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Langmuir 1997, 13, 6980-6984
Water-in-CO2 Microemulsions Studied by Small-Angle Neutron Scattering Julian Eastoe* and Beatrice M. H. Cazelles School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
David C. Steytler* and Justin D. Holmes School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K.
Alan R. Pitt and Trevor J. Wear Kodak European Research R. and D., Kodak Ltd., Headstone Drive, Harrow HA1 4TY, U.K.
Richard K. Heenan ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, OXON OX11 0QX U.K. Received August 4, 1997. In Final Form: October 7, 1997X Aggregate structures in water-in-CO2 microemulsions were studied by high-pressure small-angle neutron scattering (SANS). With liquid CO2 at 15 °C, the partially fluorinated, di-chain surfactant bis(1H,1H,5Hoctafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) stabilized single-phase microemulsions at pressures above ∼400 bar. The maximum water loading (w) investigated was 30 ([water]/[di-HCF4]), representing formation of relatively large water droplets in the microemulsion. Between w ) 5 and 30, the SANS data were consistent with a model for attractive polydisperse spherical droplets. A linear relationship between the water droplet radius (Rc) and w was found, which gave an apparent head group area for the surfactant of 87 Å2 at the water-CO2 interface. In Winsor II type microemulsions the value of Rc, measured in the presence of excess water, increased with pressure from 36 Å at 400 bar to 56 Å at 550 bar.
Introduction Carbon dioxide (CO2) is cheap, nontoxic, highly volatile, chemically inert, nonflammable, and recyclable. These properties have inspired a high level of research effort to examine CO2 as a viable replacement for various conventional solvents. However, the solubility of most compounds in CO2 is low, and recently research into overcoming this problem has mushroomed.1-17 In this * Authors to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (2) Beckman, E. J.; Hoefling, T. A.; Enick, R. M. J. Phys. Chem. 1991, 95, 7127. (3) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536. (4) a. Eastoe, J.; Robinson, B. H.; Young, W. K.; Steytler, D. C. J. Chem. Soc. Faraday Trans. 1990, 86, 2883. b. Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Heenan, R. K. J. Chem. Soc. Faraday Trans. 1994, 90, 3121. (5) Eastoe, J.; Steytler, D. C.; Bayazit, Z.; Martel, S.; Heenan, R. K. Langmuir 1996, 12, 1423. (6) 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, 217, 624. (7) Johnston, K. P.; Harrison, K. L.; Sanchez, I. C. Langmuir 1996, 12, 2637. (8) Zielinski, R. G.; Kline, S. R.; Kaler, E. W.; Rosov, N. Langmuir 1997, 13, 3934. (9) DeSimone, J. M.; Guan, Z.; Eisbernd, C. S. Science 1992, 257, 945. (10) DeSimone, J. M.; Guan, Z. Macromolecules 1994, 27, 5527. (11) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J.; Romack, T. J., Combes; J. R. Science 1994, 265, 356. (12) DeSimone, J. M.; Schaffer, K. A. Trends Polym. Sci. 1995, 3, 146. (13) DeSimone, J. M.; Maury, E. E.; Combes; J. R.; McClain, J.; Romack, T. J., Samulski, E. T.; Fulton, J. L.; Pfund, D. M., Capel, M. Langmuir 1995, 11, 4241. (14) DeSimone, J. M.; Londono, D.; Combes; J. R.; McClain, J.; Romack, T. J., Canelas, D. A.; Betts, D. E.; Wignall, G. D.; Samulski, E. T. J. Am. Chem. Soc. 1996, 118, 971.
