Switchable Pickering Emulsions Research Articles

PH-Controllable Redox Responsive Amphiphilic Viologens for Switchable Emulsions.

A pH and redox dual responsive amphiphilic viologen is synthesized, which can be reversibly transformed among the zwitterionic (SVa), monovalent anionic (SV+), and divalent anionic (SVH2+) forms upon pH variation, exhibiting pH-controllable redox responsive properties. Switchable Pickering emulsions with different droplet size and viscosity are prepared by the mixture of hydrophilic silica nanoparticles and the viologens (SV+ or SVH2+) at acidic conditions, while such combination yielded an oil-in-dispersion emulsion at neutral pH value. Not only can rapid reversible demulsification/stabilization of the Pickering emulsions be achieved by redox reactions, but the rate of redox-demulsification can also be controlled by pH trigger. The dual-responsive amphiphilic viologens have potential applications in developing intelligent colloid materials and molecular logic systems.

Switchable CO2-Responsive Janus Nanoparticle for Lipase Catalysis in Pickering Emulsion.

The use of convertible immobilized enzyme carriers is crucial for biphasic catalytic reactions conducted in Pickering emulsions. However, the intense mechanical forces during the conversion process lead to enzyme leakage, affecting the stability of the immobilized enzymes. In this study, a CO2-responsive switchable Janus (CrSJ) nanoparticle (NP) was developed using silica NP, with one side featuring aldehyde groups and the other side adsorbing N,N-dimethyldodecylamine. A switchable Pickering emulsion catalytic system for biphasic interface reactions was prepared by covalently immobilizing lipase onto the CrSJ NPs. The CO2-responsive nature of the CrSJ NPs allowed for rapid conversion of the Pickering emulsion, and covalent immobilization substantially reduced lipase leakage while enhancing the stability of the immobilization during the conversion process. Impressively, after repeated transformations, the Pickering emulsion still maintains its original structure. Following 10 consecutive cycles of esterification and hydrolysis reactions, the immobilized enzyme's activity remains at 77.7 and 79.5% of its initial activity, respectively. The Km of the CrSJ catalytic system showed no significant change compared to the free enzyme, while its Vmax values were 1.2 and 1.6 times that of the free enzyme in esterification and hydrolysis reactions, respectively.

Switchable Pickering Emulsions Stabilized via Synergistic Nanoparticles-Superamphiphiles Effects and Rapid Response to CO2/N2.

A CO2/N2-responsive emulsion provides milder reaction conditions, nontoxicity, and economic feasibility compared to other switchable surfactants. In this study, CO2/N2-responsive pickering emulsions were fabricated by using a compounded dispersion containing SiO2 nanoparticles (NPs) and superamphiphiles as the emulsifying agents. The synergistic effects of the SiO2 NPs and superamphiphiles significantly stabilized the emulsion at all of the tested concentrations and prevented complete phase separation of oil and water. The electrostatic interaction between the SiO2 NPs and superamphiphiles was disrupted after bubbling with CO2 for 30 s, resulting in the breaking of the emulsion. However, the dispersion recovered its interfacial activity after the introduction of N2 and again emulsified the emulsion. This reversible switching behavior was validated through three consecutive cycles of bubbling CO2/N2. The protonation and deprotonation of the SiO2 NPs and superamphiphiles in response to CO2/N2 facilitated reversible assembly and disassembly, which enabled the switching of the emulsions between inactive and active forms. The novel highly stable Pickering emulsions demonstrated rapid demulsification and emulsification in response to CO2/N2 and are promising for a wide range of applications.

CO2 Switchable Pickering Emulsion Stabilized by Responsive Janus SiO2 Nanoparticles for Enhanced Oil Recovery

CO₂ Switchable Pickering Emulsion Stabilized by Responsive Janus SiO₂ Nanoparticles for Enhanced Oil Recovery

A pH switchable Pickering emulsion stabilised by controlled non-conventional lanthanum sulfide nanoparticles, in situ hydrophobized with a cationic surfactant

Microemulsion mediated controlled lanthanide metal based nanoparticles are employed in the preparation of a pH responsive Pickering emulsion.

