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Study on the Phase Behavior and Molecular Dynamics Simulation of a Supercritical Carbon Dioxide Microemulsion Containing Ionic Liquid Hong-Rui Ren, Xiang-Dong Liang, Dan Zhou, and Jian-Zhong Yin* State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: 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. emulsions nowadays are fluorous nonionic surfactants.18−20 Compared with ionic surfactants, nonionic surfactants have many advantages such as better solubility in organic solvent or water and better compatibility with other surfactants, and the nonionized form makes them more stable and more difficult to be absorbed onto solid surfaces. Furthermore, using nonfluorous21−23 rather than fluorous nonionic surfactants to form scCO2 microemulsions is much more desirable because the latter ones are very expensive and toxic. However, many nonionic surfactants will become insoluble in water as the temperature rises, videlicet, the raising temperature will reduce the interaction between surfactants and water. Room-temperature ionic liquids (ILs), which are organic salts with melting points below 100 °C, are considered to be clean solvents because of their low volatility and toxicity. On account of their excellent intermiscibility with many organic/ inorganic substances and designable properties, ionic liquids have potential applications in a range of areas, such as separations,24,25 chemical reactions,26−28 and materials synthesis.29,30 scCO2 microemulsion-containing ILs may be an ideal medium to integrate the virtues of these two chemical substances and provide foundations for practical applications. For the first time, the existence of IL(1,1,3,3-tetramethylguani-

1. INTRODUCTION The microemulsion, which is a thermodynamically stable, isotropic, and transparent dispersion system composed of two immiscible liquids with the assistance of surfactants, was first discovered by Hoar and Schulman.1 Carbon dioxide in the liquid and supercritical fluid states (scCO2, critical temperature and pressure are 31.26 °C and 7.38 MPa, respectively) as a green solvent has attracted considerable attention because of its nontoxicity, nonflammability, abundant source, and adjustable solvent properties. Johnston et al.2 first introduced scCO2 into a microemulsion that contains dispersed water cores as the continuous phase utilizing a newly fabricated surfactant. Since then, numerous studies about water/scCO2 microemulsion phase behaviors under different compositions have been conducted.3−7 Because of their unique properties, such as ultralow interfacial tension, large interfacial area, and adjustable structure and property of the micropolar core, scCO 2 microemulsions have been successfully applied in separating 1,3-propanediol from dilute aqueous solution,8−10 extracting metal ions11 and macromolecules,12−14 synthesizing inorganic/ organic nanoparticles,15,16 and serving as reaction media.17 Surfactants, as the connecting bridge between the polar core and nonpolar scCO2, play a crucial role in constructing microemulsions successfully. Generally, an effective surfactant has to satisfy the requirements of sufficient solubility in CO2 and formation of aggregates. Although ionic surfactants such as bis (2-ethylhexyl) sulfosuccinate sodium salt (AOT) can form scCO2 microemulsion with a certain amount of cosolvent,4 most of the surfactants used to construct scCO2 micro© 2017 American Chemical Society

Received: Revised: Accepted: Published: 3733

February 1, 2017 March 8, 2017 March 16, 2017 March 16, 2017 DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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Industrial & Engineering Chemistry Research

ethanol were injected into the view cell separately through different microsyringes when needed. Before experiments started, the air in the view cell was displaced with CO2 for three times. After the view cell was heated to a specified temperature using a water bath, CO2 was slowly charged into the view cell through HPLC until an optically transparent single-phase solution was achieved. Then, both the HPLC and magnetic stir bar were stopped to let the system stand for enough time to ensure the stability of the microemulsion system before measuring the cloud point pressure (CPP). Subsequently, the pressure of the system was gradually lowered by adjusting the variable-volume piston (volume ranges from 0 to 1.5 mL) until the cloud point appeared and CPP datum was recorded for further analysis. Under every condition, the experiment was repeated for three times and an averaged CPP datum (combined expanded uncertainty within 6%, coverage factor k = 2) from these three data was used in this paper. 2.2. Molecular Dynamics Simulation. To obtain some insights about the microstructure of scCO2 microemulsion and evidence of existing surfactant aggregates, an all-atom molecular dynamic simulation was employed to the LS-36/[Bmim][BF4]/ H2O/CO2 system. MS-STUDIO software was used to construct the molecular geometry of surfactant, H2O, ILs, and CO2. GROMACS package (v4.6.2) was then used to compute the equilibrium progress. As for calculating molecular forces, force field parameters listed in Supporting Information Tables S1−S5 were used along with different molecule models. CO2 was represented by the EPM2 model35 and water was represented by the SPC model with intermolecular potential parameters.36 Force field parameters of [Bmim][BF4] were adopted from Balasubramanian.37 Surfactant LS-36 molecule was compiled by CHARMM force field. All molecules were put in the cubic box randomly and all the systems were computed under periodic boundary conditions (PBC) with NPT ensemble. We chose a Sarkar thermostat with a temperature relaxation time of 0.5 ps and Berendsen barostat with a pressure relaxation time of 1.0 ps, respectively. The simulations were carried out at 308.15 K and 18 MPa, which was identical to the experimental temperature and pressure in this work. The compressibility was set as 4.5 × 10−5 bar−1. The calculation of long-range Coulombic forces was performed by applying the full Ewald summation technique. The real space part of the LJ interaction was cut off at 1.2 nm. LINCS was used to constrain bond length between heavy atoms and hydrogens. Meanwhile, the stochastic dynamic (SD) leapfrog method was employed for integrating the Newtonian equations of motion.

