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Stubby Surfactants for Stabilization of Water and CO2 Emulsions: Trisiloxanes Sandro R. P. da Rocha,†,‡,§,| Jasper Dickson,‡,§ Dongman Cho,‡,§ Peter J. Rossky,†,‡,§ and Keith P. Johnston*,‡,§ Department of Chemistry and Biochemistry, Department of Chemical Engineering, and NSF-STCsEnvironmentally Responsible Solvents and Processes, University of Texas at Austin, Austin, Texas 78712-1062 Received September 25, 2002. In Final Form: January 9, 2003 Amphiphiles of a new class, with very short and bulky CO2-philic headgroups, the trisiloxane surfactants of the form ((CH3)3SiO)2Si(CH3)(CH2)3EOnR, where EO stands for ethylene oxide, are shown to be very interfacially active at the CO2-water interface, as indicated by their effectiveness in forming and stabilizing both concentrated and dilute emulsions of water and CO2. A transition in morphology from water-in-CO2 (W/C) to CO2-in-water (C/W) emulsions is induced by manipulating the surfactant hydrophilic-to-CO2philic balance (HCB) through variation of the number of EO groups. This inversion in morphology is in agreement with a minimum in interfacial tension at the water-CO2 interface. Microscopy and turbidimetry are used to follow the time evolution of the particle size and stability of dilute W/C emulsions. Dilute W/C emulsions formed with a trisiloxane surfactant containing seven EO groups show unparalleled stability against flocculation, indicating effective solvation of the trisiloxane group by CO2. Such enhanced stability suggests that the prevalence of CH3 groups on the surfactant tails favors solvation by CO2, and that the stubby nature of these CO2-philic groups minimizes tail overlap, leading to weaker intermolecular tailtail interactions and thus minimal flocculation. Consequently, the stability of dilute W/C emulsions with stubby nonfluorous trisiloxane tails is enhanced compared to the stability of those with longer, more overlapping tails.
Introduction Trisiloxane surfactants have significant technological relevance as wetting agents, foam stabilizers, lubricants, and emulsifiers.1 The exceptional surface properties of trisiloxane surfactants, some having “superwetting” ability,2 are attributed to their unique molecular architecture.3 Their general structure is commonly represented by M(D′En)M, where M is a trimethylsiloxy group, (CH3)3SiO1/2-, D′ is -O1/2Si(CH3)(R)O1/2-, R is the hydrophilic headgroup which is attached to the silicone hydrophobic tail through a -(CH2)3- spacer, and E is the oxyethylene (EO) group -(CH2CH3O)-. Figure 1 shows the gas-phaseoptimized conformation of two trisiloxane surfactant molecules with two EO units, and also that of two C12E2 hydrocarbon surfactants, both obtained under the AM1 electronic structure approximation.4 The trisiloxane tail group is similar in hydrophobicity to a linear C12H25 tail, but its volume is much largers530 Å3 compared to 350 Å3sand it is also much shorters9.7 Å compared to 15 Å.5 * To whom correspondence should be addressed. E-mail: kpj@ che.utexas.edu. † Department of Chemistry and Biochemistry. ‡ Department of Chemical Engineering. § NSF-STCsEnvironmentally Responsible Solvents and Processes. | Present address: Department of Chemical Engineering and Materials Science, Wayne State University, Detroit, MI. E-mail:
[email protected]. (1) Hill, R. M. Silicone Surfactants; Marcel Dekker: New York, 1999; Vol. 86. (2) Nikolov, A. D.; Wasan, D. T.; Chengara, A.; Koczo, K.; Policello, G. A.; Kolossvary, I. Adv. Colloid Interface Sci. 2002, 96, 325-338. (3) Svitova, T.; Hoffmann, H.; Hill, R. M. Langmuir 1996, 12, 17121721. (4) Deppmeier, B. J. e. a. SPARTAN, 5.1.3 ed.; WaveFunction Inc.: Irvine, CA, 1998. (5) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2278-2289.
Figure 1. Space-filling model of the gas-phase-optimized conformation obtained under the AM1 electronic structure approximation of two trisiloxanes with two EO units and two hydrocarbon surfactants with 12 carbon atoms in the hydrophobic tail and two EO units in the headgroup.4 The figure illustrates the larger number of possible contact points of the linear hydrocarbon-based surfactant compared to the stubby trisiloxane headgroup.
