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.

Cellulose Nanocrystal-Based Emulsion of Thyme Essential Oil: Preparation and Characterisation as Sustainable Crop Protection Tool

Essential oil-based pesticides, which contain antimicrobial and antioxidant molecules, have potential for use in sustainable agriculture. However, these compounds have limitations such as volatility, poor water solubility, and phytotoxicity. Nanoencapsulation, through processes like micro- and nanoemulsions, can enhance the stability and bioactivity of essential oils. In this study, thyme essential oil from supercritical carbon dioxide extraction was selected as a sustainable antimicrobial tool and nanoencapsulated in an oil-in-water emulsion system. The investigated protocol provided high-speed homogenisation in the presence of cellulose nanocrystals as stabilisers and calcium chloride as an ionic crosslinking agent. Thyme essential oil was characterised via GC-MS and UV-vis analysis, indicating rich content in phenols. The cellulose nanocrystal/essential oil ratio and calcium chloride concentration were varied to tune the nanoemulsions' physical-chemical stability, which was investigated via UV-vis, direct observation, dynamic light scattering, and Turbiscan analysis. Transmission electron microscopy confirmed the nanosized droplet formation. The nanoemulsion resulting from the addition of crosslinked nanocrystals was very stable over time at room temperature. It was evaluated for the first time on Pseudomonas savastanoi pv. savastanoi, the causal agent of olive knot disease. In vitro tests showed a synergistic effect of the formulation components, and in vivo tests on olive seedlings demonstrated reduced bacterial colonies without any phytotoxic effect. These findings suggest that crosslinked cellulose nanocrystal emulsions can enhance the stability and bioactivity of thyme essential oil, providing a new tool for crop protection.

Molecular Dynamics Study of Silica Nanoparticles and CO2-Switchable Surfactants at an Oil-Water Interface

Adsorbing CO2-sensitive surfactants on the surface of nanoparticles is an important strategy for preparing stimuli-responsive Pickering emulsions. However, the microscopic mechanisms are still limited, owing to a lack of intuitive understanding at the molecular level on the interactions between nanoparticle and switchable surfactants at the oil-water interface. We employed the molecular dynamics (MD) simulations to explore the mechanism behind the reversible emulsification/demulsification of a Pickering emulsion stabilized by silica nanoparticles (NPs) and CO2-switchable surfactants, named N-(3-(dimethylamino)propyl)alkyl amide (CPMA). MD results show that the protonated surfactant CPMAH+ has strong hydrophilicity, forming an adsorption layer at the oil-water interface. The ionic surfactants can be tightly adsorbed on NP surface through electrostatic interactions. Thus, the formed colloid particle has both hydrophobic and hydrophilic properties, which is a key factor in stabilizing emulsion. When CPMAH+ molecules were deprotonated to CPMA, the hydration activity of the headgroups reduced greatly, inducing a mixture with oil molecules. There are still a certain number of CPMA molecules residing at the oil-water interface due to the hydrophilic amine groups. The results from repeated simulations show that NP can either stay in the water phase or locate at the interface. Even NP was finally adsorbed on the interface and combined with CPMA or oil molecules, the adsorption configuration of CPMA on the NP surface was essentially different from that of CPMAH+. The potential of mean force confirmed that the combination between NP and CPMA is quite unstable due to the disappearance of electrostatic attraction. Different binding configurations and stability between NP and CPMA or CPMAH+ were the fundamental reason for the reversible emulsification/demulsification of Pickering emulsion.

Design of Proanthocyanidins and TiO2 Nanoparticles-Based Novel Emulsions as a Platform for UV Protection

A white oil-in-water novel emulsion stabilized by TiO2 nanoparticles with UVB shielding properties and proanthocyanidins with antioxidant activity was prepared, where the proanthocyanidins aggregated at the oil-water interface to reduce interfacial tension while TiO2 nanoparticles were dispersed in the continuous water phase to hinder droplet coalescence. It was found that the average oil droplet size was less than 10 μm and decreased with the increase of proanthocyanidins concentration, but the increase of the content of TiO2 nanoparticles had little effect on it. The combination of TiO2 nanoparticles and proanthocyanidins was versatile for oil phases with different polarities, and the resulting emulsion exhibited high stability in the face of centrifugation, heating and prolonging storage time. After encapsulating the UVA filter avobenzone in white oil, the emulsion was endowed with the ability to resist UVB and UVA. Further, the emulsion showed great free radical scavenging ability for superoxide anion radical (⋅O2 - ), hydroxyl radical (⋅OH) with the clearance rate of over 70 %, indicating the good antioxidant activity. The ingenious combination of UVB, UVA filter and antioxidant with emulsion as carrier provides a new idea for the preparation of full-band sunscreen emulsion.

