Heptane Solvent Mixtures - American

Sep 4, 2009 - Martin J. Hollamby, Kieran Trickett, Azmi Mohamed, and Julian Eastoe*. School of Chemistry, University of Bristol, Cantock's Close, Bris...
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Surfactant Aggregation in CO2/Heptane Solvent Mixtures Martin J. Hollamby, Kieran Trickett, Azmi Mohamed, and Julian Eastoe* School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.

Sarah E. Rogers and Richard K. Heenan ISIS-CCLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K. Received May 27, 2009. Revised Manuscript Received July 30, 2009 The effect of improving “solvent quality” of pure liquid CO2 with a heptane cosolvent on the phase behavior and micellization of commercially available surfactants has been explored using high-pressure small-angle neutron scattering (HP-SANS). The nonionic C12E5 was found to be highly soluble in both pure CO2 and the solvent blends, but no aggregation was detected by HP-SANS for any of the compositions studied, even up to 12 vol % surfactant. On the other hand, improving CO2 solvent quality by adding heptane above 30 vol % promoted solubility and aggregate formation with normal sodium bis(ethylhexyl)sulfosuccinate (AOT). The solvent quality index Hildebrand solubility parameter, used to predict surfactant aggregation in pure hydrocarbon solvents (Langmuir, 2008, 24 (21), 12235-12240) has been tested here for CO2-heptane mixtures. The results show how solubility and efficiency of AOT, a commercially viable, well-known, and commonly used surfactant, can be boosted in alkane-containing CO2-rich fluids compared to pure CO2 alone.

Introduction It is now appreciated that, to develop the full economic and environmental potential of liquid or supercritical CO2 (scCO2), new creative approaches to modify solvent physicochemical properties will be needed.1-4 One important method is incorporation of low molecular mass surfactants, and or amphiphilic block copolymers to act as fluid modifiers affecting surface tension, wetting, thickening, and solubilization properties. Customdesigned perfluorinated amphiphiles5,6 and polymers7,8 are the best candidates for CO2 amphiphiles. These F-compounds have high CO2 compatibility and aggregate/stabilize water-in-scCO2 (w/scCO2) microemulsions,7,9 gels,10 and disperse inorganic nanoparticles.11-13 Unfortunately, F-surfactants and F-polymers suffer from many drawbacks, including high cost and environmental persistence.14 To overcome these problems, cheaper and greener hydrocarbon CO2 surfactants are required. *Corresponding author. E-mail: [email protected]. (1) Leitner, W. Nature 2000, 405, 129–130. (2) Beckman, E. J. Ind. Eng. Chem. Res. 2003, 42, 1598–1602. (3) Adam, D. Nature 2000, 407, 938–940. (4) Kaiser, J. Science 1996, 274, 2013. (5) Eastoe, J.; Gold, S.; Steytler, D. C. Langmuir 2006, 22, 9832–9842. (6) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Langmuir 1997, 13, 6980–6984. (7) 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, 368–371. (8) Eastoe, J.; Gold, S. Phys. Chem. Chem. Phys. 2005, 7, 1352–1362. (9) McClain, J. B.; Betts, D. E.; Canelas, D. A.; Samulski, E. T.; DeSimone, J. M.; Londono, J. D.; Cochran, H. D.; Wignall, G. D.; Chillura-Martino, D.; Triolo, R. Science 1996, 274, 2049–2052. (10) Shi, C.; Huang, Z.; Kilic, S.; Xu, J.; Enick, R. M.; Beckman, E. J.; Carr, A. J.; Melendez, R. E.; Hamilton, A. D. Science 1999, 286, 1540–1543. (11) Yates, M. Z.; Apodaca, D. L.; Campbell, M. L.; McCleskey, T. M.; Birnbaum, E. R. Chem. Commun. 2001, 25–26. (12) Eastoe, J.; Hollamby, M. J.; Hudson, L. Adv. Colloid Interface Sci. 2006, 128-130, 5–15. (13) Hollamby, M. J.; Trickett, K.; Vesperinas, A.; Rivett, C.; Steytler, D. C.; Schnepp, Z.; Jones, J.; Heenan, R. K.; Richardson, R. M.; Glatter, O.; Eastoe, J. Chem. Commun. 2008, 5628–5630. (14) Renner, R. Science 2004, 306, 1887.

