Carbon Dioxide Microemulsions Research Articles

Supercritical carbon dioxide microemulsion stabilized with zwitterionic surfactant: A dissipative particle dynamics simulation study

Supercritical carbon dioxide can be regarded as a green organic solvent, which has essential application prospects. Supercritical carbon dioxide can replace the oil phase in a conventional microemulsion system. In this work, supercritical carbon dioxide in water (C/W) microemulsion is constructed by a zwitterionic surfactant (CAPB) by the Dissipative particle dynamics (DPD) simulation method. The internal structures of the C/W microemulsion can been intuitively exhibited by the translucent three-dimensional diagrams. The stretching degree of the molecular chain at the C-W interface is reflected by the end to end distance of the surfactant molecule. Research results prove that supercritical carbon dioxide is entirely wrapped by water forming C/W microemulsion when the ratio of water and carbon dioxide increases to 4:1. The CAPB molecules became more aligned with larger end to end distance after forming the C/W microemulsion. The stability studies show that the formed C/W microemulsion has better stability to resist the salt, temperature and shear rate due to the surfactant adsorption layer. The findings open an avenue in the production of C/W microemulsion, which would push forward future application in material synthesis, chemical reactions, and extraction.

Microscopic properties and stabilization mechanism of a supercritical carbon dioxide microemulsion with extremely high water content

HypothesisDeveloping the supercritical carbon dioxide microemulsion with a broad water content (W0) window can provide more possibility for designing highly efficient chemical processes, which is challenging due to the lack of comprehension about its formation mechanism. Molecular dynamics simulation method is expected to reveal the microscopic stabilization mechanism of high-W0 microemulsions. SimulationsAll-atom molecular dynamics simulations of the ternary systems with varied W0 stabilized by 4FG(EO)2 surfactant were designed according to phase behavior experiments. A systematic investigation was performed concerning the self-assembling, equilibrium morphology and detailed microstructure of the microemulsion droplet. An in-depth comparative study about the distribution of both H2O and CO2, the interfacial behaviors of 4FG(EO)2, as well as the microscopic interactions was conducted. FindingsFor the first time, direct evidence was provided for the formation of water-in-carbon dioxide microemulsion with extremely high W0 (80) under the effect of 4FG(EO)2. Furthermore, a unique interfacial phenomenon, i. e. CO2 accumulating at the interface, was revealed to be responsible for the formation and enhanced stability of the nanosized droplet with high W0. This should set a new guiding star for synthesizing and selecting effective interfacial modifiers to create high-W0 microemulsions.

Study on cloud point pressure of [Emim][Tf2N] in supercritical carbon dioxide microemulsions based on non-ionic surfactant and role of solubilized water

We studied the solubilization effect of the supercritical carbon dioxide (scCO2) microemulsion based on non-ionic surfactants (Ls-45, Ls-54, Ls-36, Dynol-604, and TMN-6) for 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) ionic liquid (IL) as well as the influencing factors by analyzing the cloud point pressure (CPP) curves, which served as a function of dissolved ILs concentration. The [Emim][Tf2N]/surfactant/scCO2 microemulsion system was characterized by small-angle X-ray scattering (SAXS), which was proved to be a useful method to provide structure and size information of microemulsion. The effect of water on [Emim][Tf2N]-in-scCO2 (ILs/CO2) microemulsions was studied. The results showed that the ILs/CO2 microemulsions could be constructed by five surfactants selected in this paper, and in the Ls-mn system, the Ls-45/[Emim][Tf2N]/scCO2 system had the lowest CPP. However, the microemulsion constructed by Dynol-604 and TMN-6 surfactants had a poor solubilization effect, and the TMN-6 system was more sensitive to temperature. Besides, small amounts of water could reduce the CPP of the system to a certain extent, making the [Emim][Tf2N]/surfactant/scCO2 microemulsion system more stable, which had a positive effect on the solubilization of [Emim][Tf2N], especially the Ls-36 system. Moreover, the size of microemulsion droplets in [Emim][Tf2N]/surfactant/scCO2 systems was obtained by using the method of SAXS experiment, which also confirmed the solubilization of [Emim][Tf2N] in scCO2 microemulsion system. The structure of ILs/CO2 microemulsions constructed with a non-ionic surfactant was speculated. This study can provide a reference for the construction of ILs/CO2 microemulsions, which was helpful in understanding the microstructure and solubilities of ionic liquids in scCO2 microemulsions.