Advances in CO2-switchable surfactants towards the fabrication and application of responsive colloids

CO₂-switchable surfactants have selective surface-activity, which can be activated or deactivated either by adding or removing CO₂ from the solution. This feature enables us to use them in the fabrication of responsive colloids, a group of dispersed systems that can be controlled by changing the environmental conditions. In chemical processes, including extraction, reaction, or heterogeneous catalysis, colloids are required in some specific steps of the processes, in which maximum contact area between immiscible phases or reactants is desired. Afterward, the colloids must be broken for the postprocessing of products, solvents, and agents, which can be facilitated by using CO₂-switchable surfactants in surfactant-stabilized colloids. These surfactants are mainly cationic and can be activated by the protonation of a nitrogen-containing group upon sparging CO₂ gas. Also, CO₂-switchable superamphiphiles can be formed by non-covalent bonding between components at least one of which is CO₂-switchable. So far, CO₂-switchable surfactants have been used in CO₂-switchable spherical and wormlike micelles, vesicles, emulsions, foams, and Pickering emulsions. Here, we review the fabrication procedure, chemical structure, switching scheme, stability, environmental conditions, and design philosophy of such responsive colloids. Their fields of application are wide, including emulsion polymerization, catalysis, soil washing, drug delivery, extraction, viscosity control, and oil transportation. We also emphasize their application for the CO₂-assisted enhanced oil recovery (EOR) process as a promising approach for carbon capture, utilization, and storage to combat climate change.

A Robust Switchable Oil-In-Water Emulsion Stabilized by Electrostatic Repulsions between Surfactant and Similarly Charged Carbon Dots

How to control the stability of oil-in-water (O/W) emulsions is one of the main topics for scientists working in colloidal systems. Recently, carbon dots (CDs) have received great interest as smart materials because of their excellent physicochemical properties and versatile applications. Herein, for the first time, advanced and switchable O/W emulsions are presented that are stabilized by the synergistic effect of cationic surfactant cetyltrimethylammonium bromide CTAB (emulsifier) and similarly charged CDs (stabilizer). In the formulated emulsion, the cationic surfactant molecules are adsorbed at the oil and water interface to decrease the interfacial tension and enrich the drops with a positive charge to ensure intensive electrostatic repulsions among them. On the contrary, cationic CDs are distributed in the water phase among the droplets to reduce the water secretion and prevent flocculation and droplet coalescence. The stabilizing effect is found to be universal for emulsions of a range of oil phases. Furthermore, the formulated emulsion is found to be switchable between "stable" and "unstable" modes by adding an equivalent of anionic surfactant sodium dodecyl benzene sulphonate (SDBS). The stabilized and switchable O/W emulsions are believed to have wide practical applications in water purification, pharmaceuticals, protein recognition, as well as catalysis.

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.

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.

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.

Redox and pH-responsive emulsions based on TiO2 nanoparticles and ferrocene derivates

We synthesized a novel surfactant ferrocene azine (FcA) and developed a redox and pH-responsive emulsion stabilized by TiO2 nanoparticles and oxidized ferrocene azine (Fc+A) with fluorescence through simple mixing instead...

The effect of chitosan molecular weight on CO2-triggered switching between emulsification and demulsification

The role of molecular weight as a key physical property of macromolecules in determining the CO₂-triggered switching characteristics of responsive emulsions prepared using CO₂-switchable macromolecules has not been studied and is the focus of the current study. In this work, CO₂-switchable chitosan of four different molecular weights is used to investigate the effect of molecular weight on CO₂-triggered switching of CO₂-responsive emulsions. The molecular weight of chitosan is shown to have an opposite effect on emulsification and demulsification by the CO₂ trigger. Before bubbling of CO₂, chitosan of higher molecular weight forms a more stable three-dimensional network structure in the continuous phase of oil-in-water (O/W) emulsions, which leads to the formation of a more stable emulsion. After bubbling of CO₂, the chitosan of higher molecular weight makes the continuous phase more viscous, which leads to an incomplete demulsification as compared with the chitosan of lower molecular weight. Experimental evidence from the measurement of conductivity, interfacial tension and rheological properties is provided to support the proposed mechanism. This work is of great significance in guiding the selection of desirable CO₂-switchable polymers for switchable emulsions of desired switching characteristics.

