Hydrothermal Synthesis and Adjustment of Liquid Crystalline Phase Behavior of Colloidal Mg-Al LDH Dispersions
|Course||Physical and chemical|
|Keywords||Layered double hydroxide Crystal growth Colloidal dispersions Mineral lyotropic liquid crystal Multiphase coexistence Electrooptic effects Nanocomposites|
Layered double hydroxides （LDHs）, also known as anionic clays or hydrotalcite-like compounds, are a class of inorganic materials with potential applications in wide areas. They are structurally related to brucite Mg（OH）2 with some certain divalent ions replaced by trivalent ones, and the net positive charges in the layers due to the replacement are compensated by exchangeable anions and water molecules in the interlayer. The distance between the layers is changeable with different anions. LDHs can be used in catalysis, adsorption, nanocomposites, drug carriers and biosensors due to their unique structural and electrical properties. More and more researchers have interests in the application of LDHs nanocomposites. By intercalating functional molecules into the gallery between the sheets, the distance between the layers can be changed and in the meantime we can obtain new materials with special capabilities. Recently, efforts have been focused on preparation of stable colloidal dispersions of monodispersed LDHs for their merits in preparation of thin films, delivery of cellar genes/drugs, stabilization of Pickering emulsions and formation of mineral liquid crystals. A typical and widely used method for preparing LDHs is coprecipitaion. It has been reported in literatures that when using non-steady coprecipitaion method to prepare LDHs, the crystalline structure of LDHs has formed at an early stage of the coprecipitaion and the following process of peptization only was used to make the particles grow up and develop their crystallinity. For a long time, crystal growth has been considered a process involving deposition of individual atoms or ions onto an existing crystal. However, a different picture about the growth of crystalline structures has emerged, in which nanoparticles serve as building blocks for construction of single crystals. This is referred as non-classical crystallization. Non-classical crystallization in organism has recently been found in crystallization of inorganic materials, which promotes us anew realizing the crystallization mechanism of LDHs and provides a new method for us to obtain stable dispersions of LDHs particles with controlled size and shapes. In our group, it has been found that it can form mineral liquid crystalline phases in colloidal dispersions of LDH plates, which has largely contributed to the field of mineral lyotropic liquid crystals. Any researches for liquid crystalline properties of colloidal LDH dispersions can contribute to the typical Onsager’s theory of liquid crystalline phase transition, and eventually it can well validate and enrich the Onsager’s theory. As we all known that the phase behavior of colloidal dispersions can be well controlled through adjusting the interactions between colloidal particles in the dispersions. In turn, the controlled liquid crystalline phase behavior can well develop the liquid crystalline phase transition theory and improve the application values of mineral liquid crystal materials. It has been known that, the application of liquid crystals in display techniques is still the major impetus of the researches in the liquid crystal field. However, what now have been used in displays mostly are small organic molecule liquid crystals. In comparison to organic molecules, the inorganic substances may have enhanced optical, electrical, and magnetic properties, and they are probably more stable and cheaper. If the mineral liquid crystals can be used in displays, a great revolution would be induced in the liquid crystal display industries. In order to examine the practicability of their application in display techniques, it is very important to investigate the electrooptic effects of these mineral liquid crystal materials. In this thesis, the effects of the introduction of an effective attraction between the colloidal LDH particles and the application of an electric field on the liquid crystalline phase behavior of LDH dispersions have been explored. I hope that it can provide an effective theoretical fundamental for the potential applications of mineral lyotropic liquid crystals and achive artificial adjustment of liquid crystalline phase transition.In this thesis, stable colloidal LDH dispersions have been prepared by a fast coprecipitaion, thoroughly washing precipitates, and a following hydrothermal treatment of the filter cake. This is a simple method which can be easily manipulated in laboratories and can obtain LDH products in large amount. The XRD, SEM, TEM, HRTEM, and AFM characterization results demonstrated that amorphous nanoparticles instead of crystalline LDH particles formed in the coprecipitation process, and the following hydrothermal treatment resulted in crystallization of the amorphous precipitates. Attachment of amorphous nanoparticles with interface nucleation was observed in the early stage of crystal growth. Thus, we propose a new crystallization mechanism for LDH nanoplates, in which both particle-mediated non-classical crystal growth and ion-mediated Ostwald ripening