Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant

Temperature-Insensitive Nonpolar Suspensions of Polyoxyethylene Alkyl Ether-Grafted Silica Nanoparticles

Nonpolar suspensions of organically modified particles exhibit a strong temperature sensitivity owing to the high-temperature-induced desorption/decomposition and the low-temperature-induced disorder/order conformational transition of the modifiers. This strong temperature sensitivity limits their applications, such as lubricants and oil-based drilling fluids, which require the suspensions to operate over a wide temperature range (e.g., 0-200 °C). We hypothesize that the introduction of a flexible ethylene oxide (EO) chain into the modifiers can disrupt the low-temperature-induced ordered conformation to improve the stability of the nonpolar suspensions. In this article, nonpolar suspensions with temperature insensitivity in the range of 5-160 °C were obtained via the covalent modification of silica NPs and the introduction of EO chains into the modifier molecules. Here, octadecyl-grafted silica NPs (C18-SiO2) and polyoxyethylene alkyl ether-grafted silica NPs (AEOn-SiO2) were synthesized and subsequently dispersed in mineral oil. The rheological properties of each suspension at different temperatures were evaluated, and the thermal stability of AEOn-SiO2 in mineral oil was investigated along with the conformational changes of the grafted chains. In the temperature range of 5-160 °C, the apparent viscosity and gel strength of the C18-SiO2 suspension changed dramatically, whereas the AEOn-SiO2 suspensions exhibited constant rheological properties over this temperature range. This temperature insensitivity of AEOn-SiO2 suspensions is attributed to the excellent thermal stability of AEOn-SiO2 in mineral oil and the disordered conformation of the EO chains upon cooling. This study provides a novel approach to preparing temperature-insensitive nonpolar suspensions, which have potential applications in the petroleum and lubricant industries.

Coal-Based Branched Vicinal Diol Ethoxylates Versus Guerbet Alcohol Ethoxylates: Role of Tertiary Hydroxyl Groups

Branched surfactants exhibit a lower surface tension, excellent low defoaming performance, and better wetting ability compared with linear surfactants, making them promising for applications in industrial cleaning. In this study, 2-hexyl-1-decene (C8 olefin dimer), obtained from the dimerization of 1-octene, was used as the hydrophobe to synthesize branched nonionic surfactants via hydroxylation and ethoxylation. The hydroxylation of the C8 olefin dimer to synthesize 2-hexyldecane-1,2-diol (C8 BD) using H2O2 and HCOOH was investigated systematically. Under the optimal reaction conditions (H2O2/C8 olefin dimer molar ratio: 1.5, HCOOH/C8 olefin dimer molar ratio: 4.0, reaction time: 10 h, reaction temperature: 50 °C), the conversion of the C8 olefin dimer and selectivity toward C8 BD were found to reach 99.96 and 79.89%, respectively. Further, branched nonionic surfactants (C8 BDEn) were synthesized via ethoxylation of C8 BD with ethylene oxide and characterized using FTIR, LCMS, 1H NMR, and 13C NMR techniques. The presence of a tertiary hydroxyl group in C8 BD increases the reactivity of the primary hydroxyl group, leading to a narrower range of products and lower residual substrate content. The physicochemical properties and surface properties of C8 BDEn with different degrees of ethoxylation at various concentrations were investigated and compared with those of commercially available Guerbet alcohol polyoxyethylene ethers (C8 GAEO9 and C6 GAEO9). The results show that, compared with C8 GAEO9 and C6 GAEO9, C8 BDE6 displayed a higher surface activity with a lower equilibrium surface tension (27.14 mN·m-1), superior wettability with a smaller contact angle (39.2°), better emulsification performance with a longer emulsification time of 548 s, and excellent foaming properties (initial foam volume of 11.6 mL). This strategy of utilizing coal-based α-olefins for the synthesis of branched nonionic surfactants presents a route to prepare value-added fine chemicals from coal-based resources.

pH/Ion Dual-Responsive Emulsion Via a Cationic Surfactant and Positively Charged Magnesium Hydroxide Nanosheets

Emulsions, formed by dispersing a liquid into another immiscible one by virtue of emulsifiers, have been widely applied in commercial applications like foods, pharmaceuticals, cosmetics, and personal care, which always confront environmental and/or toxic questions due to emulsifiers' high dosage. Recently, a study on Pickering emulsions points out a solution to stable emulsions based on the costabilizing effect of colloidal particles, which focused on surface-active particles cooperating with oppositely charged ionic surfactants. Costabilized emulsions adopting a charge-similar ionic surfactant and particles were less studied. In this article, a hexane-in-water emulsion was prepared in use of a cationic surfactant cetyltrimethylammonium bromide (CTAB) with positively charged magnesium hydroxide (MH) nanosheets at low concentrations (10-5 M and 10-2 wt %, respectively). The emulsion is stable due to the synergy by CTAB and MH nanosheets, which functions in virtue of the electric repulsion by similarly charged particles, the mechanical shielding by MH nanosheets, and restrained water drainage in lamellae between droplets due to the gelation of MH nanosheets. Moreover, the emulsion is doubly switchable within emulsification/demulsification via convenient pH or ion manipulation, a mechanism based on the breakdown and rebuilding of the costabilizing synergy. Such dual-responsive emulsions show high potential for the delicate control of drug delivery, release, and biphasic biocatalysis applications.

