8040
Langmuir 2001, 17, 8040-8043
Investigation of Nonionic Surfactant Dynol-604 Based Reverse Microemulsions Formed in Supercritical Carbon Dioxide Juncheng Liu,†,‡ Buxing Han,*,† Ganzuo Li,‡ Xiaogang Zhang,† Jun He,† and Zhimin Liu† Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, China Received May 21, 2001. In Final Form: September 17, 2001 The solubility of Dynol-604 (a surfactant) in supercritical (SC) CO2 and the phase behavior of the CO2/ water/Dynol-604 system were studied at different temperatures and pressures. The results showed that the solubility of the surfactant in SC CO2 is high and one-phase water-in-CO2 microemulsions could be formed although Dynol-604 is a nonfluorous and nonsiloxane nonionic surfactant. The solvatochromic probe studies and the UV spectrum of lysozyme in the supercritical CO2/water/Dynol-604 system further proved the existence of a water domain in the supercritical CO2 microemulsions.
1. Introduction In recent years, the use of supercritical fluids (SCFs) has offered the opportunity to replace conventional organic solvent in a variety of applications. CO2 is most attractive because it is nontoxic, nonflammable, and environmentally benign, and it has an easily accessible critical temperature and pressure (31.1 °C and 7.38 MPa). However, because of its very low dielectric constant and polarizability per volume, CO2 is a poor solvent for high molecular weight or hydrophilic molecules, such as amino acids, proteins, metal ions, and many polymers. In the past decade, several approaches have been explored to enhance the solubility of polar substances in SC CO2.1 For example, addition of polar cosolvents such as alcohols or acetone can increase the polarity of supercritical (SC) CO2. Another way to make CO2 suitable for dissolving strong polar molecules is using suitable surfactants to create reverse micelles or microemulsions. Exploration of CO2-soluble surfactants is of great importance to both pure and applied sciences. Consani and Smith2 tested the solubility of over one hundred commercially available surfactants in SC CO2 at 50 °C and 10-50 MPa. Practically all of them were insoluble or only slightly soluble, and thus they could not solubilize a significant amount of water. To overcome the problem described, Hoefling et al.3,4 have synthesized CO2-soluble surfactants. For this purpose, functional groups with low solubility parameters, low polarizability parameters, and Lewis bases such as dimethyl siloxanes, hexafluoropropylene oxides, and fluoroalkyl groups were chosen, and the synthesized surfactants displayed relatively high solubility in CO2 at moderate pressures. Solutions of * Corresponding author. Fax: +86-10-62559373. E-mail address:
[email protected]. † Chinese Academy of Sciences. ‡ Shandong University. (1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction; 2nd ed.; Butterworth: Boston, 1994. (2) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (3) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (4) Hoefling, T. A.; Beitle, R. R.; Enick, R. M.; Beckman, E. J. Fluid Phase Equilib. 1993, 83, 203.
several surfactants in CO2 were able to extract thymol blue, a hydrophilic dye, from aqueous solution. Subsequently, a repeating fluorinated ether functional group (hexafluoropropylene oxide) was incorporated into a polymer to increase the solubility in CO2.5 Poly(hexafluoropropylene oxide) is the most CO2-philic soluble polymer observed to date, and the surfactants with a perfluoroalkylpolyether tail exhibit extremely high solubility in CO2.6-10 In addition, fluorocarbon/hydrocarbon hybrid surfactants,11 as well as fluorous and nonfluorous analogues of AOT,12,13 have been synthesized to form SC CO2 microemulsions. Some of the CO2 microemulsions have been applied to produce nanoparticles,14-16 to extract metal ions17 and macromolecules,8,18,19 and to conduct chemical reactions.20,21 The nonionic surfactants C12EO3 and C12EO8 formed small aggregates in CO2 that contained approximately three to five surfactant molecules per aggregate.22 McFann (5) Hoefling, T. A.; Stofesky, D.; Reid, M.; Beckman, E. J.; Enick, R. M. J. Supercrit. Fluids 1992, 5, 237. (6) Singley, E. J.; Liu, W.; Beckman, E. J. Fluid Phase Equilib. 