Adsorption Of Plate-like Inorganic Nanoparticles At Oil/water Interface And Stability Of Pickering Emulsions

With the development of nano-technology, more attentions have been focused on the adsorption of nanoparticles at oil/water interface and the emulsions stabilized by such particles (also called Pickering emulsions). However, the former work mainly concerned the effect of particle wettability (often expressed by the three phase contact angle 0 of the particles) on the interfacial adsorption and Pickering emulsions, ignored the electrical properties of particles. In addition, the studies are mainly focused on spherical particles, studies on anisotropic plate-like particles are scarce. Therefore, in this dissertation, we choose positively charged plate-like layered double hydroxides (LDH) and negatively charged disc-like Laponite clay particles as model particles. The effects of particle electrical properties on the interfacial adsorption of the particles and the emulsions stabilized by such particles are studied in detail. The adsorption mechanisms of the particles and the stabilization mechanisms of the emulsions are also discussed here. The results are helpful for us to better understand the adsorption of nanoparticles at oil/water interface and the properties of Pickering emulsions.Using nonsteady coprecipitation method, the Mg-Al LDH with Mg/Al molar ratio 2:1 was prepared. The morphology and size of LDH were studied with AFM, TEM and light scattering technology. The TEM and AFM images of LDH show that the particles are all hexagonal plate-like with an average particle size 120 nm. The thickness of LDH is around 5 nm. The effects of salt type and dispersion pH on the emulsification efficiency of LDH have been investigated here. Using heterocoagulation technology, the LDH-Laponite composite particles were prepared and the emulsification efficiency of the composite particles was studied.The main experimental methods used in this dissertation include: measuring the particle zeta potential with a microelectrophoresis instrument; measuring the three phase contact angle of particles with the classic captive drop method; investigating the stability, structural strength of particle dispersions and the flocculation degree of particles by phase diagram establishment and rheological experiments; studying the adsorption behavior of particles at flat oil/water interface by macroscopic observation and SEM; investigating the emulsion stability against creaming and coalescence with direct visual observation; investigating the particle adsorption at emulsion droplets with TEM and Laser Scanning Confocal Microscopy (LSCM); measuring the size of emulsion droplets and flocculated particles with Microscopy and LDS technology.1. Effect of NaCl on the emulsification efficiency of LDH particlesThe effect of NaCl on the adsorption behavior of LDH particles at oil/water interface and the emulsions stabilized by LDH was investigated here. The following conclusions are drawn: (1) with the increase of NaCl concentration in LDH aqueous dispersions, the particle zeta potential gradually decreases, leading to the flocculation of LDH into large flocs. The three phase contact angle increases with increasing NaCl concentration, but the variation is very small. The structural strength of LDH dispersions is enhanced with the increase of NaCl and particle concentrations. (2) The NaCl concentration controls the adsorption behavior of LDH at flat oil/water interface: with the increase of NaCl concentration, a) the attachment energy E of flocculated particles increases, b) the particle-interface and particle-particle (at the interface) electrostatic repulsions decreases, both of which promote the particle adsorption. (3) Liquid paraffin-in-water emulsions stabilized by LDH were successfully prepared in the presence of NaCl. Stable emulsions do not separate oil phase after 3 months. All emulsions prepared cream with time and the final state of the creamed emulsion phase is gel-like. (4) The presence of salt is crucial for the formation and stability of the emulsions. Emulsions cannot be formed without salt. The addition of salt to LDH dispersions decreases particle zeta potential, leading to a) the adsorption of particles at the interface and b) the aggregation and network formation of particles adsorbed at the interface, both of which promote the formation and stability of LDH-stabilized emulsions. The formation of a three-dimensional network in continuous phase favors the stability of emulsions, but is not necessary for emulsion formation. (5) The volume fraction of oil phase, Φo, plays an important role in stabilizing the emulsions. When Φo ≤ 0.7, the emulsions formed are very stable to coalescence. At high values of Φo (≥ 0.8), the emulsions formed are very unstable to both creaming and coalescence. We did not find the phase inversion of emulsions at high values of Φo.2. Effect of dispersion pH on the emulsification efficiency of LDH particlesThe effect of dispersion pH on the adsorption behavior of LDH particles at oil/water interface and emulsions stabilized by LDH was investigated here. The following conclusions are drawn: (1) the zeta potential of LDH dispersions decreases with increasing dispersion pH, causing the flocculation of LDH particles into large flocs. The contact angle of LDH particles increases with the increase of pH, but the variation is very small. The structural strength of LDH dispersions is enhanced by increasing pH or particle concentration. (2) The adsorption of LDH particles at flat oil/water interface is thermodynamically favorable at all pH range measured. The adsorption behavior is controlled by dispersion pH. As the pH increases, the particle-interface and particle-particle (at the interface) electrostatic repulsions are well controlled, leading to electrostatic interaction tailored particle adsorption. At high pH, however, the size of particles is so large that they sediment, then particle adsorption at the oil/water interface becomes difficult. (3) Liquid paraffin-in-water emulsions stabilized by LDH particles were successfully prepared. All emulsions prepared cream with time and the final state of creamed emulsion phase is gel-like. (4) The formation of an adsorbed particle layer at the oil drops is crucial for the formation and stability of emulsions. With the increase of dispersion pH and particle concentration, the stability of emulsions improves and the drop size of emulsions decreases. In addition, the enhancement of dispersion structural strength with increasing pH and particle concentration also favors the stability of emulsions. The gel-like structure of emulsion phase is not necessary for emulsion formation, but improves the stability of emulsions. Emulsions can not be demulsified by adjusting the emulsion pH. In addition, the emulsification style greatly influences the emulsion formation and stability. (5) TEM images of the emulsion droplets confirm that a thick particle layer forms arround the surface of the emulsion droplets. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.3. Effect of salt type on the emulsification efficiency of LDH particlesThe effects of salt type and concentration on the adsorption behavior of LDH particles at oil/water interface and emulsions stabilized by LDH were investigated here. The following conclusions are drawn: (1) Salt type determines the electrokinetic properties of LDH particles. NaNO₃ is an indifferent electrolyte for LDH particles, while SO₄^2-, PO₄^3- and P₂O₇^4-. are all potential determining ions for the particles. (2) Salt addition does not obviously change the wettability of the LDH particles, but greatly influences the structural strength of LDH dispersions and the flocculation degree of LDH particles. For NaNO₃, the structural strength and flocculation degree first increase with increasing salt concentration and then remain unchanged at high salt concentrations; for Na₂SO₄, the structural strength and flocculation degree continuously increase with increasing salt concentration; for Na₃PO₄, the structural strength and flocculation degree first increase with increasing salt concentration, then slightly decrease, then increases again; for Na₄P₂O₇, the structural strength and flocculation degree first increase with increasing salt concentration, then decrease with further increases of salt concentration. (3) Liquid paraffin-in-water emulsions stabilized by LDH particles in the presence of different types of salt are successfully prepared. Salt type and concentration determine the emulsion stability. For NaNO₃, the emulsion stability first increases with increasing salt concentration and then remains unchanged at high salt concentration; for the other three salts, the emulsion stability first increases and then decreases with increasing salt concentration. (4) TEM images of the emulsion droplets confirm that the emulsions are stabilized by a thick adsorbed particle mulitilayer around the oil droplets. The adsorption of LDH particles at oil/water interface is controlled by two factors: a) the particle-particle (at the interface) and particle-interface electrostatic repulsions and b) the flocculation degree of LDH particles, both of which are determined by the electrokinetic properties of the particles. The stability of emulsions is also controlled by the same two factors: when the flocculation degree of particles is not high (Ï„₀≤1.1 Pa), the high electrostatic repulsions induced by the high zeta potential (positive or negative) impedes particle adsorption at the oil/water interface, causing bad stability of emulsions; salt addition decreases the electrostatic repulsions, promoting particle adsorption at the interface and then improving the emulsion stability. When the flocculation degree of particles is high enough (Ï„₀≥ 1.3 Pa) to effect the adsorption of LDH particles at oil/water interface, salt addition depresses the particle adsorption and then decreases the emulsion stability. (5) High electrostatic repulsions (high zeta potential of the particles) and high flocculation degree of LDH particles (large particle size) are both adverse for preparing stable emulsions. Stable emulsions are only prepared with particles of intermediate flocculation degree.4. Pickering emulsions stabilized by LDH-Laponite composite particlesPickering emulsions stabilized by LDH-Laponite composite particles were investigated here. The following conclusions are drawn: (1) the zeta potential of the composite particles decreases with increasing X_Lap and reverses to negative at X_Lap around 0.15. After that, zeta potential gradually decreases with further increases of X_Lap. (2) The phase state and structural strength of LDH-Laponite aqueous dispersions are both controlled by the electrokinetic properties of the composite particles. The phase state and structural strength of 2 wt% LDH-Laponite dispersions are special because the phase state of 2 wt% Laponite dispersion is gel. (3) The adsorption of the composite particles at flat oil/water interface is controlled by the particle-particle (at the interface) and particle-interface electrostatic repulsions. Addition of Laponite into the pure LDH dispersion or addition of LDH into the pure Laponite dispersion promotes the adsorption of the composite particles by decreasing the electrostatic repulsions. (4) Emulsions stabilized by the composite particles were successfully prepared. The fluorescent microscopic images of emulsions verify that the emulsion stability is gained by the formation of a compact particle layer around the emulsion droplets. Two factors control the emulsion stability: adsorption of the composite particles at oil/water interface and network structure of the LDH-Laponite dispersions. a) At particle concentrations ≤ 1wt%, the particle adsorption determines the emulsion stability. With the increase (0 ～ 0.1) or decrease (1 ～ 0.8) of X_Lap, the electrical potential of the composite particles decreases, promoting the adsorption of composite particles at oil/water interface and improving the emulsion stability. In addition, the enhancement of dispersion structural strength also favors the stability of emulsions; b) at the particle concentration of 2 wt%, the network structure of the mixed dispersions greatly influences the formation and stability of emulsions. At X_Lap= 0.9, emulsions can not be prepared due to the very strong network structure of the dispersions (45 Pa); at X_Lap> 0.9, the network structure of the mixed dispersions slightly decreases (20 30 Pa) and the emulsions can be prepared through emulsification. The emulsion droplet size is large while the emulsions have good stability against creaming and coalescence. This is because that the strong network structure of the mixed dispersions impedes creaming and coalescence of the emulsions.