Pluronic Induced Interparticle Attraction and Re-entrant Liquid-Liquid Phase Separation in Charged Silica Nanoparticle Suspensions

Tuning surface properties of nanoparticles by introducing charge, surface functionalization, or polymer grafting is central to their stability and applications. Here, we show that introducing non-DLVO forces like steric and hydrophobic effects in charged silica nanoparticle suspensions through interaction with a nonionic surfactant brings about interesting modulations in their interparticle interaction and phase behavior. The Ludox TM-40 negatively charged silica suspensions thus exhibit liquid-liquid phase separation driven by the onset of interparticle attraction in the system in the presence of the triblock copolymer Pluronic P123. The observed phase separations are thermoresponsive in nature, as they are associated with lower consolute temperatures and a re-entrant behavior as a function of temperature. The nanoparticle-Pluronic system thus undergoes transformation from one-phase to two-phase and then back to one-phase with monotonic increase in temperature. Evolution of the interparticle interaction in the composite system is investigated by dynamic light scattering (DLS), small angle neutron scattering (SANS), zeta potential, rheological, and fluorescence spectroscopy studies. Zeta potential studies show that the charge interaction in the system is partially mitigated through adsorption of a Pluronic micellar layer on the nanoparticle surfaces. Contrast-matching SANS studies suggest that hydrophobic interactions between the adsorbed micellar layer bring about the onset of interparticle attraction in the system. The results are unique and not reported hitherto in charged silica nanoparticle systems.

Recyclable Nonionic-Anionic Bola Surfactant as a Stabilizer of Size-Controllable and pH-Responsive Pickering Emulsions

A novel nonionic-anionic Bola surfactant (abbreviated as CH₃O(EO)₇-R₁₁-COOH) was designed and synthesized by condensation of methyl polyoxyethylene (7) ether with 12-bromododecanoic acid. In neutral aqueous solution, the surfactant behaves as a nonionic one and can stabilize oil-in-water (O/W) conventional emulsions alone and costabilize O/W Pickering emulsions with positively charged alumina nanoparticles with n-decane as the oil. In alkaline solution, the carboxylic acid group is deprotonated, becoming anionic and the surfactant is converted to Bola form, which is an inferior emulsifier and does not adsorb on particle surfaces, resulting in demulsification of both kinds of emulsions. With strong hydrophilicity, both the Bola surfactant and the bare particles return to the aqueous phase after demulsification, which is therefore recyclable and reusable in accordance with sustainable chemistry and engineering. In acidic media between pH 3 and 6, the ethyleneoxy groups tend to desorb from particle surfaces, slightly reducing the hydrophobicity of the particles. However, Pickering emulsions are still stable but their droplet size increases on lowering the pH. The Pickering emulsions are therefore pH-responsive and size-controllable. This newly designed Bola surfactant is effective in preparing smart emulsions, which are extensively applied in heterogeneous catalysis, oil product transportation, emulsion polymerization, and new material preparation.

Hydrophilic Silica Nanoparticles in O/W Emulsion: Insights from Molecular Dynamics Simulation

Previous studies have been carried out on the effect of silica nanoparticles (SNPs) on the stability of oil-water emulsions. However, the combining configuration of SNPs and oil droplets at the molecular level and the effect of SNP content on the coalescence behavior of oil droplets cannot be obtained through experiments. In this paper, molecular dynamics (MD) simulation was performed to investigate the adsorption configuration of hydrophilic SNPs in an O/W emulsion system, and the effect of adsorption of SNPs on coalescence of oil droplets. The simulation results showed: (i) SNPs adsorbed on the surface of oil droplets, and excessive SNPs self-aggregated and connected by hydrogen bonds. (ii) Partially hydrophilic asphaltene and resin molecules formed adsorption configurations with SNPs, which changed the distribution of oil droplet components. Furthermore, compared with hydrophobic asphaltene, the hydrophilic asphaltene was easier to combine with SNPs. (iii) SNPs would extend the oil droplet coalescence time, and the π-π stacking structures were formed between asphaltene and asphaltene or resin molecules to enhance the connection between oil droplets during the oil droplet contact process. (iv) Enough SNPs tightly wrapped around the oil droplet, similar to the formation of a rigid film on the surface of an oil droplet, which hindered the contact and coalescence of components between oil droplets.

Pickering Emulsions Synergistically Stabilized by Aliphatic Primary Amines and Silica Nanoparticles

Innovation in emulsion compositions is necessary to enrich emulsion formulations and applications. Herein, Pickering emulsions were prepared using silica nanoparticles and aliphatic primary amines with an oil-water ratio of 1:1 (v/v). Contact angle experiments revealed that the in situ hydrophobization of nanoparticles was caused by the surface adsorption of amine molecules. Notably, the interactions between amine compounds and the surface of silica nanoparticles were electrostatic attractions and mutual hydrogen bonding. The existence of hydrogen bonds was further confirmed by demulsification experiments using a chaotropic agent DMF and increasing temperatures. The hydrophobicity of silica nanoparticles can be effectively improved using most commercially available aliphatic primary amines such as n-hexylamine, n-octylamine, n-decylamine, dodecylamine, and tetradecylamine. The minimum concentrations of the aforementioned amines necessary for stabilizing the emulsions with 0.3 wt % silica nanoparticles are 3, 0.6, 0.3, 0.06, and 0.03 mM, respectively, decreasing significantly with increasing alkyl chain length. With the increase of the amine concentrations, the hydrophobicity of silica particles monotonically increased and finally resulted in the inversion of emulsions. The amine concentrations for emulsion phase inversion were 150, 40, 30, 20, and 20 mM, respectively, in the presence of 0.3 wt % silica nanoparticles. In this work, silica nanoparticles were hydrophobized using aliphatic primary amines. The composite stabilizers developed are useful for developing novel stimuli-responsive Pickering emulsions, while the synergistic effects introduced herein are also helpful in expanding the hydrophobization methods available for nanoparticles.

