Hydrophobicity and Charge Distribution Effects in the Formation of Bioorganoclays

Interactions of bioorganic moieties with clay minerals have attracted attention not only from the perspective of novel bioclay materials but also because they play a crucial role in our understanding of physical and chemical processes in soils. The aim of the present article is to explore the interactions responsible for the formation of a phosphatidylcholine-kaolinite bioclay by employing a series of classical molecular dynamic simulations. Detailed analysis of the structure and energies of the resulting bioclays reveals that the phosphatidylcholine molecules bind to the kaolinite surface either via their zwitterionic heads or hydrophobic aliphatic tails, depending on the kaolinite surface characteristics and the density of organic coating. The phosphatidylcholine molecules have a tendency to form irregular layers with a preferred parallel orientation of molecules with respect to the kaolinite surface. The tails exhibit varying degrees of flexibility and disorder depending on their distance from the surface and the density of surface coating. Significant differences in the binding can be spotted with respect to the two types of kaolinite basal surfaces, i.e., the hydrophobic siloxane surface, which possesses a considerable dispersion character, and the hydrophilic alumina surface, polarized by the surface hydroxyl groups.

Fe-doped TiO2/Kaolinite as an Antibacterial Photocatalyst under Visible Light Irradiation

In this work, undoped and Fe-doped TiO2 immobilized on kaolinite surface was successfully synthesized by sol-gel method with various Fe concentrations (0.05, 0.125, and 0.25 wt%). The effects of Fe doping into TiO2 lattice were thoroughly investigated by a diffuse reflectance UV-visible (DRS) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy, and X-ray diffraction (XRD). The optical band gap of undoped and Fe-doped TiO2/kaolinite is red shifted with respect to the incorporation of Fe3+ into the structure of TiO2 resulted band gap. The FTIR spectra shows a shift of peak at the wave number at 586 cm−1 and 774 cm−1 which is attribute of the Fe−O vibration as an indication of the formation of Fe-TiO2 bonds. Incorporation of Fe3+ cation into the TiO2 lattice replacing the Ti4+ ions, which induced a perturbation in anatase crystal structure, causes the change in the distance spacing of the crystal lattices dhkl(101) of 8.9632 to 7.9413. The enhanced photocatalytic performance was observed for Fe-doped TiO2/kaolinite compared with TiO2/kaolinite with respect to Escherichia coli growth inhibition in solution media under visible light irradiation. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

Simulation of Oil-Water Rock Wettability of Different Constituent Alkanes on Kaolinite Surfaces at the Nanometer Scale

Kaolinite is widely distributed in shale formations. Kaolinite has two surface types, Si–O and Al–OH, and the two surfaces have different chemical properties. The surface wettability of kaolinite minerals is closely related to the occurrence of crude oil, the migration process of crude oil, and the filling process of crude oil. In this paper, we focus on the oil-water rock wettability of different alkane hydrocarbons on the different surfaces of kaolinite and construct a model of oil and water with variation of the alkane components on the surface of tetrahedral and octahedral kaolinite. Molecular dynamics methods were used to study the morphological changes in water clusters in different alkanes on different surfaces of kaolinite and to calculate the wetting angles. Studies have shown that the octahedral kaolinite surface is strongly hydrophilic, and the water clusters become monolayers adsorbed on the surface. Water easily displaces the oil on the surface and preferentially drives low carbon number alkanes. The tetrahedral siloxane kaolinite surface is oleophilic, the water molecules in C6H14–C18H38 are clustered on the surface, and the wetting angle of the water cluster in the alkane increases with increasing carbon number. Water has difficulty displacing oil on this surface.

Luminescence spectroscopic investigations of europium complexes formed in the kaolinite-humic acid/citric acid systems

In the present study, the nature of Eu(III) complexes (Eu(III) was used as a surrogate for Am(III)) formed in kaolinite–humic acid (HA)/citric acid (CA) system was investigated by luminescence spectroscopy. In addition to the ternary system (kaolinite + Eu + L(CA/HA)), the binary system (Eu-L) was also looked at for a better understanding of the complexes formed at the kaolinite surface. The lifetime and emission spectra of Eu-L complexes on the kaolinite surface differ considerably as compared to the same in the aqueous phase. The Eu-HA aqueous complexation shows differences in the excitation spectra with similar decay lifetimes with increasing aqueous HA concentrations. The ligand-to-metal charger transfer (LMCT) in the Eu-HA excitation spectra suggests the complexation of Eu(III) with HA at pH ∼ 4. Although the mode of Eu(III) binding to the kaolinite surface in the presence of CA/HA was the same i.e. metal-bridged ternary complex formation, the local surroundings around the sorbed Eu(III) differ in the two cases. The loading of HA in the Eu-HA-kaolinite system does not have a large effect on the local structure around the sorbed Eu(III) ion, but enhances the percentage of Eu(III) uptake onto the kaolinite surface. The number of H2O molecules in the primary hydration sphere of sorbed Eu(III) differs in the Eu-HA-kaolinite and Eu-CA-kaolinite systems. In addition, Eu(III) assisted precipitation of HA was also seen using a radiometric method.

A Molecular Dynamics Study on the Mechanism of Adsorption of Phenol and P-Nitrophenol on Kaolinite Surface

Herein, we investigate the adsorption of two organic pollutants, phenol and p-nitrophenol (PNP) in dilute aqueous solution conditions on kaolinite (001) surface through classical molecular dynamics (MD) simulations. The present investigation addresses both adsorption isotherms and mechanistic issues. MD simulations at different solute concentrations generated density profiles and, thereby, adsorption isotherms. The data generated for phenol adsorption fitted both Langmuir and Freundlich isotherm models equally well. Alternatively, PNP adsorption data on the kaolinite surface followed the Langmuir model better. Overall, phenol exhibits higher adsorption capacity on kaolinite than PNP. These results confirm to the experimental observations made by earlier publications in the literature. Radial distribution functions (RDF) between various atom types on the adsorbent and molecules in the solution phase point towards a hydrogen bond dominated interaction mechanisms for organic pollutants.

A Molecular Dynamics Study on the Mechanism of Adsorption of Phenol and P-Nitrophenol on Kaolinite Surface

Herein, we investigate the adsorption of two organic pollutants, phenol and p-nitrophenol (PNP) in dilute aqueous solution conditions on kaolinite (001) surface through classical molecular dynamics (MD) simulations. The present investigation addresses both adsorption isotherms and mechanistic issues. MD simulations at different solute concentrations generated density profiles and, thereby, adsorption isotherms. The data generated for phenol adsorption fitted both Langmuir and Freundlich isotherm models equally well. Alternatively, PNP adsorption data on the kaolinite surface followed the Langmuir model better. Overall, phenol exhibits higher adsorption capacity on kaolinite than PNP. These results confirm to the experimental observations made by earlier publications in the literature. Radial distribution functions (RDF) between various atom types on the adsorbent and molecules in the solution phase point towards a hydrogen bond dominated interaction mechanisms for organic pollutants.