CO2 responsive emulsions: Generation and potential applications

Colloids and Surfaces A 582 (2019) 123919

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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa

CO2 responsive emulsions: Generation and potential applications a

c

Xueqian Guan , Dongfang Liu , Hongsheng Lu a b c

b,⁎

, Zhiyu Huang

b,⁎

T

Shengli Petroleum Administration Bureau, Dongying, Shandong 257002, PR China School of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu, Sichuan 610500, PR China School of Science, Xihua University, Chengdu, Sichuan 610039, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2responsive emulsion Demulsification CO2 switchable Emulsifier CO2 switchable solvent

Responsive emulsions can be “switched off” by external environmental stimulus or trigger, allowing a simple and effective demulsification approach and thus attracting much attention. CO2 is a green trigger for the response process because it is non-toxic, abundant, benign, non-accumulating in the system and easily removed. This review focuses on the generation, response mechanism and potential industrial applications of CO2 responsive emulsions. The CO2 responsive emulsions can be prepared using either CO2 switchable emulsifiers adsorbed at the oil-water interface or CO2 switchable solvents in continuous or dispersed phase. The stability of emulsion can be reversibly controlled by alternate addition and removal of CO2, depending on the protonation/deprotonation degree of CO2 switchable materials. The CO2 responsive emulsions show great potential in application to heavy oil transport, soil washing, emulsion polymerization and chemical reactions.

1. Introduction Responsive emulsions refer to the emulsions that “stabilization” and “destabilization” can be well controlled by external environmental stimuli. Emulsions are common in nature and daily life, [1–3] among them responsive emulsions can be used in crude oil production and transport, oil sand separation, emulsion polymerization, organic synthesis, nanoparticle preparation and so on. After their service in

⁎

applications, emulsion stability is no longer desired and additional demulsification methods are required to achieve the separation and recovery of oil and water phases. Compared with ordinary emulsions, responsive emulsions possess two advantages in application: firstly, the common demulsification methods for ordinary emulsions, including adding salt or acid, electrical deposition, filtration, ultrasound and so on, usually give rise to large energy consumption and/or material waste, and may cause a series of negative effects on the environment

Corresponding authors. E-mail addresses: [email protected] (H. Lu), [email protected] (Z. Huang).

https://doi.org/10.1016/j.colsurfa.2019.123919 Received 25 July 2019; Received in revised form 1 September 2019; Accepted 2 September 2019 Available online 03 September 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

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[4–6]. For responsive emulsions, however, the fine control of “destabilization” can be simply realized by stimulus or trigger without additives, achieving low energy consumption. Secondly, the rapid and reversible responsiveness makes emulsions reusable, which is consistent with the concept of green and sustainable development. Therefore, it is of great significance to study the responsive emulsions systematically. Recently, many kinds of responsive emulsions have been reported, including some that respond to light, [7] temperature [8,9], pH [10–13], magnetism [14,15], or Multi-responsive emulsion [16]. However, there are many limitations for these responsive emulsions, including product contamination, economic and environmental costs. On the contrary, CO2 responsive emulsions have attracted much attention because the CO2 trigger is non-toxic, abundant, environmentfriendly and easily-removed. [3,17–20] In addition to the large-scale CO2 response macroemulsion in the general sense, CO2 responsive microemulsions, [17,21] nanoemulsions [18], and pickering emulsions [20,22] have also been extensively studied. As sparged into an aqueous solution, CO2 combines with water to form carbonic acid. The decrease in pH, similar to that of the pH responsive systems, allows for certain moieties of materials in the system to undergo some type of ionic change, thus altering the stability of emulsions. The added CO2 can easily be removed by bubbling air or N2 under heating. Compared with a pH triggered switching, the switching with CO2/N2 is a non-accumulative process, without any significant changes in either macromorphology or microscopic size of emulsion. In 2005, Jessop et al. [23] first reported a CO2 switchable polarity solvent (SPS) composed of DBU/1-hexanol. Subsequently, this research group [24] developed CO2 switchable solvents, surfactants, solutes, particles, polymers and gels. These materials can be reversibly transformed between two forms with drastic differences in properties or performance from each other. These are also described as “smart’’ materials, and can be used for construction of CO2 responsive emulsions. [25–27] In general, emulsification and demulsification can be reversibly achieved based on the CO2 responsive behaviors of either emulsifiers adsorbed at the oil-water interface or solvents in continuous or dispersed phase. The CO2 responsive behavior of emulsifiers adsorbed at the oil-water interface mainly refers to the reversible changes of molecular structure and surface activity of emulsifiers driven by CO2. The common used CO2 switchable emulsifiers can be divided into several categories: amphiphiles, inorganic nanoparticles, polymer particles and Ionic Liquids (ILs). [28–30] On the other hand, the CO2 responsive behavior of solvents in either continuous or dispersed phase refers to changes of their properties with a CO2 trigger, including polarity, ionic strength, hydrophilicity, and so on. [31–35] Recent advances and potential applications of CO2 responsive emulsions will be described in the main body of this paper.