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Because of the poor and weak solvent properties of scCO2, few commercially available hydrocarbon surfactants are known to be soluble in pure scCO2, and even fewer are known to micellize.5,8,15,16 Consider the general problem: How can one induce micelle formation in a very weak uncooperative solvent, such as liquid CO2? Two strategies might conceivably be employed: (1) chemical design of new surfactants exhibiting enhanced solvophilicity, or (2) quality enhancement of the poor CO2 solvent with soluble modifiers so that standard commercially available surfactants can be employed. To date approach 1 has been extensively explored, requiring time-consuming and labor-intensive surfactant synthesis to generate CO2-compatible surfactants, with low surface energies and favorable surfactant chain-CO2 interactions.16-18 The design features of these CO2-philic compounds are consistent with the fractional free volume (FFV) approach proposed by Johnston et al.19 Gratifyingly, there have been some successes, and custom hydrocarbon surfactants bearing highly methylated t-butyl chain tips are CO2-soluble,18,20,21 forming micellar aggregates16,17 in scCO2, as well as efficiently stabilizing w/scCO2 macro- and mini-emulsions,22 and nanoparticle dispersions.13,23 More recently there has been a significant development: a new (15) DeSimone, J. M.; Keiper, J. S. Curr. Opin. Solid State Mater. Sci. 2001, 5, 333–341. (16) Eastoe, J.; Dupont, A.; Steytler, D. C.; Thorpe, M.; Gurgel, A.; Heenan, R. K. J. Colloid Interface Sci. 2003, 258, 367–373. (17) 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. (18) Gold, S.; Eastoe, J.; Grilli, R.; Steytler, D. C. Colloid Polym. Sci. 2006, 284, 1333–1337. (19) Stone, M. T.; Smith, P. G.; da Rocha, S. R. P.; Rossky, P. J.; Johnston, K. P. J. Phys. Chem. B 2004, 108, 1962–1966. (20) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 7274–7275. (21) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165–168. (22) Johnston, K. P.; Cho, D.; DaRocha, S. R. P.; Psathas, P. A.; Ryoo, W.; Webber, S. E.; Eastoe, J.; Dupont, A.; Steytler, D. C. Langmuir 2001, 17, 7191– 7193. (23) Anand, M.; Bell, P. W.; Fan, X.; Enick, R. M.; Roberts, C. B. J. Phys. Chem. B 2006, 110, 14693–14701.

