Critical Microemulsion Concentration and Molar Ratio of Water-to

Article pubs.acs.org/jced

Critical Microemulsion Concentration and Molar Ratio of Water-toSurfactant of Supercritical CO2 Microemulsions with Commercial Nonionic Surfactants: Experiment and Molecular Dynamics Simulation Xiang-Dong Liang, Yi-Fan Liu, Dan Zhou, Wen Yu, 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: The critical microemulsion concentration (cμc) and the molar ratio of water-to-surfactant (W0) of supercritical CO2 (scCO2) microemulsion that uses different nonionic hydrocarbon surfactants (LS-36, LS-45, LS-54, DYNOL-604, TMN-6) were examined at temperatures from 35 to 45 °C and pressures up to 19 MPa. The results show that the cμc mainly depends on the structure of the surfactant. The surfactant with more hydrophilic structure, such as the ethylene oxide (EO) group and hydroxyl, tends to produce a higher cμc. In addition, the cμc increases with the increase of the ratio of ethylene oxide (EO) group number to the propylene oxide (PO) group number of the surfactant. The capacity of the microemulsion system to dissolve water, which is characterized by W0, is related to the concentration and structure of surfactant. It is found that a higher solubility of surfactant in CO2 favors the system to dissolve water at lower pressure. At higher pressure, the stronger hydrophilicity of surfactant and the higher surfactant concentration are beneficial for microemulsions to contain more water. The molecular dynamics (MD) simulation, which was conducted in the NPT ensemble, shows the spontaneous evolution of a surfactant cluster and microstructure of microemulsion at different conditions. It demonstrates that the microemulsion system with more water molecules can form a larger water cluster and catch more surfactants although a few surfactants dissociate in the continuous phase. The experimental data and MD simulation results provide useful infomation for the structure regulation of the scCO2 microemulsion and expand the study to the microscopic scale.

1. INTRODUCTION

significant meaning in the application of scCO2 microemulsions. In recent years, the application of the scCO2 microemulsion mainly focused on the preparation of nanoparticles, extraction, and so forth. Wai et al.7 synthesized silver halide nanoparticles by mixing two scCO2 microemulsions which contained Ag+ and X− ions, respectively. The silver halide nanoparticles which include AgI, AgBr, and AgCl were collected by the rapid expansion of supercritical solution (RESS) method. Erkey et al.8 synthesized CuS nanoparticles using the same method, and the size of copper sulfide particles ranged from 4 to 6 nm. Wang et al.9,10 extracted heavy metal ions such as Cd2+, Co2+, and Cu 2+ from solid matrices utilizing a water-in-CO 2 microemulsion. Yin’s group11−14 successfully extracted 1,3propanediol from a dilute aqueous solution using scCO2 microemulsion. The results provide practical guidance for efficiently extracting 1,3-propanediol selectively from fermenta-

A supercritical CO2 microemulsion (reverse micelles) is a thermodynamically stable and optically transparent system that contains a nonpolar continuous CO2 phase, a polar core (typically water), and dynamic spherical aggregates of surfactant molecules surrounding a core dispersed in the continuous phase. Owing to the capacity to dissolve polar or ionic species and the novel structure, the water-in-CO2 (w/c) microemulsion has greatly expanded the application of scCO2 and been widely applied in areas such as green chemistry and technology. The surfactant, which is generally an effective CO2-phile, is an indispensable component to scCO2 microemulsion. So far, most surfactants that have been reported to form microemulsions are fluorinated surfactants,1 nonfluorous hydrocarbon surfactants,2 and mixed fluorocarbon−hydrocarbon surfactants.3−5 As compared to fluoro-surfactants, hydrocarbon surfactants are low-cost, and economically viable, and have low toxicity, which can effectively unlock the potential of scCO2based technologies.6 Hence, hydrocarbon surfactants have © 2016 American Chemical Society

