Self-adaptive Deformation Of Kaolinite And The Structural Properties Of Kaolinite Nanoscrolls

Clay mineralogy,crystallography,crystal chemistry and other theoretical knowledges were used to characterize the structure of methoxy-grafted kaolinite,kaolinite compouonds and kaolinite nanoscroll.A series of kaolinite compounds were synthesized and high-yield production preparation method of kaolinite nanoscrolls were investigated.The morphological evolution sequence of kaolinite particles was proposed.New origin model of halloysite nanotubes was proposed based on the exfoliation and the morphological evolution sequence of kaolinite.Kaolinite is a layered alumosilicate clay mineral with a hexagonal platy morphology and a size of about 0.1⁵?m in diameter and about 0.1¹0?m in thickness.Kaolinite naturally occurs in the plate form for the interlayer hydrogen bond and the distortion and adaption of tetrahedron and octahedron.The Si-O tetrahedra are linked to each other via the basal oxygens while sharing their apical oxygens with Al-O?OH?octahedra.In order to adapt to the smaller size of octahedral sheet,the tetrahedral sheet adjusts itself by changing the Si-O tetrahedra through ditrigonal rotation from the ideal hexagonal arrangement.Therefore,platy kaolinite is in a?quasi-stable?physical form or in a?metastable state?.Methoxy-grafted kaolinite is an excellent organic-inorganic compound,which serves as a precursor to prepare new organic or inorganic kaolinite hybrid materials.Direct intercalation of kaolinite with dimethyl sulfoxide,N-methylformamide and urea was achieved first.All of the above intercalated kaolinites could be grafted by methoxy groups by reacting with methanol.Methoxy groups can be grafted on the aluminol surface of the kaolinite via an Al-O-C bond,accompanied by the loss of hydrogen of methanol,which may combine with the hydroxyl groups to form water.The d_?001?of methoxy-grafted kaolinite was found to be in the range of 0.99¹.11 nm in wet state but constant at 0.86 nm for dry state.The pre-intercalated molecules of dimethyl sulfoxide,N-methylformamide and urea blocked the grafting action at first but facilitated it at a later stage.Here,the mechanism of the structural collapse of methoxy-grafted kaolinite was also proposed.Spontaneous deintercalation of small molecules such as water,ethanol,isopropanol and others was observed.Water and methanol molecules played an important role in the grafting action and also affected the structure of methoxy-grafted kaolinite.¹³C MAS NMR spectra of intercalated kaolinites confirmed that the pre-intercalated moleculeswere not displaced completely even though it cannot be detected by XRD.The hygroscopicity of the pre-intercalated molecules was found to affect the structure of methoxy-grafted kaolinite.Kaolinite layers can be exfoliated to nanoscrolls artificially in laboratory through multiple-step displacement intercalation.This study presents a novel idea to prepare large amounts of kaolinite nanoscrolls using ultrasonic assisted liquid exfoliation method.Ultrasonicationwasusedtofacilitatetheintercalationof cetyltrimethylammonium chloride and also,to promote the exfoliation process greatly.The shear forces and cavitation voids apparently stimulated the layer curling and sheared the nanoscrolls apart from the mother particles.The specific surface area of liquid exfoliated kaolinite nanoscrolls was found to be 5 times larger than that of the original kaolinite.However,specific surface area increased 10 times when ultrasonication was used.Based on this research,ultrasonic waves apparently contributed to the production of kaolinite nanoscrolls through liquid exfoliation.In the present study,the nanoscroll formation mechanism of kaolinite sheets via the intercalation–liquid exfoliation process was proposed.The driving force for kaolinite sheet to be curled nanoscroll originates from the size discrepancy of Si–O tetrahedron and Al–O?OH?octahedron.External conditions for the curling behavior include weakening of the interlayer bonds and the generation of free space.The intercalated molecules weaken the hydrogen bonding significantly,and provide space for kaolinite to roll up after their deintercalation.The displacement intercalation promoted the platy kaolinite layers spontaneously curled.Kaolinite layers get curled up into one-dimensional nanoscrolls automatically when they are exfoliated in methanol after intercalation with alkyltrimethylammonium salts.The mismatch of Si-O tetrahedral sheet and Al-O?OH?octahedral