The aqueous phase behavior and the ternary oil/water/ surfactant phase diagrams of trisiloxane surfactants show trends very similar to those of the alkyl ethoxylated CiEj surfactant class.6 Compared to a CiEj surfactant with the same hydrophobicity, the phase behavior is, however, shifted to longer EO groups.5,7 In dilute aqueous surfactant
10.1021/la026608y CCC: $25.00 © 2003 American Chemical Society Published on Web 03/13/2003
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solutions, isotropic regular and inverted micellar fluid phases or anisotropic lamellar liquid crystalline structures can be observed, depending on the number of EO units in the surfactants. Similar to CiEj surfactants, morphology inversion from oil-in-water (O/W) to water-in-oil (W/O) microemulsions can be attained by varying the number of EO units.5 Despite these similarities, trisiloxanes show enhanced interfacial activity at both the air-water (AW) and oil-water (O-W) interfaces compared to CiEj surfactants, with surface tensions (σ) on the order of 20 mN/m, and very low O-W interfacial tensions (γ < 1.0 mN/m).3,8 Its unusual surface activity may be attributed to the presence of low surface energy methyl groups attached to a highly flexible siloxane backbone, in contrast with conventional hydrocarbon surfactants in which higher surface energy methylene groups compose the surfactant tail groups.9 A trisiloxane surfactant with eight EO groups has been previously shown to have high solubility in CO2.10 At 25 °C, 18 wt % surfactant was soluble below 10 MPa. Highly methylated substituted acetylenic diol surfactants with EO headgroups, e.g., 2,4,7,9-tetramethyl-4,7-di(ethylene oxide)4.8-5-decyne (Air Products, Surfynol 465), have also been shown to be highly soluble in CO2, to lower the water-CO2 interfacial tension and to form hydrated reverse micelles in CO2.11,12 Emulsions and microemulsions of water and liquid or supercritical CO2 have recently received considerable attention due to their benign nature, and potential application in fields including cleaning,13 enzymatic catalysis,14 bioseparations,15 and nanoparticle formation.16 While microemulsions of water and CO2 are attractive due to their thermodynamic stability and nanometer size range, they require large interfacial surfactant coverage. Moreover, relatively expensive fluorinated amphiphiles are often required to stabilize microemulsions of water and CO2.17-20 On the other hand, emulsions are only kinetically stable and have much larger droplet sizes, falling in the submicrometer and micrometer size range. The amount of surfactant required, however, is much lower due to the lower interfacial area. Another attractive feature of emulsions is that both water-in-CO2 (W/C) and CO2-in-water (C/W) emulsions have been formed with moieties that present a range of solubilities in CO2,11 (6) He, M.; Hill, R. M.; Lin, Z.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1993, 97, 8820-8834. (7) Li, X.; Washenberger, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2267-2277. (8) Svitova, T.; Hill, R. M.; Smirnova, Y.; Stuermer, A.; Yakubov, G. Langmuir 1998, 14, 5023-5031. (9) Hill, R. M. In Siloxane Surfactants; Hill, R. M., Ed.; Marcel Dekker: New York, 1999; Vol. 86. (10) McFann, G. J. Formation and Phase Behavior of Reverse Micelles and Microemulsions in Supercritical Fluid Ethane, Propane and Carbon Dioxide; University of Texas at Austin: Austin, TX, 1993. (11) O’Neill, M. L.; Cao, Q.; Fang, R.; Johnston, K. P.; Wilkinson, S. P.; Smith, C. D.; Kerschner, J. L.; Jureller, S. H. Ind. Eng. Chem. Res. 1998, 37, 3067-3079. (12) Liu, J.; Han, B.; Li, G.; Zhang, X.; He, J.; Liu, Z. Langmuir 2001, 17, 8040-8043. (13) Momose, T.; Yoshida, H.; Sherverni, Z.; Ebina, T.; Tatenuma, K.; Ikushima, Y. J. Vac. Sci. Technol., A 1999, 17, 1391-1393. (14) Holmes, J. D.; Steytler, D. C.; Rees, G. D.; Robinson, B. H. Langmuir 1998, 14, 6371-6376. (15) Beckman, E. J. Science 1996, 271, 613-614. (16) Holmes, J. D.; Bhargava, P. A.; Korgel, B. A.; Johnston, K. P. Langmuir 1999, 15, 6613-6615. (17) Harrison, K.; Goveas, J.; Johnston, K. P.; Orear, E. A. Langmuir 1994, 10, 3536-3541. (18) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934-3937. (19) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 69806984. (20) da Rocha, S. R. P.; Johnston, K. P. Langmuir 2000, 16, 36903695.