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chemical structure, a triple-chain anionic surfactant (TC14) bearing a t-butyl group on each chain tip, forms hydrated reversed micellar dispersions in CO2.24 On the other hand, strategy 2, nudging the quality of CO2 with soluble modifiers to meet the solvency requirements of commercial surfactants, has received less focused attention. Nonpolar hydrocarbon additives have been shown to boost the solubility/ dispersibility of organic species and nanoparticle dispersions in scCO2-rich systems.23,25-27 However, these reports are quite limited in scope, being directed toward specific applications, and investigations of the general effects of alkane-CO2 solvent mixtures on the aggregation of common surfactants are needed. With that in mind here, aggregation of sodium bis(ethylhexyl)sulfosuccinate (AOT) and C12E5 has been explored in scCO2/heptane-d16 (C7D16) mixtures as a function of surfactant concentration and solvent mixture composition. It is known that pure AOT has a very low solubility in pure scCO2,17 whereas C12E5 is soluble, and, for related CnEm surfactants, small aggregate structures have been inferred by Fourier transform infrared (FT-IR) spectroscopy.28 Because in pure nonpolar solvents, such as heptane, both AOT and C12E5 form reversed micellar aggregates and water-in-oil microemulsions,29-32 it is reasonable to expect that modifiers could promote reversed micelle formation in scCO2-rich solvent mixtures. The approach taken here is to scan mixed solvent compositions from pure scCO2, moving toward neat alkane (n-heptane), using high-pressure small-angle neutron scattering (HP-SANS) to detect the presence of and quantify the extent of aggregation. Another idea that has been tested, related to approach 2, is to start at the alkane-rich side of the phase diagram and dose, in trace levels, compressed gas CO233,34 into AOT solutions of isooctane33 or decane.34 On the basis of fluorescence probe33 and small-angle X-ray scattering (SAXS)34 data, extensive aggregation was detected in the mixed systems. (Note, in those studies33,34 low CO2 pressures up to ∼60 bar, and temperatures between 15 and 45 °C were employed, so that the majority of systems investigated contained compressed gas CO2, different from this work where pumped CO2 is in the liquid phase.) In general, small-angle neutron scattering (SANS) is a preferable method for detecting and quantifying surfactant aggregation:5,35 SANS offers key advantages, including clean contrasting of micellar aggregates against a background solvent continuum and accurate absolute intensity calibration for quantifying the extent of aggregation (micellar volume fraction). Furthermore, there is no need to perturb the system under study by adding probes, such as is required for fluorimetry. For these reasons, (24) Hollamby, M. J.; Trickett, K. J.; Mohamed, A.; Cummings, S.; Tabor, R. F.; Myakonkaya, O.; Gold, S.; Rogers, S.; Heenan, R. K.; Eastoe, J. Angew. Chem., Int. Ed. 2009, 48, 4993–4995. (25) Choi, E.-J.; Yeo, S.-D. J. Chem. Eng. Data 1998, 43, 714–716. (26) Dobbs, J. M.; Wong, J. M.; Johnston, K. P. J. Chem. Eng. Data 1986, 31, 303–308. (27) Bell, P. W.; Anand, M.; Fan, X.; Enick, R. M.; Roberts, C. B. Langmuir 2005, 21, 11608–11613. (28) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992, 8, 377–384. (29) Kotlarchyk, M.; Huang, J. S.; Chen, S. H. J. Phys. Chem. 1985, 89, 4382– 4386. (30) Kon-no, K.; Kitahara, A. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, pp 185-231. (31) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97, 9–25. (32) Zana, R. Colloids Surf., A 1997, 123-124, 27–35. (33) Chen, J.; Zhang, J.; Han, B.; Feng, X.; Hou, M.; Li, W.; Zhang, Z. Chem.; Eur. J. 2006, 12, 8067–8074. (34) Shen, D.; Han, B.; Dong, Y.; Chen, J.; Mu, T.; Wu, W.; Zhang, J.; Wu, Z.; Dong, B. J. Phys. Chem. B 2005, 109, 5796–5801. (35) Hollamby, M. J.; Tabor, R.; Mutch, K. J.; Trickett, K.; Eastoe, J.; Heenan, R. K.; Grillo, I. Langmuir 2008, 24, 12235–12240. (36) Kaler, E. W.; Billman, J. F.; Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1991, 95, 458–462.

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HP-SANS36,37 is recognized as the method of choice for studying self-assembly and aggregation in scCO2; it has been used to provide irrefutable evidence for the presence (or absence) of reversed micelles and dispersed nanodomains of surfactant and water in high-pressure solvents.5 Extensive SANS studies under ambient pressure in normal liquid solvents (water-dioxanealkane blends) show that a variety of solvent quality indices, such as the Hildebrand solubility parameter, the Snyder polarity parameter, or the dielectric constant, can all be used to predict surfactant aggregation. This new work aims to explore the applicability of such solvency scales to high-pressure mixed CO2-alkane solvents, with an aim to increase the scope for commercial hydrocarbon surfactants in scCO2 systems.

