Effects of Fluorocarbon Surfactant Chain Structure on Stability of Water

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Effects of Fluorocarbon Surfactant Chain Structure on Stability of Water-in-Carbon Dioxide Microemulsions. Links between Aqueous Surface Tension and Microemulsion Stability Julian Eastoe,*,† Alison Paul, and Adrian Downer School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.

David C. Steytler*,‡ and Emily Rumsey School of Chemical Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Received September 14, 2001. In Final Form: January 22, 2002 Formation and stability of water-in-carbon dioxide microemulsions are described with eight related fluorinated analogues of the anionic surfactant Aerosol-OT. The aim was to identify a structure-performance relationship in CO2 with surfactants of high surface chemical purity that can also be synthesized from readily available reagents. The most effective CO2-philes from this group were sodium bis(1H,1Hperfluoropentyl)-2-sulfosuccinate (di-CF4), sodium bis(1H,1H,2H,2H-perfluorohexyl)-2-sulfosuccinate (di-CF4H), and sodium bis(1H,1H-perfluoroheptyl)-2-sulfosuccinate (di-CF6). All three of these compounds stabilized microemulsions at CO2 bottle pressure (57 bar) at 15 °C, with a w value ([water]/[surf]) of 10. A close correlation is demonstrated between limiting aqueous phase surface tension of a given surfactant at its critical micelle concentration, γcmc, and its performance in water-CO2 microemulsions, as measured by the phase transition pressure Ptrans. This finding has important implications for the rational design of CO2-philic surfactants.

Introduction Carbon dioxide, either in its supercritical (sc-CO2) or liquid states (l-CO2) represents a cheap (∼$0.05 kg-1), nontoxic, nonflammable, and environmentally responsible “green” alternative to conventional petrochemical and chlorinated solvents. The critical point is readily accessible (31.1 °C and 73.8 bar), so that variation of density with temperature and pressure offers flexibility in solvent power not so readily achieved with other solvents. Rapid solvent removal and recycling are also possible by control over evaporation and condensation. Certain low molecular weight and nonpolar materials are soluble in CO2, and it is already used routinely in various industrial processes.1-4 Despite the potential applications, little is known on the important issue of how to optimize surfactant chemical structure for CO2 dispersions. This is partly due to a lack of suitable, well-characterized compounds and also that one of the most studied surfactants is a technical grade product perfluorpolyether (PFPE) consisting of a molecular weight distribution (e.g., ref 5). Eastoe et al.6-8 and Erkey and Liu9 have shown that custom-synthesized fluorinated † To whom correspondence may be addressed. Tel.: UK + 117 9289180. Fax: UK + 117 9250612. E-mail: julian.eastoe@ bristol.ac.uk. ‡ To whom correspondence may be addressed. Tel.: UK + 1603 592033. Fax: UK + 1603 25985. E-mail: [email protected].

(1) Eckert, C. A.; Knutson, B. L.; Debendetti P. G. Nature 1996, 383, 313. (2) Smith, R. M. J. Chromatogr., A 1999, 856, 83. (3) DeSimone, J. M.; Guan, Z.; Elsbernd, C. S. Science 1992, 257, 945. (4) See: www.micell.com. (5) Zielinski, R. G.; Rosov, N.; Kaler, E. W.; Kline, S. R. Langmuir 1997, 13, 3934. (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. (7) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E. Prog. Colloid Polym. Sci. 2000, 115, 214. (8) Eastoe, J.; Downer, A.; Paul, A.; Steytler, D. C.; Rumsey, E.; Heenan, R. K. Phys. Chem. Chem. Phys. 2000, 2, 5235.

