pH-responsive Pickering Emulsions Research Articles

General destabilization mechanism of pH-responsive Pickering emulsions.

The superior stability of Pickering emulsions can be detrimental in several applications. There is therefore growing interest in using stimuli responsive particle emulsifiers that can considerably reduce the cost of demulsification. Among the possible triggers for the remote control of emulsion stability, pH sensitive systems are advantageous because of the simplicity and ease of implementation. Herein, the destabilization mechanism of pH-switchable emulsions stabilized solely by particles of different shapes and surface properties - namely - hematite, silica and polystyrene - are discussed. The surface activity of all these particles can be tuned by controlling the surface charge density via dispersing them in aqueous solutions of different pH. Hematite particles of cuboidal, spherocylindrical, peanut and ellipsoidal shapes stabilized O/W emulsions at pH 6.5, but they were completely destabilized when the pH of the continuous phase was adjusted to either 2 or 12. Similarly, the O/W emulsions stabilized by silica rods and spheres at pH 2 were destabilized upon adjusting the pH to 6.5 and 12. The detachment of particles from the droplet surfaces resulted in partially covered drops that coalesced, leading to the destabilization of emulsions as confirmed by direct visualization via optical microscopy. The destabilization mechanism of the Pickering emulsions stabilized solely by pH responsive particles was observed to be general, and was mediated by the detachment of particles because of the pH induced wettability change. Furthermore, the pH responsive behaviour was found to be reversible. Since stable emulsions are formed in several particle-water-oil systems, the possibility of conveniently changing the pH of the continuous phase can be exploited to tune the particle wettability and thus the destabilization in applications that demand controlled demulsification.

A pH responsive Pickering emulsion stabilized by fibrous palygorskite particles

Toluene/water Pickering emulsions based on fibrous palygorskite clay particles were stabilized by increasing the solid particle concentration. The emulsion droplet size reached to a plateau at 2wt.%, suggesting the development of a three dimensional network in the emulsion. This was supported by measuring the steady state shear stress-shear rate of emulsions at different concentrations of palygorskite particles. Oscillatory rheological measurements of storage modulus G′ and loss modulus G″ were then performed to verify that the emulsion system is strongly associated at concentrations greater than 2wt.%, resulting in a viscoelastic film at the o/w interface. The role which palygorskite plays in emulsion stability by forming a three dimensional network is directly confirmed by observing the micro-morphology of palygorskite particles in the emulsions using polarizing optical microscope (POM) and transmission electron microscopy (TEM). Without any chemical modification, palygorskite fibers stabilized toluene/water Pickering emulsions over a wide range of pH (pH=1.40 to 11.44). By adding HCl/NaOH, palygorskite clay-stabilized Pickering emulsions were successfully subjected to 5 emulsification–demulsification cycles, and all of the emulsions are stable for over 3months in storage. Palygorskite has been demonstrated as an effective pH responsive emulsifier with long-term stability. The addition of NaCl leads to the aggregation and flocculation of the palygorskite particles, which increases the size of the emulsion droplets; however, stable toluene/water Pickering emulsions based on palygorskite clay can still be formed even at high NaCl concentrations.

Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification

Chitosan without hydrophobic modification is not a good emulsifier itself. However, it has a pH-tunable sol-gel transition due to free amino groups along its backbone. In the present work, a simple reversible Pickering emulsion system based on the pH-tunable sol-gel transition of chitosan was developed. At pH > 6.0, as adjusted by NaOH, chitosan was insoluble in water. Chitosan nanoparticles or micrometer-sized floccular precipitates were formed in situ. These chitosan aggregates could adsorb at the interface of oil and water to stabilize the o/w emulsions, so-called Pickering emulsions. At pH < 6.0, as adjusted by HCl, chitosan was soluble in water. Demulsification happened. Four organic solvents (liquid paraffin, n-hexane, toluene, and dichloromethane) were chosen as the oil phase. Reversible emulsions were formed for all four oils. Chitosan-based Pickering emulsions could undergo five cycles of emulsification-demulsification with only a slight increase in the emulsion droplet size. They also had good long-term stability for more than 2 months. Herein, we give an example of chitosan without any hydrophobic modification to act as an effective emulsifier for various oil-water systems. From the results, we have determined that natural polymers with a stimulus-responsive sol-gel transition should be a good particulate emulsifier. The method for in situ formation of pH-responsive Pickering emulsions based on chitosan will open up a new route to the preparation of a wide range of reversible emulsions.