S0743-7463(97)00876-7 CCC: $14.00
respect, water-in-CO2 (w/c) microemulsions show promise because they are potentially “universal solvents”. In the early 1990s it was shown that fluorinated surfactants dissolve in CO2 and these observations suggested the feasibility of forming w/c microemulsions.1,2 It was also reported that partitioning of CO2-insoluble thymol blue into the CO2-rich phase of a w/c mixture was enhanced by addition of a CO2-phillic di-fluorocarbon sulfosuccinate surfactant.2 Later Johnston et al.3 studied w/c formation with a di-chain hybrid surfactant [(C7H15)(C7F15)CHSO4- Na+ or H7-F7], which can microemulsify up to its own weight of water.3 Just like related low density water-in-alkane systems,4 the H7-F7 w/c phase stability depends on pressure (P) and temperature (T). Eastoe et al.5 used high-pressure small-angle neutron scattering (SANS) to show that in a H7-F7 w/c sample, with a water loading (w ) [water]/[surfactant]) of 33 at T ) 25 °C and P ) 500 bar, spherical water droplets of radius (Rc) ∼25 Å were present. For lower pressures, the SANS was consistent with enhanced droplet clustering and, at ∼120 bar, phase separation (clouding) was observed. Johnston et al. have also shown that w/c6 and polyethylene glycolin-CO2 microemulsions7 can be formed with an ammonium carboxylate perfluoropolyether [CF3O(CF2CF(CF3)O)3CF2COO-NH4+, PFPE]. Most recently Kaler et al.8 have used SANS to confirm that spherical water droplets are definitely present in these PFPE w/c systems. Over a similar period, DeSimone’s group pioneered research into (15) DeSimone, J. M.; McClain, J.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; Londono, J. D.; Chochran, H. D.; Wignall, G. D.; ChilluraMartino, D., Triolo, R. Science 1996, 274, 2049. (16) Chillura-Martino, D.; Triolo, R.; DeSimone, J. M.; McClain, J.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; Londono, J. D.; Chochran, H. D.; Wignall, G. D. J. Mol. Struct. 1996, 383, 3. (17) DeSimone, J. M.; McClain, J.; Londono, J. D.; Wignall, G. D.; Samulski, E. T.; Chochran, H. D.; Lin, J. S.; Dobrynin, A.; Rubenstein, M.; Burke, A. M. C.; Fre´chet, J. M. J., submitted for publication in Nature.
© 1997 American Chemical Society
SANS of Water-in-CO2 Microemulsions
CO2-soluble fluoropolymers, block copolymers, and dendrimers, also with the aim of increasing solubility in CO2.9-17 Because of a lack of chemically robust and adaptable surfactants the important issue of how structure influences properties of w/c microemulsions remains largely unresolved. However, this paper reports the first stage of a study, starting with a partially fluorinated anionic surfactant (H(CF2)4CH2OCO)2CH2CHSO3- Na+ (bis(1H,1H, 5H-octafluoro-n-pentyl) sodium sulfosuccinate or diHCF4]. The compound has bulky, CO2-comptaible chains and a water-soluble head group, so on packing grounds it is expected to from reversed curvature structures in CO2.2,3 The microemulsion stability, aggregation, and interactions have been studied by SANS as a function of w from 5 to 30, and P from 400 to 600 bar. The results show that di-HCF4 behaves in a similar fashion in w/c phases to the hydrocarbon analogue AOT in well-studied water-in-oil (w/o) systems. Furthermore, strong evidence is presented for the formation of Winsor II type systems where a w/c microemulsion coexists with an excess aqueous phase. Therefore, using di-HCF4 as a keystone, various chain structures can now be investigated with the aim of optimizing the w/c microemulsion behavior. Experimental Section Chemicals. The bis(1H,1H,5H, octafluoro-n-pentyl) sodium sulfosuccinate (di-HCF4) was prepared by a similar method to that of Yoshino et al.18 Elemental microanalysis as well as 1H, 13C, and 19F NMR spectra (Jeol GX400) were consistent with the desired product at a purity of 98-99%. The surfactant was stored over refreshed P2O5 in a dessicator. The aqueous phase critical micelle concentration (cmc ) 5.0 × 10-3 mol dm-3) was measured at 25 °C by duNouy tensiometry (Kruss