Experimental Section Chemicals. AOT was purchased from Sigma Aldrich and purified before use by Soxhlet extraction (dry ethyl acetate) followed by repeated centrifugation of dispersions in dry methanol to remove any excess salts.38 Pentaethylene glycol monododecyl ether (C12E5) was purchased from Fluka and used without any further purification. Deuterated heptane (98%) was purchased from Goss Scientific and used without further purification. Small-Angle Neutron Scattering. HP-SANS experiments were carried out on the time-of-flight LOQ instrument at ISIS, U.K., where incident wavelengths are 2.2 e λ e 10 A˚, resulting in an effective Q range of 0.009-0.249 A˚-1. Absolute intensities ((5%) for I(Q) (cm-1) were determined by calibrating the received signal for a partially deuterated polymer standard and then corrected for sample transmission. An important development that has permitted these studies is a new bespoke pressurecell, discussed in more detail in the Supporting Information (maximum safe working pressure, 400 bar). Vigorous stirring was initially applied to solubilize components (where possible), and was employed in phase studies to identify cloud point pressures (Pcloud) by visual inspection. HP-SANS data were obtained in optically clear single phase regions, at 25 °C and 360 bar, as noted in the figure captions and main text, but without stirring of the samples to ensure experiments were conducted on thermodynamically stables phases, rather than stirred up mixtures. Pressure cell volume was held at 13 mL ( 1 mL, and typical count times per sample were 20 min in SANS mode and 5 min in transmission. Experiments were repeated on separate occasions, with reproducible results; furthermore, no time dependence of the phase behavior was observed in parallel off-line phase stability experiments over periods of up to 1 day. Data Analysis. As explained in the Supporting Information, the data were fitted by analytical scattering laws (form factors and, where appropriate, structure factors), employing absolute intensities to determine scale factors (SFs). These fitted SF values were used to obtain the effective aggregated surfactant volume fraction φagg knowing the composition-dependent scattering length densities F (Table S1 Supporting Information). Note that, in these systems, it is not generally true that φagg is equal to total added surfactant volume fraction φsurf; in fact, in many cases, φagg < φsurf because the surfactants are insufficiently solvophobic in the studied mixtures. In addition, taking into account the natural inaccuracies in measured absolute intensities ((5%) and the known close-coupling in the fitting models between SFs and aggregate radius, calculated aggregate volume fractions are reported to within an accuracy on the order of (20% of the value. This apparently high uncertainly is partly a result of the experimental difficulties associated with HP-SANS compared to standard SANS experiments conducted in quartz cells under ambient pressure conditions.5 Justifications for employing the (37) Eastoe, J.; Bayazit, Z.; Martel, S.; Steytler, D. C.; Heenan, R. K. Langmuir 1996, 12, 1423–1424. (38) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733–8740.

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Article Table 1. Cloud Points (Pcloud) for AOT in C7D16/scCO2 Mixtures at 25 °C and Micellar Volume Fractions Oagg Determined by Analyses of HP-SANS Profiles at 360 bar and 25 °C

Figure 1. SANS profiles for C12E5 in 19.2 vol % C7D16/scCO2 at 360 bar, 25 °C. SANS data have been subtracted for scattering from the solvent + cell background. Lines show fits to the data. structural models (C12E5 rods and AOT spheres) can be found elsewhere.31,35

Results and Discussion C12E5 Aggregation in C7D16/scCO2. Samples of different vol % C12E5 (for constant vol % C7D16) and vol % C7D16 (constant vol % C12E5) were directly prepared in the pressure cell (Figure S1), and the Pcloud and HP-SANS profiles were recorded. All samples were clear throughout the pressure range studied (70-400 bar, 25 °C), indicating high solubility of C12E5 in the solvent blends. For the lowest (4.0 vol%) concentration of C12E5 studied in the C7D16/scCO2 mixtures, there was little difference between the scattering derived from the background and that from C12E5-containing samples, indicating no detectable aggregation. With increasing C12E5 concentration (8.0 and 12.0 vol %), at constant solvent composition (19.2 vol % C7D16), an increase in SANS intensity was noted above the solvent background; this scattering was very weak, as shown in Figure 1 below. Scattering data have been fitted to a rod model, as explained in the Supporting Information. A fixed and precalculated SF was used, and, based on the known contrast and expected volume fraction, the data can be well described by Rrod = 3.5 A˚ and Lrod = 22 A˚. These parameters are similar to those previously used to account for SANS from single nonassociated C12E5 monomers in organic solvents.35 Despite complications with data fitting, particularly those introduced by scattering from the heptane-d16 dispersion (see Supporting Information), it is clear that C12E5 does not aggregate strongly in pure scCO2 or C7D16/scCO2 mixtures of any composition in the range studied, even up to 12 vol% surfactant. In previous work, aggregation was detected by SANS above 1 wt % (∼ 1.1 vol%) in pure hexane and cyclohexane.35 The results were discussed with respect to solvent quality as defined by, among other indices, the Hildebrand solubility parameter, δ, and the dielectric constant, εr. Applying the same arguments to this work, “solvent quality” δ is known to increase with pressure: for n-heptane at constant temperature over 0-300 bar, δ increases linearly by approximately 2.5%.39 It can therefore be assumed that an increase in pressure to 360 bar would increase δ by ∼3%; given δ = 15.3 MPa1/2 at ambient temperature and pressure,40 there will be an increase to ∼15.8 MPa1/2 at 360 bar. The solubility parameter of scCO2 for (39) Verdier, S.; Andersen, S. I. Fluid Phase Equilib. 2005, 231, 125–137. (40) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed.; CRC Press: Boca Raton, FL, 1990.

