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Supercritical CO2-continuous microemulsions and compressed CO2-expanded reverse microemulsions
J. of Supercritical Fluids 47 (2009) 531–536
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Review
Supercritical CO2 -continuous microemulsions and compressed CO2 -expanded reverse microemulsions Jianling Zhang, Buxing Han ∗ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
a r t i c l e
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Article history: Received 7 July 2008 Received in revised form 26 August 2008 Accepted 26 August 2008 Keywords: Microemulsion CO2 Reverse micelles Properties Applications
a b s t r a c t This review describes recent advances in the supercritical (SC) CO2 -continuous microemulsions, compressed CO2 -expanded water-in-oil microemulsions, and compressed CO2 -induced water-in-oil microemulsions, and pays more attention to tuning the aggregation behavior of surfactants in liquid solvents using compressed CO2 . The applications of these microemulsions in synthesis of nanomaterials and chemical reactions are also discussed briefly. © 2008 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supercritical CO2 -continuous microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Water-in-CO2 microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Ionic liquid-in-CO2 microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressed CO2 -expanded water-in-oil microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thermodynamics of micellizaiton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressed CO2 -assisted formation of water-in-oil microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A microemulsion is a thermodynamically stable dispersion of two immiscible fluids stabilized by surfactants. There are roughly three types of microemulsions, including water-in-oil, bicontinuous and oil-in-water microemulsions. A “water-inoil” microemulsion is formed when water is dispersed in a hydrocarbon-based continuous phase. The “water pool” character-
∗ Corresponding author. Tel.: +86 10 62562821; fax: +86 10 62559373. E-mail address: [email protected] (B. Han). 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.08.014
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ized by the water-to-surfactant molar ratio (W0 ) is usually regarded as a novel microreactor for carrying out reactions or material synthesis. Up to now, water-in-oil microemulsions have been widely used in many fields, such as chemical reactions, nanoparticle synthesis, extraction of hydrophiles and proteins, and pharmaceutical and cosmetic industry [1–3]. Supercritical fluids (SCFs) have received much attention in recent years. Supercritical (SC) or compressed CO2 , in particular, is most attractive because it is readily available, inexpensive, nontoxic, nonflammable, and has moderate critical temperature and pressure (31.1 ◦ C and 7.38 MPa). Besides, it can be easily recaptured and recycled after use. Such advantages combined with
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unique properties of SCFs such as adjustable solvent power and enhanced mass transfer characteristics have offered the opportunity to replace conventional organic solvents in a variety of applications in chemistry and reaction engineering [4–6]. However, CO2 has no dipole moment, very low dielectric constant and polarizability per volume. Consequently, high molecular weight or hydrophilic solutes such as proteins, metal ions and most polymers are only sparingly soluble or insoluble in CO2 . One effective way to make CO2 accessible for dissolving proteins and other nonvolatile hydrophilic substances is using suitable surfactants to produce SC CO2 -continuous microemulsions. During the last two decades, many research groups have studied the formation of microemulsions with SC CO2 as continuous phase and their possible applications in chemical reactions and nanoparticle synthesis [7,8]. On the other hand, compressed CO2 is quite soluble in many organic solvents and the solubility depends on temperature and pressure. Dissolution of CO2 in liquids can change the properties of the liquid solvents considerably. Therefore, the properties of liquid solvents can be tuned continuously by controlling pressure of CO2 . Recently, the fundamental researches on gas-expanded liquids (mainly traditional organic solvents) and their applications in different processes, such as tuning properties of chemical reactions, material processing, promoting polymerization, and as mobile phase of HPLC have been reviewed by Philip G. Jessop and Bala Subramaniam [9]. To combine the water-in-oil microemulsion and compressed CO2 is a very interesting topic. It is expected that the properties of the water-in-oil microemulsions can be tuned easily by the pressure and temperature of CO2 because of the tunable nature of compressed CO2 . In recent years, some researches have been carried out on the combination of compressed CO2 and water-in-oil microemulsions, including mainly compressed CO2 -expanded water-in-oil microemulsions and compressed CO2 -assisted formation of water-in-oil microemulsions, where compressed CO2 is used to tune the properties of waterin-oil microemulsions and to induce the formation of water-in-oil microemulsions, respectively. In comparison with the conventional methods to tune the properties of water-in-oil microemulsions or assist the formation of water-in-oil microemulsions (adding additives, changing pH, heating), the use of CO2 has many advantages. For example, reversible control of the properties of water-in-oil microemulsions by CO2 can be accomplished just by pressurization and depressurization; CO2 can be easily removed, which makes the post-process much easier; CO2 is environmentally benign. This review describes the recent advances in the above three types of microemulsions related with supercritical or compressed CO2 , i.e. SC CO2 -continuous microemulsion, compressed CO2 -expanded water-in-oil microemulsions, and compressed CO2 -assisted formation of water-in-oil microemulsions.
2. Supercritical CO2 -continuous microemulsions Up to now, for most of the CO2 -continuous microemulsions, water is used as the dispersed phase, forming the water-in-CO2 microemulsions. The water droplets dispersed in supercritical CO2 have been applied to the nanoparticle synthesis and chemical reactions, which showed special properties over the conventional water-in-oil microemulsions. Recently, the formation of ionic liquid-in-CO2 microemulsion was reported, of which the nanosized ionic liquid droplets are dispersed in supercritical CO2 with the aid of a perfluorinated surfactant [10]. Due to the unique features of ionic liquid, this kind of ionic liquid-in-CO2 microemulsion may find various potential applications.
