Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. 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.     

Stabilization of carbon dioxide-in-water emulsions with silica nanoparticles

Stable carbon dioxide-in-water emulsions were formed with silica nanoparticles adsorbed at the interface. The emulsion stability and droplet size were characterized with optical microscopy, turbidimetry, and measurements of creaming rates. The increase in the emulsion stability as the silica particle hydrophilicity was decreased from 100% SiOH to 76% SiOH is described in terms of the contact angles and the resulting energies of attachment for the silica particles at the water-CO(2) interface. The emulsion stability also increased with an increase in the particle concentration, CO(2) density, and shear rate. The dominant destabilization mechanism was creaming, whereas flocculation, coalescence, and Ostwald ripening played only a minor role over the CO(2) densities investigated. The ability to stabilize these emulsions with solid particles at CO(2) densities as low as 0.739 g/mL is particularly relevant in practical applications, given the difficulty in stabilizing these emulsions with surfactants, because of the unusually weak solvation of the surfactant tails by CO(2).     

Structure of microparticles in solid-stabilized emulsions

Emulsions of oil and water stabilized by adsorbed solid particles are known as solid-stabilized emulsions (often referred to as Pickering emulsions). Using confocal microscopy, we have studied the assembly of colloidal-sized polystyrene particles in poly(dimethylsiloxane)-in-water solid-stabilized emulsions. Monodisperse polystyrene particles, when included in the emulsions at low concentrations, were found to form small patches with local "hexagonal" order, separated by other particle-free domains. Polystyrene particles with different sizes (1 and 4 microm) and different wettability could simultaneously segregate to the emulsion interface; even mixtures of hydrophobic and hydrophilic solid particles were found to simultaneously segregate to the same interface.     

Pickering emulsions stabilized solely by layered double hydroxides particles: the effect of salt on emulsion formation and stability

The formation and stability of liquid paraffin-in-water emulsions stabilized solely by positively charged plate-like layered double hydroxides (LDHs) particles were described here. The effects of adding salt into LDHs dispersions on particle zeta potential, particle contact angle, particle adsorption at the oil-water interface and the structure strength of dispersions were studied. It was found that the zeta potential of particles gradually decreased with the increase of salt concentration, but the variation of contact angle with salt concentration was very small. The adsorption of particles at the oil-water interface occurred due to the reduction of particle zeta potential. The structural strength of LDHs dispersions was strengthened with the increase of salt and particle concentrations. The effects of particle concentration, salt concentration and oil phase volume fraction on the formation, stability and type of emulsions were investigated and discussed in relation to the adsorption of particles at the oil-water interface and the structural strength of LDHs dispersions. Finally, the possible stabilization mechanisms of emulsions were put forward: the decrease of particle zeta potential leads to particle adsorption at the oil-water interface and the formation of a network of particles at the interface, both of which are crucial for emulsion formation and stability; the structural strength of LDHs dispersions is responsible for emulsion stability, but is not necessary for emulsion formation.     

胶体颗粒稳定的界面及胶体颗粒在界面的相互作用

Particle-stabilized dispersions such as emulsions, foams and bubbles are catching increasing attentions across a number of research areas. The adsorption mechanism and role of these colloidal particles in stabilizing the oil-water or gas-water interfaces and how these particles interact at interfaces are vital to the practical use of these dispersion systems. Although there have been intensive investigations, problems associated with the stabilization mechanisms and particle-particle interactions at interfaces still remain to explore. In this paper, we first systematically review the historical understanding of particle-stabilized emulsions or bubbles and then give an overview of the most important and well-established progress in the understanding of particle-stabilized systems, including emulsions, foams and liquid marbles. The particle-adsorption phenomena have long been realized and been discussed in academic paper for more than one century and a quantitative model was proposed in the early 1980s. The theory can successfully explain the adsorption of solid particles onto interface from energy reduction approaches. The stability of emulsions and foams can be readily correlated to the wettability of the particles towards the two phases. And extensive researches on emulsion stability and various strategies have been developed to prepared dispersion systems with a certain trigger such as pH and temperature. After that, we discuss recent development of the interactions between particles when they are trapped at the interface and highlight open questions in this field. There exists a huge gap between theoretical approaches and experimental results on the interactions of particles adsorbed at interfaces due to demanding experimental devices and skills. In practice, it is customary to use flat surfaces/interfaces as model surfaces to investigate the particle-particle at interfaces although most of the time interfaces are produced with a certain curvature. It is shown that the introduction of particles onto interfaces can generate charges at the interfaces which could possibly account for the long range electrostatic interactions. Finally, we illustrate that particle-stabilized dispersions have been found wide applications in many fields and applications such as microcapsules, food, biomedical carriers, and dry water. One of the most investigated areas is the microencapsulation of actives based on Pickering emulsion templates. The particles adsorbed at the interface can serve as interfacial stabilizers as well as constituting components of shells of colloidal microcapsules. Emulsions stabilized by solid particles derived from natural and bio-related sources are promising platforms to be applied in food related industries. Emulsion systems stabilized by solid particles of the w/w (water-in-water) feature are discussed. This special type of emulsion is attracting increasing attentions due to their all water features. Besides of oil-water interface, particle stabilized air-water interface share similar stabilization mechanism and several applications reported in the literature are subsequently discussed. We hope that this paper can encourage more scientists to engage in the studies of particle-stabilized interfaces and more novel applications can be proposed based on this mechanism     