dium acetate, [TMG][Ac]) domains in a continuous CO2 phase with N-ethyl perfluorooctyl sulfonamide (N-EtFOSA) as surfactant was reported by Han et al.31 who conducted spectroscopic experiments. In a later study, such a system was simulated by Senapati et al.32 using MD to study the microstructure, which was proven to be consistent with small angle neutron scattering experiments on IL-in-oil microemulsion systems.33 However, studies about scCO2 microemulsion with other ILs as polar cores are seldom reported. Thus, we investigated the solubilization patterns of [Bmim][BF4], one of the most widely used ionic liquids, in an scCO2 microemulsion from a thermodynamical perspective.

2. EXPERIMENT AND SIMULATION METHODS 2.1. Experimental Section. Materials. LS-mn surfactants (dodecyl polyoxyethylene (m) polyoxypropylene (n) ether, namely LS-36, LS-45, and LS-54), 99.5% purity, were obtained from Shanghai Owen Chemicals Co. Ltd., China. 1-Butyl-3methylimidazolium tetrafluoroborate ([Bmim][BF4]) was purchased from Shanghai Cheng Jie Chemical Co., Ltd., China. Their chemical structures are depicted in Figure 1.

Figure 1. Chemical structure of surfactant LS-mn and [Bmim]BF4.

Ethanol, 99.8% analytical purity, was provided by Tianjin Fuyu Fine Chemical Co., Ltd., China. Deionized water was prepared in our laboratory. CO2, purity 99%, was supplied by Dalian Guangming Gas Co., Ltd., China. Phase Behavior Apparatus and Measuring Method. A similar variable-volume view cell (30.164 mL), compared with our previous reports,10,34 was used to observe the phase behaviors of LS-mn systems in CO2 and to measure IL solubilization. A high-performance liquid chromatography (HPLC) pump was used to infuse CO2 into the view cell, the temperature and pressure of which were monitored with a thermocouple (±0.1 °C) and a pressure transducer (DG1300BZ-A-2-40 ± 0.01 MPa), respectively. The phase behavior measuring method is the same as reported in our previous work.34 Surfactants, water, IL, and

Figure 2. CPP curves of LS-36/H2O/CO2 + [Bmim][BF4] microemulsion system: cLS‑36 = 0.008 mol/L, (a) W0 = 5, [c[Bmim][BF4]]max = 3.90 × 10−4 mol/L; (b) W0 = 10, [c[Bmim][BF4]]max = 5.34 × 10−4mol/L; (c) W0 = 12, [c[Bmim][BF4]]max = 5.54 × 10−4 mol/L. 3734

DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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Industrial & Engineering Chemistry Research