2.1. Water-in-CO2 microemulsion The design of surfactants compatible with CO2 is crucial for the formation of stable water-in-CO2 microemulsions. Early attempts by Consani and Smith to disperse water into SC CO2 phase showed that most conventional surfactants are insoluble in CO2 [11]. Later, Hoefling et al. designed the effective fluorosurfactants for compatibility with CO2 [12]. The fluorinated chains represent low cohesive energy density groups thereby promoting low solubility parameters and low polarizability. Since then, many papers on the design of CO2 -compatible surfactants and formation of water-in-CO2 microemulsions have emerged. The most effective compounds that have been reported to stabilize water-in-CO2 microemulsions are the partially or fully fluorinated surfactants [13–20], especially the fluorinated sodium bis(2-ethylhexyl)sulfosuccinate (AOT) analogue [21–25]. Although high CO2 compatibility can be achieved by fluorinated surfactants, the cost of fluorinated compounds is high and they are toxic. On consideration of the environmental and economical factors, some efforts have been made for the hydrocarbon surfactants [26–31], hybrid fluorocarbon-hydrocarbon surfactants [32–34], and O-surfactant, by incorporating oxygen into the surfactant tails [35]. Different techniques of NMR [36,37], SANS [38,39], SAXS [27], FTIR [40], electrical conductivity [41], UV–vis spectra [27,42], etc. have been used to characterize the water-in-CO2 microemulsions. The water core of water-in-CO2 microemulsion provides nanoreactors for the synthesis of nanoparticles [7,43]. The advantages of using SC CO2 as the continuous phase over conventional organic solvents for the synthesis of nanoparticles are that, in addition to being nontoxic and nonflammable, CO2 has low viscosity and high diffusion coefficient. Moreover, the stability of water-in-CO2 microemulsions depends on the density of SC CO2 . Therefore, the breakdown of the microemulsions can be accomplished simply by controlling the temperature and/or pressure of the system, leading to direct deposition of nanoparticles. Different inorganic nanoparticles have been successfully synthesized in water-in-CO2 microemulsions, including metals of Ag [44–49], Cu [50], Pd [51], and Rh [52], metal oxides of TiO2 [53] and zirconia [54], metal halides of AgI [55], silver halide [56,57], and metal sulfide of Ag2 S [58], CdS and ZnS [59,60]. Besides, Wai et al. synthesized the carbon nanotube-supported metallic nanoparticles (Pd, Rh, and bimetallic Pd–Rh) with diameters in the range 2–10 nm at room temperature [61]. Ye et al. reported the microemulsion polymerization in SC CO2 and nanoparticles with sizes less than 20 nm in diameter were obtained [62]. The water-in-CO2 microemulsions have been applied to the chemical reactions. Wai et al. reported the preparation of nanometer-sized metallic palladium particles in a water-in-CO2 microemulsion by hydrogen reduction of Pd2+ and the subsequent in situ hydrogenation of olefins catalyzed by the Pd nanoparticles in SC CO2 microemulsion [51]. The method is not limited to palladium [63–65]; other metal nanoparticles such as rhodium [52] were also synthesized and dispersed in SC CO2 microemulsion using a similar approach for various catalysis. Tsang et al. demonstrated that the stabilized water droplet dispersed in SC CO2 microemulsion is an excellent alternative solvent system to acetic acid for air oxidation of a number of alkyl aromatic hydrocarbons using Co(II) species at mild conditions [66,67]. Johnston et al. have demonstrated that organic synthesis between hydrophilic nucleophiles (Br- and water itself) and CO2 -soluble reactants can be performed in a water-in-CO2 microemulsion containing small amounts (<1.5 wt%) of surfactant, PFPE COO− NH4 + [68].
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2.2. Ionic liquid-in-CO2 microemulsion Ionic liquids have attracted much attention. They can dissolve many organic and inorganic substances, and their properties are tunable to satisfy the requirements of a variety of tasks. Many ionic liquids can be considered as cleaner solvents than conventional solvents because they are nonvolatile and can be designed to be nontoxic or low toxic. It was reported that ionic liquids could dissolve SC CO2 , whereas the solubility of ionic liquids in SC CO2 was negligible in the absence of cosolvents [69,70]. Recently, a novel ionic liquid-in-CO2 microemulsion was prepared. Three kinds of ionic liquids, 1,1,3,3tetramethylguanidinium acetate, 1,1,3,3-tetramethylguanidinium lactate, and 1,1,3,3-tetramethylguanidinium trifluoroacetate, could be dispersed in SC CO2 with the aid of surfactant N-ethyl perfluorooctylsulfonamide [10]. Gold nanoparticles were prepared in the reverse micelles with ionic liquid domains by rapid expansion of a supercritical solution into a liquid solvent. These ionic liquidin-CO2 microemulsions have some obvious advantages, especially when water sensitive compounds are involved. 