Effect of oil soluble surfactant in emulsions stabilised by clay particles

Although surfactants and particles are often mixed together in emulsions, the contribution of each species to the stabilisation of the oil-water interface is poorly understood. We report the results of investigations into the formation of emulsions from solutions of surfactant in oil and aqueous suspensions of laponite. Depending on the salt concentration in the aqueous suspensions, the laponite dispersed as individual disc-shaped particles, 30 nm in diameter, or flocculated into aggregates tens of micrometres in diameter. At the concentrations studied, the flocculated particles alone stabilized oil-in-water emulsions. Synergistic interactions between the particles and octadecylamine at the oil-water interface reduced the average emulsion drop size, while antagonistic interactions with octadecanoic acid enhanced coalescence processes in the emulsions. The state of particle dispersion had dramatic effects on the emulsions formed. Measurements of the oil-water interfacial tension revealed the origins of the interactions between the surfactants and particles.     

Some Emulsion Features

Traditionally, emulsions have been defined as consisting of two liquids, of which one is dispersed in the other as macroscopic drops, stabilized by mono‐molecular layer of surfactant at the interface. However, a large majority of commercial emulsions are more complex than so and the added elements are essential for the properties of the emulsions including their stability.

With this in mind, this treatment of emulsions is divided into emulsions with mono‐layers and multilayers at the interface. In addition, additional elements in emulsions are described; such as lyotropic liquid crystals, vesicles, microemulsion droplets and solid particles, and their potential influence on the emulsion properties is indicated.     

Effect of silica particles on stability of highly concentrated water-in-oil emulsions with non-ionic surfactant

Water-in-oil, high internal phase emulsion made of super-cooled aqueous solution containing a mixture of inorganic salts and stabilized with non-ionic surfactant (sorbitan monooleate) alone was investigated. It was not possible to produce a highly concentrated emulsion (with aqueous phase fraction = 94 wt %), stabilized with surface-treated silica, solely: we were able to form an emulsion with a maximal aqueous phase mass fraction of 85 wt % (emulsion inverts/breaks above this concentration). The inversion point is dependent on the silica particle concentration, presence of salt in the aqueous phase, and does not depend on the pH of the dispersed phase. All emulsions stabilized by the nanoparticles solely were unstable to shear. So, the rheological properties and stability of the emulsions containing super-cooled dispersed phase, with regards to crystallization, were determined for an emulsion stabilized by non-ionic surfactant only. The results were compared to the properties obtained for emulsions stabilized by surface treated (relatively hydrophobic) silica nanoparticles as a co-surfactant to sorbitan monooleate. The influence of the particle concentration, type of silica surface treatment, particle/surfactant ratio on emulsification and emulsion rheological properties was studied. The presence of the particles as a co-stabilizer increases the stability of all emulsions. Also, it was found that the particle/surfactant ratio is important since the most stable emulsions are those where particles dominate over the surfactant, when the surfactant’s role is to create bridging flocculation of the particles. The combination of the two types of hydrophobic silica particles as co-surfactants is: one that resides at the water/oil interface and provides a steric boundary and another that remains in the oil phase creating a 3D-network throughout the oil phase, which is even more beneficiary in terms of the emulsion stability.     