The self-assembly and microscopic interfacial properties of a supercritical CO2 microemulsion having hydrotropes: Atom-level observation from molecular dynamics simulation

Supercritical carbon dioxide microemulsion is a complex fluid mixture with distinct nano-scale microstructure that can be tailored to synthesize nano-particles and conduct interfacial reactions, which generally forms under high pressure. Reducing the pressure needed is significantly valuable for promoting the accessibility of the scCO2 microemulsion technology and further spurring the development of CO2-based green chemistry or chemical process. In this paper, we provide the direct evidence for the formation of 4FG(EO)2 based water in scCO2 microemulsion and study the microscopic mechanism of the positive synergistic effect of a series of sodium p-alkyl benzoate (hydrotrope) using molecular dynamics simulation method. Results show that the three additives can be effectively solubilized into the microemulsion aggregate and mainly distribute at the water-CO2 interface. Conformational analysis shows that sodium benzoate (BA0) stands at the interface with a relatively perpendicular conformation, while sodium p-ethyl benzoate (BA2) and sodium p-octyl benzoate (BA8) tend to lie on the interface by side. The packing of the surfactant molecules and interaction energy within the interfacial region are examined to reveal the specific microscopic origin of the positive synergistic effect between different hydrotropes and the surfactant in the sense of contributing to the stability of the aggregate, which is favorable for lowering the pressure needed to construct scCO2 microemulsion. This work is expected to direct the better design of the surfactant and rational choice of additives to successfully construct scCO2 microemulsion under lower pressure or even surfactant-free scCO2 microemulsion, which facilitates the application of scCO2 meeting industrial requirements.

Extraction with Water‐in‐Carbon Dioxide Microemulsions: A Case Study on Steviol Glycosides

AbstractThis work explores the use of water‐in‐supercritical carbon dioxide (scCO2) microemulsions for the extraction of polar metabolites from plants. Stevia rebaudiana Bertoni leaves and polyethylene glycol trimethylnonyl ether (TMN) surfactants were selected for a case study. A CO2‐water‐TMN 10 mixture at 35 °C and 30.0 MPa extracted 7 mg target analyte/g dry leaves. The extraction was proven to occur due to a water‐surfactant liquid solution rather than a water‐in‐CO2 microemulsion. Using a modified extraction setup, the microemulsion was created prior to extraction. TMN 6 was able to dissolve enough water in CO2 to extract steviol glycosides, in detectable but very small amounts.

Molecular dynamics simulation of supercritical CO2 microemulsion with ionic liquid domains: Structures and properties

Abstract Supercritical carbon dioxide microemulsions are great medium to combine two immiscible substances through forming nanoscale polar cores in nonpolar continuous phase with the help of proper surfactants. The properties of microemulsions could be significantly affected by their constituents and structures. In this work, molecular dynamics simulation was implemented to study supercritical carbon dioxide microemulsions containing ionic liquid [bmim][PF6] and water by adding surfactant Ls-36. Results showed that the above components could form spherical aggregates in CO2 bulk phase with [bmim][PF6] and some water as the inner core, surfactant headgroups and water as the intermediate shell, and surfactant tails as the outer shell. The microstructure information about the outer shell was further investigated by defining an angle between the surfactant tail and the normal direction of the aggregate outer surface, which ranged from 78° to 125°. The influence of the ionic liquid content on the size and structure of microemulsions was explored and the best molar ratio between the ionic liquid and surfactant was around 1.25 for getting maximum water solubility.

