Theoretical study for the interlamellar aminoalcohol functionalization of kaolinite
Introduction
Kaolinite is a hydrated aluminosilicate with chemical composition Al2Si2O5(OH)4. An individual layer of kaolinite consists of two connected sheets: a tetrahedral sheet formed from SiO4 tetrahedral sharing corners and an octahedral sheet that consists of AlO6 octahedral sharing edges. Adjacent tetrahedral and octahedral are linked by hydrogen bonds and van der Waals (vdW) forces, which induce restricted intercalation or graft of organic molecules in consecutive layers [1], [2], [3].
Clay minerals, such as kaolinite, can be combined with substances of organic molecules or biological molecules to form novel functional materials. The original hydrogen bonds of the kaolinite break when interacting with organic molecules. Two possible interaction forms exist in this situation: one is the formation of new hydrogen bonds between molecules and surface hydroxyl groups on the octahedral side as well as oxygen atoms on the tetrahedral side, and the other is the susceptibility of interlayer hydroxyl groups to graft with organic molecules [4], [5]. Organically modified clay minerals are important in a wide range of chemical applications [6], [7], [8]. These materials may act as a rigid inorganic supramolecular organizing medium and have potential as integration for functionalities and specific properties [9], [10].
Attaching molecules with free amine groups in the interlayer space of kaolinite could provide an ideal site for the attachment of a functionalized moiety. Tunny et al. [11] attempted to graft ethanolamine to the internal aluminol surfaces of kaolinite and suggested that ethanolamine (EOA) units were attached to the aluminate interlameller surface of kaolinite via ≡Al
OCH2CH2NH2 covalent linkages, with the pendant NH2 group keyed into (SiO)6 macro-ring of the adjacent silicate surface. The experimental work of Letaief et al. [12], [13] indicated that diethanolamine (DEOA) and triethanolamine (TEOA) were covalent graft, whereas ethanolamine was intercalated but not grafted on the surface. In the case of EOA–kaolinite, EOA was displaced by washing with water, and the original kaolinite was recovered. However, the grafted derivatives that resulted from esterification of the aluminol surfaces remained stable with extensive washing [12], [13]. According to these experimental results, one or two organic arms DEOA and TEOA were covalent graft with mineral, and the remaining amino group was available for further modification.
Organo-clays are a versatile area of investigation. Systemic theoretical study about the physical intercalation and chemical graft modes of aminoalcohols on kaolinite is necessary to describe the characterizations of nanohybrid materials that are derived from the kaolinite. In this work, further theoretical studies are performed to determine the interaction modes of DEOA and TEOA on kaolinite, including identifying if the modes prefer to have one covalent organic arm or two organic arms covalently bridging two vicinal structure units of the mineral. The role of hydrogen bond and vdW forces in the interaction between physically intercalated or covalent graft aminoalcohols and aluminosilicate layers are discussed. Chemical reactivities of functionalized kaolinite complexes are also analyzed.
Section snippets
Computational methods
The primitive unit cell of kaolinite was optimized by using Dmol3 program package [14]. Generalized gradient approximation (GGA) for the exchange-correlation potential (PW91) [15] is appropriate for the relatively weak interactions [16]. The threshold values of the convergence criteria were 10−5 Ha for energy, 0.002 Ha/Å for maximum force, 0.005 Å for maximum displacement, and 10−6 Ha for self-consistent field tolerance. All the atoms and unit cell parameters were relaxed in the geometry
Structural relaxations of DEOA and TEOA
The optimized geometry parameters of DEOA and TEOA molecules (Fig. 2) on free status and on K-INT-112 are listed in Table 2. Free DEOA and TEOA were in Cs symmetry. This equivalence was lost for the intercalated molecules. The DETA and TEOA on K-INT-112 had no symmetry and showed a clear distortion for molecule skeleton, which was mainly caused by the interaction between organic molecules and clay sheets. The interactions between organic molecules and kaolinite also led to the large difference
Conclusions
These theoretical studies presented the interaction modes of DEOA and TEOA on kaolinite, including the kaolinite complexes with physical intercalation and covalent graft of aminoalcohols. The calculated formation energy of these kaolinite complexes indicated that aminoalcohols preferred to exist with a combination of physical intercalation and covalent graft. This work also explored the roles of hydrogen bonds and vdW forces between aminoalcohols and interlayer of kaolinite. The amino and
Acknowledgments
We would like to thank the Major Program of National Natural Science Foundation (no. 51034006) and the National Scientific and Technological Special Project (no. 2011BAC06B13) of China for their financial support. The results described in this article are obtained on the Deepcomp7000 of Supercomputing Center in the Computer Network Information Center of the Chinese Academy of Sciences.
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