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Solubility of Ls-36 and Ls-45 Surfactants in Supercritical CO2 and Loading Water in the CO2/Water/Surfactant Systems Juncheng Liu,†,‡ Buxing Han,*,† Zhengwu Wang,‡ Jianling Zhang,† Ganzuo Li,‡ and Guanying Yang† Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Key Laboratory of Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, China Received November 28, 2001. In Final Form: January 28, 2002 The solubility of Ls-36 and Ls-45 surfactants in supercritical (SC) CO2 and the phase behavior of CO2/ water/surfactant systems were studied at different temperatures and pressures. The results showed that the solubility of the surfactants in SC CO2 is high although they are non-fluorous and non-silicone containing surfactants. Moreover, the loading water of the 0.02 M Ls-36/SC CO2 and Ls-45/SC CO2 systems, which was characterized by the molar ratio of water to surfactant, W0corr, was up to 7.9. The results of this work provide useful information for designing other low-cost CO2-philic non-fluorous and non-silicone containing surfactants.
1. Introduction Today, CO2 has emerged as a promising environmentally benign alternative for toxic organic solvents since it is inexpensive, nontoxic, nonflammable, and readily available in large quantities and has moderate critical temperature and pressure (31.1 °C and 7.38 MPa). Moreover, it can be easily recaptured and recycled after use. However, a series of obstacles to employ CO2 result from its inability to dissolve high molecular weight or hydrophilic molecules, such as proteins, metal ions, and many polymers because of its very low dielectric constant and polarizability per volume. A strategy for overcoming the poor solubility of polar solutes in CO2 is to add cosolvents (also known as a modifier or entrainer), such as low-molecular-weight alcohols.1-3 However, the solubilities of many hydrophilic solutes, such as metals or proteins, are still very low. An effective approach to solve this problem is to employ specialized supercritical (SC) CO2 soluble surfactants to produce thermodynamically stable water-in-carbon dioxide (W/CO2) microemulsions, which contain polar microaqueous domains. Exploration of CO2-soluble surfactants offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science, and materials science. Unfortunately, early attempts were unsuccessful in that conventional alkyl-functional ionic amphiphiles exhibit poor to negligible solubilities in CO2 at moderate pressure.4 Although some nonionic surfactants can exhibit reasonable solubilities in CO2 at moderate pressure, solvation of water and polar compounds has * Corresponding author. Tel: (86-10)-62562821. Fax: (86-10)62559373. E-mail address:
[email protected]. † Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences. ‡ Key Laboratory of Colloid and Interface Chemistry of State Education Ministry. (1) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction, 2nd ed.; Butterworth: Boston, MA, 1994. (2) Kim, S.; Johnston, K. P. AIChE J. 1987, 33, 1603. (3) Johnston, K. P.; McFann, G. J.; Peck, D. G.; Lemert, R. Fluid Phase Equilib. 1989, 52, 1989. (4) Consani, M. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51.
proven to be difficult.4,5 A number of research groups have subsequently investigated the design of so-called “CO2philic” surfactants that are soluble in CO2 at moderate pressure.6-11 Fluorinated acrylates6 and fluorinated ether7-9 are the most technologically successful of these new amphiphiles, and a number of studies have been published on the phase behavior of such amphiphiles as a function of their chemical structure.10,11 In addition, a fluorocarbon-hydrocarbon hybrid surfactant (C7F15CH(OSO3-Na+)C7H15, (H-F)) has been shown to dissolve in CO2.12 However, this fluorocarbon-hydrocarbon surfactant hydrolyzes over time.12,13 Johnston et al. found that the surfactant with a perfluoroalkylpolyether tail (CF3(OCF2CF(CF3))3(OCF2)3COO-NH4+ (PFPE)) could form micelles which can solubilize significant amounts of water and protein.13 Nonetheless, there are concerns about toxicity of this class of surfactants.14 Silicones (poly(dimethylsiloxanes)) are also generally considered to be CO2-philic.8 However, silicone-functional amphiphiles require higher pressure to generate a single-phase solution in SC CO2 than do fluoroacrylates or fluoroether analogues. Consequently, studies on phase behavior-structure correlation or applications for silicone-containing amphiphile/ CO2 mixtures are relatively sparse compared to their fluorinated cousins.15 The nonionic surfactants, C12EO3 and C12EO8, formed small aggregates in CO2 that con(5) Hoefling, T. A.; Stofesky, D.; Reid, M.; Beckman, E. J.; Enick, R. M. J. Supercrit. Fluids 1992, 5, 237. (6) DeSimone, J. M.; Guan, Z.; Eisbernd, C. S. Science 1992, 25, 95. (7) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (8) Hoefling, T. A.; Newman, D. A.; Enick, R. M.; Beckman, E. J. J. Supercrit. Fluids 1993, 6, 165. (9) Hoefling, T. A.; Beitle, R. R.; Enick, R. M.; Beckman, E. J. Fluid Phase Equilib. 