Behavior of Smart Surfactants in Stabilizing pH-Responsive Emulsions

Quercetin encapsulation and release using rapid CO2-responsive rosin-based surfactants in Pickering emulsions

Emulsion-based delivery systems are extensively employed for encapsulating functional active ingredients, protecting them from degradation, and enhancing bioavailability and release efficiency. Here, a CO2-responsive surfactant synthesized from rosin displays rapid responsiveness to CO2 at room temperature, transitioning reversibly switches between active and inactive states multiple times. The dual tertiary amines on the rosin rigid structure contributes to its CO2 sensitivity. When in its active cationic form, in conjunction with silica nanoparticles, it exhibits desired Pickering emulsification performance across various oil phases. In the Pickering emulsion loaded with quercetin, the encapsulation efficiency and loading efficiency reached 80.50% and 0.69%, respectively, with stability lasting at least 30 days. The system provides robust protection for quercetin against external factors, such as UV and heat, revealing sustained release effects. This study investigated the potential of using rosin-based CO2-responsive surfactants alongside nanoparticles to design stable Pickering emulsion systems for active substance encapsulation and sustained release.

Building Functional Liquid-Based Interfaces: From Mechanism to Application

Functional liquid-based interfaces, with their inhomogeneous regions that emphasize the functionalized liquids, have attracted much interest as a versatile platform for a broad spectrum of applications, from chemical manufacturing to practical uses. These interfaces leverage the physicochemical characteristics of liquids, alongside dynamic behaviors induced by macroscopic wettability and microscopic molecular exchange balance, to allow for tailored properties within their functional structures. In this Minireview, we provide a foundational overview of these functional interfaces, based on the structural investigations and molecular mechanisms of interaction forces that directly modulate functionalities. Then, we discuss design strategies that have been employed in recent applications, and the crucial aspects that require focus. Finally, we highlight the current challenges in functional liquid-based interfaces and provide a perspective on future research directions.

Realizing the Efficient Dissolution of Drag Reducer upon pH-Induced Demulsification of Inverse Polymer Emulsion

The increasing exploitation of fossil resources needs to shift people's attention to developing unconventional reservoir resources. Polyacrylamide-based emulsion drag reducers can effectively reduce the turbulence of fracturing fluid and improve oil recovery, but their release effect is poor, which limits their practical application. Here, we constructed a pH-responsive inverse polymer emulsion of poly(acrylamide-2-acrylamido-2-methylpropanesulfonic acid). Interestingly, HOA/Tween 80 exhibits remarkable pH-responsive behavior, which enables the monomer emulsion to change the type of emulsion under pH stimulation. Our results propose that the P(AM-AMPS) polymer emulsion is eluted with the same aqueous phase as the W/O P(AM-AMPS) polymer emulsion, thereby achieving the effect of drag reduction. P(AM-AMPS) can be rapidly released from the inverse polymer emulsion within 70 s upon pH stimulation, the drag reduction rate of which was 60.4%. Obviously, the inverse polymer emulsion prepared by a pH-responsive surfactant not only has good stability but also can achieve rapid release of the polymer upon pH stimulation, which supplies an interesting indication to explain why a balance exists between the stability of the emulsion and release of P(AM-AMPS).

Insights into the Efficient Release of the Polyacrylamide Drag Reducer via a pH-Responsive Inverse Polymer Emulsion

The enormous demand for petroleum consumption has resulted in the shortage of fossil resources, prompting the need to explore unconventional reservoirs. Polyacrylamide emulsion drag reducers are capable of inhibiting the turbulence of fracturing fluids for enhancing the reservoir stimulation results, but the poor dissolution efficiency of polyacrylamide emulsion drag reducers is the primary limitation to their large-scale application. Here, a pH-responsive ionic liquid surfactant, oleic acid/cyclohexanediamine (HOA/HMDA), is synthesized by using oleic acid (HOA) and cyclohexanediamine (HMDA). HOA/HMDA shows a remarkable pH-responsive behavior due to the pH-induced deconstruction of the HOA/HMDA structure. Interestingly, the HOA/HMDA-stabilized monomer emulsion exhibits an obvious pH-induced emulsion structure transformation behavior. In addition, the HOA/HMDA-stabilized monomer emulsion possesses excellent dynamic and storage stability, supporting the inverse emulsion polymerization of the polymer P(AM/AMPS/AA). The obtained P(AM/AMPS/AA) polymer inverse emulsions maintained stability for 30 days. Our finding proposes that the structure of the P(AM/AMPS/AA) polymer inverse emulsions changes with pH stimulation, which is capable of facilitating the release of polymers. P(AM/AMPS/AA) is released from the P(AM/AMPS/AA) polymer inverse emulsions within 30 s at a pH value of 12.06, along with a drag reduction rate of 62.54%. Obviously, the HOA/HMDA-stabilized P(AM/AMPS/AA) polymer inverse emulsions eliminate the contradiction between the stability and release of polyacrylamide emulsion drag reducers, which is promising for meeting the demands of reservoir stimulation.