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AOT/vol %

C7D16/vol %

Pcloud/bar

φagg  103

3.5 3.5 3.5 5.2 7.0

30.8 38.5 57.7 38.5 38.5

325 110

7 6 6 9 12

195 245

a wide range of temperatures and pressures has been calculated,41 and, for 360 bar, 25 °C, δ = 16.0 MPa1/2. Therefore, the combined solubility parameter, estimated by a volume-weighted sum for the C7D16/scCO2 mixtures is on the order of 15.815.9 MPa1/2, depending on the precise composition. The dielectric constant of liquid CO2 (εr = 1.6)42 is proportionally lower than that for cyclohexane or hexane (εr = 2.0)42 On this basis alone, with reference to previous work,35 weak aggregation of C12E5 would be expected, although it is in fact a very weak effect in heptane. The lack of any experimental aggregation detected by HP-SANS points to limitations in the solvent quality argument when applied to nonionic surfactants in CO2-containing systems. It should be realized that this is a stringent test of the solubility parameter idea, since, in truth, C12E5 has only a very weak tendency to aggregate in pure heptane. AOT Aggregation in C7D16/scCO2. Phase Behavior. Samples of AOT in C7D16/scCO2 mixtures were prepared in the pressure cell at different vol % AOT (at constant vol % C7D16) and vol % heptane (constant vol% AOT). Below 30 vol % C7D16 at 25 °C samples were cloudy (some contained lumps) throughout the pressure range (60-400 bar), indicating low AOT solubility. However, above 30 vol % C7D16, a clear-to-cloudy transition was observed at pressures (Pcloud) given in Table 1. This phase behavior is indicative of a change from a stable (P > Pcloud) to unstable, biphasic systems (P < Pcloud). A heptane-rich phase at 3.5 vol % AOT/57.7 vol % C7D16 remained clear throughout the pressure range studied, consistent with an absence of any Pcloud instability, at least up to 400 bar, which is the maximum safe working pressure of the cell employed. There are two ways by which the mixed C7D16/scCO2 solvent quality δ can be improved: increasing heptane content or increasing pressure. At constant vol % AOT, Pcloud decreases with increasing C7D16 content, which is consistent with an improvement in solvent quality. The Pcloud increases with greater AOT content, which again may be linked to the need for an improvement in solvent quality (by increasing pressure) to disperse the greater quantity of surfactant. HP-SANS. SANS profiles for constant AOT vol % in the C7D16/scCO2 mixtures are given in Figure 2, with scattering from the CO2-heptane solvent (see SI) background subtracted. Below 30 vol % heptane-d16, very little scattering was observed, consistent with the low AOT solubility suggested by the phase behavior observations: any residual SANS can be accounted for as owing to scattering from (low level) dissolved AOT monomers as seen before35 and difficulties in background subtraction (see SI). Above 30 vol % C7D16, the SANS intensity increases dramatically, indicating AOT begins to aggregate in the more favorable solvent environment. As the solvent composition shifts in favor of C7D16, the scattering contrast, ΔF = FCO2+d-hept - FAOT also (41) Allada, S. R. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 344–348. (42) Wohlfarth, C. In CRC Handbook of Chemistry and Physics, 89th ed (Internet version 2009); Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2009; p 6-148.

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Figure 2. SANS profiles for 3.5 vol % AOT in C7D16/scCO2

mixtures at 360 bar, 25 °C. Lines are fits to a Schultz distribution of polydisperse spheres, with a critical scattering-type S(Q) employed for the 30.8 vol % heptane-d16 data due to its proximity to the cloud point (see Tables 1 and S3).