1997, 128, 199. (7) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V.; Carlier, C.; Randolph, T. W. Science 1996, 271, 624. (8) Heitz, M. P.; Carlier, C.; deGrazia, J.; Harrison, K. L.; Johnston, K. P.; Randolph, T. W.; Bright, F. V. J. Phys. Chem. B 1997, 101, 6707. (9) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934. (10) Clarke, M. J.; Harrison, K. L.; Johnston, K. P.; Howdle, S. M. J. Am. Chem. Soc. 1997, 199, 6399. (11) Harrison, K. L.; Goveas, J.; Johnston, K. P. Langmuir 1994, 10, 3536. (12) Liu, Z. T.; Erkey, C. Langmuir 2001, 17, 274. (13) Eastoe, J.; Paul, A.; Nave, S.; Steytler, D. C.; Robinson, B. H.; Rumsey, E.; Thorpe, M.; Heenan, R. K. J. Am. Chem. Soc. 2001, 123, 988. (14) Ji, M.; Chen, X. Y.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631. (15) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langumuir 1999, 15, 6613. (16) Ohde, H.; Rodriguez, J. M.; Ye, X. R.; Wai, C. M. Chem. Commun. 2000, 2353. (17) Yates, M. Z.; Apodaca, D. L.; Campbell, M. L.; Birnbaum, E. R.; McCleskey, T. M. Chem. Commun. 2001, 25. (18) Beckman, E. J. Science 1996, 271, 613. (19) Cooper, A. I.; Londono, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.; Burke, A. L. C.; Frechet, J. M. J.; Desimone, J. M. Nature 1997, 389, 368.
10.1021/la010743d CCC: $20.00 © 2001 American Chemical Society Published on Web 11/28/2001
Dynol-604 Based Reverse Microemulsions
Figure 1. Schematic diagram for studying phase behavior: (1) carbon dioxide cylinder; (2) syringe pump; (3) high-pressure view cell; (4) constant temperature water bath; (5) magnetic stirrer; (6) motor for stirrer; (7) high-pressure gauge; (8) temperature controller.
found that certain nonionic surfactants solubilized excess water into CO2 when a cosurfactant was added.23 So far, the formation of SC CO2 microemulsions by nonfluorous or nonsiloxane nonionic surfactants has been seldom reported in the literature.24 In this work, the solubility of an acetylenic glycol-based nonionic surfactant Dynol-604, which is a nonfluorous and nonsiloxane surfactant, was studied in SC CO2; the phase behavior of a SC CO2/Dynol-604/water system and the solubilization of methyl orange and lysozyme were also investigated. 2. Experimental Section Materials. Dynol-604 was obtained from Air Products and Chemicals Inc. It is a nonfluorous and nonsiloxane, acetylenic glycol-based nonionic surfactant. CO2 (99.995% purity) was supplied by the Beijing Analytical Instrument factory. Methyl orange and methanol were obtained from the Beijing Chemical Agent Factory (A. R. Grade). Double distilled water was used throughout the experiments. The lysozyme (MW ) 14 300) was supplied by sino-American Biological Chemical Corporation with a purity of >98%. Apparatus and Procedures for Determining Phase Behavior. The apparatus for studying the solubility of the surfactant in supercritical CO2 and the phase behavior is shown schematically in Figure 1. It consists mainly of a high-pressure view cell with a volume of 40.0 cm3, a constant temperature water bath, a high-pressure syringe pump, a pressure gauge, a magnetic stirrer, and a gas cylinder. The high-pressure view cell is made of stainless steel with two borosilicate windows. The temperature of the water bath was controlled by using a Haake-D8 controller, and the temperature was measured by a mercury thermometer with an accuracy of better than (0.05 K. The pressure gauge was composed of a pressure transducer (FOXBORO/ICT, Model 93) and an indicator, which was accurate to (0.04 MPa in the pressure range 0-30 MPa. The procedures for studying the phase behavior of the SC CO2/Dynol-604/H2O ternary mixture are given because those for CO2/Dynol-604 are simpler. In a typical experiment, a suitable amount of the surfactant Dynol-604 was charged into the high-pressure view cell, and the air in the cell was replaced by CO2. The desired amount of double distilled water was then injected into the cell by a syringe. The cell was then placed into the constant temperature water bath. CO2 was compressed into the cell slowly by the pump after thermal equilibrium had been reached. The fluid was stirred at fixed pressure, and stirring was stopped when observing the phase behavior. The (20) Jacobson, G. B.; Lee, C. T.; Johnston, K. P. J. Org. Chem. 1999, 64, 1201. (21) Kane, M. A.; Baker, G. A.; Pandey, S.; Bright, F. V. Langmuir 2000, 16, 4901. (22) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377. (23) McFann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (24) Wilkinson, S. P.; Robeson, L. M.; Schweighardt, F. K. Eur. Pat. Appl. EP 0 830 890 A1, 1998.