Double inversion of emulsions induced by salt concentration

The effects of salt on emulsions containing sorbitan oleate (Span 80) and Laponite particles were investigated. Surprisingly, a novel double phase inversion was induced by simply changing the salt concentration. At fixed concentration of Laponite particles in the aqueous phase and surfactant in paraffin oil, emulsions are oil in water (o/w) when the concentration of NaCl is lower than 5 mM. Emulsions of water in oil (w/o) are obtained when the NaCl concentration is between 5 and 20 mM. Then the emulsions invert to o/w when the salt concentration is higher than 50 mM. In this process, different emulsifiers dominate the composition of the interfacial layer, and the emulsion type is correspondingly controlled. When the salt concentration is low in the aqueous dispersion of Laponite, the particles are discrete and can move to the interface freely. Therefore, the emulsions are stabilized by particles and surfactant, and the type is o/w as particles are in domination. At intermediate salt concentrations, the aqueous dispersions of Laponite are gel-like, the viscosity is high, and the transition of the particles from the aqueous phase to the interface is inhibited. The emulsions are stabilized mainly by lipophilic surfactant, and w/o emulsions are obtained. For high salt concentration, flocculation occurs and the viscosity of the dispersion is reduced; thus, the adsorption of particles is promoted and the type of emulsions inverts to o/w. Laser-induced fluorescent confocal micrographs and cryo transmission electron microscopy clearly confirm the adsorption of Laponite particles on the surface of o/w emulsion droplets, whereas the accumulation of particles at the w/o emulsion droplet surfaces was not observed. This mechanism is also supported by the results of rheology and interfacial tension measurements.     

Synergetic Effect of Reactive Surfactants and Clay Particles on Stabilization of Nonaqueous Oil-in-Oil (o/o) Emulsions

Although surfactants and particles are often used together in stabilization of aqueous emulsions, the contribution of each species to such stabilization at the oil-water interface is poorly understood. The situation becomes more complicated if we consider the nonaqueous oil-oil interface, i.e, the stabilization of nonaqueous oil-in-oil (o/o) emulsions by solid particles and reactive surfactants which, to our knowledge, has not been studied before. We have prepared Pickering nonaqueous simple (o/o) emulsions stabilized by a combination of kaolinite particles and a nonionic polymerizable surfactant Noigen RN10 (polyoxyethylene alkylphenyl ether). Different pairs of immiscible oils were used which gave different emulsion stabilities. Using kaolinite with equal volumes of paraffin oil/formamide system gave no stable emulsions at all concentrations while the addition of Noigen RN10 enhanced the emulsion stability. In contrast, addition of Noigen RN10 surfactant to silicon oil-in-glycerin emulsions stabilized by kaolinite resulted in destabilization of the system at all concentrations. For all systems studied here, no phase inversion in simple emulsion was observed by altering the volume fraction of the dispersed phase as compared to the known water-based simple Pickering emulsions.      

A technique to estimate the apparent surface pressure of emulsion particles using apolipoproteins as probes

The apparent pressures in the surface monolayer of emulsion particles can be estimated by comparing the absorption of an apolipoprotein to planar lipid monolayers and to emulsions. Lipids are spread at an air-water interface in a Pockels/Langmuir surface balance and the adsorption of [¹⁴C]-labeled apolipoproteins placed in the subphase is studied as a function of surface pressure using the surface radioactivity method. An apoprotein surface concentration/initial lipid surface pressure curve (Γ/gp_i) is constructed. The maximum apolipoprotein surface concentration Γ_e of emulsions is derived from standard emulsion/apolipoprotein binding isotherms. The apparent emulsion surface pressure is then estimated by comparing Γ_e to the Γ/π_i curve. Apolipoprotein A-I has been used as an example of a probe to estimate the effective surface pressure in ~1000 Å diameter egg yolk phosphatidylcholine/cholesterol/triolein emulsion particles. When the cholesterol content of emulsions is low, the surface pressure of the emulsion is about 17 dyne cm⁻¹. At high cholesterol concentrations (0.49 cholesterol/phospholipid mole ratio) the surface pressure is increased to 25 dyne cm⁻¹. The addition of the maximum amounts of apoA-I to these particles raises the effective surface pressure of the emulsion to about 30 dyne cm⁻¹ and stabilizes the particles.     

Water-in-trichloroethylene emulsions stabilized by uniform carbon microspheres

Uniform hard carbon spheres (HCS), synthesized by the hydrothermal decomposition of sucrose followed by pyrolysis, are effective at stabilizing water-in-trichloroethylene (TCE) emulsions. The irreversible adsorption of carbon particles at the TCE-water interface resulting in the formation of a monolayer around the water droplet in the emulsion phase is identified as the key reason for emulsion stability. Cryogenic scanning electron microscopy was used to image the assembly of carbon particles clearly at the TCE-water interface and the formation of bilayers in regions of droplet-droplet contact. The results of this study have potential implications to the subsurface injection of carbon submicrometer particles containing zero-valent iron nanoparticles to treat pools of chlorinated hydrocarbons that are sequestered in fractured bedrock.     