switchable surfactants with different head groups were designed and used for stabilizing emulsions, including guanidine, imidazoline, arylsubstituted acetamidine and tertiary amine. [38] By comparison, tertiary amine is more suitable for stabilizing the emulsion, mainly owing to its strongest CO2 responsiveness and commercial availability. Feng et al. [39] reported a CO2 switchable microemulsion composed of N, Ndimethyl-N-dodecyl amine (C12A), sodium dodecyl sulphate (SDS), nbutanol, n-heptane and water (Fig. 2). The surfactant SDS is able to reduce the n-heptane-water interfacial tension (IFT) to ultra-low by cooperating with co-surfactants, i.e. C12A and n-butanol. By addition of CO2, the neutral C12A could be protonated and then bind with the negatively charged SDS through electrostatic attraction, generating an oilsoluble compound. The resulting compound migrated from the oilwater interface to oil phase, inducing a drastic increase in IFT and then nearly complete phase separation. Removal of CO2 with N2 under heating, however, could neutralize the C12A and decompose the oilsoluble compound, resulting in a decrease in IFT and re-formation of a monophasic microemulsion. Our group [40,41] developed a series of CO2 responsive emulsions by introducing tertiary amines into the emulsions stabilized by sodium dodecyl benzene sulfonate (SDBS) or sodium oleate (NaOA). The CO2 responsiveness of emulsions could be well controlled by varying the alkane carbon number (ACN), the number of tertiary amine groups or the position of the hydroxyl group of tertiary amine. All the switchable surfactants mentioned above are cationic surfactants, but in some areas, such as soil washing process, cationic surfactants do not perform well because they adsorb significantly on the surface of negatively charged solid particles. Many CO2 switchable anionic surfactants are developed to address this limitation. Jessop et al. [42] found that phenolate, benzoate and carboxylate salts are typical switchable anionic surfactants. Contrary to cationic surfactants, these anionic salts are hydrophilic and surface-active when in air but are converted to nonionic forms with poor solubility and lose the surface activity when under the atmosphere of CO2. Using sodium dodecanoate as an emulsifier, the emulsion of 1-octanol and water could be stable for hours in the absence of CO2 but disrupt immediately by addition of CO2. Emulsions stabilized by phenolate and benzoate surfactants show similar CO2 responsiveness. Liu et al. [43] reported a CO2 responsive emulsion stabilized by a novel CO2 switchable anionic surfactant, 11-dimethylaminoundecyl sulfate sodium salt (DUSNa) (Fig. 3). DUSNa was synthesized by introducing tertiary amine group into a traditional anionic surfactant of sodium alkyl sulfate. When treated with CO2, the dimethylamine group was protonated to generate inactive DUS, leading to an effective destabilization of emulsion. Interestingly, DUS was re-transformed to active DUSNa after N2 treatment, and thus a stable oil-in-water (O/W) emulsion was obtained again. However, conventional small amphiphiles with CO2 responsiveness suffer serious drawbacks. Synthesis of these small amphiphiles is complicated and time-consuming. The concept of supramolecular amphiphiles (i.e. superamphiphiles), proposed by Zhang et al., [36,37] greatly simplifies the design and construction of responsive molecules. Supramolecular amphiphiles refer to amphiphiles that are constructed through noncovalent interactions or dynamic covalent bonds. The noncovalent interactions or dynamic covalent bonds can be reversible controlled by a trigger [11,44–49]. Recently, many studies have focused on the CO2 responsive emulsions based on superamphiphiles. Sun and co-workers [50] assembled a CO2 responsive superamphiphile (DOA) by simple mixing of Jeffamine D 230 and oleic acid (HOA) at a stoichiometric ratio of 1:2. D230 or HOA alone cannot stabilize the emulsion, but the formed D-OA exhibits excellent interface activity and could effectively adsorb at the oil-water interface, thus stabilizing an O/ W emulsion for more than 2 weeks (Fig. 4). Within seconds of CO2 addition, part of D-OA molecules was decomposed into neutral HOA and protonated D 230 (D2+), desorbing from the oil-water interface and leading to complete phase separation. Upon subsequent removal of CO2 by bubbling N2 at 60 ℃, D-OA was re-constructed and thus a stable

2. CO2 responsive emulsions stabilized by switchable emulsifiers 2.1. Responsive emulsions based on amphiphiles containing CO2 switchable moieties Amphiphiles are commonly divided into conventional small amphiphiles and supramolecular amphiphiles. [36,37] Conventional small amphiphiles refer to surfactants that contain both hydrophilic and lipophilic moieties, and they will adsorb at the oil-water interface to stabilize emulsions. Currently, the reported CO2 switchable amphiphiles include both cationic and anionic surfactants. In 2006, Jessop et al. [3] first reported a CO2 switchable cationic surfactant containing an amidine head group and used it as an emulsifier to stabilize emulsions (Fig. 1). The amidine with a long hydrophobic chain could be readily transformed into bicarbonate in the presence of CO2 and water, exhibiting high surface activity and thus stabilizing a hexadecane-inwater emulsion. The later removal of CO2, however, was able to reverse the reaction and “switch off” the surfactant, leading to a clear separation of the oil from the water. Subsequently, a small library of CO2 2

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Fig. 1. (top) The switching of amidine between neutral and surfactant forms. (bottom) Photographs of hexadecane-inwater emulsions containing amidine under the atmosphere of CO2 (left) or argon (right). The emulsions were placed for (A) 5 min, (B) 30 min, and (C) 24 h after 10 min of shaking. (D) Photograph of the CO2-induced emulsion after argon bubbling at 65–70 ℃ for 2 h. [3].

Fig. 2. Schematic diagram illustrating the switching mechanism of the microemulsion based on SDS and C12A. [39].

organic amines. [51–53] For instance, Hao et al. [27] developed CO2 responsive superamphiphiles by combining stearic acid with three organic amines, i.e. ethanolamine (EA), diethanolamine (DEA), and triethanolamine (TEA). Stearic acid bilayers are self-assembled and can be

emulsion was re-generated. Besides D-OA, other superamphiphiles with similar structure were also constructed and served as emulsifiers to stabilize CO2 responsive macroemulsions or microemulsions. Most of these superamphiphiles were assembled based on fatty acids and 3

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Fig. 3. CO2-triggered reversible stabilization and destabilization of an O/W emulsion using a CO2 switchable anionic surfactant DUSNa. [43].

2.2. CO2 responsive Pickering emulsions stabilized by inorganic nanoparticles

used as excellent emulsifiers for stabilizing hexadecane emulsions. Stable emulsions were formed without separating for several months. The addition of CO2 decreased the solubility of acid and disrupted the bilayers, leading to rapid destabilization of emulsion. In general, the CO2 response process of such emulsions is achieved by changes in the molecular structure of the amphiphilic molecules. There are generally tertiary amines and carboxylates. The tertiary amine species can be protonated to a surfactant species in the presence of CO2. An organic carboxylate such as sodium carboxylate becomes a carboxylic acid (non-surfactant) upon action of CO2. The reversible regulation of CO2 between surfactants and non-surfactants is key to the construction of CO2 responsive emulsions.

The emulsions described above are prepared using switchable surfactants alone, in which emulsions are only kinetically stable, and very high concentration of surfactants (> > cmc) is needed. Besides that, a CO2 responsive emulsion can also be a class of Pickering emulsions that stabilized by relatively simple nanoparticles. Surprisingly, Pickering emulsions have long-term stability against coalescence, and they possess other advantages, such as reducing foaming, decreasing skin irritation and requiring only a trace amount of a switchable surfactant (≈cmc/10) or even no surfactant. [54,55] The higher stability of the Pickering emulsion means that its demulsification is more difficult than that of the ordinary emulsion. Therefore, a Pickering emulsion that can be “switched off” will be preferred.

Fig. 4. Schematic illustration of proposed emulsification and demulsification mechanism of O/W emulsions prepared using CO2 responsive superamphiphile D-OA. [50]. 4

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Fig. 5. Photographs of n-octane-in-water Pickering emulsions stabilized by silica nanoparticles (0.5 wt %) and either an amidinebased switchable surfactant (0.3 mM; (a)-(h)) or cetyltrimethylammonium bromide (CTAB) (0.1 mM; (i), (j)). (a) and (b) Stable emulsion with amidinium; (c) and (d) after N2 bubbling at 65 °C; (e) and (f) standing for 24 h and one week after homogenization; (g) standing for one week after CO2 bubbling; (h) standing for 24 h without any treatment; (i) and (j) emulsion with CTAB before and after N2 bubbling, 24 h later. [55].