Adsorption of sterically-stabilized diblock copolymer nanoparticles at the oil-water interface: effect of charged end-groups on interfacial rheology

The RAFT aqueous emulsion polymerization of either methyl methacrylate (MMA) or benzyl methacrylate (BzMA) is conducted at 70 °C using poly(glycerol monomethacrylate) (PGMA) as a water-soluble precursor to produce sterically-stabilized diblock copolymer nanoparticles of approximately 30 nm diameter. Carboxylic acid- or morpholine-functional RAFT agents are employed to confer anionic or cationic functionality at the ends of the PGMA stabilizer chains, with a neutral RAFT agent being used as a control. Thus the electrophoretic footprint of such minimally-charged model nanoparticles can be adjusted simply by varying the solution pH. Giant (mm-sized) aqueous droplets containing such nanoparticles are then grown within a continuous phase of n-dodecane and a series of interfacial rheology measurements are conducted. The interfacial tension between the aqueous phase and n-dodecane is strongly dependent on the charge of the terminal group on the stabilizer chains. More specifically, neutral nanoparticles produce a significantly lower interfacial tension than either cationic or anionic nanoparticles. Moreover, adsorption of neutral nanoparticles at the n-dodecane-water interface produces higher interfacial elastic moduli than that observed for charged nanoparticles. This is because neutral nanoparticles can adsorb at much higher surface packing densities owing to the absence of electrostatic repulsive forces in this case.

Molecular structure-tuned stability and switchability of CO2-responsive oil-in-water emulsions

HYPOTHESIS

Pseudo-Gemini surfactants (PGS) possessing switchable and recyclable features have drawn increasing attention on generating high-performance CO₂-responsive emulsions for wide range and versatile applications. However, there is a lack of fundamental understanding on how the molecular structure of PGS affects the stability and switchability of emulsions. We hypothesize that the length and type of the spacer in PGS play a decisive role in controlling interfacial and switching properties.

EXPERIMENTS

Two series of PGS with different spacers were prepared through electrostatic association between amines and oleic acid. The interfacial activity and CO₂-responsive properties of corresponding emulsions were systematically investigated by well-designed experiments and molecular dynamics simulations.

FINDINGS

Increasing the spacer length to allow the bent configuration leads to more tight arrangement of oleic molecules, consequently improving the interfacial activity. In addition, the introduction of amino group into the spacer dramatically promotes CO₂ response of resulting PGS due to ehanced migration of the spacer from the interface to the aqueous phase after CO₂ addition. These results are inspiring in designing controllable CO₂-responsive emulsions for a wide range of industrial applications (e.g., enhanced oil recovery and oil-contaminated soil remediation).

Recent Progress toward Physical Stimuli-Responsive Emulsions

Emulsion as a fine dispersion of immiscible liquids has involved widespread applications in industry, pharmaceuticals, agriculture and personal care. Stimuli-responsive emulsions capable of on-demand demulsification or changing their properties are required in many cases such as controllable release cargo, oil recovery, emulsifiers recycle and product separation, great progress has been achieved in these areas. Among these various triggers, much effort has been made to develop physical stimuli, due to the noninvasive and environmentally friendly characteristics. Physical stimuli-responsive emulsions provide a plenty of valuable practical applications in the fields of sustainable industry, biomedical reaction, drug delivery. Here, we summarize the recent development in the field of emulsions in response to physical stimuli consisting of temperature, light, magnetic field, electrical field, etc. The preparation methods and mechanisms of physical stimuli-responsive emulsions and their applications of catalysis reaction, drug delivery, and oil recovery are highlighted in this review. The future directions and outstanding problems of the physical stimuli-responsive emulsions are also discussed. This article is protected by copyright. All rights reserved.

Environmentally Responsive Emulsions of Thermotropic Liquid Crystals with Exceptional Long-Term Stability and Enhanced Sensitivity to Aqueous Amphiphiles

We report colloidally stable emulsions of thermotropic liquid crystals (LCs) that can detect the presence of amphiphilic analytes in aqueous environments. Our approach makes use of a Pickering stabilization strategy consisting of surfactant-nanoparticle complexes (SiO₂/C_nTAB, n = 8, 12, 16) that adsorb to aqueous/LC droplet interfaces. This strategy can stabilize LC emulsions against coalescence for at least 3 months. These stabilized LC emulsions also retain the ability to respond to the presence of model anionic, cationic, and nonionic amphiphiles (e.g., SDS, C₁₂TAB, C₁₂E₄) in aqueous solutions by undergoing "bipolar-to-radial" changes in LC droplet configurations that can be readily observed and quantified using polarized light microscopy. Our results reveal these ordering transitions to depend upon the length of the hydrocarbon tail of the C_nTAB surfactant used to form the stabilizing complexes. In general, increasing C_nTAB surfactant tail length leads to droplets that respond at lower analyte concentrations, demonstrating that this Pickering stabilization strategy can be used to tune the sensitivities of the stabilized LC droplets. Finally, we demonstrate that these colloidally stable LC droplets can report the presence of rhamnolipid, a biosurfactant produced by the bacterial pathogen Pseudomonas aeruginosa. Overall, our results demonstrate that this Pickering stabilization strategy provides a useful tool for the design of LC droplet-based sensors with substantially improved colloidal stability and new strategies to tune their sensitivities. These advances could increase the potential practical utility of these responsive soft materials as platforms for the detection and reporting of chemical and biological analytes.

