Molecular Dynamics Simulations Studies On Organic Compounds In Kaolinite Interlayer

Kaolinite is one of the most widely used clay minerals.Our country is rich in kaolin resources with fine quality,which have been widely used in application areas of rubbers,plastics,coatings,paper manufacture,ceramics and environmental protections,etc.Particle size and surface properties of kaolinite are the important benchmark of kaolinite products,especially in the high-technology field,which have critical impact on the properties of kaolinite associated products.Intercalation,delamination,and surface modification are the prerequisites to achieve the nanocrystallization and surface decoration of kaolinite for its fine processings,which can enhance the grade and additional value of kaolinite products.Experimental studies have proven that only a few small orgnic compounds can directly intercalate in kaolinite interlayer,and the organic compounds with relatively large molecule size can achieve kaolinite intercalation through indirect intercalation method.Although the kaolinite/organic compounds have been extensively investigated using the X-ray diffraction,infrared and Raman spectroscopy,and nuclear magnetic resonance,etc.the interfacial structure and interactions of kaolinite/organic compounds at atomic scale still keep unclear,and the underlying force that drives the organic compounds entering in kaolinite interlayer is obscure.The representative organic compounds urea,dimethyl sulfoxide(DMSO),and dodecylamine with relatively large molecule size were selected for intercalating purified kaolinite in liquid phase to prepare kaolinite/urea,kaolinite/DMSO,and kaolinite/dodecylamine intercalation complexes.The complex models of kaolintie/organic compounds were built based on the X-ray diffraction and thermal analysis for the following molecular dynamics simulations.The interfacial structure and interaction of kaolinite interlayer surfaces and organic compounds were calculated.The adaptive biasing froce(ABF)molecular dynamics simulations were employed to calculate the thermodynamics mechanism of intercalation and surface modification of methanol pre-intercalated kaolinite interlayer by alkyl surfactants.The decomposition of free energy,namely,the interaction energies of surfactants with the interlayer environment of methanol pre-intercalated kaolintie were also calculated.Molecular dynamics simulations were used to deeply probe into the interfacial structure and interaction of intercalated organic compounds in kaolinite interlayer,and the underlying force that drives the intercalation and surface modification of kaolinite by organic compounds.The urea molecules arranged a monolayer structure in kaolinite interlayer.The C=O groups of urea functioned as H-acceptors primarily interacting with the hydroxyl groups on interlayer alumina surface,and the NH2 groups functionsed as H-donors interacting with both interlayer siloxane and alumina surfaces.The number of H-bonds formed with alumina surface was more than that with siloxane surface.In addition,the interaction energy of urea with alumina surface was greater than that with siloxane surface.Thus,the alumina surface played a major role in the intercalation and stabilization of urea in kaolinite interlayer.The DMSO arranged a monolayer structure in kaolinite interlayer.The S=O groups of DMSO functioned as H-acceptor strongly hydrogen bonded with the hydroxyl groups on octahedral surface.The tetrahedral surface attracted the methyl groups of DMSO via hydrophobic interaction.The ditrigonal cavity on tetrahedral surface and alumina octahedral vacancy on octahedral surface were the relatively energetically stable adsorption sites,where the methyls and S=O groups of DMSO can be firmly anchored.The octahedral and tetrahedral surfaces both showed attractive interactions with DMSO molecules with the interaction energy of DMSO with octahedral surface greater than that with tetrahedral surface.The relatively strong interaction with octahedral surface as well as the network of hydrogen bonds formed between S=O groups of DMSO and hydroxyl groups on octahedral surface played the key role in the kaolinite intercalation by DMSO.The intercalation free energies calculated by ABF molecular dynamics simulations showed that the methanol pre-intercalated kaolinite interlayer had relatively strong affinity to CTAC with cationic head group.The driving force for the intercalation and surface modification process was the enthalpic energy gain via the interactions of surfactants with methanol pre-intercalated kaolinite interlayer environment.The interaction energies were determined by both electrostatic and van der Waals energies with the electrostatic energy playing the important role.The strong electrostatic interactions of ionic head groups of CTAC and stearate with kaolinite interlayer environment can help in gaining more enthalpic energy allowing them to readily achieve the intercalation and surface modification compared with neutral dodecylamine.The interlayer methanol can make the kaolinite interlayer more hydrophobic for introducing surfactants enter the kaolintie interlayer throung attractive hydrophobic,electrostatic,and van der Waals force.Kaolinite/dodecylamine intercalation complex was prepared using the methanol intercalated kaolinite as the intermediate.When the dodecylamine was intercalated in kaolinite,its basal spacing was expanded from 7.2 ? to 42.9 ?.Molecular dynamics simulatios indicated that the alkyl chains of dodecylamine within the kaolinite interlayer adopted a hybrid layering structure rather than the idealized structural models such as paraffin-type structure hypothesized based on the experimental studies.The alkyl chains arranged a mixture of ordered paraffin-type-like structure and disordered gauche conformation in the middle interlayer region of kaolinite.A certain number of alkyl chains arranged in bilayer structures near to the siloxane and alumina surfaces with their alkyl chains parallel to the kaolinite basal surface.In present study,the molecular dynamics simulaitons and experiment techniques were combined to deeply explore the interfacial structure and interfacial interaction of kaolinite/organic compounds,and the underlying force that drives the organic compounds entering kaolinite interlayer.It can serve as a theoretical guidance for the experimental design of kaolinite intercalation and surface modification to prepare kaolinite products with enhanced additional value.