Zr-Based MOF-Stabilized CO2-Responsive Pickering Emulsions for Efficient Reduction of Nitroarenes

A Pickering emulsion is a natural microreactor for interfacial catalysis in which an emulsifier is critical. Recently, a metal-organic framework (MOF) has attracted attention to emulsify water-organic mixtures for constructing a Pickering emulsion. However, a few stimuli-responsive Pickering emulsions based on MOFs have been reported, and the MOF emulsifiers cannot be regenerated at room temperature. Herein, the Zr-MOF with a rodlike morphology is synthesized using ionic liquid as a modulator and then modified with n-(trimethoxysilylpropyl)imidazole (C3im) to prepare a series of functionalized Zr-MOFs (MOF-C3im). It is found that MOF-C3im is an excellent emulsifier to construct stable and CO2-responsive Pickering emulsions even at low content (>0.20 wt %). Notably, the emulsification and demulsification of the emulsions can be easily and reversibly switched by bubbling of CO2 and N2 alternatively at room temperature because CO2 and imidazole molecules anchored on the Zr-MOF underwent a reversible acid-base reaction, resulting in an obvious change in the wettability of the emulsifier. As a proof of concept, the reduction reactions of nitrobenzene have been successfully carried out in these Pickering emulsions, demonstrating the efficient integration as a microreactor for chemical reaction, product separation, and emulsifier recycling under ambient conditions. This strategy provides an innovative option to develop stimulus-responsive Pickering emulsions for sustainable chemical processes.

Pickering Emulsions and Viscoelastic Solutions Constructed by a Rosin-Based CO2-Responsive Surfactant

Designing stimulus-switch viscoelastic solutions and Pickering emulsions with reversible CO2-responsive behavior remains a challenge. A rosin-based CO2-responsive surfactant, N-cetyl-maleimidepimaric acid N,N-dimethylenediamide (C16MPAN), was synthesized and used to prepare CO2-triggered viscoelastic solutions and Pickering emulsions. This surfactant exhibited excellent CO2-responsive performance in water and formed a viscoelastic solution. This viscoelastic system was investigated by dynamic light scattering (DLS), rheology, and cryogenic transmission electron microscopy (Cory-TEM). The shear viscosity of the system increased by 3-4 orders of magnitude after bubbling with CO2 and a large number of elongated, flexible, tubular wormlike micelles were observed. Further, Pickering emulsions were prepared by C16MPAN+ synergistically with cellulose nanocrystals (CNCs), whose stability and switchability were investigated via adsorption isotherm, droplet size, contact angle, and macroscopic photographs. C16MPAN+ was adsorbed with CNCs to form mechanical barriers at the oil-water interface, making the emulsion stable for at least three months, and desorption from CNCs enabled emulsion breaking. The cycle could be switched reversibly multiple times and the particle size distribution of emulsion was basically the same. This work enriches the application of biomass resources in intelligent responsive materials.

Photo-Responsive Control of Adsorption and Structure Formation at the Air-Water Interface with Arylazopyrazoles