analogues of Aerosol-OT (sodium bis-2-ethylhexyl sulfosuccinate, AOT), such as those shown in Figure 1, can be very effective. These compounds provide a suitable platform for delineating structure-performance relationships in CO2, and here 12 different surfactants have been studied. Following the scheme given in Figure 1, these compounds are di-HCF2, di-HCF4, di-HCF6, di-CF2, diCF3, di-CF4, di-CF6, di-CF8, di-CF4H, di-CF6H, diHCF4GLU, and the cobalt salt Co(di-HCF4)2. This represents the widest study of chemical structure variation for CO2 surfactants reported to date. In the present paper surfactant purity issues have been carefully addressed to ensure reliable results. Furthermore, an important finding is highlighted for the first time: a clear correlation between surface tension of an aqueous solution and the performance of the compound in a water-in-carbon dioxide (w/c) microemulsion. This link between aqueous and CO2 phase behavior can now be used as a guide for designing new, highly efficient CO2-philes. Experimental Section Surfactant Synthesis and Surface Tension Measurements. Syntheses, purification, and chemical characterization of the fluorinated surfactants have been fully described elsewhere.6-8,10-13 Tensiometry with aqueous solutions indicated surface chemically pure surfactants. H2O of resistivity 18.2 MΩ cm was taken from either a R0100HP Purite water system or a Millipore Milli-Q Plus system, and CO2 (BOC, U.K.) was used as received. Tensiometric measurements were made with a dropvolume (DV) machine (Lauda TVT1), in the presence of trace (9) Liu, Z. T.; Erkey, C. A. Langmuir 2001, 17, 274. (10) Yoshino, N.; Komine, N.; Suzuki, J.-I.; Arima, Y.; Hirai, H. Bull. Chem. Soc. Jpn. 1991, 64, 3262. (11) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Langmuir 1999, 15, 7591. (12) Eastoe, J.; Robinson, B. H.; Fragneto, G.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1992, 88, 461. (13) Eastoe, J.; Nave, S.; Downer, A.; Paul, A.; Rankin, A.; Tribe, K.; Penfold, J. Langmuir 2000, 16, 4511.

10.1021/la015576w CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002

Structure Variation of CO2 Surfactants

Langmuir, Vol. 18, No. 8, 2002 3015 Table 1. Aqueous Phase Critical Micelle Concentrations (cmc’s) and Limiting Surface Tensions (γcmc) for Fluorinated Surfactants Shown in Figure 1a di-HCF4 di-HCF6 di-CF3 di-CF4 di-CF6 di-CF4H di-CF6H di-HCF4GLU

cmc/(mmol dm-3)

γcmc/(mN m-1)

Ptrans/bar

12 0.8 12 2 0.1 0.7 0.05 11

26.8 24.1 17.8 17.7 15.5 15.6 22.0 25.8

193 163 124 70 77 89 139 181

a Temperatures for tension measurements were as follows: diHCF4, 25 °C; di-HCF6, 40 °C; di-CF3, 25 °C; di-CF4, 30 °C; di-CF6, 40 °C; di-CF4H, 25 °C; di-CF6H, 40 °C; di-HCF4GLU, 25 °C. The phase transition pressures, Ptrans, water-in-CO2 microemulsions are for systems with [surf] ) 0.05 mol dm-3 and w ) 10 at 25 °C. Tension and phase stability data for di-HCF4, di-HCF6, and diCF4 have been taken from ref 8.

the onset of opacity as the pressure dropped through the phase boundary.

Results and Discussion

Figure 1. Surfactants used to form water-in-CO2 microemulsions. EDTA (99.5% tetrasodium salt hydrate, Sigma) employing a protocol described in detail elsewhere.13 Thermostating to (0.1° was provided by a Grant LTD6G bath; appropriate temperatures were chosen to avoid Krafft points: experiments were conducted atthe following temperatures: di-HCF4, 25 °C; di-HCF6, 40 °C; di-CF3, 25 °C; di-CF4, 30 °C; di-CF6, 40 °C; di-CF4H 25, °C; di-CF6H, 40 °C; di-HCF4GLU, 25 °C. For these fluorosurfactants, the effect of temperature on surface tension at the critical micelle concentration, γcmc , is a second-order effect. For example, with di-HCF4 the depression in γcmc is 1-2 mN m-1 over the range 25-40 °C. Water-in-Carbon Dioxide Phase Stability Measurements. The custom-built stainless steel pressure cell has been described previously.14 Unless otherwise indicated, all experiments were carried out using a starting surfactant concentration of 0.05 M. Due to CO2-hydrate formation at e10 °C, it was not possible to conduct experiments at temperatures below 15 °C. Not all the surfactants gave a stable microemulsion at this initial temperature, notably di-HCF6 and diCF6. However, after these systems were heated to ∼40 °C at ∼400 bar, single-phase dispersions were formed, which could then be cooled to 15 °C (400 bar) without inducing any phase separation. After adjustments to attain experimental conditions (T and P), samples were stirred and equilibrated for 15-20 min. Phase transitions were determined visually by adjusting pressure at fixed temperature. Above the phase transition pressure (Ptrans), a clear one-phase region was observed. When the pressure was lowered, samples became cloudy as Ptrans was approached and turned opaque below the transition point. With the stirrer switched off, the system separated to give water and surfactant at the bottom of the cell and a clear upper CO2 phase. Where no phase boundary was observed (i.e., a single-phase region existed with no applied pressure), it was possible to induce the transition by gently bleeding CO2 from the cell. For consistency Ptrans was taken as (14) Eastoe, J.; Robinson, B. H.; Steytler, D. C. J. Chem. Soc., Faraday Trans. 1990, 86, 511.