K10) and electrical conductivity. Both D2O (Fluorochem 99% D-atom) and CO2 (BOC) were used as received. The P-T phase stability for w/c microemulsions was determined visually in a high-pressure optical cell.4 Small-Angle Neutron Scattering. The LOQ time-of-flight instrument, at the CLRC Rutherford Appleton Laboratory at ISIS UK, in conjunction with a stirred high-pressure cell, were used as described previously.4,19 The measurements gave the absolute scattering crosssection I(Q) (cm-1) as a function of momentum transfer Q (Å-1) ) (4π/λ) sin(θ/2) where λ is the incident neutron wavelength (2.2 f 10 Å) and θ is the scattering angle (< 7°). A partially deuterated polymer standard was used as calibrant.20 In addition to the usual transmission, empty cell, and solvent background corrections, the raw data were also normalized for pressure-induced sample volume changes and the true cell pathlength. For each P and T, the volume was measured with a remote displacement transducer to track the piston height, and separate spectrophotometric measurements, using standard dye solutions, gave the pathlength as 1.96 mm. Data Analysis. Under these T and P conditions, the solubility of water in CO2 is negligible (0.13%) compared with the ∼5% of D2O present, and the w values were not corrected for this small effect. For liquid CO2, the neutron scattering length density may be taken as FCO2 ) 2.2 × 1010 cm-2, with 10% variation over the pressures studied. Assuming a mass density of 1 g cm-3 for di-HCF4, Fsurf is 1.8 × 1010 cm-2, and, because FD2O ) 6.4 × 1010 cm-2, the scattering comes principally from the contrast step at the D2O interface so the water radius Rc can be determined. (Fine detail about the microemulsion film can only be obtained from extensive contrast variation experiments.21) (18) Yoshino, N.; Komine, N.; Suzuki, J-I.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (19) a. King, S. M.; Heenan, R. K. “The LOQ Instrument Handbook”, Rutherford Appleton Laboratory Report RAL-TR-96-036, CCLRC, Didcot, U.K. 1996. b. Eastoe, J. In New Physico-Chemical Techniques for the Characterisation of Complex Food Systems; Dickinson, E., Ed.; Blackie: Glasgow, 1995. (20) Wignall, G. D.; Bates, F. S. J. Appl.Crystallogr. 1987, 20, 28. (21) Eastoe, J.; Hetherington, K. H.; Sharpe, D.; Dong, J.; Heenan, R. K.; Steytler, D. C. Langmuir 1996, 12, 3876.
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Figure 1. Pressure-temperature phase diagram for w/c microemulsions stabilized by di-HCF4 at 0.10 mol dm-3. The shaded area is biphasic. The I(Q) data were analyzed with the FISH fitting program,22 and from a number of different possible models it was found that a Schultz distribution of spherical particles, experiencing mutual attractive interactions,23 gave the best fits and most physically reasonable fit parameters. This scattering law is as follows:
I(Q) ) φ(FD2O - FCO2)2 [Σi Vi P(Q, Ri) X(Ri)] S(Q, ζ) (1) where the parameters φ, R, and V are the particle volume fraction, radius, and volume, respectively. The spherical form factor is P(Q), and X(Ri) is the Schultz function,23 which is characterized by an average radius (Rav) and RMS deviation (σ) of Rav/(Z + 1)1/2 with Z being a width parameter. S(Q) is the Ornstein-Zernicke structure factor, which describes a decaying particle distribution with ξ a correlation length, so that:
S(Q,ξ) ) 1 +
[
S(0)
]
1 + (Qξ)2
(2)
The parameter S(0) is related to the strength of interactions via the isothermal compressibility,23 but here it acts as an effective parameter. Because the sample composition and scattering length densities are all known, the parameters Rcav, σ/Rcav, ζ, and S(0) were adjusted to fit eq 1 to the data. For the first three of these parameters, the uncertainties may be taken as (1 Å, ( 0.02, and ( 10 Å, respectively. Initially, the scale factor φ(FD2O-FCO2)2 was input as a constant. However, for the final fits, ( 5% of the value was allowed and this is at the resolution of the high-pressure SANS method.