Smart interfaces that are responsive to external triggers such as light are of great interest for the development of responsive or adaptive materials and interfaces. Using alkyl-arylazopyrazole butyl sulfonate surfactants (alkyl-AAP) that can undergo E/Z photoisomerization when irradiated with green (E) and UV (Z) lights, we demonstrate through a combination of experiments and computer simulations that there can be surprisingly large changes in surface tension and in the molecular structure and order at air-water interfaces. Surface tensiometry, vibrational sum-frequency generation (SFG) spectroscopy, and neutron reflectometry (NR) are applied to the study of custom-synthesized AAP surfactants with octyl- and H-terminal groups at air-water interfaces as a function of their bulk concentration and E/Z configuration. Upon photoswitching, a drastic influence of the alkyl chain on both the surface activity and the responsiveness of interfacial surfactants is revealed from changes in the surface tension, γ, where the largest changes in γ are observed for octyl-AAP (Δγ ∼ 23 mN/m) in contrast to H-AAP with Δγ < 10 mN/m. Results from vibrational SFG spectroscopy and NR show that the interfacial composition and the molecular order of the surfactants drastically change with E/Z photoisomerization and surface coverage. Indeed, from analysis of the S-O (head group) and C-H vibrational bands (hydrophobic tail), a qualitative analysis of orientational and structural changes of interfacial AAP surfactants is provided. The experiments are complemented by resolution of thermodynamic parameters such as equilibrium constants from ultra-coarse-grained simulations, which also capture details like island formation and interaction parameters of interfacial molecules. Here, the interparticle interaction ("stickiness") and the interaction with the surface are adjusted, closely reflecting experimental conditions.

Charge Density Overcomes Steric Hindrance of Ferrocene Surfactant in Switchable Oil-in-Dispersion Emulsions

A ferrocene surfactant can be switched between single and double head form (FcN⁺ C₁₂ /Fc⁺ N⁺ C₁₂ ) triggered by redox reaction. FcN⁺ C₁₂ can neither stabilize an O/W emulsion alone nor an oil-in-dispersion emulsion in combination with alumina nanoparticles due to the steric hindrance of the ferrocene group. However, such steric hindrance can be overcome by increasing the charge density in Fc⁺ N⁺ C₁₂ , so that oil-in-dispersion emulsions can be co-stabilized by Fc⁺ N⁺ C₁₂ and alumina nanoparticles at very low concentrations (1×10^-7  M (≈50 ppb) and 0.001 wt %, respectively). Not only can reversible formation/destabilization of oil-in-dispersion emulsions be achieved by redox reaction, but also reversible transformation between oil-in-dispersion emulsions and Pickering emulsions can be obtained through reversing the charge of alumina particles by adjusting the pH. The results provide a new protocol for the design of surfactants for stabilization of smart oil-in-dispersion emulsions.

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.

Novel Strategy of Polymers in Combination with Silica Particles for Reversible Control of Oil-Water Interface

Hybrid smart emulsification systems are highly applicable in manipulating oil-in-water (O/W) droplets. Herein, novel switchable block polymers containing both zwitterionic and tertiary amine pendent groups were designed and synthesized to combine with charged silica particles to stabilize the O/W emulsion responsive to pH. This study was carried out in O/W emulsions stabilized with the polymer and silica particles under different pH conditions. The emulsion system was also simulated using molecular dynamics simulation to reveal the mechanism at molecular levels, thus gaining insight into the relationships between the emulsifying properties and the molecular interaction of the mixed system. Upon acidification of the continuous aqueous phase, protonated polymers with excellent hydrophilicity were induced by charged silica particles to cause rapid emulsion coalescence. In alkaline media, the mixed system conversely stabilized the O/W emulsions, cutting polymer consumption by over three-quarters. The emulsification and demulsification can be switched alternately by tuning the pH conditions. The applications exhibited excellent efficiency in separating heavy oil/water emulsions and proved the high conversion rate in emulsion polymerization. Overall, with this novel strategy to relieve tedious modifications on particle surfaces and massive consumption of polymers, the designed responsive emulsification systems can impart intelligent and controllable chemical reactivity to emulsions on demand in a more affordable and sustainable way.

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.

Pickering Emulsion Catalysis: Interfacial Chemistry, Catalyst Design, Challenges, and Perspectives

Pickering emulsions are particle-stabilized surfactant-free dispersions composed of two immiscible liquid phases, and emerge as attractive catalysis platform to surpass traditional technique barrier in some cases. In this review, we have comprehensively summarized the development and the catalysis applications of Pickering emulsions since the pioneering work in 2010. The explicit mechanism for Pickering emulsions will be initially discussed and clarified. Then, summarization is given to the design strategy of amphiphilic emulsion catalysts in two categories of intrinsic and extrinsic amphiphilicity. The progress of the unconventional catalytic reactions in Pickering emulsion is further described, especially for the polarity/solubility difference-driven phase segregation, "smart" emulsion reaction system, continuous flow catalysis, and Pickering interfacial biocatalysis. Challenges and future trends for the development of Pickering emulsion catalysis are finally outlined.