Metal ion-triggered Pickering emulsions and foams for efficient metal ion extraction

Emulsions and foams were constructed by using surfactant particles as stabilizers. Bis(2-ethylhexyl) phosphate, abbreviated as HDEHP, was used as both an oil in neutral form and an anionic surfactant in deprotonated form, DEHP^-. In the system of HDEHP/H₂O, upon adding NaOH, a portion of HDEHP was deprotonated to form DEHP^- as stabilizers of O/W emulsions. After introducing some certain metal ions, the O/W emulsions were transformed to W/O Pickering emulsions due to the generation of insoluble particles by DEHP^- and metal ions. In addition, DEHP^- could also combine with some metal ions to produce particles absorbed at air/water interface, forming ultrastable foams. Accompanied with the formation of Pickering emulsions and foams, the extraction of metal ions from water could be realized with high removal efficiency. The extractant, HDEHP, could be effectively recycled through convenient demulsification of Pickering emulsions or destruction of foams. This work provides new ideas for the construction of particle-stabilized dispersion systems and proposes methods with potential applications in industrial wastewater treatments.

Photothermally responsive Pickering emulsions stabilised by polydopamine nanobowls

Pickering emulsions with stimuli responsive properties have attracted mounting research attention owing to their potential for on-demand destabilisation of emulsions. However, a combination of biocompatibility and long-term stability are essential to efficiently apply such systems in biomedical applications, and this remains a significant challenge. To address current limitations, here we report the formation of photothermally responsive oil-in-water (o/w) Pickering emulsions fabricated using biocompatible stabilisers and showing prolonged stability. For the first time, we explore polydopamine (PDA) bowl-shaped mesoporous nanoparticles (PDA nanobowls) as a Pickering stabiliser without any surface modification or other stabiliser present. As-prepared PDA nanobowl-stabilised Pickering emulsions are shown to be pH responsive, and more significantly show high photothermal efficiency under near-infrared illumination due the incorporation of PDA into the system, which has remarkable photothermal response. These biocompatible, photothermally responsive o/w Pickering emulsion systems show potential in controlled drug release applications stimulated by NIR illumination.

Conversion of bile salts from inferior emulsifier to efficient smart emulsifier assisted by negatively charged nanoparticles at low concentrations

Bile salts (BS), one of the biological amphiphiles, are usually used as solubilizing/emulsifying agents of lipids or drugs. However, BS such as sodium deoxycholate (NaDC) can't stabilize an oil-in-water (O/W) emulsion alone due to its unusual molecular structure. In this paper we report that these emulsifiers with poor emulsifying ability can be transformed to highly efficient emulsifiers by combining with negatively charged particles (silica or montmorillonite). Both together can synergistically co-stabilize oil-in-water emulsions at extremely low concentrations (minimum 0.01 mM NaDC plus 0.003 wt% particles). Moreover, the emulsions can be reversibly switched between stable and unstable triggered by CO₂/N₂ at room temperature. This strategy is universal for emulsions containing different oils (alkanes, aromatic hydrocarbons and triglycerides) and for different BS and offers a generic model for a variety of BS of different molecular structure, which will extend their applications in more technical fields such as emulsion polymerization, biphasic catalysis and emulsion extraction.

Bile salts can be converted to efficient emulsifiers assisted by a trace amount of similarly charged nanoparticles and the emulsions formed are CO₂/N₂ switchable at room temperature.

pH-Responsive Behavior of Pickering Emulsions Stabilized by a Selenium-Containing Surfactant and Alumina Nanoparticles

Herein, we describe pH-responsive Pickering emulsions stabilized by a sodium carboxylate-derived selenium surfactant (C₁₀-Se-C₁₀·(COONa)₂) in combination with positively charged alumina nanoparticles. Unlike other bola-type carboxylate surfactants (e.g., disodium eicosanoate), C₁₀-Se-C₁₀·(COONa)₂ is soluble in water with a low Krafft temperature (36.1 °C). The emulsions are sensitive to pH variations, and efficient demulsification can be achieved by a pH trigger. The carboxylic sodium group in the C₁₀-Se-C₁₀·(COONa)₂ structure can be reversibly cycled between its anionic and nonionic states (carboxylic acid), resulting in a pH-controlled electrostatic attraction between the surfactant and alumina. The Pickering emulsion can be reversibly switched between "on" (stable) and "off" (unstable) states by pH at least four times. Compared with the emulsions stabilized by specially synthesized stimuli-responsive particles or surfactants, the method reported here is much easier to implement and requires very low concentrations of the surfactant and nanoparticles, with potential applications in the fields of biomedicine, drug delivery, and cosmetics.