Fabricating switchable Pickering emulsions by dynamic covalent copolymer amphiphiles

In this work, amphiphilic copolymers D-An based on the dynamic imine bonds were generated in-situ in the aqueous solutions by two water-soluble precursors, the random copolymer poly(N-(2-methacryloylxyethyl)-random-poly(4-(formaldehyde) phenyl methacrylate) (PNMP83-co-PBFMA4, Precursor D) and alkylamines (Precursor An, n represents the carbon number in the alkyl chain that is ranging from 4 to 10). Amphiphiles D-An formed micelles spontaneously in the aqueous solutions, which were studied by the surface tension and cryogen-transmission electron microscopy (cryo-TEM) measurements. Moreover, the CO2/N2 responsive micellization of D-An due to the reversible formation and breaking of the dynamic imine bond (-CH=N-) was confirmed by the nuclear magnetic resonance (NMR) measurements. Accordingly, the emulsification of dynamic covalent amphiphilic copolymers D-An was studied in detail. O/W Pickering emulsions with excellent stability were commonly generated by employing D-An as emulsifiers regardless of n and the nature of oil employed, which was significantly different from the weak emulsification of either precursor D or precursor An. The synergistic emulsification of D-An monomers and micelles was proved to be the major cause as evidenced by the confocal laser scanning microscopy (CLSM) method. Noticeably, the abundant alkylamines were also advantageous in stabilizing emulsions. Attributing to the stimuli-responses of amphiphiles D-An to CO2/N2, the stability of Pickering emulsions could be reversibly controlled by CO2/N2 as well.

A CO2/N2-Responsive Pickering Emulsion Stabilized by Novel Switchable Surface-Active Alumina Nanoparticles

This article reports the development of a novel switchable Pickering emulsion with rapid CO2/N2 responsiveness, which is stabilized using alumina nanoparticles hydrophobized in situ with a trace amount of a switchable superamphiphile via electrostatic interactions. With the introduction of CO2 for 30 s, the Pickering emulsion can be spontaneously demulsified with complete phase separation; the emulsion can then be reconstructed in response to N2 purging for 10 min followed by homogenization. Moreover, the stable Pickering emulsion can be stored for more than 60 days at room temperature without any visible change. The CO2/N2-responsive behavior of the switchable Pickering emulsion is attributed to the reversible desorption/adsorption of the switchable surfactants on the surfaces of the alumina nanoparticles upon the alternative bubbling of CO2 or N2. Thanks to the simple fabrication of the surfactant and the hydrophobization of the alumina nanoparticles, this research has developed an extremely facile and cost-efficient method for preparing a rapidly CO2/N2-responsive switchable Pickering emulsion. The dosage of the switchable surfactants has been significantly reduced by nearly 1500 times (from 150 to 0.1 mmol∙L−1) as compared with the dosage used in previous studies. Moreover, the as-prepared CO2/N2-responsive switchable Pickering emulsion is environmentally friendly, mild, and nontoxic; thus, it holds great potential for practical applications with considerable economic and environmental benefits, such as oil transport, fossil fuel production, environmental gases detection, and the encapsulation and release of active ingredients.

CO2-responsive surfactants for switchable Pickering emulsions with a recyclable aqueous phase

CO2-responsive Pickering emulsions were prepared with a CO2-switchable surfactant (NCOONa) and alumina particles, in which the surfactant and alumina particles could be fully recycled and reused from aqueous phase after demulsification.

Salt-Resistant Switchable Pickering Emulsions Stabilized by Mesoporous Nanosilica Hydrophobized In Situ by pH-Insensitive Surfactants.

Novel oil-in-water (O/W) Pickering emulsions (PEs) were prepared using mesoporous nanosilica in combination with a pH-insensitive cationic surfactant as a stabilizer and show an interesting sensitivity to acids and bases. Adding a suitable amount of NaOH (nNaOH/ncationicsurfactant ≥ 1) led to prompt demulsification within 10 s. Upon further adding HCl solutions (nHCl/nNaOH = 1), stable PEs re-formed after homogenization. These emulsions remained stable for over 30 days after 60 cycles, switching from stable to unstable and back to stable states, and showed a high salt tolerance. A mechanism for the switching of the Pickering emulsion (PE) to unstable and back to stable states was derived and involved anionic and neutral forms of hydroxyl groups at the mesopores of the mesoporous silica nanoparticles (MSNPs). This work reveals a switchable PE system involving a pH-insensitive surfactant, in which the species of oils and cationic surfactants can be arbitrarily selected, a feature that greatly expands the applicability of PEs.

Ambient CO2/N2 Switchable Pickering Emulsion Emulsified by TETA-Functionalized Metal-Organic Frameworks.