3. Compressed CO2 -expanded water-in-oil microemulsions 3.1. Properties The traditional organic solvents can dissolve large amounts of CO2 and expand greatly, which has been very fully described in Ref. [9]. The dissolution of CO2 in oil-continuous microemulsion with water as dispersed phase can cause a volume expansion, which is characterized by the volume expansion coefficient V = (V − V0 )/V0 , where V and V0 are the volumes of the CO2 -saturated and CO2 -free solutions, respectively. The effects of different experimental conditions on V were studied. V increases with pressure because the concentration of CO2 in the solutions is higher at higher pressure [71]; at a given pressure, V decreases with increasing chain length of the solvent and temperature [71,72]; V decreases slightly as the concentration of AOT was increased, and the variation in W0 has a negligible effect on V [73]. An important property of reverse micelles is their solubilization capacity for water as microdroplets dispersed in the oil phase. The solubilization capacity of water in reverse micelles depends on many factors, such as the nature of surfactant, solvent, temperature, and the presence of cosurfactant and electrolytes. It was demonstrated that the amount of solubilized water is increased considerably by CO2 in suitable CO2 pressure ranges [71,74]. A possible reason is that CO2 penetrates the interfacial film of the reverse micelles and results in a more rigid, hardened interfacial film, which reduces the attractive interactions between the droplets. In addition, compressed CO2 could enhance the solubilization of bovine serum albumin (BSA) in water/AOT/isooctane reverse micelles [73] and solubilization of ionic liquid [bmim][BF4 ] in TX-100 reverse micelles [75]. The microproperties and microstructures of these CO2 -expanded microemulsions were studied by FTIR and UV–vis [76], Fluorescence [77], conductivities [78] and small-angle X-ray scattering [78,79]. 3.2. Thermodynamics of micellizaiton The critical micelle concentration (cmc) values of AOT in isooctane at different CO2 pressures were measured by UV–vis spectroscopy [72]. At a fixed temperature, the cmc value decreases with increasing CO2 concentration at lower XCO2 and then increases with XCO2 after passing through a minimum. This means that addi-
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tion of CO2 is favorable to the formation of the reverse micelles in a suitable CO2 concentration range. The thermodynamics of micellization at different CO2 pressures were calculated using cmc at different temperatures. The G0m value is negative at all the experimental conditions, and a minimum occurs at a certain XCO2 value. Thereby, the formation of AOT micelles is spontaneous and thermodynamically favored, especially at XCO2 value where the minima occur. In other words, addition of CO2 at suitable pressure is favorable to the formation of the reverse micelles. The H0m is positive, indicating that formation of the reverse micelles is an endothermic process. The positive S0m value indicates that the system becomes more disordered during the micellization process in the solvent. According to the idea of enthalpy-entropy compensation, the contribution of the enthalpy (H0m ) for the Gibbs free energy is small compared to the entropy term (TS0m ), that is, the entropy change is the major driving force for the formation of the reverse micelles. 3.3. Applications The use of water droplets in the reverse micelles as an environment for nanoparticle synthesis has attracted much attention. Recovery of the nanoparticles from the reverse micelles is one of the main steps in this method, and some methods such as flocculation, evaporation to dryness or adding certain chemical reagent to cause phase separation, have been used to recover the particles, which precipitate the surfactant simultaneously. The post-process is troublesome because the products contain large amount of surfactant. A method to recover inorganic nanoparticles synthesized in reverse micelles was proposed by compressed CO2 . It was found that almost all the nanoparticles synthesized in reverse micelles could be recovered by compressed CO2 , while the surfactants remain in the solution. That is, the separation of inorganic nanoparticles from microemulsion can be realized by the easy control of CO2 pressure. By this technique, well dispersed ZnS [80,81], Ag [82] and TiO2 [83] nanoparticles with narrow size distribution were obtained. Based on this principle, different kinds of nano-scale composites were fabricated, including ZnS/MCM-41 [84], TiO2 –SiO2 [85], CdS/ZnS [86], Ag/polystyrene nanospheres [87,88], cadmium sulfide/poly(methyl methacrylate) [89] and cadmium sulfide/polyacrylamide [90]. In addition, it was revealed that the proteins can be precipitated from the reverse micelles by compressed CO2 [91–93]. Using this concept, protein nanoparticles, dendritic crosslinked enzyme aggregates [94], Ag/BSA composites [95] and ␣-chymotrypsin/polyvinylpyrrolidone composites were obtained [96]. The use of micellar water droplets as an environment for chemical reactions has also attracted much attention. In microemulsions, high concentrations of both hydrophilic and hydrophobic reactants can be dissolved simultaneously. Solubilization of immiscible reactants can lead to an increase in reaction rate. In addition, the reaction rates can be tuned by the microproperties of microemulsion. How compressed CO2 affects the catalytic activity of biocatalysts in reverse micelles is a very interesting topic, since the addition of CO2 into the reverse micellar solution can effectively change the microproperties of microemulsions. The chlorination of 1,3-dihydroxybenzene in CTAC reverse micellar solution catalyzed by chloroperoxidase (CPO) was studied at different CO2 pressures [97]. The specific activity of CPO in reverse micelles increases with increasing CO2 pressure. The reduction in the viscosity of the solution by adding CO2 is believed to be one of the main reasons for the higher specific activity of CPO in CO2 -expanded reverse micelles.