Chitosan nanoparticles as particular emulsifier for preparation of novel pH-responsive Pickering emulsions and PLGA microcapsules

In this work, we describe a novel and simple method for fabricating biocompatible microcapsules. Chitosan colloidal nanoparticle-coated micrometer-sized poly(lactic-co-glycolic acid) (PLGA) microcapsules were fabricated via a combined system of “Pickering-type” emulsion route and solvent volatilization method in the absence of any molecular surfactants. Stable oil-in-water emulsions were prepared using chitosan colloidal nanoparticles as a particulate emulsifier and a dichloromethane (CH2Cl2) solution of PLGA as an oil phase. Moreover, this stable emulsion present a good pH-responsive characteristic. The uncross-linked chitosan nanoparticles coated PLGA microcapsules were fabricated by the evaporation of CH2Cl2 from the emulsion, and the cross-linked chitosan nanoparticles coated PLGA microcapsules were prepared by cross-linking with glutaraldehyde and evaporation of CH2Cl2. The two types of microcapsules were characterized in terms of size, morphology using scanning electronic microscope (SEM), optical microscope, and so on. These observations confirm the robust nature of these cross-linked microcapsules. Moreover, a possible mechanism for the formation of these microcapsules was proposed. The combined system of Pickering emulsion and solvent volatilization opens up a new route to fabricate a variety of microcapsules.

Alkaline lignin extracted from furfural residues for pH-responsive Pickering emulsions and their recyclable polymerization

Recycling of waste leads to a decrease of environmental pollution and landfill. In this work, we describe an interesting application of furfural residues in fabricating pH-responsive Pickering emulsions and their stable emulsion polymerization. Alkaline lignin extracted from furfural residues is soluble in basic water. However, in acidic conditions, lignin becomes insoluble and particles are formed, which could be used as an effective particular emulsifier for reversible styrene-in-water Pickering emulsions. The emulsions will break when the pH value of the aqueous phase increases. The emulsion system based on a small amount of lignin, 0.05 wt% in water, exhibits good pH-responsivity. Lignin-coated polystyrene (PS) microparticles and pure PS microparticles have been fabricated using the emulsion template. Furthermore, this lignin emulsifier can be recirculated after polymerization. Finally, we propose a cheap and recyclable route for industrial emulsion polymerization. This work opens up new possibilities for agricultural residues as a valuable and recyclable emulsifier in green chemistry.

An Integrated Multifunctional Nanosystem from Command Nanoparticles and Enzymes

A hierarchical nanosytem combines pH-responsive Pickering emulsion and specific enzymatic reactions (see image). Multiple signals received from the surrounding environment and processed by the integrated nanosystem are used for the direct (without an external computer or energy source) regulating and turning on and off of a range of properties of the integrated system (electrical conductivity, transport of ions, water- and oil-soluble molecules)

Doubly pH-Responsive Pickering Emulsion

A pH-responsive Pickering emulsion has been designed on the basis of commercially available alumina-coated silica nanoparticles (Ludox CL silica particles) and potassium hydrogen phthalate (KHP). KHP was found to bind to cationic particle surfaces at pH values between 3.5 and 5.5, enabling the resulting surface-active particles to stabilize emulsions of xylenes in water. Above and below this pH range, the system demulsifies, resulting in a reversible Pickering emulsifier having two pH-controlled, reversible transitions.

Synthesis of bilayer oleic acid-coated Fe 3O 4 nanoparticles and their application in pH-responsive Pickering emulsions

Fe 3O 4 nanoparticles coated with oleic acid bilayer (a diameter about 12 nm) were synthesized. The structure and composition of the particles were analyzed by TEM, FTIR and TGA. The TGA experiments of the bilayer-coated particles show a distinct two-stage mass loss. Partition experiments show that the modified Fe 3O 4 nanoparticles are affected by aqueous dispersion pH and ion strength. Accordingly, the Pickering emulsions stabilized by modified Fe 3O 4 particles are also sensitive to pH and ion strength. The phase inversion of the emulsions occurs when 1.00 < pH < 12.05 , and no phase inversion occurs after complete desorption of the second layer surfactants when pH > 13.50 . The phase inversion of emulsions also can be adjusted by the ion strength. In interfacial adsorption experiments, the hydrophobic Fe 3O 4 nanoparticles form particle clusters, while the hydrophilic particles form uniform multilayers.