1993, 83, 203. (10) Newman, D. A.; Hoefling, T. A.; Beitle, R. R.; Beckman, E. J. J. Supercrit. Fluids 1993, 6, 205. (11) Singley, E. J.; Liu, W.; Beckman, E. J. Fluid Phase Equilib. 1997, 128, 199. (12) Harrison, K. L.; Goveas, J.; Johnston, K. P. Langmuir 1994, 10, 3536. (13) Johnston, K. P.; Harrison, K. L.; Clarke, M. J.; Howdle, S. M.; Heitz, M. P.; Bright, F. V. Science 1996, 271, 624. (14) Jonnston, K. P.; Randolph, T. W.; Bright, F. V.; Howdle, S. M. Science 1996, 272, 1726. (15) Fink, R.; Beckman, E. J. J. Supercrit. Fluids 2000, 18, 101.
10.1021/la011721u CCC: $22.00 © 2002 American Chemical Society Published on Web 03/16/2002
Surfactant Solubility in CO2 Chart 1. The Structures of Ls-36 and Ls-45 Surfactants
Langmuir, Vol. 18, No. 8, 2002 3087 Table 1. Solubility of Ls-36 and Ls-45 Surfactants in SC CO2 at Different Pressures and Temperatures and the Density of the Fluid at Dissolution Pressure Surfactant of Ls-36 308.2 K
tained approximately 3-5 surfactant molecules per aggregate.16 Mcfann et al. found that certain nonionic surfactants solubilized excess water into CO2 when a cosurfactant was added.17 So far, SC CO2/non-fluorous and SC CO2/non-silicone containing nonionic surfactants systems which can absorb a significant amounts of water have been seldom reported in the literature.18,19 In this work, we found that Ls-36 and Ls-45 surfactants shown in Chart 1,were soluble in SC CO2 and the Ls-36/ SC CO2 and Ls-45/SC CO2 systems could solubilize a significant amount of water at accessible conditions, although they are non-fluorous and non-silicone containing surfactants. In addition, they also have advantages of low toxicity and low price compared with fluorous surfactants. The results of this work provide useful information for designing other low-cost CO2-philic non-fluorous and nonsilicone containing surfactants. 2. Experimental Section 2.1. Materials. Ls-36 and Ls-45 surfactants were obtained from Henkel Coporation of Germany. CO2 (99.995% purity) was supplied by the Beijing Analytical Instrument factory. The water used was twice distilled, and its conductivity was 0.9 × 10-6 S cm-1. 2.2. Apparatus. To investigate the solubility of the surfactants in SC CO2 and the phase behavior of SC CO2/ surfactants/H2O systems, a high-pressure stainless steel view cell of 40 cm3 was used, which was similar to that used previously.20 The view cell was immersed in a water bath which was controlled by a Haake-D8 controller, and temperature was measured by accurate mercury thermometers 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 of 0-34 MPa. The chemicals in the cell were stirred by a magnetic stirrer. To determine the density of the mixed fluid and obtain more accurate density data, a light stainless steel sample cell (40.0 cm3 in volume and 321.470 g in weight) without windows was used to replace the heavy view cell. The density of the fluid was calculated from the mass of the fluid and the volume of the sample cell. 2.3. The Procedures for Phase Behavior. The procedures for studying phase behavior of SC CO2/Lsseries surfactants/H2O ternary system is described because that for SC CO2/surfactant system is simpler. In a typical experiment, a suitable amount of the Ls-36 or Ls45 surfactant was charged into the high-pressure view cell, and the air in the cell was replaced by CO2. The desired amount of twice 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 syringe pump after thermal equilibrium (16) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377. (17) Mcfann, G. J.; Johnston, K. P.; Howdle, S. M. AIChE J. 1994, 40, 543. (18) Liu, J. C.; Han, B. X.; Li, G. Z; Zhang, X. G.; He, J.; Liu, Z. M. Langumir 2001, 17, 8040. (19) Liu, J. C.; Han, B. X.; Zhang, J. L.; Li, G. Z.; Zhang, X. G.; Wang, J.; Dong, B. Z. Chem.sEur. J., in press. (20) Liu, J. C.; Han, B. X.; Li, G. Z; Liu, Z. M.; He, J. Fluid Phase Equilib. 2001, 187-188, 247.