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4. Compressed CO2 -assisted formation of water-in-oil microemulsions Some copolymers can form reverse micelles in organic solvents, and different methods to induce micellization, such as changing temperature and adding salts, have been studied [98]. The formation of copolymer reverse micelles induced by CO2 was investigated. It was discovered that compressed CO2 can facilitate the formation of reverse micelles of the copolymer (EO)27 (PO)61 (EO)27 (P104) in p-xylene [99]. The pressure at which reverse micelles begin to form is defined as critical micelle pressure (cmp). The effect of structure of PEO–PPO–PEO copolymers on reverse micelle formation induced by compressed CO2 was further studied and it was found that both the EO ratios and the molecular weights are important for reverse micelle formation [100]. Kho et al. also reported the preparation of pressure-tunable fluorinated microemulsions at water to surfactant molar ratios of 20 and temperatures up to 45 ◦ C using CO2 expansion [101]. The unique advantage of this kind of reverse micelles is that the formation and breaking of reverse micelles can be repeated easily by controlling the pressure. The research on the applications of this kind of microemulsion is very interesting. The hydrolysis of benzoyl chloride (BzCl), p-nitrophenyl chloroformate (NPhCl) is performed in the P104/H2 O/p-xylene/CO2 microemulsions [102]. It was revealed that the pseudo-first-order rate constants kobs = k[H2 O] at W0 = 3.08 is almost ten times faster than that at W0 = 1.03 for the hydrolysis of BzCl, and the effect of W0 on the rate constant of NPhCl is smaller than that of BzCl. The kobs of NPhCl hydrolysis is larger than that of BzCl at W0 = 2.2, and becomes smaller at W0 = 2.2. This was explained by the difference in reaction mechanism of the two substrates. As the hydrolysis of the substrates is sensitive to the polarity and nucleophilicity of the water, the variation of W0 will affect the kinetics of the hydrolysis. Nanometer-sized gold particles were synthesized by the reduction of HAuCl4 with KBH4 in the CO2 -induced microemulsion of (EO)27 (PO)61 (EO)27 /p-xylene/CO2 /H2 O [102]. Since the breaking of the reverse micelles can be accomplished simply by the venting of CO2 , the solubilized Au nanoparticles in the reverse micelles can be precipitated after releasing CO2 , while the surfactant remains in the organic phase. The excellent reversibility of the reverse micelle system is a unique feature and it is advantageous for the recovery of the nanoparticles synthesized in the reverse micelles. The effects of the molar ratio of the reductant to HAuCl4 , the concentration of the reactants, and the molar ratio of water to EO segments (W0 ) in the reverse micelles on the size of the gold particles were studied. 5. Others Except for the above water-in-oil microemulsions, some recent work reveals that compressed CO2 also has remarkable effects on other surfactant systems such as emulsions and aqueous micellar solutions. It was found that the addition of compressed CO2 into turbid water/oil/surfactant emulsion could induce the transparency of system [103]. It is known that both microand nanoemulsions are transparent. Sedimentation analysis was used to differentiate between them because a microemulsion is thermodynamically stable whereas a nanoemulsion is not. It was demonstrated that the transparent emulsion was not thermodynamically stable. Therefore, the transparent emulsion is a nanoemulsion with submicrometer droplet size. This kind of CO2 induced nanoemulsion has the following special advantages: (1) the surfactant concentration can be very low; (2) the nanoemulsions can be formed in a wide range of water-to-oil volume ratios;
(3) formation and breakage of the nanoemulsions are reversible and can be controlled by pressurization and depressurization; (4) the gas can be easily removed by reducing the pressure; (5) the method is environmentally benign. The applications of CO2 -induced nanoemulsions were explored in the preparation of cross-linked porous polystyrene materials as well as the enhanced oil recovery. Besides, it was demonstrated that CO2 could reduce the cloud point temperature (CPT) of the aqueous micellar solutions considerably [104]. On the basis of this finding, a new route to separate phenol or vanadium ion from water by combination of Triton X-100 and CO2 was developed. Also, gold nanoparticles synthesized in Triton X-100 micellar solutions could be recovered using CO2 while the surfactant remained in the solution. This is attractive because recovery of gold nanoparticles is very convenient. This separation method has some unusual advantages, such as high separation efficiency, simple posttreatment process, and lower separation temperature, which is especially advantageous when temperature-sensitive substances are involved. 6. Conclusions and perspectives Study on the microemulsions with SC CO2 as continuous phase and tuning the properties of water-in-oil microemulsions using compressed CO2 are interesting topics. Some future researches on these are in the following: • For the microemulsions with SC CO2 -continuous phase, much work have been done on the formation of the water-in-CO2 microemulsions. Additional work should be carried out especially using lower cost surfactants and reducing operation pressure. Moreover, other CO2 -continuous microemulsions with the different polar cores should be designed, particularly, the ionic liquid-in-CO2 microemulsions. The ionic liquid droplets have many advantages, for example, they can dissolve many organic and inorganic substances, and their properties are tunable to satisfy a variety of tasks. So the ionic liquid-in-CO2 microemulsions will show many unique features and can satisfy many different requirements of applications. • Most of the applications of SC CO2 -continuous microemulsions in nanomaterial synthesis is limited to the simple preparation of inorganic nanoparticles. On consideration of the practical applications of nanomaterials, more attentions should be paid to the preparation of nano-scale composites using SC CO2 -continuous microemulsions, maybe in combination with other techniques or principles. • Tuning the properties of surfactant solutions using CO2 is a new topic and the CO2 -expanded water-in-oil microemulsions are new type of microemulsions, which exhibit unique properties in the synthesis of nanomaterials and chemical reactions because the properties of the microemulsions can be tuned continuously by pressure and the operation pressure is relatively low. Their applications can be extended to the synthesis of a variety of nanomaterials and carrying out chemical reactions. It is expected that different processes will be tested in the future. Their performance and applicability should be evaluated aiming to the industrial scale. • CO2 is a nonpolar molecule, which is very different form the conventionally used cosurfactants, such as alcohols. The mechanism for CO2 to stabilize reverse micelles may be very different from that of conventionally used cosurfactants. This is an interesting topic to be studied further, and the progress on this will contribute greatly to colloid and interface science.