Received: March 16, 2016 Accepted: July 22, 2016 Published: August 2, 2016 3135

DOI: 10.1021/acs.jced.6b00231 J. Chem. Eng. Data 2016, 61, 3135−3143

Journal of Chemical & Engineering Data

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native state remains intact in the water pool. More recently, formation of an ionic liquid-in-CO2 microemulsion was tested via MD simulation,30 and the results were in good agreement with the concentration of each component measured by Zhang.15 Nevertheless, most of the simulations were focused on the microemulsion with fluorinated surfactants, the selfassembly of hydrocarbon surfactants has been seldom reported in the literature31 and more studies are expected. To further investigate scCO2 microemulsion technology in the areas of nanomaterials synthesis, ionic or highly polar species extraction, and specialized reaction, the structure parameters and regulation of a scCO2 microemulsion should be examined in detail. In this paper, we examined the cμc of five nonionic hydrocarbon surfactants (LS-36, LS-45, LS-54, DYNOL-604, TMN-6) and the molar ratio of water-tosurfactant (W0) of each microemulsion system. All of them are commercially available and have a certain solubility in scCO2, and it has been demonstrated that they all form a stable microemulsion in scCO2.19,24,25,32 The results of the phase behavior of the surfactants/water/scCO2 ternary system with different molar concentrations of surfactant and water are presented. The molecular dynamics (MD) simulation of LS36/water/CO2 system was performed to analyze the microstructure and the generative process of the scCO2 microemusion. The effect of W0 on the distribution of water in the microemulsion was also studied.

tion broth in industry. Recently, novel microemulsions with ionic liquid domains have been studied. The combination of scCO2 and ionic liquids is a promising application prospect. Han et al.15 created the scCO2 reverse micelles with an ionicliquid core in which salts such as methyl orange, CoCl2, and HAuCl4 were soluble. Zhang et al.16 synthesized metal−organic framework (MOF) nanospheres in IL-in-CO2 microemulsion with fluorinated surfactant N-EtFOSA. The MOF spheres prepared by this method combine the advantages of both microporous and mesoporous materials, and the material has great applications in many fields such as gas separation and catalysis. It is necessary to study the microstructure of the microemulsion so as to broaden the application of scCO 2 microemulsions. The concentration of surfactant is an essential parameter for formation of a microemulsion. Otake17 tested the cμc of six fluorinated surfactants through the interfacial tension method. Zhou18 examined the cμc of an anionic surfactant (AOT) with ethanol as a cosurfactant. However, no work has been done to explore the cμc of scCO2 microemulsions with nonionic hydrocarbon surfactants up to now. Han et al.19 studied the structural information on the nonionic hydrocarbon surfactant (Ls-54)-based water-in-CO2 reverse micelles by small-angle X-ray scattering (SAXS) and indicated that the radii of the reverse micelles are in the range from 20.4 to25.2 Å at different pressures and W0. Eastoe et al.20,21 used highpressure small-angle neutron scattering (SANS) to test the effect of hydrotrope on the shape transition of scCO 2 microemulsions. The results showed that the microemulsion’s water droplet has a significant elongation when a small amount of hydrotrope is added. Sagisaka and co-workers22,23 investigated nanostructures of water-in-CO2 microemulsion such as interfacial properties and surfactant films by SANS; the relationships between nanostructure and the structural parameters such as surfactant chain length, W0, and CO2 density were also discussed. Johnston et al.24 performed highpressure dynamic light scattering (DLS) measurement to reveal the hydrodynamic diameter (Dh) of water-in-CO2 microemulsions with methylated branched hydrocarbon surfactants (TMN ethoxylated nonylether surfactants) and found that the Dh increased with a decrease in temperature (at constant CO2 density) or an isothermal decrease in density. Moreover, UV− visible spectrometry was widely used to prove the existence of polar domains in the scCO2 microemulsion.15,24−26 However, structural properties of the microemulsion are still hard to get directly and vividly. Hence, much effort has been devoted to explore the microstructure of microemulsion through molecular dynamics simulation (MD). In 1999, Cummings27 reported the first molecular dynamics simulation of the self-assembly of dichain fluorinated surfactants in scCO2 into stable, spherical aggregates. The simulation results exhibit the potential of molecular dynamics simulation for the study of the scCO2 microemulsion system. Considering the effect of the initial conditions on the formation of the micelle, Berkowitz28 performed a series of simulations of fluorinated surfactants (PFPE)/water/CO2 ternary systems, and they found that either the molecules were initially distributed randomly or, on a regular lattice, the system would gradually form a spherically shaped microemulsion configuration from the start. Senapati29 reported the MD simulations of quaternary mixtures of protein/water/CO2/fluoro-surfactants, and manifested the protein was entrapped inside the aqueous pool of the scCO2 microemulsion. The results also indicated that the protein