In recent years, metal-organic frameworks (MOFs) have been explored as emulsifiers for the fabrication of Pickering emulsions and then used for hybrid material synthesis and interface catalysis. Nevertheless, stimuli-responsive Pickering emulsions stabilized by MOFs have been rarely reported so far, although they are of great importance for fundamental research studies and practical applications. Herein, for the first time, triethylenetetramine (TETA)-functionalized MOFs (ZIF-90/TETA) have been designed, synthesized, and used for fabricating CO2-/N2-response Pickering emulsions. It is shown that even at the ZIF-90/TETA content of 0.25 wt %, the functional MOF can still efficiently emulsify n-hexane and water to form a high internal phase Pickering emulsion. Importantly, the Pickering emulsion can be easily and reversibly switched between emulsification and demulsification by bubbling of CO2 and N2 alternatively at atmospheric pressure. The possible mechanism of the CO2/N2 switchable emulsion is investigated by zeta potential, water contact angle, interfacial tension, 13C NMR spectroscopy, and an optical microscope. It is found that the acid-base reaction of CO2 with TETA anchored on the surface of ZIF-90 leads to the production of hydrophilic ammonium bicarbonate and carbamate, which results in the emulsification of the Pickering emulsion. However, when N2 is bubbled to remove CO2, the reverse reaction takes place to cause the demulsification of the Pickering emulsion. Moreover, the CO2/N2 switchable Pickering emulsion has been successfully used as a microreactor for Knoevenagel reactions to demonstrate a highly efficient integration of chemical reaction, product separation, and ZIF-90/TETA recycling for a sustainable chemical process.

Switchable Pickering emulsions stabilized by polystyrene-modified magnetic nanoparticles

Switchable Pickering emulsions were prepared using polystyrene-modified magnetic nanoparticles (PS-MNPs) as stabilizers in mixtures consisted of a polar solvent, such as chloroform, and water. For this purpose, bare magnetic nanoparticles (bMNPs) were firstly synthesized but they were not able to stabilize emulsions prepared with chloroform. Consequently, the surface of the bMNPs was modified with polystyrene via radical polymerization to provide the nanoparticles more hydrophobic character. This resulted in stable water-in-oil (W/O) and oil-in-water (O/W) Pickering emulsions from water/chloroform mixtures. Variables affecting the coating step, including reaction time and amount of styrene in the polymerization mixture, were studied. The resulting material was characterized in terms of composition and size by infrared spectroscopy (IR-ATR), transmission microscopy (TEM) and dynamic light scattering (DLS). Next, different percentages of aqueous and oil phases were tested obtaining switchable W/O and O/W Pickering emulsions, depending on the percentages of each phase. To demonstrate what type of emulsion was formed with each percentage, the emulsions were observed under the optical microscope and both the drop and the methylene blue tests were performed. Finally, to evaluate the stability and morphology of the Pickering emulsions obtained by the PS-MNPs, the effect of the temperature, pH value and salt was also investigated in detail.

In Situ Measurements of Interactions between Switchable Surface-Active Colloid Particles Using Optical Tweezers.

Switchable surface-active colloid particles are critical to the preparation of switchable Pickering emulsions, which are widely involved in multitudinous fundamental and practical fields, such as biomedical, food products, and spinning cosmetics. The stability of switchable surface-active particles relies on the full understanding of interaction forces between individual colloid particles quantitatively. In this work, a dual-laser optical tweezers instrument was applied to measure the interaction forces between silica particles coated with a common cationic surfactant (cetyltrimethylammonium bromide, CTAB) in water, and all of the measured forces can be well fitted with the theoretical model derived from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. It was revealed that the minimum surface distance to engender the interaction forces between silica particles was closer progressively with the increase of CTAB concentrations, suggesting that the introduction of CTAB molecules in the solution thinned the electric double layer. In addition, the minimum surface distance between surface-inactive silica particles further decreased compared to surface-active states, although the ζ-potential has returned to the initial value of bare silica in pure water when the molecular ratio of 1:1 anionic surfactant (sodium dodecyl sulfate, SDS) was added into the solution to switch the surface-active silica particles to surface-inactive states. Our results provide a considerate methodology for quantifying the interaction forces and investigating the switchable behaviors of CTAB molecules from the adsorption to desorption at the particle-water interfaces, which provide vital foresights into the stabilization mechanism of switchable surface-active colloid particles and the further development of switchable Pickering emulsions.

CO2-Switchable Pickering Emulsion Using Functionalized Silica Nanoparticles Decorated by Amine Oxide-Based Surfactants

Herein, we describe a novel CO2-switchable oil-in-water Pickering emulsion stabilized by functionalized silica nanoparticles with a trace amount of myristylamidopropyl amine oxide (C14PAO), which is commercially available, and readily biodegradable. C14PAO in the current system has been demonstrated to be CO2-responsive. Upon alternately bubbling CO2 and N2 under mild conditions (30 °C, 40 mL min–1), C14PAO is reversibly switched between cationic and nonionic forms, and is thereby adsorbed on or desorbed from the surface of the particles. In this way, interfacially active particles are formed and adsorbed on the surface of oil droplets, stabilizing the emulsion (CO2), or disrupted and desorbed from the surface of oil droplets, breaking the emulsion (N2). As compared to the traditional acid/base cycle, switching the current system with CO2/N2 multiple times does not lead to any evident changes in either macroscopic appearance or microscopic size. Moreover, this CO2-responsive Pickering emulsifier can be re...