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Review
Supercritical CO2 -continuous microemulsions and compressed CO2 -expanded reverse microemulsions Jianling Zhang, Buxing Han ∗ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
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Article history: Received 7 July 2008 Received in revised form 26 August 2008 Accepted 26 August 2008 Keywords: Microemulsion CO2 Reverse micelles Properties Applications
a b s t r a c t This review describes recent advances in the supercritical (SC) CO2 -continuous microemulsions, compressed CO2 -expanded water-in-oil microemulsions, and compressed CO2 -induced water-in-oil microemulsions, and pays more attention to tuning the aggregation behavior of surfactants in liquid solvents using compressed CO2 . The applications of these microemulsions in synthesis of nanomaterials and chemical reactions are also discussed briefly. © 2008 Elsevier B.V. All rights reserved.
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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supercritical CO2 -continuous microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Water-in-CO2 microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Ionic liquid-in-CO2 microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressed CO2 -expanded water-in-oil microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thermodynamics of micellizaiton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Compressed CO2 -assisted formation of water-in-oil microemulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction A microemulsion is a thermodynamically stable dispersion of two immiscible fluids stabilized by surfactants. There are roughly three types of microemulsions, including water-in-oil, bicontinuous and oil-in-water microemulsions. A “water-inoil” microemulsion is formed when water is dispersed in a hydrocarbon-based continuous phase. The “water pool” character-
∗ Corresponding author. Tel.: +86 10 62562821; fax: +86 10 62559373. E-mail address: [email protected] (B. Han). 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.08.014
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ized by the water-to-surfactant molar ratio (W0 ) is usually regarded as a novel microreactor for carrying out reactions or material synthesis. Up to now, water-in-oil microemulsions have been widely used in many fields, such as chemical reactions, nanoparticle synthesis, extraction of hydrophiles and proteins, and pharmaceutical and cosmetic industry [1–3]. Supercritical fluids (SCFs) have received much attention in recent years. Supercritical (SC) or compressed CO2 , in particular, is most attractive because it is readily available, inexpensive, nontoxic, nonflammable, and has moderate critical temperature and pressure (31.1 ◦ C and 7.38 MPa). Besides, it can be easily recaptured and recycled after use. Such advantages combined with
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unique properties of SCFs such as adjustable solvent power and enhanced mass transfer characteristics have offered the opportunity to replace conventional organic solvents in a variety of applications in chemistry and reaction engineering [4–6]. However, CO2 has no dipole moment, very low dielectric constant and polarizability per volume. Consequently, high molecular weight or hydrophilic solutes such as proteins, metal ions and most polymers are only sparingly soluble or insoluble in CO2 . One effective way to make CO2 accessible for dissolving proteins and other nonvolatile hydrophilic substances is using suitable surfactants to produce SC CO2 -continuous microemulsions. During the last two decades, many research groups have studied the formation of microemulsions with SC CO2 as continuous phase and their possible applications in chemical reactions and nanoparticle synthesis [7,8]. On the other hand, compressed CO2 is quite soluble in many organic solvents and the solubility depends on temperature and pressure. Dissolution of CO2 in liquids can change the properties of the liquid solvents considerably. Therefore, the properties of liquid solvents can be tuned continuously by controlling pressure of CO2 . Recently, the fundamental researches on gas-expanded liquids (mainly traditional organic solvents) and their applications in different processes, such as tuning properties of chemical reactions, material processing, promoting polymerization, and as mobile phase of HPLC have been reviewed by Philip G. Jessop and Bala Subramaniam [9]. To combine the water-in-oil microemulsion and compressed CO2 is a very interesting topic. It is expected that the properties of the water-in-oil microemulsions can be tuned easily by the pressure and temperature of CO2 because of the tunable nature of compressed CO2 . In recent years, some researches have been carried out on the combination of compressed CO2 and water-in-oil microemulsions, including mainly compressed CO2 -expanded water-in-oil microemulsions and compressed CO2 -assisted formation of water-in-oil microemulsions, where compressed CO2 is used to tune the properties of waterin-oil microemulsions and to induce the formation of water-in-oil microemulsions, respectively. In comparison with the conventional methods to tune the properties of water-in-oil microemulsions or assist the formation of water-in-oil microemulsions (adding additives, changing pH, heating), the use of CO2 has many advantages. For example, reversible control of the properties of water-in-oil microemulsions by CO2 can be accomplished just by pressurization and depressurization; CO2 can be easily removed, which makes the post-process much easier; CO2 is environmentally benign. This review describes the recent advances in the above three types of microemulsions related with supercritical or compressed CO2 , i.e. SC CO2 -continuous microemulsion, compressed CO2 -expanded water-in-oil microemulsions, and compressed CO2 -assisted formation of water-in-oil microemulsions.
2. Supercritical CO2 -continuous microemulsions Up to now, for most of the CO2 -continuous microemulsions, water is used as the dispersed phase, forming the water-in-CO2 microemulsions. The water droplets dispersed in supercritical CO2 have been applied to the nanoparticle synthesis and chemical reactions, which showed special properties over the conventional water-in-oil microemulsions. Recently, the formation of ionic liquid-in-CO2 microemulsion was reported, of which the nanosized ionic liquid droplets are dispersed in supercritical CO2 with the aid of a perfluorinated surfactant [10]. Due to the unique features of ionic liquid, this kind of ionic liquid-in-CO2 microemulsion may find various potential applications.