Water-in-carbon dioxide emulsions stabilized with hydrophobic silica particles

W/C emulsions were stabilized using hydrophobic silica particles adsorbed at the interface, resulting in average droplet diameters as low as 7.5 microm. A porous cross-linked shell was formed about a hydrophilic (colloidal and fumed) silica core with a trifunctional silylating agent, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)triethyoxysilane, to render the particles CO(2)-philic. The stability of emulsions comprising equal weights of CO(2) and water was assessed with visual observations of settling fronts and the degree of emulsion coalescence, and the average drop size was measured by optical microscopy. The effect of CO(2) density on both emulsion stability and droplet size was determined quantitatively. The major destabilizing mechanism of the emulsions was settling, whereas Ostwald ripening and coalescence were not visible at any density, even over 7 days. Flocculation of the settling droplets did not occur, although gelation of the emulsions through particle interactions resulted after longer periods of time. CO(2)-philic particles offer a new route to highly stable W/C emulsions, with particle energies of attachment on the order of 10(6)kT, even at CO(2) densities as low as 0.78 g ml(-1). At these low densities, surfactants rarely stabilize emulsions as the result of poor surfactant tail solvation.     

Synergistic stabilization of emulsions by a mixture of surface-active nanoparticles and surfactant

The stability and rheology of tricaprylin oil-in-water emulsions containing a mixture of surface-active hydrophilic silica nanoparticles and pure nonionic surfactant molecules are reported and compared with those of emulsions stabilized by each emulsifier alone. The importance of the preparation protocol is highlighted. Addition of particles to a surfactant-stabilized emulsion results in the appearance of a small population of large drops due to coalescence, possibly by bridging of adsorbed particles. Addition of surfactant to a particle-stabilized emulsion surprisingly led to increased coalescence too, although the resistance to creaming increased mainly due to an increase in viscosity. Simultaneous emulsification of particles and surfactant led to synergistic stabilization at intermediate concentrations of surfactant; emulsions completely stable to both creaming and coalescence exist at low overall emulsifier concentration. Using the adsorption isotherm of surfactant on particles and the viscosity and optical density of aqueous particle dispersions, we show that the most stable emulsions are formed from dispersions of flocculated, partially hydrophobic particles. From equilibrium contact angle and oil-water interfacial tension measurements, the calculated free energy of adsorption E of a silica particle to the oil-water interface passes through a maximum with respect to surfactant concentration, in line with the emulsion stability optimum. This results from a competition between the influence of particle hydrophobicity and interfacial tension on the magnitude of E.     

Silicone-protein surfactants; stability of water-in-silicone oil emulsions

Small scale water-in-silicone oil emulsions were readily prepared using high speed mixers. Two surfactant systems were studied: a comb-type silicone-polyether surfactant, and a surfactant system employing a mixture of the surface active protein human serum albumin (HSA, in the internal phase) and an alkoxysilane-modified silicone TES-PDMS in the silicone oil (continuous phase). Little difference in particle sizes was noted between the two surfactant types for a given mixing protocol, but dual-blade turbulent mixing led to relatively monodisperse particles of approximately 2–5 m in diameter while high speed Dremel mixers led to bimodal particle distributions. Prior to spontaneous demulsification of the latter emulsions stabilized by HSA/TES-PDMS (the 3225C emulsions remain stable), they proved very difficult to break. The addition of dibutyltin dilaurate to the HSA/TES-PDMS-stabilized emulsions led to catastrophic collapse of the emulsion and formation of a silicone elastomer at the bulk water/oil interface. This makes unlikely the possibility that silicone elastomers, formed by protein-catalyzed crosslinking of the alkoxysilane in albumin/TES-PDMS-stabilized emulsions, are involved in stabilizing the emulsion. The nature of the stabilization of the interface is discussed.     

Pickering emulsions with controllable stability

We prepare solid-stabilized emulsions using paramagnetic particles at an oil/water interface that can undergo macroscopic phase separation upon application of an external magnetic field. A critical field strength is found for which emulsion droplets begin to translate into the continuous-phase fluid. At higher fields, the emulsions destabilize, leading to a fully phase-separated system. This effect is reversible, and long-term stability can be recovered by remixing the components with mechanical agitation.     