Study on the Phase Behavior and Molecular Dynamics Simulation of a Supercritical Carbon Dioxide Microemulsion Containing Ionic Liquid

We investigated the solubilization effect of a supercritical carbon dioxide microemulsion based on LS-mn (LS-36, LS-45, and LS-54) surfactants for 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) as well as the influencing factors by analyzing the cloud point pressure (CPP) curves, which served as a function of dissolved IL concentration. Results show that increased water content (W0) could enhance [Bmim][BF4] dissolution. Under the same conditions, microemulsion systems consisting of LS-54, LS-45, and LS-36 have the gradually decreased the dissolving capacity for [Bmim][BF4]. However, variation of the surfactant concentration has little influence on dissolving [Bmim][BF4]. In addition, introducing appropriate amounts of ethanol could decrease the CPP of the system, which did not bring significant enhancement on [Bmim][BF4] dissolution. Molecular dynamics (MD) simulation was implemented to give an insight about the microstructure and prove the formation of the microemulsion.

Silver nanoparticles synthesis using H2 as reducing agent in toluene–supercritical CO2 microemulsion

A new technique to synthesize silver nanoparticles (AgNPs) with controllable size was developed using hydrogen as reducing agent in toluene–supercritical carbon dioxide (toluene–scCO2, the mixture ratio of toluene:CO2 is about 20%, v/v) microemulsion. The method involves both process of AgNPs formation (nucleation and crystal growth) at high pressure and thermal incubation at ordinary pressure in the micro-water pool of the microemulsion. Silver ions were reduced by hydrogen (H2) in the micro-reactor of toluene–scCO2 microemulsion, and the size of AgNPs can be controlled by adjusting molar ratio of water to surfactant. The size and morphology of AgNPs were characterized by DLS, TEM and AFM. The AgNPs capped with surfactant were proved by the ATR-FTIR. The relationship between the wavelength of maximum absorption and size of AgNPs was discussed, which was consistent with the MIE theory. This method is better than traditional microemulsion method to synthesize AgNPs by comparing AgNPs prepared using different methods. This method is not only able to obtain AgNPs with controllable size, but also avoids use of fluorinated reagents and reducing agent residual.

Phase Behavior of Supercritical CO2 Microemulsions with Surfactant Ls-36 and Selective Solubilization of Propane-1,3-diol

Supercritical carbon dioxide (scCO2) microemulsion systems were formed in a custom-made volume-variable optical phase equilibrium cell, by employing Ls-36 (dodecyl(poly(ethylene-methylethylene glycol)) ether) as a surfactant, ethanol (EtOH) as a cosolvent, and propane-1,3-diol (1,3-PDO) as a disperse phase in the CO2 continuous phase. The phase behaviors of the ternary system of Ls-36 + CO2 + EtOH, quaternary system of Ls-36 + CO2 + EtOH + H2O, and quinary system of Ls-36 + CO2 + EtOH + 1,3-PDO + H2O were investigated in the temperature range of 303.05 K to 323.25 K and pressure range of 7.81 MPa to 18.99 MPa were studied. Thermodynamically stable microemulsion can be formed by controlling the operating pressure and temperature to selectively extract the 1,3-PDO from its dilute aqueous solution. It was found that a higher concentration of Ls-36 was more beneficial to the solubilization of 1,3-PDO. The results may provide useful thermodynamics data for industrial design and a feasible basis and practical g...

A study of the mechanism of enhancing oil recovery using supercritical carbon dioxide microemulsions

Supercritical carbon dioxide (scCO2) microemulsion was formed by supercritical CO2, HA sodium bis(2-ethylhexyl) sulfosuccinate (AOT, surfactant) and C2H5OH (co-surfactant) under pressures higher than 8 MPa at 45°C. The fundamental characteristics of the scCO2 microemulsion and the minimum miscibility pressure (MMP) with Daqing oil were investigated with a high-pressure falling sphere viscometer, a high-pressure interfacial tension meter, a PVT cell and a slim tube test. The mechanism of the scCO2 microemulsion for enhancing oil recovery is discussed. The results showed that the viscosity and density of the scCO2 microemulsion were higher than those of the scCO2 fluid at the same pressure and temperature. The results of interfacial tension and slim tube tests indicated that the MMP of the scCO2 microemulsion and crude oil was lower than that of the scCO2 and crude oil at 45 °C. It is the combined action of viscosity, density and MMP which made the oil recovery efficiency of the scCO2 microemulsion higher than that of the scCO2 fluid.