increases, being one factor leading to increased I(Q). To establish the effect of solvent composition on the aggregation, SANS data were fitted to a model representing Schultz polydisperse spheres, as previously used in hydrocarbon solvents29,35 and pure scCO2.17 Given the low intensity of the scattering (and problems with known coupling between the SF, polydispersity, and radius in the Schultz model), in this case, the radius and polydispersity were fixed at R = 15 A˚, σ/R = 0.10, again being consistent with previous analyses.35 For 19.2 and 23.1% heptane systems, the normal Guinier approach yields 15 ( 2 A˚, which justifies the use of the fixed value in the analysis to find out micellar volume fractions φagg. In any case these values are all self-consistent and within the experimental error associated with determining the size of such small objects. A small attractive structure factor, S(Q), was required for the 30.8 vol% C7D16 sample due to proximity to the Pcloud boundary (Table 1). Fits are shown as lines on Figure 2, micellar volume fractions φagg are given in Table 1, and more detail on fitted parameters are in Table S3. Because of the low dielectic constant of these CO2-heptane mixtures, it can be assumed that the AOT aggregates to generate reversed micelles, and this is supported by the absence of any strong repulsive electrostatic S(Q) peaks in the recorded SANS profiles. The level of alkane required to trigger aggregation obtained previously33 is similar to that seen here (30 vol% heptane), and the aggregate size observed by SANS is smaller than that reported in ref 33. This points to the particular strength of HP- SANS5,35 for studying small aggregate structures in unconventional solvent systems, as compared to indirect and invasive methods such as fluorescence quenching, which may be strongly influenced by local environmental factors. At constant C7D16/scCO2 composition, SANS intensity increases with AOT addition, indicating increased aggregation, especially since the contrast does not change appreciably. Data shown in Figure 3 have been fitted to the Schultz polydisperse sphere model, again fixing R = 15 A˚, σ/R = 0.10, but with the addition of a small attractive structure factor, S(Q), for the higher wt % AOT samples (proximity to Pcloud). These abrupt changes in aggregation seen for the AOT systems are quite surprising given the extensive aggregation seen in ambient pressure nonpolar solvents.29,35 It might have been imagined that a smooth transition from slight to extensive aggregation at low to high heptane content would have been observed: this does not appear to the case, as there appears to be a critical heptane content between 20 and 30% . It is well-known 12912 DOI: 10.1021/la901897w

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Figure 3. SANS profiles for different concentrations of AOT in 38.5 vol % C7D16/scCO2 at 360 bar, 25 °C. Lines are fits to a Schultz distribution of polydisperse spheres with fixed R = 15 A˚, σ/R = 0.10. A critical scattering-type S(Q) was employed for both 5.2 and 7.0 vol % AOT data due to its proximity to Pcloud (see Table S3).

Figure 4. Comparison between fitted SANS profiles from AOT aggregates in C7D16/scCO2 at 360 bar, 25 °C and in cyclohexaned12 (C6D12) at ambient pressure, 25 °C. Lines show fits to models representing a Schultz distribution of polydisperse spheres with attractive S(Q) where necessary as described in the text.

that, with low SANS intensities, data fitting is complicated by errors in all parameters (and the close coupling between them). With this in mind, a comparison between HP-SANS from AOT in the C7D16/scCO2 mixtures at 360 bar, 25 °C and ambient pressure SANS from AOT in pure cyclohexane-d12 (C6D12) at the same concentration is shown in Figure 4. The critical aggregation concentration (CAC) for AOT in C6D12 (Hildebrand solubility parameter, δ = 16.8 M Pa1/2; dielectric constant, εr = 2.0)35,40,42 is thought to be as low as 0.225 mM.29 On the basis of solvent quality arguments, the CAC in C7D16 (δ = 15.3 M Pa1/2, εr = 1.9)40,42 might be expected to be at least similar, or even lower. However, these discrepancies in CAC are unlikely to have any significant effect on the SANS, given the high AOT concentrations employed (3.5 vol % ≈ 80 mM). In these experiments C6D12 was used because there are reliable literature data on the CAC in that solvent, and also for economic reasons since it is cheaper than d-heptane. The scattered intensity is a factor of approximately 8-10 higher (depending on the CO2-heptane solvent composition) for the ambient, pure C6D12 sample. Clearly the contrast difference will have an influence on I(Q) since it is proportional to (ΔF)2. For 57.7 vol % C7D16/scCO2, ΔF = 4.1  1010 cm-2; for C6D12, ΔF = 6.1  1010 cm-2; therefore a difference in intensity of a factor of ∼2 would be expected. This still leaves a 4-fold discrepancy between SANS intensity for the pure nonpolar solvent C6D12 and the high pressure mixed C7D16/CO2 samples. Langmuir 2009, 25(22), 12909–12913