318.2 K
solubility (M)
pressure (MPa)
density (g/cm3)
solubility (M)
pressure (MPa)
density (g/cm3)
0.01 0.02 0.03 0.04 0.05
11.10 14.50 16.73 18.71 19.74
0.765 0.823 0.843 0.856 0.861
0.01 0.02 0.03 0.04
15.31 17.56 19.78 21.33
0.759 0.794 0.816 0.827
Surfactant of Ls-45 308.2 K
318.2 K
solubility (M)
pressure (MPa)
density (g/cm3)
solubility (M)
pressure (MPa)
density (g/cm3)
0.01 0.02 0.03 0.04
12.97 16.73 19.32 21.05
0.796 0.845 0.870 0.878
0.01 0.02 0.03
16.46 20.11 22.37
0.785 0.829 0.848
had been reached. The fluid was stirred at fixed pressure, and stirring was stopped when observing the phase behavior. The pressure was increased gradually until the surfactant-rich phase disappeared completely and the system became a homogeneous transparent single phase. This pressure is defined as dissolution pressure. This procedure was repeated three times at each condition, and the uncertainty for the dissolution pressure was (0.05 MPa. 2.4. The Procedures for Measuring Density. The experimental procedures for measuring the mixture fluid density at the dissolution pressure were similar to that for studying phase behavior. As an example, we describe the procedure for determining the density of a Ls-36 surfactant/SC CO2/H2O ternary mixture. A suitable amount of Ls-36 was charged into the sample cell and the air in the sample cell was replaced by CO2, and then the desired amount of twice distilled water was charged into the sample cell by syringe. As the system had reached thermal equilibrium, the sample cell was shaken and CO2 was slowly charged into the system by using the syringe pump until the dissolution pressure was reached. After the mixed fluid was stirred for 1 h and stabilized for 30 min at dissolution pressure, the sample cell was removed and its weight was determined. The mixture fluid density was obtained on the basis of the volume of the sample cell and the mass of the fluid. 3. Results and Discussion 3.1. The Solubility of Ls-36 and Ls-45 in Pure SC CO2. Table 1 lists the solubility data of Ls-36 and Ls-45 surfactants in SC CO2 at different pressures and temperatures. The results demonstrate that the surfactants are quite soluble in CO2 (0.04-0.05 M or about 4 wt % in the solution) at easily accessible temperatures and pressures. As expected, the solubility increases with increasing pressure. The main reason is that the density of CO2 increases with the increase of pressure, and thus the solvent power of SC CO2 is stronger at the higher pressures. The solubility of Ls-36 is higher than that of Ls-45 at the same condition. This is understandable considering the fact that Ls-36 has three ethylene oxide (EO) groups and six propylene oxide (PO) groups, and Ls-45 has four EO groups and five PO groups, which can be known from Chart 1. The PO group is more CO2-philic than the EO
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Figure 3. Loading water of 0.02 M Ls-36/H2O/SC CO2 and 0.02 M Ls-45/H2O/SC CO2 systems at 308.2 K and different pressures. Figure 1. Dependence of solubility of Ls-36 and Ls-45 in SC CO2 on fluid density (d) at 308.2 K.
Figure 2. Effect of temperature on the solubilities of Ls-36 and Ls-45 in SC CO2 at different pressures.