°C, 45 °C) were plotted as a function of dissolved IL concentration. Results are shown in Figure 2−Figure 4, the horizontal coordinate is the concentration of dissolved [Bmim][BF4], the vertical coordinate is CPP, and dash lines indicate the maximum amounts of dissolved [Bmim][BF4]. It needs to be emphasized that the notion of the maximum amount of dissolved [Bmim][BF4] used in this paper should be interpreted as the dissolved amount when the system pressure reaches the maximum pressure limit of the experimental apparatus. In other words, if the pressure continues to rise, [Bmim][BF4] may continue to dissolve, but is not the focus of this paper. Maximum amounts of dissolved [Bmim][BF4] indicated by the vertical dash lines in Figure 2−Figure 4 are expressed in terms of concentration and listed in the captions along with the compositions of different systems. Take W0 = 10 for demonstration, as shown in Figure 2b. When the temperature is constant, the CPP is relatively invariant with the concentration of [Bmim][BF4] over a certain range. At higher concentration, a little higher pressure is required to attain a one-phase system. In addition, under the same pressure, the system will solubilize less [Bmim][BF4] as the temperature increases. When W0 = 5 and W0 = 12, systems show similar patterns as shown in Figure 2a,c. When LS-45 and LS-54 are used as surfactant, we obtain some results that have great similarities with those of systems using surfactant LS-36, as illustrated in Figure 3 and Figure 4. It can be seen from the pictures that the change in temperature does not significantly affect microemulsions’ capacity to solubilize [Bmim][BF4]. 3.2. Influence Factors of LS-mn/H2O/CO2 Microemulsion Solubilizing [Bmim][BF4]. Water Content W0. For further investigating the influence of W0 on [Bmim][BF4] solubilization, CPP curves of microemulsion systems composed of different W0 were compared under three different temperatures as depicted in Figure 5. From Figure 5 panels a and b it can be seen that increased W0 can enhance the solubilization of [Bmim][BF4] in the system. After a certain amount of [Bmim][BF4](2 × 10−4 mol/ L) is dissolved, the slopes of CPP curves of systems with W0 = 5 become very steep, showing a similar pattern to the slope of the curve of the ionic dye (methylene blue) solubilized in an scCO2 microemulsion38 and indicate the systems are extremely sensitive to the variation of the dissolved [Bmim][BF4] amount. In other words, for the purpose of solubilizing equally more [Bmim][BF4] into the system, an increase in cloud point pressure of the system W0 = 5 was much more than that of the system W0 = 10, both of which contained identical [Bmim][BF4] amounts (c[Bmim][BF4] ≥ 2 × 10−4 mol/L]). Consequently, CPP curves of W0 = 5 and W0 = 10 intersect with each other

3. RESULTS AND DISCUSSION 3.1. Phase Behavior of LS-mn/[Bmim][BF4]/H2O/CO2 Systems. To determine the maximum amount of dissolved IL for different systems investigated in the following parts. A series of experiments were conducted and CPP curves of these systems composed of different W0 (Figure 2) or surfactants (Figure 2−Figure 4) under different temperatures (35 °C, 40

Figure 3. CPP curves of LS-45/H2O/CO2 + [Bmim][BF4] microemulsion system: cLS‑45 = 0.008 mol/L, W0 = 10, [c[Bmim][BF4]]max = 6.87 × 10−4 mol/L.

Figure 4. CPP curves of LS-54/H2O/CO2 + [Bmim][BF4] microemulsion system: cLS‑54 = 0.008 mol/L, W0 = 10, [c[Bmim][BF4]]max = 8.40 × 10−4 mol/L.

Figure 5. CPP curves of LS-36/H2O/CO2 + [Bmim][BF4]: cLS‑36 = 0.008 mol/L. 3735

DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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Figure 6. CPP curves of LS-mn/H2O/CO2 + [Bmim][BF4]: cLS‑36 = cLS‑45 = cLS‑54 = 0.008 mol/L, W0 = 10.