2. MATERIALS AND METHODS 2.1. Materials. LS-mn surfactants (dodecyl polyoxyethylene (m) polyoxypropylene (n) ether33,34) LS-36, LS-45, and LS-54 were obtained from Shanghai Owen Chemicals Co. Ltd., China. The surfactant poly(ethylene glycol) 2,6,8-trimethyl-4-nonyl ether (TMN-6) with purity 90% was purchased from Sigma. The residual water, which accounts for 10% of TMN-6, was removed by drying the surfactant over excess amounts of anhydrous MgSO4 for a month followed by centrifuging. The surfactant DYNOL-604 (2,5,8,11-tetramethyl-6-dodecyn-5,8diol ethoxylate) was purchased from Sanky Chemicals of Shanghai. Their chemical structures were depicted in Figure 1. CO2 (purity 99%) was supplied by Dalian Guang Ming Gas Corporation of China. Double-distilled water was prepared in this laboratory. 2.2. Phase Equilibrium Measurement. Phase behavior measurements were similar to our work reported previously.31

Figure 1. Chemical structures of DYNOL-604, LS-mn, and TMN-6. 3136



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parameters40 were used for the LS-36. The CO2 molecules and water molecules were described by the EPM2 model41 and the SPC model,42 respectively. All MD simulations were carried out until the system became stable enough. The Force field parameters were listed in Table S1−S5 in Supporting Information.

The schematic diagram of a homemade volume-variable optical phase equilibrium apparatus is illustrated in Figure 2.

3. RESULTS AND DISCUSSION 3.1. The Phase Behavior and the cμc of Different Surfactants. The cμc is a primary and fundamental property Table 1. Cloud-Point Pressures of Surfactant/H2O/CO2 System concentration

Figure 2. Schematic diagram of phase behavior test apparatus: (1) CO2 cylinder; (3) filter; (4) cooling coil; (5) HPLC pump; (8) surge tank; (11) thermocouple; (12) view cell; (13) magnetic stirrer; (16) variable-volumn piston; (2)(7)(14) pressure sensor; (6)(9)(10)(15) needle valve; (17) water bath.