Fig. 6. (left) The switching of the surfactant AZO-B4 triggered by CO2 and light. (right) Schematic illustration demonstrating the dual CO2 and light responsiveness for n-octane-in-water Pickering emulsions stabilized by silica nanoparticles and trans-AZO-B4. [61].

(MPAGN), and used to prepare a kind of CO2 responsive Pickering emulsions with silica nanoparticles. All of these surfactants are cationic surfactants that could adsorb on the surface of silica nanoparticles via electrostatic interactions. In addition, nonionic surfactants can also be used to prepare CO2 responsive Pickering emulsions with silica nanoparticles. Zhang et al. [60] found that the adsorption of a conventional polyoxyethylene nonionic surfactant (C12E9) through hydrogen bonding on the surface of silica nanoparticles could be significantly affected by CO2, making the particles hydrophobized in situ and become excellent stabilizers for Pickering emulsions. This Pickering emulsion could be effectively “switched on and off” by addition and removal of CO2. Pickering emulsion can also be designed to respond to two different triggers, thus achieving a precise control of emulsion properties. Binks et al. [61] developed a dual stimuli-responsive Pickering emulsion with CO2 and light triggers (Fig. 6). The silica nanoparticles were hydrophobically modified by a switchable surfactant AZO-B4 containing both a charged amine group and an azobenzene group, thus stabilizing the

2.2.1. Non-covalently modified particles for CO2 responsive Pickering emulsions Binks et al. [55] first reported a CO2 responsive Pickering emulsion using silica nanoparticles and a switchable surfactant as the stabilizer (Fig. 5). The negatively charged silica nanoparticles are too hydrophilic to stabilize the emulsion alone. Under the atmosphere of CO2, the particles were hydrophobized in situ by adsorption of cationic headgroups (i.e. amidinium) of surfactant through electrostatic interactions, thereby stabilizing Pickering emulsions. When CO2 was removed, however, approximately 81% of the surfactant molecules became inactivated and desorbed from particle surfaces, occurring extensive coalescence and thus complete demulsification. Moreover, when CO2 was added to the mixture again, the stability of the emulsions was reestablished. Zhang et al. [56–58] designed the tertiary amine or amine oxide-based surfactants instead of amidine-based surfactants to prepare Pickering emulsions with similar CO2 responsive behaviors. Rao et al. [59] synthesized a bio-based rigid surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)-propylamine imide 5

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Fig. 7. (top) Photographs demonstrating the CO2 responsiveness for n-decane-in-water Pickering emulsions stabilized by alumina nanoparticles (0.5 wt %) in combination with 0.3 mM SDS and 0.3 mM DDAA. (bottom) Micrographs of the corresponding stable emulsions. [62].

surfactant-free responsive Pickering emulsions (Fig. 8). In the absence of CO2, the contact angle (θ) of amidine grafted silica particles (P1) was measured to be around 66°, and thus P1 could stabilize O/W emulsions. In contrast, amidine and phenyl co-grafted silica particles (P2) possessed a more hydrophobic surface (θ = 89°) and was able to stabilize a W/O emulsion. The addition of CO2, however, could protonate the amidine groups and thus depress the wettability of both P1 and P2 particles, resulting in rapid destabilization of the emulsions. In both cases, the removal of CO2 by bubbling air or N2, could reverse the behaviors observed under CO2 atmosphere and cause the re-generation of stable O/W or W/O emulsions.

responsive Pickering emulsion. The emulsion can be rapidly transformed between stabilization and destabilization via CO2 trigger, while the droplet size and rheological properties of the emulsion can be well controlled by light trigger without changing composition. Zhang et al. [25] prepared a CO2 and redox dual responsive Pickering emulsion based on the modified silica nanoparticles with Se-containing tertiary amine. Similarly, the reversible phase separation of emulsions is primarily ascribed to a CO2-controllable electrostatic attraction, while a well-controlled change in the droplet size is attributed to a redox-tunable hydrogen bonding. In both cases, the dual responsiveness is beneficial to a precise control of emulsion properties. CO2 responsive Pickering emulsion can also be developed using a switchable surfactant as the demulsifier. Cui et al. [62] prepared a CO2 responsive Pickering emulsion based on a switchable demulsifier, N’dodecyl-N, N-dimethylacetamidine (DDAA) (Fig. 7). DDAA could be reversibly transformed between cationic amidinium and nonionic amidine upon alternatively treating with CO2 and N2. The Pickering emulsion stabilized by positively charged alumina nanoparticles in combination with SDS, cannot respond to CO2. Under the atmosphere of CO2, when introducing DDAA into the emulsion, SDS bound with the cationic DDAA to form ion pairs and then desorbed from particle surfaces. As a result, alumina particles were rendered hydrophilic and the emulsion was readily destabilized. After removal of CO2, however, DDAA molecules were re-converted into neutral forms, resulting in the collapse of ion pairs and re-establishment of the in situ hydrophobization of particles. A stable Pickering emulsion was re-generated after homogenization. This simple demulsification/re-stabilization process can be repeated several times. The non-covalent bond action (mostly electrostatic action) can surface-hydrophobize the particles (nano-silica, etc.) to give them a stable emulsion property. The CO2 responsive substance is capable of achieving a change in chargeability under the action of CO2 (from a neutral molecule to a positively charged molecule or from a negatively charged molecule to a neutral molecule). Therefore, CO2 can regulate the non-covalent bond between different charged particles and CO2 responsive substances, and finally can construct a CO2 response pickering emulsion.

