The positions of inner hydroxide groups and aluminium ions in exfoliated kaolinite as indicators of the external chemical environment

Abstract

Kaolinite as a remarkable industrial raw material has notable structural features despite its simple chemical composition (Al2O3·2SiO2·2H2O). We report here a systematic development of a coordination chemical model for the [6Al-6(OH)] honeycomb-like unit of kaolinite's octahedral sheet, which was proposed to be the adsorption site for small molecules from earlier studies. The coordination environment of the Al(3+) ions was completed with outer sphere groups from both octahedral and tetrahedral sheets. Dangling bonds were terminated by additional Al(3+) and Si(4+) ions with hydroxide and oxide groups from the second coordination sphere versus simple protonation. A cage of Na(+) and Mg(2+) ions rendered the computational model to be charge neutral. In this exfoliated kaolinite model, the inner hydroxide groups and the adjacent Al(3+) ions have compositionally the most complete environments with respect to the crystal structure. Thus, their atomic positions were used as a benchmark for the level of theory dependence of the optimized structures. We evaluated the performance of a representative set of density functionals, basis sets, point-charges, identified pitfalls and caveats. Importantly, the structural changes during optimization of periodic and cluster models suggest pliability for the exfoliated kaolinite layers, which is influenced by the external chemical environment.

Thin-walled nanoscrolls by multi-step intercalation from tubular halloysite-10 Å and its rearrangement upon peroxide treatment

“…The Hnano sample shows scrolls with significantly varied curvature that are connected with occasionally partially curled plates ( Figure 2, Hnano). The formation of thin-walled nanoscrolls can be envisioned as an unwinding mechanism triggered by the adsorption of intercalating agents that make the TOsheets flexible and pliable [43]. During the delamination process, small scrolls and plates peel off from the thick-walled halloysite tubes that can form various combinations of secondary structures as a function of their size, crystallinity, and presence of transition metal ion contaminants.…”

“…The positions and ratios of the Si/O, Si/O/Si and Al/O/Si bands are also altered, since upon intercalation and delamination, the Al-O/Si-O distances and the position of the cations change considerably[42,43,50] relative to the crystalline structures.The replacement intercalation process resulted in minor spectral changes between the H and Hnano samples. The 3620 cm -1 band was blue-shifted by 6 cm -1 , which is an indication of the formation of weaker H-bonds with the isOH groups.…”

Surface Characterization of Mechanochemically Modified Exfoliated Halloysite Nanoscrolls

“…Theoretical models describing the origin of morphological features 12 and computational chemical models have already been successful in complementing the interpretation of FT-IR, Raman, and NMR spectroscopic results. [13][14][15][16] The catalytic properties in connection with the semiconductor-like nature of the kaolinite group also show a close correlation with the morphology. It was reported recently 17 that the photochemical activity of the kaolinite shows correlation with the TO-layer structure and morphology rather than the previously assumed degree of dispersion of transition metal oxide contaminants and co-minerals.…”

“…[39][40][41][42] In order to rationalize the changes in the characteristic OH bands of the exfoliated nanoscroll samples, the positions of these groups relative to each other and to the crystallographic 'ab'plane of the TO-layer need to be taken into account. Interpretation of the spectral changes is aided by recent computational chemical results [14][15]43 for exfoliated kaolinites using both periodic and molecular cluster models and validated level of theory. According to these calculations, one of the three sHOgroups on the Al-centered octahedron folds in to adopt a parallel arrangement with respect to the TO-plane, while the other two have vertical, erected positions as in the crystalline sample.…”

Preparation and characterization of polyurethane-imide/kaolinite nanocomposite foams

“…31 The interlayer space is unsymmetrical, which leads to the formation of hydrogen bonds between consecutive layers, providing a large cohesive energy. [32][33][34][35][36][37][38][39] Then the materials with higher molecular weight tend to insert into the interlayer of kaolinite by replacing these small molecules. Therefore, only small and highly polar molecules can directly intercalate into the interlayer of kaolinite, such as dimethylsulfoxide (DMSO), deuterated dimethylsulfoxide, formamide, N-methylformamide, imethylformamide, acetamide, pyridine N-oxide, potassium acetate, methanol and octadecylamine.…”