A pH-responsive TiO2-based Pickering emulsion system for in situ catalyst recycling

Developing methods for efficient product/catalyst separation and catalyst recycling is meaningful in multi-phase catalytic reactions. Here, we reported a pH-responsive emulsion system stabilized by interfacially active TiO2 nanoparticles for achieving in situ product/catalyst separation and catalyst recycling. In this system, emulsification and demulsification process could be easily engineered through tuning the pH values. The emulsion droplets were destroyed completely at a pH value of 3–4, and the solid catalyst distributed in the aqueous phase could be used to the next reaction cycle after removal of the organic product and adjusting the pH to 7–8. Such a pH triggered switchable Pickering emulsion catalytic system not only shows good recyclability of the solid catalyst but also high catalytic efficiency, and could be recycled more than 10 cycles.

CO2-triggered switchable Pickering emulsion stabilized by guanidine-functionalized silica particles

ABSTRACTThe guanidine group-modified silica particles were used as emulsifier to obtain a CO2-responsive Pickering emulsion. To compare the wettability effect of the particles on the stability of the emulsion, both guanidine and alkyl chain were attached on the surface of silica particles. The influences of tension, particles concentration, oil-water fraction, NaCl concentration, and CO2 on Pickering emulsion properties were investigated. Although the particles did not decrease the surface and interfacial tensions of the air/oil-water interfaces, they attached on the oil–water interfaces and stabilized the emulsions at room temperature for at least 4 weeks. Addition of salt increased the emulsion stability and induced phase inversion at high salt concentration. The stabilization–destabilization cycles of the emulsion could be successively controlled by alternative CO2/heating triggers due to the protonation-deprotonation of guanidine groups on the particle surfaces.

CO2/N2 triggered switchable Pickering emulsions stabilized by alumina nanoparticles in combination with a conventional anionic surfactant

Switchable n-decane-in-water Pickering emulsions were prepared using positively charged alumina nanoparticles in combination with a trace amount of the anionic surfactant sodium dodecyl sulfate (SDS) and equal moles of a CO2/N2 switchable surfactant.

Switchable Pickering Emulsions Stabilized by Awakened TiO2 Nanoparticle Emulsifiers Using UV/Dark Actuation.

In this work, switchable Pickering emulsions that utilize UV/dark manipulation employ a type of smart TiO2 nanoparticle as emulsifiers. The emulsifiers can be awakened when needed via UV-induced degradation of grafted silanes on TiO2 nanoparticles. By tuning the surface wettability of TiO2 nanoparticles in situ via UV/dark actuation, emulsions stabilized by the nanoparticles can be reversibly switched between the water-in-oil (W/O) type and oil-in-water (O/W) type for several cycles. Due to the convertible wettability, the smart nanoparticle emulsifiers can be settled in either the oil phase or the water phase as desired during phase separation, making it convenient for recycling. The present work provides a facile and noninvasive method to freely manipulate the formation, breakage, and switching of the emulsion; this method has promising potential as a powerful technique for use in energy-efficient and environmentally friendly industries.

Switchable Pickering emulsions stabilized by silica nanoparticles hydrophobized in situ with a conventional cationic surfactant.

A stable oil-in-water Pickering emulsion stabilized by negatively charged silica nanoparticles hydrophobized in situ with a trace amount of a conventional cationic surfactant can be rendered unstable on addition of an equimolar amount of an anionic surfactant. The emulsion can be subsequently restabilized by adding a similar trace amount of cationic surfactant along with rehomogenization. This destabilization-stabilization behavior can be cycled many times, demonstrating that the Pickering emulsion is switchable. The trigger is the stronger electrostatic interaction between the oppositely charged ionic surfactants compared with that between the cationic surfactant and the (initially) negatively charged particle surfaces. The cationic surfactant prefers to form ion pairs with the added anionic surfactant and thus desorbs from particle surfaces rendering them surface-inactive. This access to switchable Pickering emulsions is easier than those employing switchable surfactants, polymers, or surface-active particles, avoiding both the complicated synthesis and the stringent switching conditions.

CO2-responsive diethylaminoethyl-modified lignin nanoparticles and their application as surfactants for CO2/N2-switchable Pickering emulsions

CO2/N2-triggered water-dispersible/collectable lignin-g-DEAEMA nanoparticles were successfully synthesized and were used to prepare emulsified/demulsified switchable Pickering emulsions by gas processing under nearly neutral conditions.

Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized In Situ with a Switchable Surfactant

Hin und her: Der CO2/N2-Trigger eines schaltbaren Tensids (neutrales Amidin/kationisches Amidinium) wurde durch In-situ-Hydrophobisierung in Wasser auf mineralische Nanopartikel übertragen. Schaltbare Öl-in-Wasser-Pickering-Emulsionen wurde durch die Verwendung negativ geladener Siliciumdioxid-Nanopartikel und einer geringen Menge des schaltbaren Tensids erhalten. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.