group.21 Thus the solubility increases with the number ratio of the PO group to the EO group in the surfactants. In other words, the solubility of surfactants can be tuned by the ratio of m/n in Chart 1 to some extent. In addition, it is more likely that the oligomeric nature of the surfactants is also the primary factor besides of the CO2philic PO groups contributing to the high solubility of Ls-series surfactant in supercritical CO2. Figure 1 shows the dependence of the solubility on the density of the fluid. The solubility increases exponentially with density, which has been shown by the solubility of many other solutes in SC CO2. 3.2. Effect of Temperature on the Solubility. Figure 2 illustrates the effect of temperature on the solubility of Ls-36 and Ls-45 in SC CO2. As shown in the figure, the solubility decreases with the increase of temperature at fixed pressure. Temperature affects the solubility in two opposite ways. The density or solvent power of CO2 decreases with increasing temperature, which is not favorable to the dissolution of the solutes. On the other hand, the volatility of the solute increases with temper(21) Mawson, S.; Bott, R. H.; Jonhston, K. P.; O’neill, M. L.; Robeson, L. M.; Smith, R. D.; Wilkinson, S. P. Eur. Patent Appl. EP 0 814112 A3, 1997.
Table 2. Loading Water of 0.02 M Ls-36/SC CO2 and 0.02 M Ls-45/SC CO2 Systems at Different Pressures and Temperatures concn (M)
T (K)
0.02 0.02 0.02 0.02 0.02 0.02 0.02
308.2 308.2 308.2 308.2 308.2 318.2 318.2
Ls-36 Surfactant 16.17 0.835 17.20 0.847 18.87 0.860 21.56 0.885 23.13 0.893 18.98 0.806 20.14 0.819
0.02 0.02 0.02 0.02 0.02
308.2 308.2 308.2 308.2 308.2
Ls-45 Surfactant 18.86 0.863 20.12 0.875 20.35 0.879 21.87 0.886 23.37 0.895
P (MPa)
density (g/cm3)
W0CO2
W0corr
W0total
3.6 3.7 4.0 4.3 4.4 5.1 5.2
2.0 4.1 6.0 6.9 7.4 2.1 4.7
5.6 7.8 10.0 11.2 11.8 7.2 9.9
4.0 4.1 4.1 4.3 4.4
2.1 4.2 6.3 7.2 7.9
6.1 8.3 10.4 11.5 12.3
ature, which is favorable to the increase of the solubility. The effect of temperature on the solvent power of CO2 is dominant at our experimental conditions. Thus the solubility is reduced by increasing temperature. 3.3. Loading Water of Ls-36/CO2 and Ls-45/CO2 Systems. The loading water of Ls-36/SC CO2 and Ls45/SC CO2 systems can be characterized by the molar ratio of water to surfactant. CO2 itself has a noticeable affinity for water, far more than that of ethane or propane.22 So the corrected amount of water, W0corr, which is solubilized in the surfactant aggregates, can be obtained by subtracting that in the bulk CO2 from the total amount of loading water, W0total. The amount of water in the bulk CO2 (W0CO2) can be considered as the same as that dissolved in pure CO2 at the same temperature and pressure, which is also listed in Table 2. The dependence of W0corr on dissolution pressure at 308.2 K is shown in Figure 3. As shown in Figure 3, W0corr increases with the increase of dissolution pressure. One can also know from Figure 3 that W0corr of the Ls-45/H2O/SC CO2 system is more sensitive to pressure than the Ls-36/H2O/SC CO2 system. At the lower pressures, W0corr for the Ls-36/SC CO2 system is larger, while at higher pressure, the Ls-45/SC CO2 system can load more water. This can be explained in the following. The W0corr depends mainly on the solubility and the structure of the surfactants. The Ls-36 is more soluble than Ls-45, which is favorable to loading water. On the (22) Wiebe, R. Chem. Rev. 1941, 29, 475.
Surfactant Solubility in CO2
other hand, Ls-36 contains fewer hydrophilic EO groups, which is not favorable to enhancing the loading of water. At lower pressure, the solubility of Ls-45 is too low and cannot form effective aggregates to solubilize a significant amount of water. Thus the effect of the solubility of the surfactants on W0corr is dominant. At higher pressures, the solubilities of the both surfactants are high enough, and the effect of hydrophilic EO groups becomes dominant. 4. Conclusion The non-fluorous and non-silicone containing nonionic surfactants Ls-36 and Ls-45, which contain PO and EO
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groups, are soluble in SC CO2, and a significant amount of water can be solubilized in the Ls-36/SC CO2 and Ls45/SC CO2 systems at easily accessible conditions. The results of this work provide useful information for designing low-cost CO2-philic non-fluorous and non-silicone containing surfactants. Acknowledgment. The authors are grateful to Ministry of Science and Technology (G20000781) and the National Natural Science Foundation of China (20133030) for the financial support. LA011721U