Dynamic Covalent Nanoparticles for Acid-Responsive Nonaqueous Pickering Emulsions

Acid-responsive nonaqueous (glycerol in n-decane) Pickering emulsions were prepared using preferentially oil-wetted dynamic covalent silica (SiO₂-pDB) nanoparticles as the Pickering emulsifiers. The acid-responsive Pickering emulsifier SiO₂-pDB was prepared based on a Schiff base reaction between amino silica (SiO₂-NH₂) and p-decanoxybenzaldehyde (pDBA). The formation of SiO₂-pDB was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis. The preferentially oil-wetted character of SiO₂-pDB was indicated by contact angle measurement. Stable nonaqueous Pickering emulsions were prepared using preferentially oil-wetted SiO₂-pDB as the Pickering emulsifier. However, after adjusting the nonaqueous Pickering emulsions to an acidic environment, complete phase separation occurred. In the acidic environment, preferentially oil-wetted SiO₂-pDB decomposed into hydrophilic SiO₂-NH₂ and hydrophobic pDBA due to the decomposition of the dynamic imine bond in the SiO₂-pDB. Then, the hydrophilic SiO₂-NH₂ and hydrophobic pDBA desorbed from the two-phase interface, resulting in complete phase separation of the initially stable nonaqueous Pickering emulsions. The acid-responsive nonaqueous Pickering emulsions show great potential in application in water sensitive systems, such as oil-based drilling fluids.

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 (n_NaOH/n_{cationic surfactant} ≥ 1) led to prompt demulsification within 10 s. Upon further adding HCl solutions (n_HCl/n_NaOH = 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.

pH-Responsive Pickering emulsions stabilized solely by surface-inactive nanoparticles via an unconventional stabilization mechanism

Using solely highly hydrophilic particles to stabilize emulsions, especially high internal phase emulsions, has always been an important challenge. Here pH-responsive Pickering emulsions stabilized by a low concentration of bare highly hydrophilic Ludox CL nanoparticles without surface modification or addition of surfactants are developed at neutral pH. The dispersed nanoparticles can be transformed into an aggregate state with a network-like structure near the isoelectric point, which contributes to the stabilization of the emulsions. Moreover, the vdW attraction between particles and droplets also plays a key role in the formation of emulsions, which can make the aggregated nanoparticles adsorb tightly around the droplets rather than penetrate the oil-water interface. The formed protective armor and network-like aggregates separate droplets from each other to prevent coalescence. At a low nanoparticle concentration (0.5 wt%), a high internal phase emulsion can be formed and can last up to half a year. This system can emulsify not only the hydrocarbon oil but also the fluoroalkane oil phase. Finally, organic-inorganic composite particles are fabricated using the template action of the Pickering emulsions. The method of preparing composite particles is more convenient than the traditional Pickering emulsion polymerization which often requires the modification of the surface of the hydrophilic particles or the addition of auxiliary monomers. This study provides a simple green strategy for the preparation of a more stable Pickering emulsion stabilized by surface-inactive nanoparticles and will broaden the scope of applications.

Transition between a Pickering Emulsion and an Oil-in-Dispersion Emulsion Costabilized by Alumina Nanoparticles and a Cationic Surfactant

The transition between a novel oil-in-dispersion emulsion and an oil-in-water (O/W) Pickering emulsion triggered by pH was achieved using alumina nanoparticles in combination with a cationic surfactant. In acidic and neutral aqueous media, positively charged particles and the surfactant both at very low concentrations costabilize an oil-in-dispersion emulsion with the surfactant adsorbed at droplet interfaces and particles dispersed in the aqueous phase between the droplets. In alkaline media, however, particles become negatively charged and are hydrophobized in situ by adsorption of the surfactant to become surface-active and stabilize an O/W Pickering emulsion. The transition between the two is also possible by lowering the pH. The transformation can be achieved several times in a mixture of 0.1 wt % nanoparticles and 0.01 mM surfactant. This transition is significant, since particles can be made to either adsorb at the oil-water interface, which is beneficial for applications like biphasic catalysis, or remain dispersed in the aqueous phase, which is favorable for their recovery and reuse.

Redox-Responsive Oil-In-Dispersion Emulsions Stabilized by Similarly Charged Ferrocene Surfactants and Alumina Nanoparticles

A redox-responsive oil-in-dispersion emulsion was developed by using a cationic ferrocene surfactant (FcCOC₁₀N) and Al₂O₃ nanoparticles, in which the required concentrations of FcCOC₁₀N and Al₂O₃ nanoparticles are as low as 0.001 mM (≈0.005 cmc) and 0.006 wt %, respectively. Rapid demulsification can be successfully achieved through a redox trigger, resulting from the transition of FcCOC₁₀N from a normal cationic surfactant form into a strongly hydrophilic Bola type form (Fc⁺COC₁₀N). Moreover, Fc⁺COC₁₀N together with the particles almost resides in the aqueous phase and can be recovered after the reduction reaction. Not only the amount of surfactant and nanoparticles are significantly reduced but also the emulsifier (surfactant and alumina) can be recycled and reused from the aqueous phase, which is a sustainable and economical strategy for various applications.