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.

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 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.

Molecular dynamics simulation of CO2-switchable surfactant regulated reversible emulsification/demulsification processes of a dodecane-saline system

CO2-Switchable surfactants are of great potential in a wide range of industrial applications related to their ability to stabilize and destabilize emulsions upon command. Molecular dynamics simulations have been performed to reveal the fundamental mechanism of the reversible emulsification/demulsification processes of a dodecane-saline system by a CO2-switchable surfactant that switches between active (i.e., N'-dodecyl-N,N-dimethylacetamidinium (DMAAH+)) and inactive (i.e., N'-dodecyl-N,N-dimethylacetamidine (DMAA)) forms. The density profiles indicate that DMAAH+ could increase the oil-water interfacial thickness to a greater extent compared to DMAA. DMAAH+ could sharply reduce the interfacial tension of the dodecane-saline system, while DMAA only exhibits a limited decrease, which is in accordance with the experimental observation that DMAAH+/DMAA can reversibly emulsify/demulsify alkane-water systems. Our simulations showed that both the number and lifetime of hydrogen bonds (HBs) between DMAA and water are almost equal to those between DMAAH+ and water. In DMAA, the N atom connecting with the alkyl tail acted as a HB acceptor, while the N atom attached by a proton in DMAAH+ acted as a HB donor. Furthermore, the HBs between DMAAH+ and HCO3- at the interfaces are relatively limited. Hence, it is deduced that the HBs are insufficient to achieve the CO2-switchability of DMAA/DMAAH+. The Lennard Jones and coulombic potentials between DMAA/DMAAH+ and other species show that the coulombic potentials between DMAAH+ and water or anions (i.e., Cl- and HCO3-) sharply decrease with the increase of DMAAH+ and are much lower than those in models with DMAA. The enhanced coulombic interactions between DMAAH+ and anions lead to a remarkable reduction in interfacial tension and the emulsification of the alkane-saline system. Therefore, coulombic interactions are of crucial importance to the reversible emulsification/demulsification processes regulated by CO2-switchable surfactants, namely DMAAH+/DMAA.

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.

Emulsions Stabilized with Alumina-Functionalized Mesoporous Silica Particles

Alumina-functionalized ordered mesoporous silica SBA-15 particles have been proposed to stabilize Pickering emulsions. Functionalization of SBA-15 particles have been performed by depositing alumina using a two-step synthesis (first, silica condensation, followed by alumina precipitation). Three different Al to Si ratios have been prepared. The calcined materials have been characterized by TEM, SEM, XRD, N₂ physisorption, and zeta potential, in order to determine key physicochemical properties, and the alumina localization. The emulsifying and stabilizing properties of the calcined particles have been evaluated for water/toluene-based Pickering emulsions.

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.

Templating Interfacial Nanoparticle Assemblies via in Situ Techniques

In situ surface modification of nanoparticles has a rich industrial history, but in recent years, it has also received increased attention in the field of directed self-assembly. In situ techniques rely on components within a Pickering emulsion system, such as amphiphiles that act as hydrophobizers or ionic species that screen charges, to drive the interfacial assembly of particles. Instead of stepwise procedures to chemically tune the particle wettability, in situ methods use elements already present within the system to alter the nanoparticle interfacial behavior, often depending on Coulombic interactions to simplify operations. The surface modifications are not contingent on specific chemical reactions, which further enables a multitude of possible nanoparticles to be used within a given system. In recent studies, in situ methods have been combined with external means of shaping the interface to produce materials with high interfacial areas and complex geometries. These systems have facilely tunable properties, enabling their use in an extensive array of applications. In this feature article, in honor of the late Prof. Helmuth Möhwald, we review how in situ techniques have influenced the development of soft, advanced materials, covering the fundamental interfacial phenomena with an outlook on materials science.

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.

Controllable CO2-responsiveness of O/W emulsions by varying the alkane carbon number of a tertiary amine

A series of CO₂-responsive oil-in-water (O/W) emulsions were prepared by introducing a hydrophobic tertiary amine (TA) with a varying alkane carbon number (ACN) into the emulsion stabilized by sodium dodecyl benzene sulfonate (SDBS).