| In industry of exploration of oil, oil-based drilling fluid is the mud using oil as the continuous phase. The advantages of selecting an oil-based drilling fluid include: superior hole stability, excellent lubrication, enhanced shale inhibition, high temperature stability. As a result, oil-based drilling fluid is a better selection for drilling in deep well, high emperature well and complex conditions. For environment protection, organoclay was widely used in oil-based drilling fluid. In previous researches, the most intensively studied process is the dispersion and sol-gel transition of spherical particles in nonpolar suspensions.Suspensions of colloidal particles in nonpolar solvents exhibit a diverse range of rheological properties, varying from simple viscous fluids to highly elastic gels. However, the anisometric such as rod-like, plate-like particles have received less attention. The properties of suspensions consisting of anisometric particles differ from the corresponding ones of spherical particles because interactions between anisometric particles depend on their mutual orientation. Therefore, it is meaningful to understand the interaction between particles in nonpolar solvents for enhancing the stability and the rheological behavior of the dispersions.Layered double hydroxides (LDHs), known as hydrotalcite-like clays (HT1c), are a class of inorganic lamellar materials consisted of positively charged brucite-like layers and charge-balancing interlayer anions which are generally exchangeable. In recent years, there has been increased focus on the delamination of LDHs to form highly dispersed monolayer colloidal phase due to their salient characteristics potential applications in many fields. They studied the effects of the types of surfactants, the solvent polarity and delamination methods on the process of exfoliation and physical and chemical properties of the dispersions after exfoliation. However, it is not entirely clear in the following aspects. Firstly, most of LDHs delamination formed in polar solvent as a result of exfoliation preferential occurrence in a high polar solvent. However, dispersions of particles in nonpolar solvents especially environmentally friendly have many applications. Secondly, the previous works are more interested in investigating the process of delamination than the dispersions possessing enhanced rheological properties due to the exfoliation. Thirdly, it is also worth understanding the interaction between particles in nonpolar solvents for improving the stability and the rheological behavior of the dispersions. In addition, emulsions stabilized by such particles (Pickering emulsions) have been attracted more and more attentions because of their lower toxicity, friendly to environment, strong emulsion stability and so on. Therefore, the investigation of the exfoliation of LDHs in nonpolar solvent and the Pickering emulsions stabilized by exfoliated LDHs is desired.Based on the above discussion, in this dissertation, LDHs intercalated with different types of surfactants, nonpolar solvents (liquid paraffin, toluene, white oil) are investigated. First, the swelling and exfoliation behavior of LDHs particles intercalated with surfactants in liquid paraffin. The existence of nanoscale layers achieved by delamination drastically affects the stability and the rheological behavior of colloidal suspensions. Second, we found that the delaminated nanosheets can be interacted with organophilic montmorillonites (OMMTs) in nonpolar solvent. In addition, Pickering emulsions stabilized by exfoliated LDHs and pristine LDHs are prepared. The exfoliation, physical and chemical properties of the dispersions after exfoliation, and the emulsions properties are investigated systematically by visual observation, rheology, optical microscope, transmission electron microscope (TEM), X-ray diffraction (XRD), Small-Angle X-ray Scatting (SAXS) and Laser-induced fluorescent confocal micrograph (LFCM) experiments. The present dissertation includes three topics desctibed as follows.1. Delamination and Sol/Gel Formation of Layered Double Hydroxides in Liquid ParaffinThis paper reports the dispersion behavior of LDHs particles intercalated with different organic surfactant anions in liquid paraffin. Among them, LDHs containing dodecyl sulfonate exhibits best swelling and exfoliation behavior, which lead to the formation of semi-transparent colloidal suspension. Further addition of particles results in a viscous gel. Results from XRD and TEM confirm that unilamellar nanosheets are formed in this system. Rheological measurements demonstrate that the dispersions exhibit enhanced viscosity and rheological properties. Dynamic oscillatory shear measurements indicate the suspension undergoes a transition from a fluid-like sol to a solid-like gel with increasing particle concentration. The complex viscosities versus low angular frequency data are well fitted by a power-law function and the shear-thinning exponent provides a gauge for the extent of LDHs exfoliation. The SAXS confirm that the gel formation is induced by the formed network, which is consisted of "tactoid" microstructure formed by loosely stacked nanosheets.2. Interaction between Exfoliated LDHs with Organophilic Montmorillonites and Pickering Emulsion Stabilized by Exfoliated LDHsWe found delaminated nanosheets can be interacted with OMMTs in nonpolar solvent. The addition of delaminated nanosheets results in dramatic enhancements to the dispersion rheological properties. Low frequency storage modulus, low shear viscosities and effective yield stress all increased, indicating network was formed between particles, and it displaying a promising potential in oil base drilling fluids, paint and so on. Rheological property of the mixed system is related to the LDHs/OMMTs ratio. When the LDHs fraction is in the range of 0.1～0.4, the mixed system exhibited the enhanced rheological properties. Results demonstrated that the network formed by mixed particles is strongest at this LDHs/OMMTs ratio. When the LDHs fraction is higher than 0.4, the Rheological property of the mixed system became weak. In addition, we investigated the influence of exfoliated LDHs on Pickering emulsions stabilized with it. Results proved that comparing to the Pickering emulsions stabilized with pristine LDHs, Pickering emulsions stabilized with exfoliated LDHs showed rasied stability, smaller dropet diameter and lower polydispersity. On the one hand, an important consequence of exfoliation is that more colloidal particles are able to stabilize much interface compared to simple emulsions with the same particle concentration. On the other hand, unabsorbed particles may form networks in the continuous phase that trap droplets, which render the systems showing enhanced long-term stability.3. Highly stable and Gel-like Pickering Emulsions Stabilized by LDHs Prepared by UltrasonicationWe studied the influence of emulsification process on the packing of LDH particles at aqueous/oil phase interface and the properties of the resulting Pickering emulsions. Emulsions prepared by ultrasonication display superior long-term stability and gel-like characteristics at the dispersed phase volume fraction well below the random close packing limit, whereas emulsions with same compositions prepared by vortex mixing show some extent of sedimentation and liquid-like behaviors. Rheological measurements demonstrate that the zero-shear elastic modulus and yield stress of gel-like emulsions exhibit power-law dependences on particle concentration and independence on aqueous/oil phase ratio. Optical microscopy results reveal the droplets become strongly adhesive and heterogeneous percolating network is formed among neighboring droplets. LFCM further confirm that the droplet adhesion is due to particle layers bridging opposite interfaces. In contrast, homogeneous, isolated and densely packed droplets are present in emulsions prepared by vortex mixing, which results in these systems are dominantly viscous like the suspending fluid. This study shows that emulsification process can be used as a trigger to modify long-term stability and rheology of solid-stabilized multiphase mixtures, which greatly expands their potential technological applications. |
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