are involved in formation of crystalline LDH particles. The size of LDH particles can be effectively adjusted. Increasing hydrothermal treatment time and temperature resulted in increase of LDH particle diameters. The particle thicknesses were not influenced by treatment time at a certain temperature and increased slightly by treatment at higher temperature. Addition of electrolyte NaCl is unfavorable for crystal growth of LDH particles.In colloidal LDH dispersions, liquid crystalline phase transition will occur when the particles exceed a certain concentration. In this thesis, the liquid crystalline phase behavior of mixed LDH/PEG dispersions has been explored. The AFM, IR, and XRD characterization results indicated that the addition of PEG molecules has not changed the shape, diameter, thickness, IR spectroscopy, and crystalline structure of the LDH particles. PEG molecules were not adsorbed on the LDH particles, and they only changed the property of the solvent which induced an effective attraction among the LDH particles. In comparison to the pure LDH dispersions, more complicated phase behavior in the mixed PEG/LDH dispersions was observed. At certain concentrations of LDH and PEG, a four-phase equilibrium and even a five-phase equilibrium appeared, including an isotropic upper phase, a sedimentation bottom phase, and two or three birefringent middle phases. One of the liquid crystalline phases must be nematic, and the phase separation proceeds via nucleation and growth. In the five-phase coexistence system, the new phase grown upon the sedimentation phase has a positional order, and it may be a columnar phase due to the Bragg reflections. It seems paradoxical with the Gibbs’ phase rule for the multiphase coexistence in a binary system. The particles polydispersity and gravity reconcile the contradiction. We explain the experimental phenomena by the interplay between a PEG-induced effective attraction, LDH particles polydispersity, and the sedimentation in the gravitational field. The effects of an electric field on the liquid crystalline phase behavior of the colloidal nematic and isotropic LDH dispersions were studied with polarized light microscopy and optical detection system observations. The results demonstrated that these dispersions showed strong electrooptic effects. In the nematic dispersions, the LDH plates can align more orderly and show stronger birefringence by the action of an electric field. In a nematic dispersion with concentration of 20 wt%, we observed a threshold voltage 1 V, below which there was no optical response. Electric field can induce forming liquid crystalline phase in the isotropic dispersions with concentration lower than the I-N phase transition threshold. The initial response times of these dispersions are all less than 1 s. The induced birefringence rapidly decreases after removing the electric field, and then gradually decreases to the original state of these dispersions. The process is reversible. A square-pulse electric field can be successively applied on these dispersions, and the reversible switching can proceed in a few tens of seconds. There is no optical response in very dilute LDH dispersions and gel samples. The results of the electrooptic effects in LDH dispersions clearly demonstrated that this family of mineral liquid crystals may have the potential applications in simple on-off switching and low-frequency display technology.Moreover, in this thesis, intercalated LDH nanocomposites have been prepared using a new method different from the traditional ones. Amorphous precipitates were first synthesized by a non-steady coprecipitation method, and then the fresh precipitates were separately added into the aqueous solutions of SDS, SDBS, citrate sodium, and stearate sodium. We can eventually obtain LDH nanocomposites after some certain hydrothermal treatment time. The XRD, TEM and TGA characterization results demonstrated that the anions of SDS, citrate sodium, and stearate sodium have successfully intercalated into the interlayer of the LDH, which produced LDH nanocomposite particles with shapes different from the regular hexagonal plate-like LDH particles grown up in water. Needle-like LDH nanocomposite particles were obtained from the amorphous particles hydrothermal treated in the SDBS solution, but the anions have not intercalated into the interlayer of the LDH. The growth mechanism of the LDH nanocomposite particles from amorphous particles involved particle attachment and Ostwald ripening. Furthermore, intercalated LDH nanocomposites have also been prepared using a traditional ion-exchange method. We first synthesized stable LDH dispersions, and then modified the LDH particles with SDBS. The distance between the layers of the LDH increased because the chlorine anions have been replaced by the long SDBS ones. The increased distance between the layers was favorable for intercalating fluorescent dye molecules such as rhodamine and coumarin.In summary, stable LDH dispersions in large amount and intercalated LDH nanocomposites have been successfully synthesized from amorphous precursors using hydrothermal treatment method; and the liquid crystalline phase behavior of the LDH dispersions has been successfully adjusted through the introduction of an effective attraction and the application of an electric field.