Redox and Doubly pH-Switchable Pickering Emulsion

In this work, a novel Pickering emulsion is stabilized by silica nanoparticles functioned with a redox and pH-responsive surfactant FA-DMDA-Ox that is prepared simply by direct neutralization of ferrocenecarboxylic acid (FA) and N,N-dimethyldodecylamine (DMDA) and exhibits redox and doubly pH-switchable behavior. Here, the Pickering emulsion can be stabilized easily by combining hydrophilic silica nanoparticles with less than 0.1 wt % FA-DMDA-Ox. After adding Na₂SO₃ and H₂O₂ alternately, the demulsification and emulsification of this Pickering emulsion are controlled reversibly. Moreover, the emulsion is switched "off" upon the addition of HCl and switched "on" upon the addition of NaOH and is also switched off upon the addition of NaOH and switched on upon the addition of HCl, which demonstrate the doubly pH-switchable behavior. Based on the analysis of ζ-potential, contact angle, and adsorbed amount of silica nanoparticles, the pH and redox-switchable mechanism of the Pickering emulsion are analyzed. Here, the redox-switchable behavior is induced by the reversible adsorption and desorption of FA-DMDA-Ox on the surface of silica nanoparticles. The pH-switchable behavior is driven by the controllable dispersion systems of silica nanoparticles and FA-DMDA-Ox because of the doubly pH switchability of FA-DMDA-Ox. More importantly, upon adding fresh oil after removing the original oil, the Pickering emulsion is recycled three times. Hence, the multiswitchable Pickering emulsion can be expected to be treated as a multifunctional material in practical applications, such as oil or wax removal in the petroleum industry.

pH-Responsive Pickering emulsion stabilized by polymer-coated silica nanoaggregates and applied to recyclable interfacial catalysis

We first synthesized a diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-(trimethoxysilyl)propyl methacrylate] (PtBMA-b-PTMSPMA) through reversible addition–fragmentation chain transfer (RAFT) living radical polymerization and grafted it onto fumed silica by converting the PTMSPMA segment to silanol and the PtBMA segment to polymethylacrylic acid (PMAA) in the presence of trifluoroacetic acid in order to obtain PMAA brush-coated silica nanoaggregates P-Si. TEM, DLS, FTIR, and TGA results confirmed the successful modification of the starting materials. The nanoaggregates flocculated and stabilized a toluene-in-water Pickering emulsion at low pH, while the nanoaggregates were well dispersed in water and broke the emulsion under both neutral and basic conditions. Alternatively, the addition of acid/base induced emulsification/demulsification cycles that were sustained for several cycles. Moreover, when the P-Si was mixed with Rh-loaded silica, Rh-Si, the mixture had the same pH-responsive Pickering emulsion behavior as the single P-Si. This Pickering emulsion system can be used in the biphasic interfacial catalytic hydrogenation of olefins and had excellent yields under a hydrogen atmosphere. The yield of Pickering emulsion catalysis rapidly reached more than 99% in 3 h, while that of the demulsified mixture failed to reach 20% in 4 h, which verified the promotion of catalysis by the Pickering emulsion. Base-induced demulsification can be used to separate the products and recycle the catalyst. This pH-responsive Pickering emulsion catalytic system was capable of several cycles of reuse, and there was no significant decrease in catalytic efficiency even after eight cycles.

We synthesized a diblock copolymer and grafted it onto fumed silica in the presence of trifluoroacetic acid to obtain a pH-responsive Pickering emulsion system stabilized by polymer-coated nanoaggregates, P-Si.

Stimuli-Responsive Biomass Cellulose Particles Being Able to Reversibly Self-Assemble at Fluid Interface

Stimuli-responsive surface-active microcrystalline cellulose (MCC) particles are obtained by interaction with conventional cationic surfactants such as cetyltrimethylammonium bromide (CTAB) in aqueous media, where MCC are in situ hydrophobized by adsorption of the cationic surfactant in water via electrostatic interaction and with the in situ hydrophobization removed by adding an equimolar amount of an anionic surfactant such as sodium dodecyl sulfate (SDS). The trigger is that the electrostatic interaction between the oppositely charged ionic surfactants is stronger than that between the cationic surfactant and the negative charges on particle surfaces, or the anionic surfactant prefers to form ion pairs with the cationic surfactants and thus making them desorbed from surface of MCC. Reversible O/W Pickering emulsions can then be obtained by using the MCC in combination with trace amount of a cationic surfactant and an anionic surfactant, and the anionic surfactant with a longer alkyl chain is more efficient for demulsification. With excellent biocompatibility, biodegradability, and renewability, as well as low toxicity, the biomass cellulose particles that can be made stimuli-responsive and able to reversibly self-assemble at fluid interface become ideal biocompatible particulate materials with extensive applications involving emulsions and foams.

Enzyme-Adsorbed Chitosan Nanogel Particles as Edible Pickering Interfacial Biocatalysts and Lipase-Responsive Phase Inversion of Emulsions

Here, a simple food-grade Pickering emulsion system is prepared and adopted for biphasic biocatalytic reactions. The chitosan nanogels were prepared with strong dispersion of chitosan aggregates approaching neutral pH and then used as the particle emulsifiers to produce oil-in-water Pickering emulsions. The chitosan nanogel exhibited high affinity to negatively charged lipase. As a result of increasing the biphasic interfacial area and loading amount on the oil-water interface, the catalysis activity of lipase and recycling and pH stability were highly enhanced through colorimetric determination of p-nitrophenol (the hydrolysis product of p-nitrophenyl palmitate). A general strategy was proposed to obtain stimulus-responsive Pickering emulsions that can undergo phase inversion. The in situ modification of the wettability of chitosan nanogel could be attributed to the interaction between nanogel and free fatty acids, which was triggered by lipase hydrolysis. This would permit a rapid and controlled release of hydrophobic active components in response to enzymatic triggers.

