Study On The Dispersion, Emulsion And Hollow Colloidosome Of LDH Nanoparticles

Layered double hydroxides (LDHs) is a general name of a series of hydroxides which have crystal structure similar like hydrotalcite and are composed of more than one metal ion. They are new inorganic materials that have promising application potential. LDHs layers contain permanent positive charge. Anionic ions can be adsorbed between the layers so as to maintain the charge balance. The distance between the layers differs for different anions. In water, the anions diffuse from the layers of LDHs particles. So the particles have positive charge and can be dispersed in water to form stable colloidal aqueous dispersion. The LDHs colloidal aqueous dispersion is of importance in fields of biological science, catalysis, oil-gas development, water environment purification, etc. Our research group has examined the rheology, the ability to stablize emulsion and foam and the liquid crystal behavior of LDHs aqueous dispersion. We have also successfully utilized LDH positive sol as a finishing agent for drilling mud in exploiting of oil-gas field. The investigation of the stability of LDHs dispersing system is the fundamental of the above mentioned research. The environmental condition and the properties of the particle are important factors for the stability of LDHs dispersion, which can affect the values of LDHs in theoretical investigation and application. Take the Pickering emulsion stabilized by LDHs nanoparticles, the stability of the LDHs dispersion can directly affect the self-assembling of LDH particles at oil-water interface, thus affecting the stability of the resulted emulsion. These properties are fundamentals of forming a higher level ordered structure of LDH particles. Based on the above-mentioned consideration, we examined the stability of MgAl-LDH aqueous dispersion system, the emulsion stability and the LDH hollow colloidosome prepared from the Pickering emulsion template method. The research is of importance for completing LDH theoretical system and extending its application value.This dissertation includes the following sections:1. Double flocculation in LDH-sodium polyacrylate aqueous dispersion The this section, the effect of sodium polyacrylate (PAAS) on the stability of aqueous dispersion of MgAl-LDH nanoparticles has been studied. The adsorption of PAAS on LDH particles was determined by total organic carbon (TOC) analysis and the electrical properties of LDH particles were characterized byζpotential measurement. The experimental results show that the LDH-PAAS mixtures appeared to be flocculation-dispersion-reflocculation as the PAAS concentration increased from 0.006 to 2.400 mM in a 1.0 wt% LDH aqueous dispersion. At first, theζ, potential of LDH particles changed from positive to zero and then to negative with the increase of PAAS concentration, which was due to the adsorption of PAAS on LDH surfaces. As a result, the electrostatic repulsion between LDH particles firstly decreased and then increased with the increase of PAAS concentration. Therefore, the PAAS-LDH mixture first flocculated and then dispersed. When the PAAS concentration further increased, the polymer adsorbed not only on LDH particle surfaces, but also in the particle layers. This caused theζ, potential of LDH particles further negatively increased. The depletion attraction between LDH particles should be the major reason for the reflocculation of the aqueous dispersion. The examination of FTIR spectra of LDH, PAAS, and LDH after PAAS adsorption suggests that the adsorption of PAAS on LDH particles was mainly through the interaction between the-COO" in PAAS molecules and the LDH particles.2. Pickering emulsion stabilized by amorphous LDH particlesThree different types of LDH aqueous dispersion were prepared. The morphology, size and structure of LDH particles, and the stability of the Pickering emulsion prepared from the LDH dispersion were compared. The results suggest that:The hydroxides synthesized from co-precipitation are amorphous. The high shear force dispersed the precipitates in water as a loccular cluster, but assisted little for the LDH crystallization. At the same rotation speed, the size of the obtained loccular cluster decreased with increasing the dispersing time. A hydro-thermal treatment can help the amorphous precipitates to grow to crystallinzed hexogenal particles. NaCl has the same effects on the stabilities of amorphous and crystalline LDH particle dispersions. NaCl can introduce the flocculation of LDH particles, increasing the adsorption energy of the particles at oil-water interface. The introduction of NaCl can also facilitate to form the three-dimension network of LDH particles in the phase, thus effectively improving the ability of LDH particles for stabilizing emulsion. Compared with the crystallinzed LDH particles, amorphous LDH particles have higher hydrophilicity and weaker adsorption ability at oil-water interface. Additionally, because the amorphous loccular cluster is soft, the strength of the networks formed by amorphous LDH particles at oil/water interface and in the aqueous phase are also weak. Therefore, when the NaCl concentration is the same, the ability to stabilize the Pickering emulsion is better for the crystalline LDH particle than the amorphous LDH particle.3. Preparation of LDH hollow colloidosome by the Pickering emulsion template methodHollow colloidosomes consisting of plate-like MgAl-LDH nanoparticles have been prepared by a facile route from a Pickering emulsion. The particles were firstly adsorbed onto the surface of paraffin oil-in-water emulsion droplets. After the core oil was dissolved in the surrounding bulk liquid, using solvents that are miscible with both the internal and external phases of the droplets, hollow colloidosomes were formed. In this process, we find that the diameters of the colloidosomes are significantly reduced compared to those of the emulsion droplets. The reduction in the diameter is caused by rearrangement of the LDH platelets. That is, the platelets changed their orientation from lying flat on the emulsion droplet surface to standing erect in a dense, face-to-face connecting pattern in the colloidosome shell. The main reason for the particle rearrangement is the increase of the attractive forces among the particles due to the reduced polarity of the solvents used during colloidosome preparation.The use of a soft template in our study not only overcomes the disadvantages of hard template-based approaches, which require multistep processes and high temperatures, but also introduces the reorganization of the anisotropic particles on the curved surfaces because of the changed polarity of the environment. This synthetic approach suggests a bright prospect for the use of Pickering emulsions in fabricating hollow constructions with special structures and applications.The rearrangement of plate-like particles on the spherical surface, which has not been previously reported, may serve as a novel model for studying the liquid crystal behavior of colloidal particles on curved interfaces. The hollow colloidosomes, with LDH nanoplatelets standing erect in the shell, have advantages both in permeability control and preparation of multi-functional microcapsules.