Results and Discussion Pressure-Temperature Phase Stability. The P-T phase stability map for di-HCF4 w/c microemulsions with different w values is shown in Figure 1. The single phase microemulsions at high pressures were all transparent, and temperatures 0.05 Å-1, the ratio of intensities high:low concentration was 2 ( 0.1. This result indicates both that the droplet structure is invariant in this range and that the cmc in CO2 is negligible compared with the overall di-HCF4 concentration. Because of the weaker SANS signals it was inefficient to carry out experiments at any lower dilutions. Variation of w. The effect of w on the SANS profiles is shown in Figure 3. All the experiments were done at 500 bar, except for w ) 30 which was at 566 bar (Pc ≈ 540 bar), and it is anticipated that higher w values could be achieved above 600 bar. The model fits are also shown, with calculated parameters given in Table 2. For some of the samples, it was not possible to fit a reliable (25) Gradzielski, M.; Langevin, D.; Farago, B. Phys. Rev. E 1996, 53, 3900. (26) Eastoe, J.; Hetherington, K. H.; Sharpe, D.; Steytler, D. C.; Egelhaaf, S.; Heenan, R. K. Langmuir 1996, 13, 2490.
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Table 2. Fitted Parameters to SANS Data from Water-in-CO2 Microemulsionsa w
R c, Å
σ/Rc
S(0)
ζ/Å
5 10 15 20 25 30
12.0 13.0 18.6 26.4 30.9 35.8
0.40 0.29 0.27 0.17 0.21 0.19
10 82 52 154 636 400
1000b 150 155 376 1000b 1000b
a [di-HCF4] ) 0.10 mol dm-3; T ) 15 °C; and P ) 500 bar, except when w ) 30 for which P ) 566 bar. b Indicates that the correlation length ζ was fixed.
Figure 5. Porod plot of high-pressure SANS data of Figure 3. Asymptote represents a constant area per molecule. Key to values of w: (b) 30; (O) 25; (9) 20; (0) 15.
Figure 4. Water droplet radius Rc versus w for di-HCF4 w/c microemulsions using values from Table 2.
correlation length and so it was fixed at values suggested by trial calculations. The region where S(Q) becomes significant can be clearly seen in Figure 3 by the characteristic rise in scattering at low Q. As w increases, the change in intensity at low Q and crossover at high Q are characteristic of an increase in droplet size, which is also clear from the fitted radii. The apparent high polydispersity for w ) 5 may just be a consequence of the weak signal, but for the other samples, σ/Rc is similar to that found in w/o systems.21,25,26 Above w ) 15, both S(0) and ζ consistently increase, indicating development of more attractive interdroplet interactions. A plot of Rc versus w is shown in Figure 4. The linear dependence is to be expected for spherical droplets because Rc ) (3VD2O w)/ah, where VD2O is the molecular volume of water and ah, the effective area per headgroup at the interface. The linear fit gave a value of ah ) 87 ( 5 Å2 from the slope, and the intercept of 4-5 Å is consistent with the finite size of the polar groups in a dry di-HCF4 reversed micelle. For w/o droplets, ah can also be estimated from high Q SANS data, by assuming a sharp interface and applying the Porod equation26:
{I(Q)Q4}Qf∞ ) 2π(FD2O - FCO2)2 Σ
(3)
where Σ is the total area per unit volume. Another requirement here is that the cmc in CO2 is low and that all N di-HCF4 molecules are at the D2O interface, then ah ≈ Σ/N. As can be seen in Figure 5 above Q ∼ 0.12 Å-1, the data are essentially asymptotic, suggesting these approximations are reasonable. The marked line represents a molecular area of 98 Å2, and given an uncertainty of (10 Å2, this value is in agreement with the previous result.