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Clearly this can only be explained by either proportionally less aggregation (i.e., an enhanced CAC in the mixed solvent) or a smaller micelle size. On the basis of the shape and form of the curves and the fitted parameters, micelle size changes alone do not explain the differences. Perhaps the discrepancy is due to pressure effects on the CAC. Measurements have been carried out to elucidate the CAC of AOT in supercritical liquids,43 and to study the effect benzene/CO2 mixture composition on this quantity.44 For the supercritical liquids studied (ethane, propane), the observed CAC was a factor of 10 higher (∼30 mM) than for ambient pressure alkanes (∼0.3 mM), which could provide some explanation for the discrepancies observed here. For the benzene/ CO2 mixtures, at lower benzene content (30 vol%) the CAC of AOT was seen to increase, although, at 50 vol % benzene, the CAC decreased back to the value in pure benzene. The results presented here can only be reconciled by allowing a significant fraction of the AOT to be unaggregated in the mixed CO2-heptane solvents, compared to typical pure alkanes.

Conclusions The effects of improving solvent quality of liquid CO2 by addition of a nonpolar hydrocarbon cosolvent on properties of two common surfactants have been explored. The nonionic C12E5 was found to be highly soluble in both pure CO2 and the solvent blends, but no aggregation was seen at any of the compositions studied by HP-SANS, even with the addition of 12 vol % surfactant. On the other hand, heptane added above 30 vol % in CO2 promotes solubility and aggregate formation of the anionic AOT; below this critical amount of heptane, AOT solubility is very low and aggregation cannot be detected. The difference in behavior between the two surfactants can be accounted for in terms of the relative CO2-philicities of the surfactants: the C12E5 is apparently too solvophillic in the CO2-heptane mixtures, tending to dissolve readily as free monomers rather than aggregate as micelles. On the other hand, the ionic groups on AOT are sufficiently solvophobic to drive aggregation of reversed micelles in the mixed solvent media. Recently,35 it has been shown that solvent quality indices allow the prediction of surfactant aggregation in polar/nonpolar hydrocarbon solvents; here, the approach is tested with CO2. Estimates for the solubility parameter of the CO2/heptane mixtures are (43) Olesik, S. V.; Miller, C. J. Langmuir 1990, 6, 183–187. (44) Giddings, L. D.; Olesik, S. V. Langmuir 1994, 10, 2877–2883.

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similar to those of ambient-pressure alkanes (in which C12E5 is known to aggregate, albeit weakly), and the mixture dielectric constant is proportionally lower, which would be expected lead to enhanced aggregation.35 This indicates general limitations of applying solvent-quality type arguments to high-pressure CO2 systems. However, providing a means to boost the solubility and aggregation of a common anionic surfactant such as AOT represents a useful development in scCO2 technology, with two benefits. First, it increases the potential for application of commercially available surfactants in liquid CO2 (e.g., possibility of dispersing water, stabilizing nanoparticle dispersions, viscosity enhancements). Second, this work clearly shows that strategy 1 mentioned in the Introduction, namely specialized surfactant design for CO2, as exemplified by ref 24, is indeed necessary to realize the potential of CO2 fluid technologies. The recently studied custom designed CO2-philes, bearing the same or similar sodium sulfonate headgroup as AOT will aggregate in pure CO2 without the need for added cosolvents or solvent modifiers,5,8,16,17,24 whereas ∼30% by volume of alkane cosolvent is required before common AOT aggregates in the mixed solvent. Hence, strategy 2, addition of modifiers for CO2 permitting the use of readily available surfactants, appears to have certain limitations, the most significant being the need for high cosolvent levels. With these new results in mind, efforts should be redoubled in the quest for the “holey grail” of commercially viable CO2philic surfactants for realizing processing applications of scCO2, including catalysis through cleaning to enhanced oil recovery and carbon capture. Acknowledgment. K.T., M.H., and A.M. thank EPSRC, Kodak, the Malaysian government, and the School of Chemistry at the University of Bristol for the provision of Ph.D. scholarships. The ISIS-STFC Neutron Scattering Facility (formerly CCLRC) is thanked for the provision of beam-time and grants towards consumables and travel. The EPSRC is thanked for provision of funding under Grants EP/C523105/1 and EP/F020686. Supporting Information Available: Details of calculations and model fitting; details of pressure cell; SANS profiles and discussion for C12E5 samples not shown in the main paper; SANS profiles and discussion for heptane/CO2 solvent mixtures; tables of fitted parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la901897w

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