2.1. Water-in-CO2 microemulsion The design of surfactants compatible with CO2 is crucial for the formation of stable water-in-CO2 microemulsions. Early attempts by Consani and Smith to disperse water into SC CO2 phase showed that most conventional surfactants are insoluble in CO2 [11]. Later, Hoefling et al. designed the effective fluorosurfactants for compatibility with CO2 [12]. The fluorinated chains represent low cohesive energy density groups thereby promoting low solubility parameters and low polarizability. Since then, many papers on the design of CO2 -compatible surfactants and formation of water-in-CO2 microemulsions have emerged. The most effective compounds that have been reported to stabilize water-in-CO2 microemulsions are the partially or fully fluorinated surfactants [13–20], especially the fluorinated sodium bis(2-ethylhexyl)sulfosuccinate (AOT) analogue [21–25]. Although high CO2 compatibility can be achieved by fluorinated surfactants, the cost of fluorinated compounds is high and they are toxic. On consideration of the environmental and economical factors, some efforts have been made for the hydrocarbon surfactants [26–31], hybrid fluorocarbon-hydrocarbon surfactants [32–34], and O-surfactant, by incorporating oxygen into the surfactant tails [35]. Different techniques of NMR [36,37], SANS [38,39], SAXS [27], FTIR [40], electrical conductivity [41], UV–vis spectra [27,42], etc. have been used to characterize the water-in-CO2 microemulsions. The water core of water-in-CO2 microemulsion provides nanoreactors for the synthesis of nanoparticles [7,43]. The advantages of using SC CO2 as the continuous phase over conventional organic solvents for the synthesis of nanoparticles are that, in addition to being nontoxic and nonflammable, CO2 has low viscosity and high diffusion coefficient. Moreover, the stability of water-in-CO2 microemulsions depends on the density of SC CO2 . Therefore, the breakdown of the microemulsions can be accomplished simply by controlling the temperature and/or pressure of the system, leading to direct deposition of nanoparticles. Different inorganic nanoparticles have been successfully synthesized in water-in-CO2 microemulsions, including metals of Ag [44–49], Cu [50], Pd [51], and Rh [52], metal oxides of TiO2 [53] and zirconia [54], metal halides of AgI [55], silver halide [56,57], and metal sulfide of Ag2 S [58], CdS and ZnS [59,60]. Besides, Wai et al. synthesized the carbon nanotube-supported metallic nanoparticles (Pd, Rh, and bimetallic Pd–Rh) with diameters in the range 2–10 nm at room temperature [61]. Ye et al. reported the microemulsion polymerization in SC CO2 and nanoparticles with sizes less than 20 nm in diameter were obtained [62]. The water-in-CO2 microemulsions have been applied to the chemical reactions. Wai et al. reported the preparation of nanometer-sized metallic palladium particles in a water-in-CO2 microemulsion by hydrogen reduction of Pd2+ and the subsequent in situ hydrogenation of olefins catalyzed by the Pd nanoparticles in SC CO2 microemulsion [51]. The method is not limited to palladium [63–65]; other metal nanoparticles such as rhodium [52] were also synthesized and dispersed in SC CO2 microemulsion using a similar approach for various catalysis. Tsang et al. demonstrated that the stabilized water droplet dispersed in SC CO2 microemulsion is an excellent alternative solvent system to acetic acid for air oxidation of a number of alkyl aromatic hydrocarbons using Co(II) species at mild conditions [66,67]. Johnston et al. have demonstrated that organic synthesis between hydrophilic nucleophiles (Br- and water itself) and CO2 -soluble reactants can be performed in a water-in-CO2 microemulsion containing small amounts (<1.5 wt%) of surfactant, PFPE COO− NH4 + [68].
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2.2. Ionic liquid-in-CO2 microemulsion Ionic liquids have attracted much attention. They can dissolve many organic and inorganic substances, and their properties are tunable to satisfy the requirements of a variety of tasks. Many ionic liquids can be considered as cleaner solvents than conventional solvents because they are nonvolatile and can be designed to be nontoxic or low toxic. It was reported that ionic liquids could dissolve SC CO2 , whereas the solubility of ionic liquids in SC CO2 was negligible in the absence of cosolvents [69,70]. Recently, a novel ionic liquid-in-CO2 microemulsion was prepared. Three kinds of ionic liquids, 1,1,3,3tetramethylguanidinium acetate, 1,1,3,3-tetramethylguanidinium lactate, and 1,1,3,3-tetramethylguanidinium trifluoroacetate, could be dispersed in SC CO2 with the aid of surfactant N-ethyl perfluorooctylsulfonamide [10]. Gold nanoparticles were prepared in the reverse micelles with ionic liquid domains by rapid expansion of a supercritical solution into a liquid solvent. These ionic liquidin-CO2 microemulsions have some obvious advantages, especially when water sensitive compounds are involved. 