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).

A Dual pH‐ and Light‐Responsive Spiropyran‐Based Surfactant: Investigations on Its Switching Behavior and Remote Control over Emulsion Stability

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual‐responsive adjustability of its surface‐active characteristics. The switching mechanism of the system relies on the reversible conversion of the non‐ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH‐dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli‐responsive behavior enables remote‐control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH‐dependent manipulation of oil‐in‐water emulsions.

Multilayer-Stabilized Water-in-Water Emulsions

Water-in-water (W/W) emulsions are of interest for various applications due to their inherent biocompatibility, ultralow interfacial tensions, and large interface thickness. However, it is still challenging to prepare stable W/W emulsions with tailored phase architectures compared to oil-in-water (O/W) and water-in-oil (W/O) emulsions. Here, we report a multilayer-stabilized W/W emulsion composed of poly(ethylene glycol)/dextran in the presence of DNA strands. The W/W emulsions present onion-ring-like structures, which are interpreted by a nanofluid film model. Emulsion behavior, e.g., stability, interface tension, etc., can be controlled by the type of DNA (single or double strands), DNA concentration, and volume fraction of dispersed phase. Our findings could broaden the preparation of novel emulsions for potential applications in emulsion polymerization, new media of homogeneous catalysis, and DNA transportation of water-in-water media.

CO2-responsive Pickering emulsions stabilized by soft protein particles for interfacial biocatalysis

Pickering emulsions are emulsions stabilized by colloidal particles and serve as an excellent platform for biphasic enzymatic catalysis. However, developing simple and green strategies to avoid enzyme denaturation, facilitate product separation, and achieve the recovery of enzyme and colloidal particle stabilizers is still a challenge. This study aimed to report an efficient and sustainable biocatalysis system via a robust CO₂/N₂-responsive Pickering oil-in-water (o/w) emulsion stabilized solely by pure sodium caseinate (NaCas), which was made naturally in a scalable manner. The NaCas-stabilized emulsion displayed a much higher reaction efficiency compared with conventional CO₂/N₂-responsive Pickering emulsions stabilized by solid particles with functional groups from polymers or surfactants introduced to tailor responsiveness, reflected by the fact that most enzymes were transferred and enriched at the oil-water interface. More importantly, the demulsification, product separation, and recycling of the NaCas emulsifier as well as the enzyme could be facilely achieved by alternatively bubbling CO₂/N₂ more than 30 times. Moreover, the recycled enzyme still maintained its catalytic activity, with a conversion yield of more than 90% after each cycle, which was not found in any of the previously reported CO₂-responsive systems. This responsive system worked well for many different types of oils and was the first to report on a protein-based CO₂/N₂-responsive emulsion, holding great promise for the development of more sustainable, green chemical conversion processes for the food, pharmaceutical, and biomedical industries.

Recyclable and re-usable smart surfactant for stabilization of various multi-responsive emulsions alone or with nanoparticles

A novel multi-responsive surfactant (abbreviated as N⁺-8P8-N) was synthesized, in which one octyl trimethylamine group (quaternary ammonium) and one octyl dimethylamine group are connected to a benzene ring through ether bonds. This novel surfactant can stabilize conventional oil-in-water (O/W) emulsions alone, and O/W Pickering emulsions and novel oil-in-dispersion emulsions together with oppositely and similarly charged nanoparticles, respectively. In all cases rapid demulsification can be achieved through either pH or CO₂/N₂ triggers, by which the surfactant is reversibly converted between a normal cationic surfactant form (N⁺-8P8-N) and a strongly hydrophilic and surface-inactive bola form (N⁺-8P8-NH⁺). Notably, the bola form N⁺-8P8-NH⁺ dissolves in the aqueous phase alone or together with nanoparticles after demulsification without contamination of the oil phase, and the aqueous phase can be recycled many times triggered by pH or CO₂/N₂ in accordance with the principle of green chemistry. This newly designed re-usable smart surfactant is significant for the development of various temporarily stable emulsions, which are extensively applied in emulsion polymerization, new material synthesis, heterogeneous catalysis and oil transportation.