (Matusik et al., 2012). Materials prepared from kaolinite by intercalation or grafting have https://doi.org/10.1016/j.clay.2020.105692 Received 27 March 2020; Received in revised form 20 May 2020; Accepted 20 May 2020 ⁎ Corresponding author. ⁎⁎ Corresponding author at: School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, PR China. E-mail addresses: eva.scholtzova@savba.sk (E. Scholtzová), daniel.tunega@boku.ac.at (D. Tunega). Applied Clay Science 193 (2020) 105692 0169-1317/ © 2020 Elsevier B.V. All rights reserved. T