Hongfei Cheng

Intercalation of dodecylamine into kaolinite and its layering structure investigated by molecular dynamics simulation

Dodecylamine was successfully intercalated into the layer space of kaolinite by utilizing the methanol treated kaolinite-dimethyl sulfoxide (DMSO) intercalation complex as an intermediate. The basal spacing of kaolinite, measured by X-ray diffraction (XRD), increased from 0.72 nm to 4.29 nm after the intercalation of dodecylamine. Also, the significant variation observed in the Fourier Transform Infrared Spectroscopy (FTIR) spectra of kaolinite when intercalated with dodecylamine verified the feasibility of intercalation of dodecylamine into kaolinite. Isothermal-isobaric (NPT) molecular dynamics simulation with the use of Dreiding force field was performed to probe into the layering behavior and structure of nanoconfined dodecylamine in the kaolinite gallery. The concentration profiles of the nitrogen atom, methyl group and methylene group of intercalated dodecylamine molecules in the direction perpendicular to the kaolinite basal surface indicated that the alkyl chains within the interlayer space of kaolinite exhibited an obvious layering structure. However, the unified bilayer, pseudo-trilayer, or paraffin-type arrangements of alkyl chains deduced based on their chain length combined with the measured basal spacing of organoclays were not found in this study. The alkyl chains aggregated to a mixture of ordered paraffin-type-like structure and disordered gauche conformation in the middle interlayer space of kaolinite, and some alkyl chains arranged in two bilayer structures, in which one was close to the silica tetrahedron surface, and the other was close to the alumina octahedron surface with their alkyl chains parallel to the kaolinite basal surface.

Thermal stability of styrene butadiene rubber (SBR) composites filled with kaolinite/silica hybrid filler

Styrene butadiene rubber (SBR) composites filled with fillers, such as modified kaolinite (MK), precipitated silica (PS), and the hybrid fillers containing MK and PS, were prepared by melt blending. The kaolinite sheets were finely dispersed in the SBR matrix around 20–80xa0nm in thickness and reached the nano-scale. The SBR composites with fillers exhibited excellent thermal stability compared to the pure SBR. The thermal stability of SBR composites was improved with the increasing of MK mass fraction. When MK hybridized with PS, kaolinite sheets were covered by the fine silica particles and the interface between filler particles and rubber matrix became more indistinct. SBR composite filled by hybrid fillers containing 40xa0phr MK and 10xa0phr PS became more difficult in decomposition and was better than that of 50xa0phr PS/SBR and 50xa0phr MK/SBR in thermal stability. Therefore, the hybridization of the fine silica particles with the kaolinite particles can effectively improve the thermal stability of SBR composites.

Thermal behavior analysis of kaolinite–dimethylsulfoxide intercalation complex

The thermal behavior of kaolinite–dimethylsulfoxide intercalation complex was investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis, X-ray diffraction (XRD) analysis, and Fourier-transform infrared (FT-IR) spectroscopic analysis. The samples gradually heated up to different temperatures were studied by XRD and FT-IR. The kaolinite–dimethylsulfoxide intercalation complex is stable below 130xa0°C. With the rise in the temperature, the relative intensity of the 1.124-nm peak gradually decreased and disappeared at 200xa0°C, however, the intensity of the 0.714xa0nm peak increased in the XRD patterns. In the infrared spectra, the appearance of methyl bands at 3018, 2934, 1428, and 1318xa0cm−1 indicates the presence of intercalated dimethylsulfoxide, the intensities of these bands decreased with the temperature rising and remained until around 175xa0°C, which agree with the XRD and TG–DSC data.

Thermal behavior of kaolinite–urea intercalation complex and molecular dynamics simulation for urea molecule orientation