2.3. CO2 responsive Pickering emulsions using polymer particles as the emulsifiers CO2 switchable polymers containing both hydrophilic and lipophilic moieties can be served as particle-based emulsifiers for stabilizing Pickering emulsions. Cunningham et al. [64,65] synthesized CO2 responsive cellulose nanocrystals (CNCs), and then used to prepare CO2 responsive toluene-in-water Pickering emulsions (Fig. 9). The CO2 responsive CNCs with high interfacial activity were prepared by grafting poly(N-3-(dimethylamino) propyl methacrylamide) (PDMAPMAm) and poly(N, N-(diethylamino)ethyl methacrylate) (PDEAEMA) to the CNC surface via nitroxide-mediated polymerization (NMP). When bubbling CO2 into emulsions, DMAPMAm or DEAEMA group was protonated and the corresponding polymer chains were hydrophilized. Consequently, the grafted CNCs shuttled back into the aqueous phase and the emulsions were broken. The removal of CO2 by bubbling N2, however, would hydrophobized the grafted CNCs and a stable Pickering emulsion was obtained again after adding oil into the aqueous solution. The emulsification and demulsification processes can be repeated for several cycles. Zhu et al. [66] modified water-insoluble lignin via atom transfer radical polymerization (ATRP) grafting of DEAEMA, and used lignin-gDEAEMA as the CO2 responsive surfactant for stabilizing a decane-inwater Pickering emulsion. Bubbling CO2 was able to break the emulsion by hydrophilization of lignin-g-DEAEMA, while bubbling N2 led to the re-stabilization of the emulsion by recovering the lignin-based emulsifier. Besides the bio-sourced Pickering emulsifiers, the polymer particles can also be constructed by emulsion polymerization of functional monomers. Zhu group [67] prepared zwitterionic polymer particles using surfactant-free emulsion copolymerization (SFEP) of 2-(diethylamino)- ethyl methacrylate (DEAEMA) and sodium methacrylate

2.2.2. Covalently modified particles for CO2 responsive Pickering emulsions Introduction of CO2 switchable functional groups into nanoparticle surfaces through covalent bonding is another important way to prepare CO2 responsive Pickering emulsions. Xu et al. [63] functionalized two silica particles by grafting amidine and/or phenyl moieties to stabilize 6

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Fig. 8. Images of O/W and W/O Pickering emulsions stabilized by more hydrophilic P1 particles (left) and more hydrophobic P2 particles (right), respectively. Samples were stained using a water-soluble green dye [63]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

the PDEAEMA particles alone, however, the separated two phases induced by CO2 were not restored to a stable emulsion after N2 bubbling because the raised solution pH was also lower than the isoelectric point (IEP) of the particles and then the cationic charge from the particle surface was not completely removed. The presence of SMA moieties is able to balance the residual protonated DEAEMA moieties. Armes and

(SMA) with N, N’-methylenebis(acrylamide) (MBA) as a cross-linker, and then used these particles as emulsifiers for preparing CO2 responsive Pickering emulsions of n-dodecane (Fig. 10). The addition of CO2 induced the protonation of DEAEMA units and thus rapid demulsification, while bubbling N2 brought about the deprotonation of DEAEMA units and efficient re-emulsification. When the emulsifier was

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Fig. 9. (top) The switching of PDEAMA- and PDMAPMAm-grafted CNC by a CO2 trigger. (bottom) Photographs demonstrating the CO2 responsiveness for toluene-inwater Pickering emulsions stabilized by the grafted CNCs. [65].

Fig. 10. (left) Schematic illustration of the CO2 switchability of P(DMAEMA-co-SMA) particles. (right) Photographs of the CO2 dependence of n-dodecane-in-water emulsions based on P(DEAEMA-co-SMA) particles. [67].

coworkers [26] developed CO2 responsive Pickering emulsions stabilized by poly(2-(diethylamino)ethyl methacrylate) (PDEA) based latexes, wherein the latter was synthesized via emulsion copolymerization of DEA using monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) as a reactive stabilizer and divinylbenzene as a cross-linker. O/W Pickering emulsions of either n-dodecane, sunflower oil, isononyl isononanoate, or isopropyl myristate could be generated when homogenized at pH 10. After CO2 bubbling, all emulsions were broken completely due to the transformation of the PEGMA-PDEA latexes to cationic microgels. Shieh et al. [68] designed dual CO2 and temperature responsive diblock copolymers of poly(N,N-dimethylaminoethyl methacrylate)-b-poly(N-iso-propyl acrylamide) (PDMAEMA-bPNIPAAM), and then used to prepare CO2 responsive n-octane-in-water emulsions. The emulsification ability of the block copolymers can be affected by CO2 and temperature, and the formed emulsions show a completely opposite behavior of CO2 responsiveness at 25 and 40 ℃. In addition to inducing the reversible phase separation of emulsion, CO2 responsive emulsifiers can also promote the reversible phase transition of emulsion. Wang et al. [69] developed a water-in-ethyl acetate Pickering emulsion using nanoparticles as the emulsifier, producing the controllable inversion of emulsions between W/O and O/W types through synergetic CO2 and light stimuli. The nanoparticles were synthesized through ATRP grafting PDEAEMA onto a polydopamine (PDA) particle surface. The original nanoparticles are hydrophobic and oil-wetted, and tend to stabilize a W/O emulsion. Within 1 min of CO2

sparging, the W/O emulsion was converted to a stable O/W emulsion because the addition of CO2 allowed the PDA-PDEAEMA nanoparticles to become protonated, hydrophilic and water-wetted. Removing CO2 with N2 under heating, however, could reconvert the O/W emulsion back to a W/O emulsion. In addition, in the presence of CO2, the O/W emulsion was reversibly converted to a W/O emulsion under 2.0 W NIR light through the synergetic effect of contraction and deprotonation. The CO2-responsive pickering emulsion based on polymer particle construction is mainly achieved by the change of the hydrophilic and hydrophobic properties of the responsive group (usually a tertiary amine group) in the polymer by CO2. The tertiary amine groups in the polymer can become hydrophilic under the action of CO2, thereby causing the amphiphilic polymer nanoparticles of the stable emulsion to become hydrophilic particles, ultimately resulting in oil-water phase separation of the emulsion. After removal of the CO2, the polymer particles can in turn change from hydrophilic to amphiphilic and stabilize the emulsion.

2.4. CO2 responsive emulsions stabilized by Ionic Liquids (ILs) Apart from the emulsions proposed above, CO2 responsive emulsions can also be prepared using reactive Ionic Liquids (ILs) as the stabilizer. ILs are neoteric materials and possess unique features including non-volatility, nonflammability, chemical and thermal stability, negligible vapor pressure, and tunable solvent power. [70–72] Wang 8

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Fig. 11. (left) Chemical structures of the anions and cations of ILs. (right) Photographs of the switching of W/O microemulsions composed of n-pentanol, ILs and water. [73].