Synthesis of Zirconia with Nanoporous Structure by a Supercritical Carbon Dioxide Microemulsion Route

Supercritical carbon dioxide (scCO2) fluid as a green solvent for processing can markedly reduce the use of organic solvents. Nanoporous clusters of zirconia (ZrO2) were synthesized in a [Zr4+](aq)/scCO2 microemulsion using hydrazine solution as the precipitating agent, and the resulting nanostructures (surface area, morphology) under different reaction temperatures and pressures were determined by BET and TEM. The inter droplet separation of microemulsions increases with the reaction pressure at a fixed temperature, giving different networked nanostructures. The closed pore diameters were in the range of 5–150 nm (mesopores and macropores) at 17.3 MPa and 80°C.

Solubilization of Oxymatrine in Water-in-Supercritical Carbon Dioxide Microemulsions

The solubilization amount of oxymatrine in ammonium carboxylate perfluoropolyether (PFPE-NH4) water-in-supercritical carbon dioxide microemulsions with a water-to-surfactant molar ratio of (10.2, 14.8, and 21.0) was determined at temperatures of (308.15, 313.15, 318.15, 323.15, and 328.15) K and a density of 0.822 g·mL−1 using a static apparatus with a sampling part. The determined solubilization amount of oxymatrine was 2 orders of magnitude higher than the solubility of oxymatrine in supercritical carbon dioxide and about 1/6 to 1/5 of the solubility of oxymatrine in bulk water. It was indicated that the solubilization amount of oxymatrine increased with an increase of the water-to-surfactant molar ratio and temperature in all experimental conditions.

Continuous Tuning of Cadmium Sulfide and Zinc Sulfide Nanoparticle Size in a Water‐in‐Supercritical Carbon Dioxide Microemulsion

The size and size dispersion of cadmium sulfide and zinc sulfide semiconductor nanoparticles can be continuously tuned over a wide range of values by adjusting the density of the fluid phase in water-in-supercritical CO2 microemulsions. The average size of the ZnS nanoparticles decreases linearly from approximately 9.1 to 1.9 nm with increasing fluid density from 0.86 to 0.99 g cm(-3) at a water-to-surfactant ratio (W value) of 10. At a W value of 6, the particle size can be tuned from 7.0 to 1.5 nm in the same density range. In the case of CdS nanocrystals, the size varied from 7.1 to 2.0 nm when the W value was 10 and from 4.0 to 1.3 nm when the W value employed was 6, in the same density range. Monodispersive CdS and ZnS nanoparticles were synthesized by chemical reaction of cadmium or zinc nitrate with sodium sulfide, using two water-in-supercritical CO2 microemulsions as nanoreactors followed by protection with a fluorinated-thiol stabilizer. The stabilizer is introduced at 6 and 16 minutes after the mixing of the two microemulsions where the intensity of the characteristic absorption peak due to the quantum confinement properties of the CdS and ZnS nanoparticles (280 and 360 nm) reaches a maximum, respectively. The supercritical CO2 microemulsion method represents a simple approach to use a density-tunable solvent for synthesizing size-controlled semiconductor nanoparticles over a broad range of values.

Continuous Tuning of Silver Nanoparticle Size in a Water‐in‐Supercritical Carbon Dioxide Microemulsion

Choose your size: The size of silver nanoparticles synthesized in a supercritical CO2 microemulsion varies linearly with the density of the fluid phase and can be tuned over a large range of values. The effect is observed at different water-to-surfactant molar ratios and surfactant concentrations. The figure shows two superimposed monolayers of the silver nanoparticles.

Designed CO2‐Philes Stabilize Water‐in‐Carbon Dioxide Microemulsions

Wrapping up water: Two non-fluorinated CO2-philic surfactants are used to generate water in carbon dioxide microemulsion dispersions. Detailed analyses of high-pressure small-angle neutron scattering data provide evidence for formation of spherical D2O nanodroplets (red), stabilized by surfactant interfacial monolayers (lines), and dispersed in CO2 (yellow in the schematic scattering length density (sld) profile). Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2006/z600397_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Effect of cosurfactant on water solubilization in supercritical carbon dioxide microemulsions