type LS-36

It mainly consists of a 30.164 mL volume high-pressure stainless steel view cell capped on two sapphire windows, a volume-variable vessel with volume range from 0 to 1.5 mL, a constant temperature water bath that can heat up to 80 °C, a high performance liquid chromatography (HPLC) pump, and a CO2 storage tank. The fluid pressure was measured with pressure transducer (DG1300-BZ-A-2-40, ±0.01 MPa accuracy), and the temperature was measured by a thermocouple with the accuracy of ±0.1 °C. In a typical experiment, a certain amount of surfactant and water was injected into the view cell; air in the cell was replaced with CO2 three times. The cell was then sealed and placed in the temperature-controlled water bath. At a fixed temperature, CO2 was slowly introduced into the cell with the HPLC pump, and the fluids were stirred at the same time. A clear homogeneous transparent single-phase solution was formed at a certain pressure, and the stirring was stopped when the phase behavior was observed. At that moment, the pressure was slowly decreased by adjusting the piston until the system became slightly turbid, then the pressure was recorded as the cloud-point pressure (CPP). The process was repeated three times at each condition and the data of cloud-point pressure at each experimental condition were averaged with an expanded uncertainty within 6% (coverage factor k = 2). 2.3. Simulation Details. The simulations steps of different conditions were performed by using simulation package GROMACS 4.6.2.35 The NPT ensemble was used for all the simulations with the stochastic velocity rescale thermostat proposed by Sarkar et al.36 and the Berendsen barostat.37 The thermostat relaxation time was set to 0.5 ps and the barostat thermostat was set to 1 ps with a compressibility of 4.5 × 10−5 bar−1. Periodic boundary conditions were employed on cubic computational boxes in all directions. The MD equations of motion were integrated using the stochastic dynamics algorithm in gromacs with 1 fs time step. To avoid the effect of initial conditions and get a higher accuracy, the molecules were first distributed randomly and an all-atom was carried out unrestrained. The real space part of the Ewald sum38 and Lennard-Jones interactions were cut off at 1.2 nm. The Linear Constraint Solver (LINCS) algorithm was used to constrain bond lengths between heavy atoms and hydrogens. The CHARMM-27 potential39 was used to model molecules in this article. CHARMM General Force Field (CGenFF)

LS-45

LS-54

DYNOL-604

TMN-6

pressure (MPa)

mol/L

35 °C

40 °C

45 °C

0.000462 0.00138 0.00161 0.00186 0.00231 0.00324 0.00462 0.000488 0.00195 0.00219 0.00245 0.00269 0.00293 0.00317 0.00391 0.00488 0.000501 0.002 0.0025 0.00301 0.00351 0.00401 0.0045 0.00501 0.00651 0.000755 0.00226 0.00302 0.0034 0.00377 0.00453 0.00604 0.01132 0.000596 0.00179 0.00239 0.00298 0.00358 0.00417 0.00596

8.57 9.26 9.47 9.06 9.44 9.97 10.21 8.23 9.33 9.61 9.93 10.21 10.41 10.88 11.43 11.54 8.41 9.30 9.79 10.39 11.19 11.79 11.39 12.01 13.00 7.92 7.99 8.15 8.09 8.08 8.22 8.33 9.14 8.30 8.43 8.55 8.55 10.00 10.79 11.69

9.63 10.43 10.76 10.32 10.79 11.34 11.94 9.34 10.48 10.81 11.32 11.69 11.70 12.39 12.88 13.10 9.10 10.46 11.20 11.83 12.62 13.29 12.91 13.55 14.52 8.21 8.90 9.28 9.10 9.08 9.25 9.45 10.57 9.33 9.66 9.85 9.79 11.10 12.00 13.24

10.81 11.60 12.00 11.53 12.09 12.79 13.55 10.28 11.74 12.28 12.77 13.14 13.15 13.88 14.31 14.66 9.72 11.94 12.62 13.40 14.24 15.00 14.46 15.18 15.92 8.30 9.90 10.41 10.14 10.16 10.27 10.79 11.92 10.32 10.75 10.97 11.03 12.31 13.23 14.43

of a scCO2 microemulsion. It defines the minimum amount of surfactant that is required to form a microemulsion. The concentration of surfactant needed to generate the microemulsion should be higher than the cμc value. Since the cloud point pressure (CPP) is one of the properties that can reflect the stability of the microemulsion, the cμc was tested by 3137



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Figure 3. Effects of concentration of surfactants on CPP of surfactant/water/CO2 ternary system at a fixed concentration of water (0.0184 mol/L). (a) LS-36/water/CO2; (b) LS-45/water/CO2; (c) LS-54/water/CO2; (d) DYNOL-604/water/CO2; (e) TMN-6/water/CO2.