SW additive DMEA in the aqueous phase. However, the synthetic amine-based additives suffer serious drawbacks. Most of amines are harmful to human beings and the environment. To address this limitation, oligochitosan has been proposed to serve as an environmentally friendly SW because it’s a natural product. [76] In the presence of oligochitosan, the ionic strength of crude oil emulsions or PS latexes can be reversibly changed upon alternatively treating with CO2 and N2, thus controlling the stability.

et al. [73] reported a class of CO2 responsive W/O microemulsions containing n-pentanol as oil phase and ILs as the emulsifier (Fig. 11). The CO2 responsive ILs composed of an N-alkyl-N, N-dimethylethylamine cation [CnDMEA]+ (n = 8, 10, 12, 14, and 16) and azole-based anions, including imidazolium ([Im]−), trizolium ([Triz]−) and pyrazolium ([Pyr]−). Upon the addition of CO2, the azole-based anions of the ILs reacted with CO2 and water, promoting an increase in solution ionic strength and eventual oil-water phase separation via salting-out effect. For all microemulsions containing different ILs, reversible transition from W/O single phase to complete phase separation could be achieved upon alternate addition and removal of CO2, mainly due to the reversible change of ionic strength in the systems. ILs can also be used to prepare Pickering O/W emulsions with silica nanoparticles. The CO2 responsive ILs, N-alkylimidazole bicarbonates ([Cnim]+HCO3−, n = 6,8,10,12,14), could adsorb on the surface of nanoparticles to modify hydrophobicity, thus stabilizing the responsive Pickering emulsions. [74]

3.2. CO2 responsive emulsions prepared using switchable hydrophilicity solvents (SHSs) A SHS, defined by Jessop et al., [32,33,77–79] is a hydrophobic solvent possessing poor miscibility with water when under air/N2 but a hydrophilic solvent possessing complete miscibility with water when under an atmosphere of CO2. The polarity or miscibility of a SHS with water can be reversibly switched via a CO2 trigger, so that it can be applied to the preparation of CO2 responsive emulsions. Several simple tertiary amines are selected because they are commercially available, hydrolytically stable and not prone to bioaccumulation. Our group [80] designed a kind of CO2 responsive surfactant-free microemulsions (SFMEs) consisting of N, N-dimethylcyclohexylamine (DMCHA), DMEA, and water (Fig. 13), in which DMCHA and DMEA were used as the oil phase and amphisolvent, respectively. In the pre-Ouzo region, the single-phase DMCHA-rich domain (SFME-I) and water-rich domain (SFME-II) coexist. The initial addition of CO2 led to a distinct oil-water phase separation for SFME-I system. With the continuous introduction of CO2, DMCHA and DMEA in the upper phase were gradually protonated and transferred to the lower phase, resulting in a gradual decrease in the volume of the upper phase. Finally, the two liquid phases were fused into a single-phase aqueous solution of ammonium bicarbonates. On the other hand, a clear phase separation was not observed when CO2 was introduced into the O/W SFME-II system, while the protonation of DMCHA and DMEA after CO2 treatment also induced the formation of a single-phase aqueous solution of bicarbonates. The two SFMEs have excellent CO2 responsive performance. Sun et al. [81] prepared a CO2 responsive emulsion using a mixture of paraffin oil and DMCHA as oil phase. The paraffin oil-in-water emulsion, stabilized by a conventional surfactant sodium dodecyl benzene sulfonate (SDBS), becomes more stable in the presence of DMCHA. After addition of CO2, neutral DMCHA was protonated and transferred from internal emulsion droplets to the water phase, leading to a dramatic increase in ionic strength. Most of the paraffin oil and water separated from the emulsion within 20 min of CO2 sparging. Interestingly, a transparent middle phase microemulsion formed in this case, primarily due to the synergistic interactions between SDBS and protonated DMCHA. Jessop et al. [82] dissolved a polymer in DMCHA, and allowed it to disperse in an aqueous SDS solution, forming a CO2 responsive O/W emulsion. The addition of CO2 promoted DMCHA to partition into the water phase, increasing ionic strength of water phase and thus allowing the polymer

3. CO2 responsive emulsions prepared using switchable solvents The reversible transition between stabilization and destabilization of emulsions can also be achieved using CO2 switchable solvents. The CO2 responsive behaviors of solvents in the continuous or dispersed phases commonly refer to changes of their properties triggered by CO2, such as polarity, ionic strength and hydrophilicity. Next, we describe in detail the emulsions prepared by CO2 switchable solvents, which include switchable water (SW), switchable hydrophilicity solvent (SHS) and switchable ILs. 3.1. CO2 responsive emulsions prepared using switchable water (SW) Switchable water is defined as an aqueous solvent with switchable ionic strength, usually an aqueous solution of hydrophilic amine. [31,34,75] The addition of CO2 into the aqueous solution will promote the protonation of the dissolved amine, resulting in the formation of charged species and a sharp increase in ionic strength. This change can be reversed upon removal of the CO2. Jessop et al. [19] prepared a CO2 responsive dodecane-in-water emulsion stabilized by SDS in the presence of SW additive (Fig. 12). The conventional surfactant SDS alone is not CO2 switchable but becomes CO2 switchable in the presence of N, N-dimethylethanolamine (DMEA). In the absence of CO2, DMEA has almost no influence on the stability of the dodecane-in-water emulsion with SDS. Upon the addition of CO2, DMEA was protonated into its bicarbonate salt, resulting in an increase in ionic strength. The stability of the emulsion was greatly reduced by the high ionic strength, and an apparent phase separation occurred within 2 h. After removal of CO2 by bubbling N2, however, the neutralization of DMEA and the consequent decrease in ionic strength promoted the re-formation of a stable emulsion. Similarly, the aggregation and re-dispersion of polystyrene (PS) latexes could also be reversibly controlled by CO2 when adding the 9

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Fig. 12. CO2-triggered reversible breaking and redispersion of a dodecane-in-water emulsion using SDS and a SW additive DMEA. [19].

interfacial tension between C16mimCl and decanol. The droplets of microemulsion possess extremely low Laplace pressures, companied by a very soft surfactant interface. After CO2 bubbling, the [tria123] anion in [bmim][tria123] reacted with CO2, inducing an increase in the anion volume by almost 50% and thereby a change in surfactant rigidity. The microemulsion was replaced by a turbid emulsion within 5 min of CO2 sparging. This turbid system could be restored to the transparent clear microemulsion when CO2 was removed by bubbling N2 on gentle heating. 4. Potential applications of CO2 responsive emulsions Emulsion is widely used in a variety of industries, such as spanning food, petroleum, cosmetics, biomedicine and pharmaceuticals. Depending on the application, fine control over stabilization and destabilization of an emulsion may be demanded. The following section reviews the potential applications of CO2 responsive emulsions, including heavy oil transport, soil washing, emulsion polymerization and chemical reactions.