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including poly(fluorooctylacrylate)s (PFOAs),21 poly(fluoropolyether)s (PFPEs),22 fluorocarbons,23 poly(dimethylsiloxane)s (PDMSs),24 and hydrocarbon-based surfactants including poly(propylene oxide)s (PPOs) and poly(butylene oxide)s (PBOs),23 poly(ether carbonate)s,25 and more recently an AOT analogue.26 Stabilization of dilute W/C emulsions, even for short periods of time, has so far been a considerable challenge. Some success has been obtained with PPO-based copolymers.27 Note that PO moieties have pendant -CH3 groups which are believed to enhance solute solubility in CO2.11 The low viscosity of CO2, relative to that of conventional solvents, provides less shear to disrupt droplets during emulsion formation. In addition, emulsions may become unstable due to the low viscosity for several reasons.28 The droplet collision frequency increases, enhancing flocculation. The drainage of thin films of continuous phase between droplets increases, leading to more rapid coalescence.23,29 Creaming and sedimentation rates also increase. Even for PFPE-based surfactants, which form stable W/C microemulsions due to highly CO2-philic tails,20,30 and for PDMS-based surfactants, flocculation of both dilute and concentrated W/C emulsions has always been observed.22,24 At higher water-dispersed phase volume fractions, hindered sedimentation of W/C emulsions leads to emulsions stable for more than 1 day.24 Emulsions are also stabilized by gelation due to the attractive forces between particles that raise the viscosity of the emulsion.24,31 An interesting strategy to guide the choice of the CO2-phile is based on its cohesive energy density.11 Low cohesive energy density tails often exhibit increased solvation by CO2 and reduced solute-solute interactions, which translate to enhanced solubility in CO2.11 For hydrocarbons, this can be accomplished by increasing the degree of branching, upon substitution of a -CH2- by, e.g., a -CH(CH3)- group,32 similarly to what is found in trisiloxane surfactants.9 For example, McFann found that surfactants with highly methylated and branched hydrocarbon tails along with nonionic EO headgroups were highly soluble in CO2.10 Using such an approach, we have been recently able to form dilute W/C emulsions with a hydrocarbon-based anionic surfactant, an AOT analogue, sodium bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate (AOTTMH).26 It may be expected that, in addition to increasing the solubility in CO2, branching can also reduce the attractive interactions between the AOT-TMH tail groups, (21) Chillura-Martino, D.; Triolo, R.; McClain, J. B.; Combes, J. R.; Betts, D. E.; Canelas, D. A.; DeSimone, J. M.; Samulski, E. T.; Cochran, H. D.; Londono, J. D.; Wignall, G. D. J. Mol. Struct. 1996, 383, 3-10. (22) Lee, C. T.; Psathas, P. A.; Johnston, K. P.; deGrazia, J.; Randolph, T. W. Langmuir 1999, 15, 6781-6791. (23) da Rocha, S. R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. J. Colloid Interface Sci. 2001, 239, 241-253. (24) Psathas, P. A.; da Rocha, S. R. P.; Lee, C. T.; Johnston, K. P.; Lim, K. T.; Webber, S. Ind. Eng. Chem. Res. 2000, 39, 2655-2664. (25) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165-168. (26) Johnston, K. P.; Cho, D. M.; DaRocha, S. R. P.; Psathas, P. A.; Ryoo, W.; Webber, S. E.; Eastoe, J.; Dupont, A.; Steytler, D. C. Langmuir 2001, 17, 7191-7193. (27) da Rocha, S. R. P.; Harrison, K. L.; Johnston, K. P. Langmuir 1999, 15, 419-428. (28) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Priciples and Practice, 2nd ed.; Butterworth: Stoneham, MA, 1994. (29) Becher, P. Encyclopedia of Emulsion Technology, 1st ed.; Marcel Dekker, Inc.: New York, 1988; Vol. 3. (30) Lee, C. T.; Bhargava, P.; Johnston, K. P. J. Phys. Chem. B 2000, 104, 4448-4456. (31) Babak, V. G.; Steve, M.-J. J. Dispersion Sci. Technol. 2002, 23, 1-22. (32) 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, 988-989.
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Figure 2. Schematic diagram indicating the relationship between formulation variable and emulsion morphology and interfacial tension (γ). The trends shown are for a nonionic surfactant system.