3. Compressed CO2 -expanded water-in-oil microemulsions 3.1. Properties The traditional organic solvents can dissolve large amounts of CO2 and expand greatly, which has been very fully described in Ref. [9]. The dissolution of CO2 in oil-continuous microemulsion with water as dispersed phase can cause a volume expansion, which is characterized by the volume expansion coefficient V = (V − V0 )/V0 , where V and V0 are the volumes of the CO2 -saturated and CO2 -free solutions, respectively. The effects of different experimental conditions on V were studied. V increases with pressure because the concentration of CO2 in the solutions is higher at higher pressure [71]; at a given pressure, V decreases with increasing chain length of the solvent and temperature [71,72]; V decreases slightly as the concentration of AOT was increased, and the variation in W0 has a negligible effect on V [73]. An important property of reverse micelles is their solubilization capacity for water as microdroplets dispersed in the oil phase. The solubilization capacity of water in reverse micelles depends on many factors, such as the nature of surfactant, solvent, temperature, and the presence of cosurfactant and electrolytes. It was demonstrated that the amount of solubilized water is increased considerably by CO2 in suitable CO2 pressure ranges [71,74]. A possible reason is that CO2 penetrates the interfacial film of the reverse micelles and results in a more rigid, hardened interfacial film, which reduces the attractive interactions between the droplets. In addition, compressed CO2 could enhance the solubilization of bovine serum albumin (BSA) in water/AOT/isooctane reverse micelles [73] and solubilization of ionic liquid [bmim][BF4 ] in TX-100 reverse micelles [75]. The microproperties and microstructures of these CO2 -expanded microemulsions were studied by FTIR and UV–vis [76], Fluorescence [77], conductivities [78] and small-angle X-ray scattering [78,79]. 3.2. Thermodynamics of micellizaiton The critical micelle concentration (cmc) values of AOT in isooctane at different CO2 pressures were measured by UV–vis spectroscopy [72]. At a fixed temperature, the cmc value decreases with increasing CO2 concentration at lower XCO2 and then increases with XCO2 after passing through a minimum. This means that addi-
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tion of CO2 is favorable to the formation of the reverse micelles in a suitable CO2 concentration range. The thermodynamics of micellization at different CO2 pressures were calculated using cmc at different temperatures. The G0m value is negative at all the experimental conditions, and a minimum occurs at a certain XCO2 value. Thereby, the formation of AOT micelles is spontaneous and thermodynamically favored, especially at XCO2 value where the minima occur. In other words, addition of CO2 at suitable pressure is favorable to the formation of the reverse micelles. The H0m is positive, indicating that formation of the reverse micelles is an endothermic process. The positive S0m value indicates that the system becomes more disordered during the micellization process in the solvent. According to the idea of enthalpy-entropy compensation, the contribution of the enthalpy (H0m ) for the Gibbs free energy is small compared to the entropy term (TS0m ), that is, the entropy change is the major driving force for the formation of the reverse micelles. 3.3. Applications The use of water droplets in the reverse micelles as an environment for nanoparticle synthesis has attracted much attention. Recovery of the nanoparticles from the reverse micelles is one of the main steps in this method, and some methods such as flocculation, evaporation to dryness or adding certain chemical reagent to cause phase separation, have been used to recover the particles, which precipitate the surfactant simultaneously. The post-process is troublesome because the products contain large amount of surfactant. A method to recover inorganic nanoparticles synthesized in reverse micelles was proposed by compressed CO2 . It was found that almost all the nanoparticles synthesized in reverse micelles could be recovered by compressed CO2 , while the surfactants remain in the solution. That is, the separation of inorganic nanoparticles from microemulsion can be realized by the easy control of CO2 pressure. By this technique, well dispersed ZnS [80,81], Ag [82] and TiO2 [83] nanoparticles with narrow size distribution were obtained. Based on this principle, different kinds of nano-scale composites were fabricated, including ZnS/MCM-41 [84], TiO2 –SiO2 [85], CdS/ZnS [86], Ag/polystyrene nanospheres [87,88], cadmium sulfide/poly(methyl methacrylate) [89] and cadmium sulfide/polyacrylamide [90]. In addition, it was revealed that the proteins can be precipitated from the reverse micelles by compressed CO2 [91–93]. Using this concept, protein nanoparticles, dendritic crosslinked enzyme aggregates [94], Ag/BSA composites [95] and ␣-chymotrypsin/polyvinylpyrrolidone composites were obtained [96]. The use of micellar water droplets as an environment for chemical reactions has also attracted much attention. In microemulsions, high concentrations of both hydrophilic and hydrophobic reactants can be dissolved simultaneously. Solubilization of immiscible reactants can lead to an increase in reaction rate. In addition, the reaction rates can be tuned by the microproperties of microemulsion. How compressed CO2 affects the catalytic activity of biocatalysts in reverse micelles is a very interesting topic, since the addition of CO2 into the reverse micellar solution can effectively change the microproperties of microemulsions. The chlorination of 1,3-dihydroxybenzene in CTAC reverse micellar solution catalyzed by chloroperoxidase (CPO) was studied at different CO2 pressures [97]. The specific activity of CPO in reverse micelles increases with increasing CO2 pressure. The reduction in the viscosity of the solution by adding CO2 is believed to be one of the main reasons for the higher specific activity of CPO in CO2 -expanded reverse micelles.