sheet of kaolinite leads to rolling up of the layer until reaching a stable morphology.Kaolinite nanoscrolls are hollow tubes with outer face of tetrahedral sheet and inner face of octahedral sheet.The inner and outer diameters of kaolinite nanoscrolls have specific values.Since layer scrolls up toward the c axis and distance of the five-layer atomic plane which is composed of tetrahedral sheets and octahedral sheets on the c axis is fixed,radius of curvature of the initial point is determined when such size difference is perpendicular to the c axis.The cross-section of nanoscroll is an Archimedes spiral with a=r-12.15 and a 0.86 nm pitch in geometric.The different adaption types and discrepancy degree between tetrahedron and octahedron generate various curling forces in different directions.The curling directions of the plates and the lumen diameter of the nanoscrolls depend heavily on the crystallographic characteristics of the kaolinite.The relative curling force are as follows,[3-10]>[100]=[1-10]>[110].Three different typical curling morphologies can be observed:unilateral curling?single direction?,trilateral curling?triangle curling?,and four-side curling?rhombus curling?.Among them,the first one is the most common,and is the final stable state while the latter two are in the intermediate exfoliation state.The length of kaolinite nanoscroll is determined by the initial particle size.Evolution sequences of kaolinite from nanolayers to nanoscrolls were discussed during the intercalation and liquid exfoliation and structural features of kaolinite nanoscrolls were disclosed.Modification-intercalation-liquid exfoliation of kaolinite could be summarized as the following morphological evolution sequences:3D original kaolinite particles?quasi-2D small kaolinite particles?2D kaolinite nanosheets?2D semi-curled kaolinite nanosheets?1D kaolinite nanoscrolls.Generally,original“3D”and“quasi-2D”kaolinite transforms to 2D structure and finally into the 1D nanoscroll structure.This paper presents a systematic analysis of morphological changes of kaolinite minerals and proposes the concept of self-adaptive deformation of minerals.Kaolinite nanoscrolls with a diameter of²0 to 100 nm and length of⁰.25 to 2?m were prepared from the exfoliation of kaolinite precursor.New interlayer space and lumens could be obtained by delamination and exfoliation of platy kaolinite while forming nanoscrolls.The morphological and structural differences between synthetic kaolinite nanoscrolls and natural halloysite nanotubes were invesigated using scanning electron microscopy,transmission electron microscopy,X-ray diffraction,Fourier transform infrared spectroscopy,thermo gravimetry-differential scanning calorimetry,nitrogen adsorption-desorption techniques.Transmission electron microscopy showed that kaolinite nanoscrolls are thinner and longer than halloysite nanotubes.Kaolinite nanoscrolls exhibited nitrogen adsorption-desorption isotherms and pore size distribution curves similar to those of halloysite but the specific surface area and pore volume of the kaolinite nanoscrolls were found to be 2 times higher than those of natural tubular halloysite.The dehydroxylation of kaolinite nanoscrolls occurred at463?,which is between the dehydroxylation temperature of kaolinite and that of halloysite.Properties such as large BET specific surface areas,total pore volumes and the average pore radius of synthetic kaolinite nanoscrolls and naturally occurring tubular halloysite nanotubes were compared for potential applications as nano-containers,nano-reactors,nanofillers and adsorbents.The current comparative study of kaolinite nanoscrolls and halloysite nanotubes suggests that the former could possibly be substituted for the latter for improving some applications.Due to the complex geological movements and epigenetic environment,the structure and morphology of clay minerals in nature are diverse.Halloysite is also a member of the kaolinite-serpentine group of 1:1 clay minerals found in many locations although it is not as common as kaolinite.The dominant morphology of halloysite is tubular and therefore,it is often called as halloysite nanotubes?HNTs?.Previous studies suggest that halloysite can be the former mineral of kaolinite in the clay wreathing sequence.However,based on this research,it is reasonable to pose that kaolinite can be converted to halloysite under certain weathering conditions.A new origin model of halloysite nanotubes were proposed based on the exfoliation process of layer kaolinite.