Table 2. Cμc of Five Surfactants and Corresponding Pressures at Different Temperatures pressure (MPa) categories

cμc (mol/L)

35 °C

40 °C

45 °C

TMN-6 DYNOL-604 LS-36 LS-45 LS-54

0.00269 0.00300 0.00161 0.00271 0.00400

8.56 8.15 9.49 10.41 11.79

9.84 9.28 10.76 11.70 13.28

10.96 10.44 12.03 13.17 14.99

Figure 5. Comparison of CPP of LS-45, TMN-6, and DYNOL-604 at 35 °C and different concentrations.

investigated at the temperature range from 35 to 45 °C and pressures up to 18 MPa. The experimental data are given in Table 1. The concentration of a surfactant is defined as the initial concentration with unit of mol/L. For the experiment, a certain amount of surfactants was added in the view cell, and the total volume, which is also the volume of the supercritical microemulsion system of this equilibrium system, was previously calibrated. Therefore, the concentration was calculated by the mole number of surfactants divided by the volume of the system. Since the maximum volume of the piston is 1.5 mL, and in typical measurements the most regulation was less than 0.5 mL, which is far less than the total volume of system (30.16 mL), the changes of the concentration of the

Figure 4. Comparison of CPP of Ls-mn series surfactant at 35 °C and different concentrations.

investigating the changes of CPP. The CPPs of the surfactant/ water/scCO2 ternary systems with a fixed amount of water (0.0184 mol/L) and different concentrations of surfactant were 3138



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was also examined by MD simulations, was later displayed to deepen the understanding of the generative process of the microemulsion. Table 2 lists the cμc data of the five surfactant/water/CO2 systems at different pressures and temperatures. The results show that cμc almost has no change under our experimental temperatures; however, the differences in the structure of surfactants can cause significant changes in cμc. As can be seen from Table 2 and Figure 4, the cμc of the LS-mn series is related to the ratio of the ethylene oxide (EO) group to the propylene oxide (PO) group, which is also equal to the ratio of m/n. The cμc and the CPP correspond to the increase with the increase of the ratio of m/n (0.5, 0.8, 1.25). The reason might be that the aggregation number increases as the ratio of m/n increases when the concentration of surfactant in the system reaches the cμc. The result can be explained as follows. The hydrophilicity of nonionic surfactants is due to the existence of hydrogen bonds formed between surfactants and water molecules. The surfactants with more hydrophilic EO groups, which have the same tail chain structure, can form more hydrogen bonds between polar heads of the surfactant and water core. In other words, the water core can attract more surfactants when the surfactant contains more EO groups. On the other hand, the solubility of surfactants in CO2 increase with the tail chain length,24 which can be approximately considered to the PO group numbers of the LS-mn surfactant. For example, LS-36 has a higher solubility in CO2 than LS-45 and LS-54.12,14,31 The increased solubility of surfactant in CO2 leads to the decrease of aggregation number, which can reflect the degree of aggregation. Hence, the two forces eventually led to the above results. Since the PO group exhibits CO2 affinity,44 the pressure (CO2 density) required to dissolve the surfactant with a lower ratio of m/n is lower. Consequently, the CPP corresponds to the cμc increase with a decrease of the PO group number. The results from Table 2 and Figure 5 show that LS-45 and TMN-6 have similar cμc and so does the aggregation number, although at different CPPs. TMN-6 includes approximately eight EO groups and a hydroxyl group, which makes it a better choice to form the strong hydrogen bond with the water core than LS-45 that contains only four EO groups. However, the three branched hydrocarbon tails make TMN-6 more soluble in CO2 than LS-45 in experimental pressure and temperature. The relatively high CO2-philes tail chain is unfavorable to the aggregation of surfactant. Hence, the integrated nature of the polar head and the tail chain resulted in the similar cμc between the two surfactants. From the structure analysis of the surfactants, we found that DYNOL-604 has the same EO group numbers with LS-45. One might expect that DYNOL-604 exhibits a lower aggregation number than LS-45, since DYNOL-604 is more soluble in CO2 compared to LS-45 which can be observed from Figure 5. However, the cμc of DYNOL-604 is slightly higher than that of LS-45, which means the two hydroxyl groups and unsaturated bond of DYNOL-604 make it a better hydrophile than LS-45. 3.2. The Capacity to Dissolve Water of the Surfactant/ CO2 Systems. The molar ratio of water-to-surfactant (W0) is also an important parameter of scCO2 microemulsion. The W0 indicates the size of the water cluster and the capacity of the microemulsion system to load water. The cloud point pressures for the CO2/surfactant/water ternary systems were measured at different W0 and temperatures from 35 to 45 °C. The experimental data was listed in Table 3.