when the concentration of [Bmim][BF4] is close to 2.89 × 10−4 mol/L. This means when pressure is lower than the crossover pressure, compared with systems with W0 = 10, systems with W0 = 5 have a better dissolution capacity of [Bmim][BF4]. When pressure is higher than the crossover pressure, the comparison result is contrary. Besides, when temperature is raised up to 45 °C, curves of W0 = 10 and W0 = 12 also intersect as shown in Figure 5c. The above phenomenon probably occurs because the dissolution capacity of the microemulsion system for IL depends on the water core’s polarity and potential for dissolving more polar substance to the maximum polarity that the system can undertake under a certain pressure. When W0 is higher, the water core’s size becomes larger and has stronger polarity, which is in favor of IL dissolution. However, under the same pressure (lower than the crossover pressure), the system with lower W0 has greater potential for dissolving more polar substances, which consequently has a better dissolution capacity for IL. Comprehensively, if the system with high W0 can be stable when the pressure is high enough, its strong polarity within the microwater environment plays a dominant role in dissolving the IL. Surfactant Type. In this section, solubilization of [Bmim][BF4] in H2O/CO2 microemulsion systems formed with surfactant LS-36, LS-45, and LS-54 under three different temperatures were studied. Under a certain temperature, CPP curves of systems containing different surfactants were plotted in one figure to better reveal the influence of different LS-mn surfactants on [Bmim][BF4] dissolution as illustrated in Figure 6. Figure 6 shows that the microemulsion systems can dissolve more [Bmim][BF4] with increasing m/n values of the LS-mn series surfactants within the pressure range in this paper. Just as the maximum horizontal coordinate of every CPP curve in each figure indicates, which is the maximum dissolved concentration of [Bmim][BF4] when using LS-36, LS-45, and LS-54, respectively. This is probably because the stronger interaction between water and surfactant that has more oxyethylene (EO) groups as the m/n value increases could make the water core more stable and facilitate [Bmim][BF4] dissolution. In addition, CPP curves of microemulsion systems formed with LS-45 and LS-54 intersect with each other after dissolving c[Bmim][BF4] = 5.15 mol/L [Bmim][BF4] in Figure 6a−c. This possibly implies that the solubility and molecular composition of the surfactant in microemulsion systems also affect a dissolving IL. For example, LS-45 has a higher solubility in scCO2 than LS-54, which will favor IL dissolution because lower pressure is needed to form one phase. However, LS-45 possesses fewer EO groups compared with LS-54, thus less

water will be solubilized in the polar core because of the relatively weaker hydrogen bonding interaction between LS-45 head groups and water, which is adverse to IL dissolution. When the system pressure is low, surfactant solubility plays the leading role in affecting IL dissolution, as indicated by the CPP curves of LS-45 systems under crossover points. As pressure is increased to a level under which LS-54 can achieve enough dissolution, because it has more EO groups, a much more solid combination between LS-54 and water will dominate the IL dissolution. Combining Figure 6 panels a, b, and c shows that the slope of the CPP curve when using LS-36 becomes steeper as the temperature rises. The slope of the CPP curve can be interpreted as the pressure increment of dissolving unit amount of IL. Comparing the CPP curve of LS-36 with that of LS-45, we can find that these two curves approach to each other as temperature increases and then intersect, as indicated by Figure 6c. This means systems using LS-36 become less capable of dissolving IL as temperature increases because a larger pressure increment is needed for dissolving a unit amount of IL, which may be explained by the solubility decrement of LS-36. Surfactant Concentration. The influence of surfactant concentration on [Bmim][BF4] solubilization in the H2O/ CO2 microemulsion was studied with systems composed of different LS-36 concentrations under different temperatures when the amount of water was 0.008 mol/L. CPP curves are shown in Figure 7 and Figure 8. Figure 7 shows that CPP becomes higher as the concentration of LS-36 increases when a certain amount of IL is dissolved, which backs up the explanation of the slope change of LS-36 CPP curves in the

Figure 7. CPP curves of LS-36/H2O/CO2 + [Bmim][BF4]: T = 35 °C, cLS‑36 = 0.006 M, 0.08 M, 0.01 M. 3736

DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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Industrial & Engineering Chemistry Research

Figure 8. CPP curves of LS-36/H2O/CO2 + [Bmim][BF4]: T = 35, 40, 45 °C, cLS‑36 = 0.006 M, 0.01 M.

3.3. MD simulation. The molecular dynamics simulation on LS-36/[Bmim][BF4]/H2O/CO2 system containing 12 LS36, 3 [Bmim][BF4], 160 H2O and 12000 CO2. Figure 10 illustrates the spontaneous evolution of system from the random mixture to an ordered polar core surrounded by surfactant in continuous scCO2. The aggregation process is similar to that for tertiary system of water in CO2 microemulsions. The small aggregations are clustering faster in the first 2 ns. Then they slowly fused to form two to three reverse micelles (RM) at around 24 ns. Next, these small aggregations gathered together to form a single RM at around 28 ns with unique size and one free LS-36 dissociate and reassociate in the continuous CO2 phase until 83 ns. Finally all the surfactants kept assembly during the last 55 ns. Since the water and surfactant have limited solubility in the scCO2 microemulsion, we could see in Figure 10f that there are still several water molecules and surfactant in a disperse state. The water core, also called the “water pool” in some papers, is well shielded by surfactant molecules. Thus, we give the direct evidence for the formation of polar cores with IL and water in the CO2 phase through MD simulation. RDFs for the [BF4]-LS-36 and [Bmim]-LS-36 were plotted in Figure 11a. Average density distribution of H2O and CO2 around the last O atom in PO group and the middle C atom in tail chain was calculated separately as depicted in Figure 11b. From the g(r) function it can be seen that the water core is within a radius of 2.5 nm, which is composed of 127 water molecules. Figure 11b shows the phase separation of H2O in the core region at about 2.5 nm with CO2 bulk phase around it.