Langmuir, Vol. 17, No. 26, 2001 8041
Figure 2. Schematic diagram of the apparatus for density measurement: (1) gas cylinder; (2, 4, 6, and 8) valves; (3) syringe pump; (5) pressure gauge; (7) sample cell; (9) water bath; (10) temperature controller. pressure was increased gradually until the Dynol-604-rich phase disappeared completely and the system became a homogeneous transparent single phase. This pressure is defined as the dissolution pressure. This procedure was repeated three times at each condition, and the repeatability for the dissolution pressure was (0.03 MPa. At dissolution pressure the solution became turbid as the pressure decreased slightly. Apparatus and Procedures for Determining the Density. The phase behavior measurements discussed above could obtain the volume concentration of the surfactant and water. To obtain the concentration in weight percent, we determined the density of the fluids at the same conditions as those in the phase behavior study. The schematic diagram of the apparatus is shown in Figure 2. The main difference, compared with the setup for studying the phase behavior, was that the high-pressure view cell in Figure 1 was replaced by a sample bomb of 40.0 cm3, which was much lighter than the view cell. Thus, the sample bomb could be weighed accurately. The volume of the sample bomb was calibrated accurately by a gravimetric technique using water. The estimated accuracy of the volume calibration was better than (0.1%. A Mattler PM1200 balance with sensitivity of 0.001 g was used for the determination of the weight of the sample bomb. In a typical experiment, a suitable amount of Dynol-604 was charged into the sample bomb and the air in the sample bomb was replaced by CO2, and then the desired amount of water was charged into the sample bomb. Valve 6 was then closed. The sample bomb was connected to the system as shown in Figure 2. After the system had reached thermal equilibrium, the sample bomb was shaken and CO2 was slowly charged into the system by using the pump until the dissolution pressure was reached. After the pressure of the system was stabilized for about 2 h, valve 6 was closed. The sample bomb was removed, and its weight was determined. The density of the fluid could be easily obtained on the basis of the volume of the sample bomb and the mass of the fluid. Procedures for Studying the Solubilization of Methyl Orange and Lysozyme. The solubilization of methyl orange and lysozyme in the microemulsions was studied by a UV method, which was similar to that reported previously.25 The apparatus consisted mainly of a gas cylinder, a high-pressure pump, a pressure gauge, a temperature-controlled high-pressure sample cell, and valves and fittings. The UV-vis spectrometer was produced by Beijing Instrument Company (TU-1201), and the data were collected by a digital computer. The high-pressure cell had two quartz windows, and the internal volume and the path length of the cell were 1.5 cm3 and 1.2 cm, respectively. Only the procedure for studying the solubilization of methyl orange is described because that for lysozyme is similar. A desired amount of a solution of methyl orange in methanol was injected into the sample cell using a 0.1 mL syringe. Gaseous CO2 was then flushed slowly through the sample cell to remove the methanol. Dynol604 (0.024 g) was loaded into the sample cell, and the air in the (25) Lu, J.; Han, B. X.; Yan, H. K. Phys. Chem. Chem. Phys. 1999, 1, 3269.
8042
Langmuir, Vol. 17, No. 26, 2001
Liu et al.