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emulsion formation. For the trisiloxane surfactants in question, an optimum surfactant balance for the C-W interface can be identified from the knowledge of the emulsion phase behavior and γ as a function of the number of EO repeat units, or the hydrophilic-to-CO2-philic balance (HCB).27 We have also shown that deviations from Bancroft’s rule can be accentuated for CO2/water emulsions due to the low viscosity and high solute diffusivity in CO2 compared to conventional oils.23 The kinetic stability of emulsions can be explained in terms of interfacial tension gradients that oppose film drainage between approaching droplets.36 Such gradients can be suppressed if surfactant molecules can diffuse fast enough to the monolayer region. The diffusion of surfactants through CO2 is much faster than through a more viscous fluid such as water. Also, fluids with low viscosity enhance the frequency of particle collisions, which also accelerates the rate of flocculation. Sedimentation or creaming increases with a decrease in viscosity. Finally, higher viscous stress from the aqueous side of the interface during droplet breakup favors the formation of CO2 droplets compared to the formation of water droplets in CO2. Experimental Section
leading to stabilization of the emulsion droplets against flocculation. The reduced interactions will result from the lower cohesive energy density and the lower packing ability of CH3 groups relative to CH2 groups. The objective of this study is to demonstrate that a new surfactant class, with a stubby (short and bulky tail group) CO2-phile, namely, the trisiloxanes, can be successfully employed in the formation of nonflocculated concentrated and dilute emulsions of water and CO2. For a trisiloxane series with varying degrees of hydrophilicity, we report emulsion capacity, stability as characterized by both visual observations and in situ video enhanced microscopy and turbidimetry, and the surfactant interfacial activity, probed by high-pressure tensiometry. The lack of flocculation for dilute W/C emulsions with trisiloxane surfactants, in contrast with previous results for other surfactant classes, is explained in terms of the differences in surfactant structure, in particular, the stubby, weakly interacting tails with little overlap. Theory We have previously established27 that, similarly to oil/ water systems,33 there exists a strong relationship between the morphology of emulsions of CO2 and water, and surfactant phase behavior and interfacial tension (γ). For equal amounts of water and oil (in our case CO2) and above the critical microemulsion concentration for systems forming aggregates,33 the continuous phase of an emulsion will be the one in which the surfactant is most soluble.34 This is known as Bancroft’s rule. γ goes through a minimum at the transition in curvature from O/W to W/O emulsions, which can be accomplished upon changing a so-called formulation variable.33,35 At this point, the surfactant is said to be balanced. The concept of formulation variable and its relation with γ and emulsion morphology is depicted in the diagram shown in Figure 2. Information regarding the affinity of new CO2-philic moieties to CO2, and the surfactant preference for either CO2 or water, can thus be obtained by studying CO2/water (33) Binks, B. P. Langmuir 1993, 9, 25-28. (34) Ruckenstein, E. Langmuir 1996, 12, 6351-6353. (35) Psathas, P. A.; Janowiak, M. L.; Garcia-Rubio, L. H.; Johnston, K. P. Langmuir 2002, 18, 3039-3046.
Materials. Trisiloxane surfactants of the form
((CH3)3SiO)2Si(CH3)(CH2)3EOnR synthesized by Dow Corning were used as received. The structures of the surfactants are summarized in Table 1. All surfactants are hydroxyl terminated (R ) OH). CO2 was purchased from Matheson with 99.99% purity and was further purified by an oxygen trap. Deionized, Nanopure II (Barnstead) water, with a surface tension of 72 mN/m, was used in all experiments. Phase Behavior. A variable-volume view cell with front and side sapphire windows allowed for observation of the cloud point (CP) pressure at constant composition as described earlier.11 Starting from a single-phase system, the CP pressure is defined as the pressure observed at the onset of turbidity, upon slowly reducing the pressure. Note that the present setup allows for variation of the system pressure independently of temperature. It comprises a subsection of a more elaborate setup schematically shown in Figure 3. CP pressures for M(D′E7)M as a function of temperature are shown in Figure 4. For M(D′E24)M at 0.1 wt % and 318 K, the observed CP is 24.3 MPa. High-Pressure Tensiometry. The interfacial tension (γ) between CO2 and water in the presence of surfactant was determined using a pendant-drop tensiometer, as described in detail elsewhere.27 Briefly, the shape of a water pendant drop at the end of a capillary tube inserted into a variable-volume cell containing CO2 and surfactant, previously saturated with water, is digitized using a CCD camera-computer setup. With the knowledge of the droplet shape and the pressure difference across the interface, γ can be calculated using the Laplace equation. The reported equilibrium values for γ are those when the variation between several subsequent measurements was approximately 0.05 mN/m, the estimated experimental error. In most cases, the time required for equilibration was less than 60 s after injection of the droplet into the variable-volume cell. The values for γ for the trisiloxanes with n ) 2, 7, 12, and 24 are shown in Table 1. They are also plotted in Figure 5 as a function of the concentration (wt %) of EO in the molecule. Interfacial tension was determined at 0.1 wt % surfactant, 318 K, and 23 MPa, except for M(D′E24)M, where the reported value was at 26 MPa. For this surfactant, a higher pressure was required to remain above the cloud point pressure. For M(D′E24)M, γ was also measured at 32 MPa. Both values at 26 and 32 MPa were the same within experimental uncertainty. (36) Ivanov, I. B. Pure Appl. Chem. 1980, 52, 1241-1262.
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Table 1. Summary of Surfactant Structures, Interfacial Tension at the CO2-Water Interface, and Emulsion Curvature, Formation, and Stabilitya surfactant
EO concn (wt %)
γ (mN/m)
observed curvatureb
% emulsification
stabilityc
M(D′E2)M M(D′E7)M M(D′E12)M M(D′E18)M M(D′E24)M M(D′E30)M
25 54 62 77 82 85
3.0 0.8 1.0
W/C C/W C/W C/W C/W C/W