Table 3. Cloud-Point Pressures of Surfactant/H2O/CO2 system pressure (MPa) concentration mol/L

W0

35 °C

40 °C

45 °C

LS-36

0.0084

0.0084

LS-54

0.0084

TMN-6

0.011

DYNOL-604

0.006

12.15 12.56 13.34 14.37 17.34 12.80 14.2 14.50 15.00 17.20 13.70 14.11 15.19 17.31 13.94 13.94 15.03 17.11 8.33 8.81 9.09 9.67 11.03 13.03 15.01 9.74 10.34 11.66 14.00 16.34

13.53 14.12 15.06 15.99 18.93 14.69 15.96 16.10 16.39 18.30 15.40 15.90 16.79 18.80 15.37 15.55 16.42 18.55 9.45 10.17 10.48 11.21 12.45 14.45 16.61 11.10 11.79 12.95 15.54 17.10

15.02 15.60 16.56 17.58

LS-45

5.8 6.8 7.8 8.8 10.8 5.8 7.8 8.8 9.8 11.8 5.8 6.8 7.8 8.8 3 5 7 9 3.1 7.5 9.3 11.8 14.3 16.8 18.4 5 6.5 8 9.5 11

type

0.011

16.53 17.2 17.31 18.00 17.10 17.50 19.00 16.57 16.83 17.98 10.79 11.59 11.90 12.55 13.97 15.86 18.13 12.42 13.25 14.34 16.77 18.57

surfactant can be ignored when the volume of the piston changes. As shown in Figure 3, the area above the cloud-point pressure curve (solid line) represents the homogeneous region, and below the curve is the heterogeneous region in which the microemulsion is not formed. As expected, the pressure required to dissolve the surfactant increases with increasing concentration of the surfactant at given temperatures. That is because the density of CO2 increases with the increase of pressure, and the solvent power of CO2 is improved as well. In Figure 3 panels a−d, the CPP curve first rises sharply and then it rises slowly. However, in Figure 3e, the trend of the curve is in contrast to the others. Of great importance is that we find the CPP curve has a strong fluctuation in a narrow range: the pressure first goes through a local maximum, follows a slightly decrease, and then increases again. The tendency of the CPP curve after the maximum point is different from previous development, which can reveal that the structural property of the system changes. Since the local maximum is the first point of the narrow range, we choose this point as the critical point, and the concentration corresponding to the critical point can be defined as the critical microemulsion concentration (cμc, mol/ L).17 When the concentration of the surfactant is lower than the cμc, the surfactants exist as a state of molecules or ions, referred to as the monomer. At the concentration moves well above the cμc, the system can be regarded as being in a state of micellar aggregation because of the molecular association.17,43 The process of micellar aggregation at the atomic level, which 3139



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Figure 6. Loading water of surfactant/water/CO2 system at different temperatures and concentrations: (a) 0.0084 M LS-36/water/CO2; (b) 0.0084 M LS-45/water/CO2; (c) 0.0084 M LS-54/water/CO2; (d) 0.011 M DYNOL-604/water/CO2; (e) 0.011 M TMN-6/water/CO2.