The thermal behavior of kaolinite–urea intercalation complex was investigated by thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD), and fourier transform infrared spectroscopy (FTIR). In addition, the interaction mode of urea molecules intercalated into the kaolinite gallery was studied by means of molecular dynamics simulation. Three main mass losses were observed at 136xa0°C, in the range of 210–270xa0°C, and at 500xa0°C in the TG–DSC curves, which were, respectively, attributed to (1) melting of the surface-adsorbed urea, (2) removal of the intercalated urea, and (3) dehydroxylation of the deintercalated kaolinite. The three DSC endothermic peaks at 218, 250, and 261xa0°C were related to the successive removals of intercalated urea with three different distribution structures. Based on the angle between the dipole moment vector of urea and the basal surface of kaolinite, the three urea models could be described as follows: (1) Type A, the dipole moment vector is nearly parallel to the basal surface of kaolinite; (2) Type B, the dipole moment vector points to the silica tetrahedron with the angle between it and the basal surface of kaolinite ranging from 20°to 40°; and (3) Type C, the dipole moment vector is nearly perpendicular to the basal surface of kaolinite. The three distribution structures of urea molecules were validated by the results of the molecular dynamics simulation. Furthermore, the thermal behavior of the kaolinite–urea intercalation complex investigated by TG–DSC was also supported by FTIR and XRD analyses.

Thermal decomposition behavior and de-intercalation kinetics of kaolinite/quaternary ammonium salt complexes

Kaolinite/quaternary ammonium salt complexes were prepared by intercalation and displacement of kaolinite–N-methylformamide (Kaol–NMF) with methanol (Me) and quaternary ammonium salt. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetry and differential scanning calorimetry (TG–DSC) analysis. The d-values of the kaolinite/quaternary ammonium salts complexes increase with the alkyl chain length of the quaternary ammonium salts. Based on the results and the available evidence pointing toward the interlayer structure of kaolinite intercalation complexes, the most possible structural model for the kaolinite/quaternary ammonium salt intercalation complexes was proposed. For the kaolinite–dodecyl trimethyl ammonium chloride, kaolinite–trimethyl tetradecyl ammonium chloride and kaolinite–hexadecyltrimethylammonium chloride, the intercalation molecules are oriented perpendicular to the kaolinite surface in a single layer. However, for kaolinite–stearyl trimethyl ammonium chloride, the cationic head of intercalated stearyl trimethyl ammonium chloride molecules may be partially hydrated and arrayed aslant in the interlayer space of kaolinite with an inclination angle of 35°. Thermal analysis results revealed that the thermal decomposition of kaolinite/quaternary ammonium salt complexes occurs in two main steps. The function of the most probable mechanism, activation energy E and pre-exponential factor were obtained by mutual authentication using KAS and Ozawa methods, Satava integral method and Achar–Brindley–Sharp–Wendworth methods. The average activation energy E of four kaolinite/quaternary ammonium salt intercalation complexes is 108.147, 153.478, 125.723 and 88.008xa0kJxa0mol−1, respectively. The optimized mechanism function for de-intercalation process of quaternary ammonium salt is f(α)xa0=xa01xa0−xa0α and G(α)xa0=xa0−ln(1xa0−xa0α).

Structural Model and De-Intercalation Kinetics of Kaolinite-Methanol-Sodium Stearate Intercalation Compound

Kaolinite-methanol-sodium stearate intercalation compound (Ka-MeOH-SS) was prepared by intercalation of kaolinite with dimethyl sulfoxide (DMSO) followed by displacing of DMSO with methanol and methanol by sodium stearate. The sample was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis (thermogravimetry (TGA) and differential scanning calorimetry (DSC)) and transmission electron microscopy (TEM). The results show that the basal spacing of Ka-MeOH-SS was enlarged to 45.5-47.9 A and parts of its layers cocked and curled. Stearate ions were grafted on the methoxy groups and arrayed aslant in the interlayer space of kaolinite with an inclination angle of 47.45o to 52.17o. Sodium ions were adsorbed around the SiO4 tetrahedron and interlayer carboxylate anions, whereas some of them entered into the ditrigonal hole of tetrahedral sheets. The thermal de-intercalation of Ka-MeOH-SS includes three steps and the activation energy E is 98.3 kJ mol−1, logarithm of pre-exponential factor lg(A / s-1 is 9.71 and the mechanism function is f(α) = −ln (1 − α) and G(α) = 1 − α.

Raman spectroscopy of coal component of Late Permian coals from Southern China.

The chemical structural characterization of four samples (M-1, M-5, VS, and BaS) from Southern China was studied by Raman spectroscopy with curve-fitting analysis. Several Raman parameters, e.g., full width at half maximum (FWHM) and intensity ratio (ID1/IG), were obtained. Vitrinite (VS) and barkinite (BaS) were separated from the same coal sample, separately. The results showed that nine bands were assigned from the Raman spectra. Two typical bands, G and D1, have broad peaks, which showed that all the samples have poor order in chemical structure. Barkinite has higher disorder in chemical structure than vitrinite.