Fig. 13. CO2-Controlled changes of properties and appearance of the W/O microemulsion SFME-I. SFME-I was prepared using DMCHA, DMEA, and water, and samples (b, c) were stained by Nile red [80]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

4.1. Heavy oil transport particles to aggregate together. The effect of CO2 switching solvent on emulsion stability is different from that of CO2 switching surfactant and CO2 responsive polymer. Under the action of CO2, SW can instantly increase the ionic strength in the emulsion system, leading to emulsion demulsification, while SHS affects the stability of the emulsion through the change of its own hydrophilic and hydrophobic properties.

Because of the extremely high viscosity of heavy oil, pipeline transport is very difficult and thus additional pumping energy is required. Current technologies for reducing viscosity of heavy oils include heating, dilution and emulsification. [84–87] However, methods of heating and dilution have the limitations of expensive economic cost or scarcity of diluents and lights oils. Emulsification method is designed to disperse the oil droplets into an aqueous phase, significantly decreasing the viscosity of heavy oil and thus making the pipeline transport quite feasible. [88,89] Emulsion stability is essential during transport but no longer desired when at downstream. Therefore, a heavy oil-in-water emulsion that can be “switched off” is preferred. Jessop et al. [3] reported a CO2 responsive heavy oil emulsion based on a long-chain acetamidine (Fig. 15). Without any additive, the crude oil existed in a fairly stable emulsion when shaken with water, presumably due to the presence of indigenous surfactants in the oil. When introducing hexadecylamidine into the system, the amidine became surface-active under the atmosphere of CO2 and the emulsion remained

3.3. CO2 responsive emulsions prepared using Ionic Liquids (ILs) CO2 responsive emulsions can also be constructed using CO2 responsive ILs as either dispersed or continuous phase. Hatton et al. [83] prepared a CO2 responsive non-aqueous microemulsion composed of 1Butyl-3-methylimidazolium triazolide ([bmim][tria123]), 1-Hexadecyl3-methylimidazolium chloride (C16mimCl), decanol and cyclohexane (Fig. 14), in which C16mimCl and decanol were employed as the surfactant and co-surfactant, respectively. The [bmim][tria123]-in-cyclohexane microemulsion was formed primarily because of the ultralow 10

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Fig. 14. (left) Absorption and desorption of CO2 in pure IL [bmim][tria123]. (right) Photographs of the switching of bicontinuous microemulsions composed of [bmim][tria123], C16mimCl, decanol and cyclohexane. [83].

surfactants when in air but were destabilized when treated with CO2. For the heavy oil with a high acid number (TAN), Jessop group [93] proposed another scenario for generating CO2 responsive heavy oil emulsions with low viscosities (Fig. 16). The indigenous surfactants present in heavy oil, such as naphthenic acids, could be activated by deprotonation with the neutral amidines and then stabilize an O/W emulsion. When arriving at the transport terminal, the indigenous surfactants were deactivated after addition of CO2, leading to a clear separation of the heavy oil from the water. The separation efficiency of the heavy oil from water with CO2 is slightly better in amidine-based system than those in Na2CO3 or DBU-based systems. Subsequently, our group [94,95] used several tertiary amine as organic alkalies to deprotonate naphthenic acids in heavy oil, creating CO2 responsive superamphiphiles and thereby stabilizing O/W emulsions. For instance, the heavy oil-in-water emulsion showed a low viscosity and a good

stability. After the pipelining is done, the amidine converted back to the uncharged form and then emulsion was easily broken by addition of argon. Sun et al. [50] prepared CO2 responsive emulsions of diluted crude oil using D-OA as the stabilizer. The formed low-viscosity emulsions exhibit good stability without water separation after standing for 24 h. After bubbling CO2 for 10 s, a clear destabilization occurred after standing for 10 min and most of the water was separated from the oil phase after 24 h. Saliu et al. [90] used bis(3-aminopropyl) terminated poly(ethylene glycol) as the CO2 responsive emulsifier for preparing the crude oil-in-water emulsion, which could be destabilized after bubbling CO2. Our group [91,92] prepared CO2 responsive heavy oil emulsions stabilized by different polymer emulsifiers. One polymer surfactant was synthesized by copolymerizing with DMAEMA and acrylamide (AM), and another was synthesized with DMAEMA and butyl methacrylate (BMA). Emulsions could be successfully stabilized by both polymer

Fig. 15. Photographs of crude oil-in-water emulsions containing either amidine and CO2 (left), amidine and argon (center), or only argon (right). The emulsions were placed for (A) 5 min, (B) 30 min, (C) 60 min and (D) 15.5 h after 10 min of shaking. [3]. 11

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Fig. 16. Schematic diagram illustrating the switching mechanism of crude oil-in-water emulsions prepared using naphthenic acids and a base amidine. [93].

realizing an apparent oil separation from the water phase. In this case, the concentration of oil and surfactant remaining in the aqueous phase dropped by 95% and 99.5%, respectively. A CO2 responsive emulsion stabilized by DUSNa, reported by Liu et al., [43] was also utilized for retrieving oil and recycling surfactant. When mixing oil contaminated quartz sand and DUSNa solution, an opaque emulsion and the cleaned sand with around 3.2% of the total oil were obtained after stirring for 2 h at ambient temperature. Upon CO2 treatment, the emulsion was broken and around 92.1% of the total oil was separated from the water phase. The inactive form of DUS cannot dissolve in oil or water, but is precipitated in the aqueous phase, and thus the separated oil is almost surfactant-free. In addition, around 95.2% of DUSNa in the separated water phase could be retrieved and reused over three cycles by treatment with N2. Our group [101] separated the diesel oil from oily cuttings using three switchable water (SW) additives, i.e. DMEA, TEA, and DEA. When mixing SW and oily cuttings, naphthenic acids in diesel oil could be activated by SW and thus an O/W emulsion was stabilized. The cleaned cuttings could be separated simply by filtration. The emulsion was destabilized by addition of CO2 and then oil was separated from the water. Recycling of SW could be easily done after addition of N2. Liu group [39] proposed that a CO2 responsive microemulsion could also be applied in soil washing, achieving the efficient recovery of oil from oilpolluted sand by simply bubbling CO2.