Both these ah values are consistently higher than for AOT at the w/o interface (60 Å2).23 For the dialkyl sulfate H7-F7 at the w/c interface, a relatively low packing density of 140 Å2 per molecule was also determined by SANS.5 Based on these observations, it appears that film packing requirements for microemulsion formation are generally lower in CO2 systems compared with hydrocarbons. The origin of this phenomenon may be the higher miscibility of water and CO2, and therefore a lower tension for the surfactant-free interface compared with hydrocarbons. This idea should be confirmed by high-pressure tensiometry experiments.7 Furthermore, because the covalently bonded radius of the F atom is nearly twice that of hydrogen (71 versus 37 pm), the higher effective volume of fluorocarbon chains may also be a factor contributing to the higher molecular areas. Winsor II Type Systems. When sufficient D2O was added to make an effective w of ∼75, phase separation occurred. After 30 min, a milky interface separated surfactant-containing water-rich and transparent CO2continuous phases, reminiscent of Winsor II systems commonly observed in w/o microemulsions. The SANS profiles (Figure 6) from the upper CO2 phase as a function of pressure indicate dispersed D2O droplets. The intensities increased consistently and reproducibly with pressure. To check that the system had reached equilibrium, a sample was left overnight. This sample gave essentially identical SANS curves as before. The fitted lines represent P(Q) for Schultz spheres, with the radii indicated, and a polydispersity σ/Rc of 0.275. At 400 bar, the average radius is 36 Å, which is similar to that for w ) 30 in Figure 3. Making a comparison between both the high Q Porod scattering and P(Q) at low Q for the two samples, the di-HCF4 concentration can be estimated as 3 × 10-3 mol dm-3 in the Winsor II system. Therefore, because dilution had a large effect on the structure factor (Table 1), this low concentration is consistent with the apparent absence of attractive S(Q). If a constant area per molecule of 87 Å is assumed, then the fits indicate that w effectively increases from 30 at 400 bar to around 50 at 550 bar. Under appropriate conditions w/c microemulsion droplets clearly can coexist with excess aqueous phase, and it is interesting to see that pressure affords some control over the preferred droplet size. Conclusion. Droplet w/c microemulsions can be formed with the partially fluorinated dichain surfactant
6984 Langmuir, Vol. 13, No. 26, 1997
Figure 6. SANS from the upper CO2-continuous phase of Winsor II type systems as a function of pressure. Lines are fits to the model described in the text.
di-HCF4. The systems investigated here behave in a straightforward way, rather akin to AOT in conventional w/o systems, and with low density solvents such as propane.4 In particular, SANS shows that, up to w ) 30,
Eastoe et al.
the water core radius depends directly on the water loading w via Rc (Å) ≈ w + 4.7. This dependence is consistent with both a low concentration of free monomer di-HCF4 in CO2 and a constant area per head group at the interface of ∼90 Å2. The proximity of the samples to the lowpressure phase boundary means that there are significant interdroplet interactions that increase in magnitude as the cloud point is approached and decrease with dilution. Very similar behavior has also been observed recently with PFPE-stabilized w/c microemulsions.8 In the presence of excess water, di-HCF4 appears to form Winsor II type systems, and the SANS measurements show that the droplet radius in the upper microemulsion phase consistently increases with pressure. The results with di-HCF4 suggest that studies of structurally related molecules will help to unveil the links between surfactant structure and the efficiency of microemulsifying water in CO2. Further work to establish these links is now in progress. Acknowledgment. We thank EPSRC for support via grant GR/L05532 and allocation of neutron beam-time at ISIS. Both Kodak and the University of Bristol are thanked for providing chemicals and analytical facilities. LA970876S