Effect of reaction temperature on intercalation of octyltrimethylammonium chloride into kaolinite

The structural property, thermal behavior, and morphology of octyltrimethylammonium chloride–kaolinite complexes prepared at different reaction temperatures were studied by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry–differential scanning calorimetry, and scanning electron microscope. The present study demonstrated that the arrangement model of octyltrimethylammonium cations (OTAC+) within the kaolinite interlayer space was independent of reaction temperature. The alkyl chains adopted a similar rigid paraffin-bilayer arrangement with different tilted angles. Although the intercalation led to an increased number of gauche conformers, the number of nonlinear conformers remained constant with increasing temperature. With increasing temperature, the number of trans conformers continuously augmented and resulted in decreased gauche/trans ratio. Therefore, the molecular environment remained solid like. Simultaneously, the surfactant packing density gradually increased, along withxa0the decreasing water content in the organoclays. This effect improved thermal stability and hydrophobicity. The thermal decomposition processes of the kaolinite–OTAC+ complex can be divided into four steps. Furthermore, SEM images showed that the morphology of these complexes was strongly dependent on the given temperature. In general, increasing the temperature within the limited given temperature (≤70xa0°C) promoted the transformation from platy layers to nanoscrolls. Most of the transformed nanoscrolls were acquired in the products prepared at 70xa0°C, and further increasing in temperature decreased the nanoscrolls yield. Nevertheless, the packing density increased in the process, thereby demonstrating that the packing density not only promoted nanoscrolls transformation but also prevented the progress.

Thermodynamic Mechanism and Interfacial Structure of Kaolinite Intercalation and Surface Modification by Alkane Surfactants with Neutral and Ionic Head Groups

Intercalation and surface modification of clays with surfactants are the essential process to tailor the clays surface chemistry for their extended applications. A full understanding of the interaction mechanism of surfactants with clay surfaces is crucial to engineer clay surfaces for meeting a particular requirement of industrial applications. In this study, the thermodynamic mechanism involved in the intercalation and surface modification of methanol preintercalated kaolinite by three representative alkane surfactants with different head groups, dodecylamine, cetyltrimethylammonium chloride (CTAC), and sodium stearate, were investigated using the adaptive biasing force accelerated molecular dynamics simulations. In addition, the interaction energies of surfactants with an interlayer environment (alumina surface, siloxane surface, and interlayer methanol) of methanol preintercalated kaolinite were also calculated. It was found that the intercalation free energy of CTAC with a cationic head group was relatively larger than that of stearate with an anionic head group and dodecylamine with a neutral head group. The attractive electrostatic and van der Waals interactions of surfactants with an interlayer environment contributed to the intercalation and surface modification process with the electrostatic force playing the significant role. This study revealed the underlying mechanism involved in the intercalation and surface modification process of methanol preintercalated kaolinite by surfactants, which can help in further design of kaolinite-based organic clays with desired properties for specific applications.

Spectroscopic characterization and solubility investigation on the effects of As(V) on mineral structure tooeleite (Fe6(AsO3)4SO4(OH)4·H2O)

Tooeleite is an unique ferric arsenite sulfate mineral, which has the potential significance of directly fixing As(III) as mineral trap. The tooeleite and various precipitates were hydrothermally synthesized under the different of initial As(III)/As(V) molar ratios and characterized by XRD, FTIR, XPS and SEM. The crystallinity of tooeleite decreases with the amount of As(V). The precipitate is free of any crystalline tooeleite at the level of that XRD could detect when the ratio of As(III)/As(V) of 7:3 and more. The characteristic bands of tooeleite are observed in 772, 340, 696 and 304 cm(-1), which are assigned to the ν₁, ν₂, ν₃ and ν₄ vibrations of AsO₃(3-). These intensities of bands gradually decreases with the presence of As(V) and its increasing. An obviously wide band is observed in 830 cm(-1), which is the ν₁ vibration of AsO₄. The result of XPS reveals that the binding energies of As₃d increase from 44.0 eV to 45.5 eV, which indicates that the amount of As(V) in the precipitates increases. The concentrations of arsenic released of these precipitates are 350-650 mg/L. The stability of tooeleite decreases by comparison when the presence of coexisting As(V) ions.