stability when the oil/water volume ratio, content of DMEA and NaCl in water were 65:35, 0.5 wt% and 0.2 wt%, respectively. This emulsion could also be broken by addition of CO2. In addition, our group [96] fabricated similar crude oil-in-water emulsions based on naphthenic acids and DMCHA, and then compared the demulsification efficiency of CO2 and organic acids. For emulsions with a low crude oil content (5 wt %), CO2 has a demulsification efficiency over 90%, realizing almost completely separation of the heavy oil from the water. For emulsions with a high crude oil content (70 wt%), however, the high viscosity of emulsion and a rising CO2 gas release pressure make a CO2 demulsification efficiency of only 56% and thus emulsion cannot be demulsified completely. In contrast, replacing CO2 with organic acid, especially citric acid and oxalic acid, the demulsification efficiency was increased to more than 90%. The main reason for this difference is that the acidity of organic acids is stronger than that of carbonic acid (formed by reaction of CO2 and water). However, the crude oil with a much low naphthenic acids content (TAN = 0.23 mg KOH/g), reported by Saliu et al. [90], could also be emulsified by indigenous surfactants and later demulsified by the addition of CO2. Some organic bases, such as di-Npropylamine, DBU, piperidine and diisopropylamine, were discovered to effectively stabilize O/W emulsions by activating naphthenic acids, while some other amines, including EA, TEA, dimethylethylendiamine, and dicyclohexylamine, were less effective. Effective emulsification is primarily attributed to the interaction of organic bases with naphthenic acids, rather than asphaltenic film formation/rupture, even at relatively low doses of naphthenic acids. Both the charged naphthenic acids and the organic salts can adsorb at the water-oil interface, playing an important role in the decrease of IFT and emulsification process. Within few minutes of CO2 bubbling, the emulsions could be “switched off” completely.

4.3. Emulsion polymerization Emulsion polymerization is a common technology originally used to synthesize CO2 responsive polymer latexes or microgels. Compared to bulk polymerization method, it has the advantages of fast reaction rate, high conversion, low viscosity and strong temperature control ability. [3,13] Particles remain suspended after emulsion polymerization. In some applications, however, subsequent coagulation of latexes is necessary, mainly because this could save energy and transport costs or the forms of polymer resins are desired for further application. Unfortunately, traditional latex destabilization methods, including addition of salts, acids for anionically stabilized latexes, or bases for cationically stabilized latexes, are not ideal because of salt accumulation in the system or the impossibility of re-dispersing aggregated particles. Therefore, responsive latexes that can be aggregated and re-dispersed using a CO2 trigger is preferred. Responsive polymer latexes can be prepared via emulsion polymerization using CO2 switchable materials, including CO2 switchable surfactants, initiators, comonomers and solvents. Jessop et al. [102,103] first developed emulsion polymerization of styrene (St) or methyl methacrylate (MMA) using Long-chain alkyl amidine-based switchable surfactants, producing particles of PS and poly(methyl methacrylate) (PMMA) with sizes ranging from 50 to 350 nm (Fig. 18). Under the atmosphere of CO2, the neutral amidine became a cationic surfactant and then adsorbed onto the particle surface, thus stabilizing PS or PMMA latexes through electrostatic repulsion. The later loss of CO2 by N2 and gentle heat, however, could “switch off” the surfactant and then decrease the electrostatic barrier between particles, leading to aggregation of particles. Moreover, the re-dispersion of the particles is

4.2. Soil washing In the process of exploration, transport or utilization of petroleum, frequent accidents and oil spills create severe soil contamination. Currently, soil washing has become a preferred treatment option because it’s fast and environmentally accepted. [97,98] In particular, surfactant-enhanced soil washing is a versatile technique to remove the petroleum contaminant from the soil. [99] Surfactants could decrease the oil-water interfacial tension as well as attraction between oil and soil, thereby improving the solubility of oil contaminant [100]. The mixture of surfactant aqueous solution and oil usually exists in a stable O/W emulsion, which is difficult to separate oil from water. Therefore, a CO2 responsive emulsion that can be “switched off” is preferential. Jessop et al. [42] used three kinds of switchable anionic surfactants, i.e. phenolate, benzoate and carboxylate surfactants, to wash oil contaminated sand, and then used CO2 to treat the formed emulsion to further separate oil (Fig. 17). At 50 ℃, all surfactants can remove at least 90% of North Sea crude oil from the surface of Ottawa sand. Among them, sodium phenolate surfactants have the highest oil removal efficiency. After washing, two or three distinct layers were discovered: the solid sand, an opaque emulsion and sometimes a thin oil layer. The wash mixture was decanted and then treated with CO2, 12

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Fig. 17. Proposed scheme for soil washing using CO2 switchable anionic surfactants at 50 °C. [42].

and 1.6–5.3 wt % of amidine-containing comonomer (2-methyl-1-(4vinylbenzyl)-1,4,5,6-tetrahydropyrimidinium Bicarbonate) (Fig. 19). The PS particles surfaces covalently bounded the amidine moieties, which allowed them to be “switched on or off” by a CO2 trigger. Consequently, the latex particles could be easily coagulated after adding small amount of NaOH and then re-dispersed after CO2 bubbling. The coagulation and redispersion processes can be repeated for several cycles. What’s more, the same approach could be employed to produce reversibly coagulatable and re-dispersible latexes of St-butyl acrylate (BA), St-MMA, and St-acrylonitrile copolymer. This research group [107] also developed the emulsion polymerization of St using new synthetic (N-amidino)dodecyl acrylamide (DAm) as a switchable comonomer. However, about 20% of DAm was observed to be hydrolyzed during polymerization. In order to address the hydrolysis issue and

more effective when a CO2 switchable initiator 2, 2′-azobis[2-(2-imidazolin-2-yl)propane] (VA-061) and a switchable surfactant are used simultaneously. This aggregation and re-dispersion of PS or PMMA particles can be achieved for several cycles without any salt accumulation. Aryl amidine- and tertiary amine-based CO2 switchable surfactants have also allowed a similar way to prepare PS or PMMA latexes, achieving reversible coagulation and redispersion of latexes. [104,105] For the emulsion polymerization based on switchable surfactants, however, the surfactants only physically adsorb onto the latex particle surface and can be readily washed off. To overcome this limitation, a comonomer containing CO2 switchable group can be employed as a surfactant for emulsion polymerization, providing robust anchoring on the particle surface. Zhu et al. [106] prepared a reversibly coagulatable and re-dispersible PS latex via soap free emulsion polymerization of St

Fig. 18. Reversible coagulation and redispersion of PS latex in the presence of a CO2-switchable surfactant and initiator. [106]. 13

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Fig. 19. (top) Synthesis routes of a CO2 switchable amidine-based comonomer and its emulsion polymerization with St. (bottom) Reversible coagulation and redispersion of the formed PS latex. [106].

microgels with a hydrodynamic diameter of about 580 nm. The later addition of N2 to remove CO2 could induce the restoration of latex particles. This transition process can be repeated for at least 10 cycles without remarkable change in the size of the swollen microgels. Zhu and coworkers [118] prepared dual O2 and CO2 responsive microgels through SFEP of O2 responsive 2,3,4,5,6-pentafluorostyrene (FS) and CO2 responsive DEA in the presence of N,N′-methylenebis(acrylamide) (BisAM) cross-linker. The particles dispersed in aqueous solution can respond to O2 and CO2, showing significant volume expansion.