Figure 3. Solubility of Dynol-604 in supercritical carbon dioxide. Table 1. Solubility of Water in SC CO2 (WCO2) and in the SC CO2/Dynol-604 (0.016 g/mL) System (Wt) T (K)
P (MPa)
density (g/cm3)
Wt (wt %)
Wco2 (wt %)
Wc (wt %)
308.15 308.15 308.15 313.15 313.15
16.17 18.23 21.87 18.56 19.94
0.788 0.797 0.805 0.763 0.770
0.23 0.45 0.60 0.26 0.42
0.17 0.18 0.19 0.20 0.21
0.06 0.27 0.41 0.06 0.21
Figure 4. Absorbance of CO2 + Dynol-604 (0.016 g/mL); CO2 + H2O + methyl orange; and CO2 + Dynol-604 (0.016 g/mL) + methyl orange systems at 21.87 MPa and 308.15 K.
sample cell was replaced by CO2.The desired amount of double distilled water was then injected into the sample cell with a 0.01 mL syringe. After thermal equilibrium had been reached, CO2 was compressed into the sample cell slowly by the syringe pump until the desired pressure was reached. The solution was stirred for at least 30 min and equilibrated for 1 h before recording the spectrum. All the measurements were carried out at 308.15 K.
3. Results and Discussion 3.1. Solubility of Dynol-604 in SC CO2. The solubility of Dynol-604 in SC CO2 was studied in the temperature range 308.15-333.15 K and at pressures up to 26 MPa. The results are shown in Figure 3. The solubility increases with increasing pressure and decreases with increasing temperature. The results in Figure 3 indicate that the surfactant is soluble in SC CO2 and the solubility can be as high as 5 wt % at easily accessible temperatures and pressures. 3.2. Loadings of Water in the CO2/Dynol-604 System. CO2 itself has a noticeable affinity for water, far more than that of ethane or propane.26 The water is distributed between the surfactant aggregates and CO2 in such a way as to minimize the overall free energy of the system. Eventually, as more water is added, a second phase forms. In this work we determined the solubility of water in SC CO2 (WCO2) and in CO2/Dynol-604 mixtures (Wt). Table 1 lists the values of Wt (wt %) and WCO2 (wt %). The corrected solubility Wc (wt %) is also given in the table, which is obtained by subtracting WCO2 from Wt at the same temperature and pressure. Obviously, Wc can be considered approximately as the quantity of the water in the reverse micelles. The results of Wc suggest that water domains are present in the reverse micelles, which has been further proved by the solubilization of methyl orange and lysozyme and will be discussed in the following sections. As shown in Table 1, Wc increases with the increase of pressure, and reducing temperature favors loading of water into the one-phase Dynol-604 based microemulsions. 3.3. Solvatochromic Probe Studies. The absorbance maximum of a solvatochromic probe is sensitive to the local environment about the probe. To confirm that a water domain exists and to characterize its nature, a solvato(26) Wiebe, R. Chem. Rev. 1941, 29, 475.
Figure 5. Absorbance of methyl orange in the CO2 + Dynol604 (0.016 g/mL) + H2O system as a function of added water at 21.87 MPa and 308.15 K. (/) The amounts of MO added in the system (3.28 × 10-6 g) are two times that (1.64 × 10-6 g) for other experiments. Table 2. Maximum Absorbance of MO (λmax) in Microemulsions at 308.15 K and 21.87 MPa and Various Compositionsa exp no.
Dynol-604 (g/mL)
1 2 3 4 5 6 7
0.016
a
Wt (wt %)
Wc (wt %)
MO in the cell (g × 106)
λmax (nm)
absorbance
0.04 0.26 0.41 0.41
1.64 1.64 1.64 1.64 1.64 3.28
408 416 428 428
0.16 0.28 0.29 0.51
0.19 0.016 0.016 0.016 0.016 0.016
0.23 0.45 0.60 0.60
Each entry is the average of three trials.