Unique Oil-in-Brine Pickering Emulsion Using Responsive Antipolyelectrolyte Functionalized Latex: A Versatile Emulsion Stabilizer

A simple and straightforward approach to synthesize oil-in-water (O/W) emulsions under high salinity and temperature using zwitterion-functionalized latexes are presented in this work. First, well-defined functionalized latexes were synthesized by emulsifier-free emulsion copolymerization in the presence of precursor sulfobetaine comonomer using brine as a continuous phase. The surface-functionalized latex particles were then characterized by DLS, SEM, TEM, XPS, and TGA. The functionalized latex exhibited antipolyelectrolyte behavior in high salinity brine and at high temperatures. The effects of salinity, temperature, and pH on the long-term stability of the particles were investigated. Further, to evaluate the potential in high salinity brine and high temperature, the saltphilic functionalized latexes were utilized to stabilize the oil/brine (O/W) interface without any other additives. The latex enabled the formation of a stable Pickering emulsion system with low solid content (<0.02% w/w) in the presence of 50% v/v n-decane. The functionalized latexes were self-assembled at the O/W interface as a spherical colloidosome in high salinity brine through hydrophobic interactions and irreversible adsorption. The supraparticles were imaged with SEM, providing an insight that the exterior of the emulsion droplets is stabilized by the saltphilic latex particles, forming a protective layer at the oil-water interface through electrostatic repulsion. The antipolyelectrolyte latex can be utilized as a novel emulsion stabilizer, which can provide a versatile alternative for applications in a complex environment such as high salinity, temperature, and low or high pH.

Multistimuli-Responsive Pickering Emulsion Stabilized by Se-Containing Surfactant-Modified Chitosan

Particle-stabilized emulsions that can respond to external stimuli have attracted significant concerns due to their intelligent-controlled stability, whereas particle-stabilized Pickering emulsions responding to multistimuli but based on biomass have been rarely reported. Here, a multistimuli-responsive Pickering emulsion was developed using the modified chitosan as stabilizer. Due to electrostatic attraction, Se-containing anionic surfactant, sodium 11-(butylselenyl)undecylsulfate (C₄SeC₁₁S), can bind with CS at an acidic pH and form CS-C₄SeC₁₁S complexes which can further self-associate to form micrometer-sized particles with the character of partially hydrophobicity. Therefore, at pH < pK_a, an oil-in-water Pickering emulsion can be formed using CS-C₄SeC₁₁S particles as stabilizers and can spontaneously respond to redox, ion, and pH. First, with the addition of oxidation, the hydrophilicity of C₄SeC₁₁S was enhanced, and thus, hydrophobic association of CS-C₄SeC₁₁S decreased, leading to the disruption of CS-C₄SeC₁₁S particles. Hence, the emulsion destabilized. The demulsification process is closely related with the dosage of oxidant and the oxidation time. Second, introduction of a competitive ion (e.g., CTAB) could break the binding between C₄SeC₁₁S and CS, leading to the disruption of particle emulsifier. Thereby, demulsification occurred. Third, with sequentially increasing/decreasing pH, the emulsion can be switched from stable to unstable and then to stable again accordingly. Such a unique pH-responsive behavior has never been discovered in other pH-responsive Pickering emulsions. All of the stimuli-responsive behaviors were reversible. Upon alternately adding oxidant/reductant, CTAB/C₄SeC₁₁S, or base/acid, the current emulsion can be reversibly switched off (destabilization) and on (stabilization). Such a Pickering emulsion may be a good candidate as a vehicle of functional ingredient.

Phase Inversions Observed in Thermoresponsive Pickering Emulsions Stabilized by Surface Functionalized Colloidal Silica

In this study, the emulsification performance of functionalized colloidal silica is explored with the aim to achieve phase inversion of particle-stabilized (Pickering) emulsion systems. An increased understanding of inversion conditions can facilitate surfactant-free emulsion fabrication and expand its use in industrial applications. Phase inversion was achieved by adjusting the temperature but without changing the composition of the emulsion formulation. Silica nanoparticles modified with hydrophobic propyl groups and hydrophilic methyl poly(ethylene)glycol (mPEG) groups are used as emulsifiers, enabling control of the wettability of the particles and exploration of phase inversion phenomena, the latter due to the thermoresponsiveness of the attached PEG chains. The phase inversion conditions as well as the reversibility of the emulsion systems were examined at varying electrolyte concentrations and pH values of the suspensions. Transitional phase inversions, from oil-in-water and water-in-oil and back, were observed in functionalized silica particle-stabilized butanol emulsions at distinct temperatures. The phase inversion temperature was affected by electrolyte concentration and pH conditions due to salting-out effects, PEG-silica interactions, and the effects of the particle surface charge. Investigations of phase inversion conditions, temperature, and hysteresis effects in Pickering emulsions can improve the theoretical understanding of these phenomena and facilitate the implementation of low-energy emulsion preparation.