simplify the synthesis steps, commercially available comonomer were then employed in emulsion polymerization. Cunningham et al. [108,109] used a CO2 switchable comonomer DEAEMA and initiator VA-061 for surfactant-free emulsion copolymerization (SFEP) of St or MMA, preparing readily coagulated and re-dispersed polymer latexes. Zhu group [110] prepared CO2 re-dispersible latexes via SFEP of MMA and BA with the same comonomer. The resulting latexes can be easily coagulated by adding small amount of NaOH. The re-dispersibility of the latexes driven by CO2, however, is dependent on the glass transition temperature (Tg), which is adjusted by the MMA/BA ratio in copolymer. For high MMA content latexes, Tg was higher than ambient conditions and the latexes treated with NaOH could be well re-dispersed by bubbling CO2 and sonication. For high BA content latexes, however, Tg was lower than ambient temperature, and the addition of NaOH resulted in the fusion of individual particles without re-dispersibility. DMAEMA, [111–113] DMAPMAm [114], 2-(dimethylamino)ethyl methacrylate (DMA) [115], 11-(diethylamino)undecyl 2bromo-2-methylpropanoate (BrC11 N) [116] and N-methacryloyl-11aminoundecanoic acid [117] were also reported to be used as CO2 switchable comonomers for preparing readily coagulated and re-dispersed polymer latexes via emulsion polymerization. Responsive polymer latexes were also reported to be synthesized by emulsion polymerization using CO2 switchable solvents. Jessop and coworkers [19] applied switchable water (SW) to emulsion polymerization of St. The role of SW in the preparation of CO2 responsive emulsions has been described in section 3.1. For PS latexes containing SW, the addition of CO2 resulted in a remarkable rise of the ionic strength and thus allowed the latexes to coagulation, while the later removal of CO2 decreased the ionic strength and thus brought about the re-dispersion of the aggregated PS latexes. Besides polymer latexes, CO2 responsive emulsions can also be used to synthesize polymer microgels via emulsion polymerization. Armes et al. [26] prepared polymer microgels based on the CO2 responsive latexes. The latexes were synthesized via SFEP of DEA using a PEGMA stabilizer and a divinylbenzene (DVB) cross-linker. Within 2 min of CO2 sparging, a latex-to-microgel swelling transition was observed, forming

4.4. Chemical reactions Emulsion, especially microemulsion, has proved useful as a microreactor for chemical reactions. Taking into account product separation and emulsion recycling, an emulsion that can be “switched off” by a trigger is preferred for chemical reactions. Wang et al. [73] used CO2 responsive water-in-ILs microemulsions as reaction template media for the Knoevenagel condensation reaction (Fig. 20). Malononitrile and benzaldehyde could react in microemulsion droplets to form 2-benzylidenemalononitrile, in which the microemulsions exhibit excellent catalytic activity. After bubbling CO2 for 10 min, the microemulsion was destabilized and the products suspended in the water phase. The resulting products could be separated from the reaction mixture simply by filtration, and the microemulsion system could be readily recycled upon N2 bubbling. In addition, the biphasic biocatalyses such as the hydrolysis of olive oil and the esterification of octanol with oleic acid can also be performed in CO2 responsive Pickering oil-in-water emulsions. [119] In the Pickering emulsions, the enzyme displays a high reaction efficiency and can be simply recycled and re-used upon alternatively treating with CO2 and N2. 5. Conclusions and perspectives Emulsion is one of the most commonly used materials in a variety of industries. Depending on the application, effective and rapid destruction of emulsions stability may be required after their service. Recently, 14

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Fig. 20. (top) Reaction of benzaldehyde with malononitrile. (bottom) The coupling of the chemical reaction, product separation and recycling of the n-pentanol/ [C12DMEA][Im]/H2O microemulsion. [73].

important for investigating the mechanism of CO2 response in emulsions. Lastly, the exploitation of CO2 responsive emulsions is mainly confined to the academic lab bench, rarely entered the industrial research to become true green technologies. Furthermore, the potential applications of CO2 responsive emulsions are primarily concentrated in the oil industry, seldom involving other industries. It’s essential to implement this green idea instead of a less green process and develop new applications. Future work includes the development of design principles for CO2 responsive emulsions, a search for more examples to enrich the responsive emulsion systems, and true application of this green idea in more industries.

responsive emulsions have attracted much attention and possess broad application prospects, mainly due to their tunable stability by triggers, allowing a simple demulsification approach with low economic cost. CO2 responsive emulsion is considered as an innovative and green class of stimuli responsive emulsions, employing a greener trigger that is abundant, non-accumulating in the system and easily-removed. This article provides a comprehensive summary of CO2 responsive emulsion systems, ranging from the basic response mechanism to potential industrial applications. The CO2 responsive emulsion is usually prepared with a CO2 switchable emulsifier or solvent. The responsive materials could be protonation or deprotonation by addition or removal of CO2, resulting in a dramatic change of emulsion stability. However, the recent CO2 responsive emulsion systems still have somewhat disadvantages. Firstly, the responsiveness of many systems is only evaluated by the visual change of emulsion stability, which just indicates that the emulsion can respond to CO2. However, the protonation degree of materials, additive amount of CO2, persistence and inevitable losses after cycle, effects of material properties and local environment on the responsiveness, and bioaccumulation in the environment are somewhat unclear and require a more rigorous analysis. If CO2 responsive emulsion systems are to be adopted industrially, then more precise control over responsiveness will be necessary to enhance the efficiency and recyclability of response process. Secondly, more green, efficient and inexpensive switchable materials need to be searched to prepare CO2 responsive emulsions. Currently, the employed switchable functional groups for responsive emulsions are mainly amidine or amine groups. High cost manufacturing of CO2 switchable materials containing amidine functional group, and hydrolytic instability of amidine group indicate that CO2 responsive emulsions based on amidine are not anticipated for practical application. CO2 switchable materials containing amine group are readily synthesized and even commercially available, but they may pose safety and environmental risks in terms of toxicity, flammability, volatility and bioaccumulation. Therefore, further improvements are needed for construction of more green and efficient responsive emulsions, such as the design of amines incorporating other functional groups and the exploration of new switchable ILs. Fundamental research on CO2 responsive emulsions is also necessary, such as whether CO2 and responsive groups can fully react, how is the CO2 removal process achieved? And whether the properties of the emulsion after CO2 removal will change. These basic studies are

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