Figure 7. Loading water of five surfactant/CO2 systems at 35 °C and 0.0084 mol/L.

Figure 8. Effects of surfactant concentration on the capacity of loading water of DYNOL-604/CO2 system at 35 °C.

As shown in Figure 6, a clear one-phase solution was observed above the solid line for each temperature. The cloud point pressure increases with W 0 at fixed surfactant concentration and temperature. The results obtained indicate that the stability of the mircoemulsion decreases with increasing concentration of water; subsequently, the pressure necessary to stable the system increases. Nevertheless, the rising tendency of the CPP curve of each system is slightly different. At a fixed W0, the pressure increases with the increase of the temperature because the density or solvent power of CO2 decreases with increasing temperature. The effect of surfactant structure on the CPP of five systems at the same concentration of surfactant (0.0084 mol/L) and

temperature (35 °C) are shown in Figure 6. The result indicates that DYNOL-604 is readily dissolved in CO2, especially compared to the remaining four, due to its unique structure mentioned above. In addition, certain amounts of water can be loaded into the surfactant/CO2 systems. As shown in Figure 7, the CPP curve of the LS-36 and LS-45 intersect as the W0 increases, similar to that of LS-54 and TMN-6. That is because the W0 mainly depends on the solubility and the structure of the surfactants. Taking LS-36 and LS-45 for example, LS-36 is more easily dissolved in CO2 than LS-45, which is favorable for a system to load water. From another point of view, LS-45 has more EO groups and is a stronger hydrophile to interact with water. At lower pressure, 3140



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Figure 9. Snapshots of the time evolution of 6 LS-36/100 H2O/12000 CO2 system at 35 °C and 17.34 MPa. Blue and green particles are water molecules and surfactant molecules, respectively, the CO2 molecules are not shown for clarity. The snapshots are taken at (a) 0, (b) 18, (c) 27, (d) 36, (e) 51, and (f) 54 ns.

Figure 10. Snapshots of LS-36/H2O/CO2 ternary systems at different conditions. The dissociated surfactants were marked in red. (a) 12 LS-36/120 H2O/12000 CO2 at 35 °C and 18.87 MPa; (b) 12 LS-36/140 H2O/12000 CO2 at 35 °C and 21.56 MPa; (c) 12 LS-36/160 H2O/12000 CO2 at 35 °C and 23.13 MPa.

Figure 8. At lower pressure, the system with a lower concentration of surfactant can load more water, which means the solubility of surfactants in CO2 plays the crucial role in this condition. At higher pressure, the system with higher concentration of surfactant is more capable of dissolving water. The main reason is that the solubility of the surfactants in both concentrations is large enough at higher pressure, the microemulsion with a larger scale surfactant cluster has the competence to capture more water. Considering CO2 can dissolve a small amount of water without surfactant,45 the water in the microemulsion can be classified as the water in the bulk CO2 and as water in the water cluster. The distribution regulation of water in a microemulsion is discussed in the next part, again through the MD simulation. 3.3. Molecular Dynamics Simulation of LS-36/Water/ CO2 Ternary System. Since the MD simulation can provide direct evidence of the formation of nanometer-sized aggregation vividly and it is helpful to explore the content of a microstructure on a relatively detailed atomic level,28 we chose LS-36 as an example and examined the LS-36/water/ CO2 ternary system under different conditions by MD. The reliability of the MD simulation was shown in Figure S2−S3 in the Supporting Information. Figure 9 illustrates the sponta-

Table 4. Detailed Information of the Systems (a), (b), and (c) in Figure 10 system

aggregation number of surfactant

percentage of water cluster (%)

calculation time (ns)

(a) (b) (c)