Temperature and CO2 Dual-Responsive Pickering Emulsions Using Jeffamine M2005-Modified Cellulose Nanocrystals

Cellulose nanocrystals (CNCs) with excellent biodegradability are promising biomaterials for use as responsive Pickering emulsifiers. However, the high hydrophilicity of CNCs limits their emulsification ability. Some existing studies have utilized complicated covalent modification procedures to increase the hydrophobicity of CNCs. To simplify the modification process, we prepared hydrophobically modified CNCs (CNCs-M2005) via simple and controllable electrostatic interactions with thermosensitive M2005. The obtained CNCs-M2005 exhibited temperature and CO₂ dual-responsive properties. Subsequently, stable oil/water Pickering emulsions were prepared using the partially hydrophobic CNCs-M2005 at 20 °C. However, demulsification occurred when the temperature increased to 60 °C. This temperature-induced demulsification resulted from the dehydration of polyethylene oxide and polypropylene oxide, causing the aggregation of the CNCs-M2005, as shown by dynamic light scattering and transmission electron microscopy experiments. In addition, demulsification was also achieved after bubbling CO₂, which was attributed to the dissociation of the partially hydrophobic CNCs-M2005. The temperature and CO₂ dual-responsive biosafe Pickering emulsions open up opportunity for the design of intelligent food, cosmetic, and drug delivery systems.

Surfactant-Mediated Assembly of Amphiphilic Janus Spheres

We investigate how amphiphilic Janus particle assembly structures, including clusters and striped two-dimensional (2D) crystals, are influenced by the addition of surfactant molecules. Janus particles are fabricated using silica particles coated with Au on one side, which is further modified with a hydrophobic self-assembled monolayer. Analysis on the cluster assembly structures suggests that in addition to hydrophobic attraction, van der Waals (VDW) attraction plays a significant role in the assembly process, which is modulated by the Au coating thickness. This is manifested by the cluster formation induced primarily by VDW forces when the hydrophobic attraction between particles is diminished by adding the surfactant. In the 2D crystal case, sodium dodecyl sulfate (SDS) and Tween 20 show opposite trends in how they affect assembly structures and particle dynamics. SDS shortens the stripes in 2D crystals and accelerates the rotation of particles, whereas Tween 20 extends the straight stripes and slows down the particle rotation. We interpret the results by considering SDS adsorption on the Au-coated hemisphere of the Janus particles and Tween 20 forming hydrogen bonds with the silica hemisphere of Janus particles. Our study offers a simple approach to change the assembly structures of Janus particles, and it provides principles and guidance for potential applications of Janus particles coupled with small amphiphilic molecules.

Redox-Responsive Pickering Emulsions Based on Silica Nanoparticles and Electrochemical Active Fluorescent Molecules

In this paper, we report a novel redox-responsive water-in-oil Pickering emulsion stabilized by negatively charged silica nanoparticles in combination with a trace amount of redox switchable fluorescent molecule ferrocene azine (FcA), in which ferrocene serves as a redox-sensitive group and anthryl unit serves as a fluorescence emission center. By alternately adding oxidants and reducing agents at a moderate condition, the amphiphilicity of silica nanoparticles changes because of the adsorption of Fc⁺A and the desorption of FcA on the silica surface. On the one hand, the stability of emulsions can be transformed between stable and unstable at ambient temperature via redox trigger and the regulation process can be cycled at least three times. On the other hand, the fluorescent intensity of the FcA molecule can be regulated by redox stimuli; thus, the change in fluorescent behavior of the emulsion droplets is observed upon redox cycles, which makes it useful in the fluorescent label of stimuli-responsive Pickering emulsions. This work provides a deep understanding of the regulation mechanism of Pickering emulsions upon redox stimuli and opens the new way for in situ fluorescent label of stimulus-responsive Pickering emulsions without introducing additional fluorescent molecules.

Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

A novel CO₂/N₂ switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO₂/N₂ switchable surfactant [ N'-dodecyl- N, N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil-water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N₂ into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil-water interface. Bubbling CO₂ into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

Amphiphilic bromelain-synthesized oligo-phenylalanine grafted with methoxypolyethylene glycol possessing stabilizing thermo-responsive emulsion properties

A thermo-responsive amphiphile was developed from oligo-phenylalanine [oligo(Phe)]. The hydrophobic moiety of the amphiphile, oligo(Phe) was synthesized via reverse hydrolysis catalyzed by bromelain in dimethyl sulfoxide and dioxane solutions. The production of oligo(Phe) increased by 80.7% by screening suitable reaction conditions. The average degree of polymerization of oligo(Phe) was determined to be four by ¹H NMR. By grafting with aldehyde-ended methoxypolyethylene glycol (mPEG), oligo(Phe) was converted to amphiphilic oligo(Phe)-mPEG. The surface tension of oligo(Phe)-mPEG solution increased with decreasing chain length of the mPEG moiety. Cytotoxicity studies showed oligo(Phe)-mPEGs are biocompatible. On varying temperature, a reversible phase transition of oligo(Phe)-mPEG solutions could be observed. N-octane-in-water emulsions and 0.5% beta-carotene containing squalene-in-water emulsions stabilized by oligo(Phe)-mPEGs occurred at 25 °C but de-emulsification took place at >40 °C. Emulsification could be restored once the separated mixture cooled and re-homogenized. The emulsification/de-emulsification cycling could be repeated many times. The time required for de-emulsification decreased with elevated temperature but increased with a reduced concentration of oligo(Phe)-mPEGs and a reduction in the chain length of the mPEG moiety.