Atomic scale structures of interfaces between kaolinite edges and water

Cheng

et al. 2021

American Mineralogist

Self Cite

In this paper, we report a first-principles Molecular Dynamics (FPMD) study of interfacial structures and acidity constants of goethite. The pKa values of the groups on (010), (110), and (021) surfaces (space group Pbnm) are derived with the FPMD based vertical energy gap technique. The results indicate that major reactive groups include ≡Fe2OH2 and ≡FeOH2 on (010), ≡FeOH2, ≡Fe3OLH, and ≡Fe3OUH on (110), and ≡FeOhH2 and ≡Fe2OH on (021). The interfacial structures were characterized in detail with a focus on the hydrogen bonding environment. With the calculated pKa values, the point of zero charges (PZCs) of the three surfaces are derived and the overall PZC range of goethite is found to be consistent with the experiment. We further discuss the potential applications of these results in future studies toward understanding the environmental processes of goethite.

Section: Discussionmentioning

confidence: 99%

Interfacial structures and acidity constants of goethite from first-principles Molecular Dynamics simulations

Zhang

Cheng

et al. 2021

American Mineralogist

Self Cite

“…The disordered interlayer structure and the difficulty in locating H atoms, however, make the detailed picture be hard to obtain using the experimental diffraction or spectroscopic data. The molecular simulations have been developed to a powerful tool to successfully describe the interfaces of clay minerals-water/organics in terms of structure at atomic scale and energetics (Cygan et al, 2009; Heinz and Ramezani-Dakhel, 2016; Liu et al, 2015; Liu et al, 2012; Sena et al, 2015; Szczerba et al, 2014; Zeng et al, 2003; Zhang et al, 2017; Zhou et al, 2014). The interface of dickite-NMF was investigated using the quantum chemistry method.…”

Section: Introductionmentioning

confidence: 99%

Interfacial structure and interaction of kaolinite intercalated with N -methylformamide insight from molecular dynamics modeling

Zhang

Gao

et al. 2018

Applied Clay Science

The evolution of basal spacing and interfacial structure of kaolinite-N-methylformamide (NMF) complexes during the intercalation process were difficult to obtain using experimental methods. In present study, a series of kaolinite-NMF complex models with various numbers of NMF molecules in the interlayer space were constructed to mimic the progressive stage of the intercalation process of kaolinite intercalated by NMF. The MD simulations were performed on these models to explore the evolution of basal spacing and interfacial structure of kaolinite-NMF complexes during the intercalation process. It was found that the basal spacing of complex was stabilized at 11 Å during the intercalation process, where the molecular plane of NMF oriented at small angles with respect to the interlayer surface with the C=O groups and N-H bonds pointing toward the octahedral and tetrahedral surfaces, respectively, due to the hydrogen bonding interactions. The basal spacing can be enlarged to larger values with the prerequisite of overcoming the energy barrier. With the increase of basal spacing during the intercalation process, the NMF were rearranged as a pillar with the molecular planes orienting at higher angles with respect to the interlayer surface, and then developed to disordered bilayer structure. For the interfacial interaction of kaolinite-NMF complex, both the octahedral surface and tetrahedral surface showed binding affinity to the NMF, which is the driving force for the intercalation of NMF in kaolinite. The octahedral surface displays stronger binding affinity to the NMF in terms of the H-bonds and energetics compared to the tetrahedral surface partially due to the highly active surface hydroxyl groups. The present study provides insight into the basal spacing evolution, and interfacial structure and interaction of kaolinite-NMF complexes, which can enhance the understanding of kaolinite intercalated by small molecules.

“…Siloxane surfaces normally 46 behave as adsorbing region for foreign molecules such as waters and cations, e.g. In previous studies, FPMD simulations were employed to investigate the structures of edge-water 81 interfaces for kaolinite and montmorillonite (Churakov, 2007;Liu et al, 2012a;Liu et al, 2012b). 82…”

Section: Introduction 25mentioning

confidence: 99%

Acidity of edge surface sites of montmorillonite and kaolinite

Geochimica et Cosmochimica Acta

Sprik

et al. 2013

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204

160

Acid-base chemistry of clay minerals is central to their interfacial properties, but up to 10 now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles 11 molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of 12 surface groups on (010)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 13 and 1:1-type clay minerals, respectively. It shows that ≡Si-OH and ≡Al-OH 2 OH groups of kaolinite 14 have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each 15 mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of 16 titration curves, indicating that ≡Si-OH and ≡Al-OH 2 OH groups are the major acidic sites responsible to 17 pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is 18 investigated and it is found that Mg substitution increases the pKas of the neighboring ≡Si-OH and ≡Si-19 OH 2 groups by 2~3 pKa units. Furthermore, our calculation shows that the pKa of edge ≡Mg-(OH 2 ) 2 is 20 as high as 13.2, indicating the protonated state dominates under common pH. Together with previous 21 adsorption experiments, our derived acidity constants suggest that ≡Si-O-and ≡Al-(OH) 2 groups are the 22 most probable edge sites for complexing heavy metal cations.