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4. Compressed CO2 -assisted formation of water-in-oil microemulsions Some copolymers can form reverse micelles in organic solvents, and different methods to induce micellization, such as changing temperature and adding salts, have been studied [98]. The formation of copolymer reverse micelles induced by CO2 was investigated. It was discovered that compressed CO2 can facilitate the formation of reverse micelles of the copolymer (EO)27 (PO)61 (EO)27 (P104) in p-xylene [99]. The pressure at which reverse micelles begin to form is defined as critical micelle pressure (cmp). The effect of structure of PEO–PPO–PEO copolymers on reverse micelle formation induced by compressed CO2 was further studied and it was found that both the EO ratios and the molecular weights are important for reverse micelle formation [100]. Kho et al. also reported the preparation of pressure-tunable fluorinated microemulsions at water to surfactant molar ratios of 20 and temperatures up to 45 ◦ C using CO2 expansion [101]. The unique advantage of this kind of reverse micelles is that the formation and breaking of reverse micelles can be repeated easily by controlling the pressure. The research on the applications of this kind of microemulsion is very interesting. The hydrolysis of benzoyl chloride (BzCl), p-nitrophenyl chloroformate (NPhCl) is performed in the P104/H2 O/p-xylene/CO2 microemulsions [102]. It was revealed that the pseudo-first-order rate constants kobs = k[H2 O] at W0 = 3.08 is almost ten times faster than that at W0 = 1.03 for the hydrolysis of BzCl, and the effect of W0 on the rate constant of NPhCl is smaller than that of BzCl. The kobs of NPhCl hydrolysis is larger than that of BzCl at W0 = 2.2, and becomes smaller at W0 = 2.2. This was explained by the difference in reaction mechanism of the two substrates. As the hydrolysis of the substrates is sensitive to the polarity and nucleophilicity of the water, the variation of W0 will affect the kinetics of the hydrolysis. Nanometer-sized gold particles were synthesized by the reduction of HAuCl4 with KBH4 in the CO2 -induced microemulsion of (EO)27 (PO)61 (EO)27 /p-xylene/CO2 /H2 O [102]. Since the breaking of the reverse micelles can be accomplished simply by the venting of CO2 , the solubilized Au nanoparticles in the reverse micelles can be precipitated after releasing CO2 , while the surfactant remains in the organic phase. The excellent reversibility of the reverse micelle system is a unique feature and it is advantageous for the recovery of the nanoparticles synthesized in the reverse micelles. The effects of the molar ratio of the reductant to HAuCl4 , the concentration of the reactants, and the molar ratio of water to EO segments (W0 ) in the reverse micelles on the size of the gold particles were studied. 5. Others Except for the above water-in-oil microemulsions, some recent work reveals that compressed CO2 also has remarkable effects on other surfactant systems such as emulsions and aqueous micellar solutions. It was found that the addition of compressed CO2 into turbid water/oil/surfactant emulsion could induce the transparency of system [103]. It is known that both microand nanoemulsions are transparent. Sedimentation analysis was used to differentiate between them because a microemulsion is thermodynamically stable whereas a nanoemulsion is not. It was demonstrated that the transparent emulsion was not thermodynamically stable. Therefore, the transparent emulsion is a nanoemulsion with submicrometer droplet size. This kind of CO2 induced nanoemulsion has the following special advantages: (1) the surfactant concentration can be very low; (2) the nanoemulsions can be formed in a wide range of water-to-oil volume ratios;
(3) formation and breakage of the nanoemulsions are reversible and can be controlled by pressurization and depressurization; (4) the gas can be easily removed by reducing the pressure; (5) the method is environmentally benign. The applications of CO2 -induced nanoemulsions were explored in the preparation of cross-linked porous polystyrene materials as well as the enhanced oil recovery. Besides, it was demonstrated that CO2 could reduce the cloud point temperature (CPT) of the aqueous micellar solutions considerably [104]. On the basis of this finding, a new route to separate phenol or vanadium ion from water by combination of Triton X-100 and CO2 was developed. Also, gold nanoparticles synthesized in Triton X-100 micellar solutions could be recovered using CO2 while the surfactant remained in the solution. This is attractive because recovery of gold nanoparticles is very convenient. This separation method has some unusual advantages, such as high separation efficiency, simple posttreatment process, and lower separation temperature, which is especially advantageous when temperature-sensitive substances are involved. 6. Conclusions and perspectives Study on the microemulsions with SC CO2 as continuous phase and tuning the properties of water-in-oil microemulsions using compressed CO2 are interesting topics. Some future researches on these are in the following: • For the microemulsions with SC CO2 -continuous phase, much work have been done on the formation of the water-in-CO2 microemulsions. Additional work should be carried out especially using lower cost surfactants and reducing operation pressure. Moreover, other CO2 -continuous microemulsions with the different polar cores should be designed, particularly, the ionic liquid-in-CO2 microemulsions. The ionic liquid droplets have many advantages, for example, they can dissolve many organic and inorganic substances, and their properties are tunable to satisfy a variety of tasks. So the ionic liquid-in-CO2 microemulsions will show many unique features and can satisfy many different requirements of applications. • Most of the applications of SC CO2 -continuous microemulsions in nanomaterial synthesis is limited to the simple preparation of inorganic nanoparticles. On consideration of the practical applications of nanomaterials, more attentions should be paid to the preparation of nano-scale composites using SC CO2 -continuous microemulsions, maybe in combination with other techniques or principles. • Tuning the properties of surfactant solutions using CO2 is a new topic and the CO2 -expanded water-in-oil microemulsions are new type of microemulsions, which exhibit unique properties in the synthesis of nanomaterials and chemical reactions because the properties of the microemulsions can be tuned continuously by pressure and the operation pressure is relatively low. Their applications can be extended to the synthesis of a variety of nanomaterials and carrying out chemical reactions. It is expected that different processes will be tested in the future. Their performance and applicability should be evaluated aiming to the industrial scale. • CO2 is a nonpolar molecule, which is very different form the conventionally used cosurfactants, such as alcohols. The mechanism for CO2 to stabilize reverse micelles may be very different from that of conventionally used cosurfactants. This is an interesting topic to be studied further, and the progress on this will contribute greatly to colloid and interface science.
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