chromic probe study was performed in this work. Methyl orange (MO) was selected as the solvatochromic probe, which has absorbance in the range 400-470 nm (depending on the environment). The absorbance profiles of these experiments are illustrated in Figures 4 and 5. The absorption band of MO cannot be observed for SC CO2/ MO, SC CO2/MO/Dynol-604, and SC CO2/MO/water systems, as can be known from Figure 4. It can be deduced that the solubility of MO in SC CO2 and SC CO2/Dynol604 and SC CO2/water mixtures is extremely low. However, the absorption band can be observed for SC CO2/ MO/Dynol-604/water mixtures, as shown in Figure 5, indicating the existence of methyl orange in the water domain region of the reverse micelles. Table 2 lists the absorbance maximum (λmax) at different conditions. The results in Table 2 and Figure 5 show that λmax increases
Dynol-604 Based Reverse Microemulsions
with Wc. The λmax is in the range 408-428 nm, while the λmax of MO in ordinary water is 464 nm. These results suggest that the polarity of the environment of the solubilized MO is intermediate between those of bulk water and alkanes; that is, the polarity of the water domains in the reverse micelles is lower than that of the bulk water, which was also discussed for other reverse micelles.27 As expected, the polarity of the environment of solubilized MO increases with Wc (wt %). Thus, the formation of microemulsions is proven by the UV spectra. Water can hydrogen bond to anionic headgroups such as sulfonates and sulfosuccinates to enhance the formation of reverse micelles.28 For block copolymers with a poly(ethylene oxide) block, small amounts of water can cause the formation of a large stable aggregate.29 The acetylenic glycol-based Dynol-604 has a hydroxy group, which can form hydrogen bonds with water. Thus, the water may act as a “glue” to bond the surfactant headgroups together, and thus act as a driving force for the aggregation. The change of the absorption value was also studied with different loadings of water. As can be seen in Table 2 or Figure 5, when 1.64 × 10-6 g of MO is added to the cell, the absorption value of MO solubilized in the Dynol604 based water-in-CO2 microemulsions with a Wc of 0.26 wt % was higher than that of microemulsions with a Wc of 0.04 wt %. The reason is that the ability of the SC CO2 microemulsions to dissolve MO increases with the increase of loading water. However, the absorption of MO solubilized in the Dynol-604 based water-in-CO2 microemulsions with a Wc of 0.26 wt % was approximately the same as that for the microemulsions with a Wc of 0.41 wt %. The reason is that the microemulsions with the Wc of 0.41 wt % may not be saturated by MO. As expected, the absorption of the microemulsions with the Wc of 0.41 wt % increases significantly as the amount of MO added to the cell increased to 3.28 × 10-6 g, which indicates that more MO was solubilized in the Dynol-604 based water-in-CO2 microemulsions. 3.4. Solubilization of Lysozyme in the Microemulsions. Lysozyme has an absorption at about 275 nm.30 Similarly to the case for MO, experiments showed that SC CO2 or SC CO2/Dynol-604 or SC CO2/water mixtures could not dissolve the biomacromolecule. Figure 6 shows the absorption spectrum of the SC CO2/Dynol-604/water (27) Lay, M. B.; Drummond, C. J.; Thistlethwaite, P. J.; Grieser, F. J. Colloid Interface Sci. 1989, 128, 602. (28) Eicke, H. F.; Christen, H. H. Chim. Acta 1978, 61, 2258. (29) Cogan, K. A.; Gast, A. P. Macromolecules 1990, 23, 745. (30) Zhang, H. F.; Lu, J.; Han, B. X. J. Supercrit. Fluids 2001, 20, 65.
Langmuir, Vol. 17, No. 26, 2001 8043
Figure 6. Absorption of Dynol-604/water/CO2 and Dynol-604/ water/CO2/lysozyme systems with a WC of 0.41 wt % at 21.87 MPa and 308.15 K (the concentration the Dynol-604 is 0.016/ mL).
system and the SC CO2/Dynol-604/water/lysozyme microemulsion system at 308.15 K and 21.87 MPa. The concentration of the surfactant in the system is 0.016 g/mL with a Wc of 0.41 wt %. The absorption band at 228 nm is assigned to the absorption of the surfactant. The absorption band of the CO2/Dynol-604/water/lysozyme system at 275 nm is attributed to the absorption of lysozyme.30 Obviously, lysozyme is solubilized in the microemulsions, which further proves the existence of water domains. 4. Conclusion The limitation of carbon dioxide as a processing fluid originates partially from the low solubility of polar compounds. The development of CO2-philic amphiphiles is one of the effective methods to solve this problem. In this work, the solubility of Dynol-604, which is a nonfluorous and nonsiloxane, acetylenic glycol-based nonionic surfactant, is measured. The results indicate that the solubility of the surfactant in supercritical CO2 is high. The phase behavior studies of the SC CO2/Dynol-604/water system indicate that water-in-CO2 microemulsions can be formed, which is further proved by the solvatochromic probe study and the UV spectrum of lysozyme in the SC CO2/Dynol-604/water system. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (Grant 20133030) and Ministry of Science and Technology for the financial support (Grant G20000781). LA010743D