7 9 10

74.16 78.57 83.75

57 39 21

the higher solubility of the surfactant in CO2 is favorable for the system to load water. When the system pressure is high enough to produce sufficient solubility for the two surfactants, the hydrophilic structure of the surfactant, which can be represented by the EO group numbers, plays a dominant role in determining the ability to load water. The results are similar to Han’s study,32 in which a series tests were performed to study the capacity of loading water of the microemulsion system that uses LS-45 and LS-36. The number of PO groups and EO groups of surfactant allows the system to load water in different conditions. To investigate the effect of different surfactant concentrations on the capacity of dissolving water in the same system, we performed a set of experiments with DYNOL-604 shown in 3141



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combination of experiment and simulation can provide an important method to explore the important information on structure regulation of self-assembled surfactants and expand the study to the microscopic scale.

neous evolution of the LS-36/water/CO2 system from a complete random distribution of the molecules to an ordered aggregate of LS-36 that dispersed in the continuous (CO2) phase. The system gradually became stable as the calculation went on, and the single aggregate of six surfactants started to form for the first time at around 36 ns. The water core is surrounded by surfactants, while the surfactant tails penetrate the carbon dioxide. The tense aggregation of a certain amount of surfactant and water molecules is seen to be the strong hydrogen bonding interaction between surfactant’s headgroup and water core. To investigate the distribution of water in the microemulsion and the effects of water amount on surfactant aggregation, a series of simulations were performed as shown in Figure 10. The more detailed information on the aggregation was listed in Table 4. All the simulations were carried out at the conditions derived from the experimental data of Han’s group.32 The percentage of water cluster reflects the distribution of water in the microemulsion. The results show that the system with more water molecules can form a larger water cluster and catch more surfactants around in a short time, although a few surfactants dissociate in the continuous phase over the entire computation time. This phenomenon proves that the stronger hydrogen bond can be formed when the system contains a bigger water cluster; therefore, surfactants can be assembled more easily. In addition, the computation time required to stablize the system decreases as the water amount increases, which reveals that the system containing more water is easier to stabilize.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00231. Force field parameters of LS-36; atom types; bond parameters; nonbonded parameters; angle parameters; additional figures and tables.(PDF)

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AUTHOR INFORMATION

Corresponding Author

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

The authors thank the National Natural Science Foundation of China (20976026, 20976028, 21376045, 21506027), Doctoral Fund of Ministry of Education of China (20120041110022), and Chinese Postdoctoral Science Foundation (2015M571307) for financial support. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS A series of experiments to test the cμc and W0 of five commercially available nonionic hydrocarbon surfactants (LS36, LS-45, LS-54, DYNOL-604, and TMN-6) at different conditions was performed. The results suggest that the structure of surfactant, which mainly contains the PO groups, EO groups, and branched chain, plays an important role in determining the cμc. The surfactants with more hydrophilic groups, such as EO group and hydroxyl, tend to have a higher cμc. The ability to load water of the surfactant/CO2 ternary systems is related to the categories and concentration of surfactants; the pressure and temperature required to stablize the system are also involved. The study of W0 demonstrates that the solubility of surfactant in CO2 favors the reverse micelles to load water at lower pressure, while high surfactant concentration and the strong hydrogen bond between the hydrophilic structure of the surfactant and water can be dominant in determining the ability of the system to solubilize water at higher pressure. The molecular dynamics simulation (MD) of self-assembly reverse micelles showed the dynamic aggregation on a relatively detailed atomic level. The results show the evolution of LS-36/H2O/CO2 ternary system from a completely random distribution of the molecules to a stable surfactant cluster containing water. The spontaneous evolution can be seen as the dynamic aggregation of the microemulsion. It is also found that the microemulsion system with more water molecules can form a larger water cluster and catch more surfactants. The system is also easier to stabilize when it contains more water. In conclusion, the cμc and W0 provide the basis for designing desired a microemulsion system and useful information for the structure regulation of a scCO2 microemulsion. The formation process of reverse micelles and the microstructure of aggregation can be obtained through MD simulation. The 3142



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Critical Microemulsion Concentration and Molar Ratio of Water-to

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