Water solubilization in supercritical carbon dioxide (SC CO 2) microemulsions prepared using F-AOT (fluorinated-sodium bis (2-ethylhexyl) sulfosuccinate) analogues has been compared in the presence of cosurfactant n-butanol. At pressure 44 MPa and 35 °C, the addition of n-butanol to di-HCF4 (sodium bis(1 H,1 H,5 H, octafluoropentyl) sulfosuccinate)/SC CO 2 microemulsion enhanced the water solubilization from W 0 = 30 to 45. Increasing the amount of cosurfactant further enhanced the water solubilization. At R = 10, a maximum amount of water W 0 = 49 was dissolved in di-HCF4/SC CO 2 system which is highest reported so far in SC CO 2 microemulsions. W 0 and R represent the molar ratio of water- and n-butanol-to-surfactant, respectively. F-AOT/SC CO 2 microemulsions were characterized by pressure dependent water solubilization phase diagram and high pressure UV absorption spectroscopy.

Phase behavior of water-in-supercritical carbon dioxide microemulsion with sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate

The fluorinated analogues of AOT surfactant, sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentanol) sulfosuccinate, which has CO 2-philic chains and a hydrophilic head group, was synthesized for forming water-in-CO 2 microemulsion. The cloud point of this surfactant was measured and the supercritical fluid-phase behavior of water-in-CO 2 microemulsion was investigated by using a variable-volume view cell apparatus. It was found that the phase behavior of microemulsion is affected by the concentrations of water and surfactant in CO 2, molar ratio of water to surfactant ( W o = [water]/[surfactant]), and the amount of CO 2. From this experiment, we found out another cloudy point which exists above the homogeneous microemulsion region. We defined this point as “upper cloud point” and general cloud point as “lower cloud point. The region which exists between these two cloud points was defined as “stable region of microemulsion”. Conditions for the formation of water-in-CO 2 microemulsion were obtained at temperatures up to 370.15 K.

Nanoparticle formation in rapid expansion of water-in-supercritical carbon dioxide microemulsion into liquid solution

Silver nanoparticles were produced in the Rapid Expansion of a Supercritical Solution into a Liquid SOLVent (RESOLV). Perfluorinated surfactant-stabilized water-in-CO 2 microemulsion was used to dissolve silver salt for the rapid expansion. It was found that well-behaved nanocrystalline silver particles could be obtained when the reductive solution at the receiving end of the rapid expansion was adjusted to be highly basic. The results suggest that the pre-expansion reverse micelles in the microemulsion might be playing a significant role in the formation of the nanoparticles and that such a role could be affected by the basicity of the receiving solution in RESOLV. The apparent correlation between the parameters of the pre-expansion microemulsion and the properties of the produced nanoparticles is discussed.

FTIR analysis of water in supercritical carbon dioxide microemulsions using monofunctional perfluoropolyether surfactants

We describe perfluoropolyether (PFPE) surfactants which are capable of stabilising the water/CO2 interface and present FTIR spectroscopic evidence for the formation of water in supercritical carbon dioxide microemulsions. A wide variety of single chain surfactants of differing chain lengths but similar structure has been screened and the effect of the surfactant chain length on the water uptake was studied. The ammonium carboxylate of the PFPE surfactant Krytox FSL™ with an average molecular weight of 2500 g mol−1 was demonstrated to be the surfactant capable of dissolving the most water out of all the tested surfactants and hence to have the optimum chain length.

Synthesis of Silver and Copper Nanoparticles in a Water-in-Supercritical-Carbon Dioxide Microemulsion

Nanometer-sized silver and copper metal particles can be synthesized by chemical reduction of Ag+ and Cu2+ ions dissolved in the water core of a water in supercritical fluid carbon dioxide microemulsion. Sodium cyanoborohydride and N,N,N‘,N‘-tetramethyl-p-phenylenediamine are effective reducing agents for synthesizing these metal nanoparticles in the microemulsion. Formation of the metal nanoparticles was monitored spectroscopically using a high-pressure fiber-optic reactor equipped with a CCD array UV−vis spectrometer. Silver and copper nanoparticles synthesized in the microemulsion showed characteristic surface plasmon resonance absorption bands centered at 400 and 557 nm, respectively. Diffusion and distribution of the oxidized form of the reducing agent between the micellar core and supercritical CO2 appeared to be the rate-determining step for the formation of the silver nanoparticles in this system.