previous section. A comparison of Figure 8 with Figure 2b shows that the maximum dissolved amount of [Bmim][BF4] is the same as the dash lines in these figures indicate, which means the increase of surfactant concentration does not effectively improve the system’s ability to dissolve [Bmim][BF4]. This may be because the increase of surfactant concentration does not effectively improve the size of the water core, so the capacity to dissolve IL remains unchanged. Ethanol. From the previous studies it has been found that microemulsion systems have limited capacity to dissolve [Bmim][BF4] and high operating pressures. To improve the above situation, ethanol(C2H6O, EtOH) was added into the microemulsion systems. As shown from Figure 9, [Bmim][BF4] solubilization into H2O/CO2 microemulsions containing 0.008 mol/L LS-36, W0 = 10, and different amounts of ethanol at 35 °C were studied.

4. CONCLUSION We managed to solubilize [Bmim][BF4] into scCO2 microemulsions composed of LS-mn surfactants, opening up a new gate for combining ILs with CO2. The influence of a series of factors, such as W0, surfactant type, surfactant concentration, and cosurfactant ethanol on the microemulsions’ dissolving capacity for [Bmim][BF4] was investigated. Hereby, we are able to provide fundamental thermodynamic data and structure regulating rules for future formation of scCO2 microemulsions containing merely ILs as polar cores, based on which many more practical applications can be explored. Moreover, the microstructure of H 2 O/CO 2 microemulsion containing [Bmim][BF4] was directly provided by performing molecular dynamic simulations for the purpose of proving formation of a microemulsion from a microperspective.

Figure 9. CPP curves of LS-36/H2O/CO2 + [Bmim][BF4] + ethanol: T = 35 °C, cLS‑36 = 0.008 mol/L, W0 = 10.

The results show that adding 0.11 mol/L ethanol can significantly reduce the CPP of the system, which stays in the relatively lower pressure range as the concentration of dissolved [Bmim][BF4] increases. However, adding ethanol has little influence on maximum amount of dissolved [Bmim][BF4]. When added ethanol is 0.23mol/L, in a certain [Bmim][BF4] concentration range, the CPP is also lower than that of the system without ethanol. But as the [Bmim][BF4] concentration continues to increase, the CPP will soar up to much higher value than the system without ethanol. In general, only adding a certain amount of ethanol can reduce the CPP. 3737

DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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Figure 10. Time evolution of the system of [Bmim][BF4]/Ls-36/H2O/CO2. Snapshots were taken at (a) 0 ns, (b) 8 ns, (c) 10 ns, (d) 24 ns, (e) 28 ns, (f) 83 ns. Color scheme: red = [Bmim]; dark blue = [BF4]; light blue = free LS-36; green = assembled LS-36; CO2 was omitted for clarity.

Figure 11. (a) RDF for [Bmim][BF4]-LS-36 in system. (b) Average density of H2O and CO2 around the last O atom in PO group and the middle C atom in the tail chain.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (21376045, 21506027), Petrochemicals Joint Fund of National Natural Science Foundation of China and China National Petroleum Corporation (U1662130), Chinese Postdoctoral Science Foundation (2015M571307), and Open Project Program of State Key Laboratory of Catalysis (Dalian Institute of Chemical Physics, Chinese Academy of Sciences. N-15-01).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00452.

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Force field parameters of LS-36; atom types; bond parameters; angle parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-411-84986274. E-mail: [email protected].

REFERENCES

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ORCID

Jian-Zhong Yin: 0000-0003-4529-3743 Notes

The authors declare no competing financial interest. 3738

DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739

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DOI: 10.1021/acs.iecr.7b00452 Ind. Eng. Chem. Res. 2017, 56, 3733−3739