Influence of microgel architecture and oil polarity on stabilization of emulsions by stimuli-sensitive core-shell poly(N-isopropylacrylamide-co-methacrylic acid) microgels: Mickering versus Pickering behavior?

Charged poly(N-isopropylacrylamide-co-methacrylic acid) [P(NiPAM-co-MAA)] microgels can stabilize thermo- and pH-sensitive emulsions. By placing charged units at different locations in the microgels and comparing the emulsion properties, we demonstrate that their behaviors as emulsion stabilizers are very different from molecular surfactants and rigid Pickering stabilizers. The results show that the stabilization of the emulsions is independent of electrostatic repulsion although the presence and location of charges are relevant. Apparently, the charges facilitate emulsion stabilization via the extent of swelling and deformability of the microgels. The stabilization of these emulsions is linked to the swelling and structure of the microgels at the oil-water interface, which depends not only on the presence of charged moieties and on solvent polarity but also on the microgel (core-shell) morphology. Therefore, the internal soft and porous structure of microgels is important, and these features make microgel-stabilized emulsions characteristically different from classical, rigid-particle-stabilized Pickering emulsions, the stability of which depends on the surface properties of the particles.     

A novel formulation of the pickering emulsion stabilized with silica nanoparticles and its thermal resistance at high temperatures

Stabilization of emulsions with solid particles can be used in several fields of oil and gas industry because of their higher stability. Solid particles should be amphiphilic to be able to make Pickering emulsions. This goal is achieved by using surfactants at low concentrations. Oil-in-water (o/w) emulsions are usually stabilized by surfactant but show poor thermal stability. This problem limits their applications at high-temperature conditions. In this study, a novel formulation for o/w stabilized emulsion by using silica nanoparticles and the nonionic surfactant is investigated for the formulation of thermally stable Pickering emulsion. The experiments performed on this Pickering emulsion formula showed higher thermal stability than conventional emulsions. The optimum wettability was found for DME surfactant and silica nanoparticles, consequently, in that region; Pickering emulsion showed the highest stability. Rheological changes were evaluated versus variation in surfactant concentration, silica concentration and pH. Scanning electron microscopy images approved the existence of a rigid layer of nanoparticle at the oil-water interface. Finally, the results show this type of emulsion remains stable in harsh conditions and allows the system to reach its optimum rheology without adding any further additives.     

Interfacial layers of stimuli-responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) microgels characterized by interfacial rheology and compression isotherms

Stimuli-sensitive emulsions stabilized by microgel particles consisting of poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) and being responsive to both pH and temperature have been investigated with respect to the visco-elastic properties of the interfacial layer. Properties of the interfacial layer were probed by means of shear and dilatational rheology as well as by compression isotherms and are related to the microgel packing at the interface as visualized by cryogenic scanning electron microscopy. The corresponding pH dependent emulsion stability is strongly correlated with the visco-elastic properties of the microgel covered oil-water interface. At high pH when the microgels are charged, a structure of partially interconnected microgels is found that provides elastic, soft gel-like interfaces. At low pH, however, the uncharged microgels are densely packed and the interface is rather brittle. Obviously, these pH dependent visco-elastic properties of the microgel layer at the oil-water interface play a determining role in the stability of emulsion droplets and allow us to prepare very stable emulsions when the microgels are charged and to break the emulsion by changing the pH.     

Emulsions stabilised by food colloid particles: role of particle adsorption and wettability at the liquid interface

We study the effect of the particle wettability on the preferred type of emulsion stabilised solely by food colloid particles. We present results obtained with the recently developed gel trapping technique (GTT) for characterisation of wettability and surface structuring of individual food colloid particles adsorbed at air-water and oil-water interfaces. This method allows us to replicate a particle monolayer onto the surface of polydimethylsiloxane (PDMS) without altering the position of the particles. By observing the polymer surface with scanning electron microscopy (SEM), we are able to determine the contact angle of the individual particles at the initial liquid interface. We demonstrate that the GTT can be applied to fat crystal particles, calcium carbonate particles coated with stearic acid and spray-dried soy protein/calcium phosphate particles at air-water and oil-water interfaces. Subsequently, we prepare emulsions of decane and water stabilised by the same food colloid particles and correlate the wettability data obtained for these particles to the preferred type of emulsions they stabilise.