a. Surface Tensions of Aqueous Solutions. Surface tension measurements were used to ascertain surface chemical purity of all surfactants, as well as determine critical micelle concentrations (cmc’s) and limiting surface tensions at the cmc γcmc. The results are listed in Table 1 (data for di-HCF4, di-HCF6, and di-CF4 have been taken from ref 8). This tension γcmc is characteristic of the efficiency of a given surfactant, and as shown before,11,15 it is strongly linked to the hydrophobic chain chemical structure. For surfactants with cmc’s typically below 1 mmol dm-3, the slow rate of adsorption may be a factor and dynamic surface tension effects can become significant. Methods and techniques, described elsewhere,11,13 were used for minimizing these problems to obtain representative equilibrium adsorption isotherms. There are no surprises in the variations in cmc with chain length (Table 1). However, interesting patterns emerge for the limiting tensions γcmc. Within a series increasing chain length leads to a lower γcmc, for the diHCFn and di-CFn series increasing n from 4 to 6 gives rise to a ∼2 mN m-1 lowering. This trend has also been found for straight-chain hydrocarbon Aerosol-OT relatives16 and may be attributed to a slight increase in chain density (decrease in molecular area) in the surface film for longer chain length surfactants. (Although the reason for the trend with di-CFnH is unclear, since the results have been checked with two batches.) On the other hand chain termination, H-CF2 vs F-CF2, has a more dramatic effect on surface properties, for example, the terminal-H compound, di-HCF4, which displays γcmc ∼9 mN m-1 higher than that for the equivalent CF3-terminated analogue, di-CF4. b. Water-in-CO2 Microemulsions. Of 12 different linear chain fluorosuccinates investigated, 9 formed w/c microemulsions with water content w ) 10; these were di-HCF4, di-HCF6, di-CF3, diCF4, diCF6, diCF4H, diCF6H, di-CF4GLU, and the cobalt salt Co-HCF4. Interestingly, neither of the shortest chain compounds, di-CF2 and di-HCF2, we tested nor the long chain di-CF8 formed stable microemulsions. On the other hand di-CF4, di-CF6, and di-CF4H were very effective, and gave w ) 10 microemulsions at CO2 bottle pressure (57 bar) at 15 (15) Pitt, A. R.; Morley, S. D.; Burbidge, N. J.; Quickenden, E. L. Colloids Surf., A 1996, 114, 321-335. (16) Nave, S.; Eastoe, J.; Penfold, J. Langmuir 2000, 16, 8733.

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di-CF6H showed a higher Ptrans than di-CF4H under equivalent conditions but also had a higher limiting surface tension. As shown, there is a distinction between the behavior of H-CF2-tipped surfactants (higher γcmc and Ptrans) and that for the class of F-CF2 compounds, which on the whole perform best in CO2. Conclusions

Figure 2. Correlation between Ptrans in w/c systems and γcmc at the a/w interface: di-CF6 (solid triangle); di-CF4H (gray diamond); diCF3 (solid circle); di-CF4 (solid diamond); di-HCF6 (open triangle); di-CF6H (gray triangle); di-HCF4 (open diamond); di-HCF4GLU (gray square). Phase transition pressures were measured at 25 °C for w ) 10 microemulsions and 0.05 mol dm-3 surfactant concentration. The line is a guide to the eye.

°C. This indicates a strong chemical specificity and is consistent with an optimum chain length for w/c formation. Phase Boundary Reproducibility. Initial measurements using unpurified surfactants resulted in poor reproducibility. Given the organic purity of the diesters and surfactants (NMR and mass spectrometry8), excess electrolyte from the sulfonation step was suspected as the main culprit. Calculations based on mass-percentage composition (assuming an average salt composition of Na2S1.5O4) showed that a 1% contamination of the raw solid surfactant with salt was sufficient to produce up to a 0.5 mol dm-3 electrolyte contamination in the aqueous phase of a w ) 10 microemulsion. Extraction and centrifugation steps, described fully elsewhere11,13 were then introduced. Reproducibility of newly made up samples using a given surfactant batch of fully purified surfactant was within (5 bar. For example, repeat experiments with di-HCF4 using different batches generally gave results within about (3 bar. The phase boundaries obtained for this newly purified di-HCF4 were about 200 bar lower than those found in our earlier work.6 The possible presence of divalent cation contaminants in those samples (e.g., Mg2+ from drying over MgSO4, a procedure which is now not used in the syntheses) would move the phase boundary to higher pressures, and this may in part be the cause of the discrepancy. For our high-purity diHCF4 there is good agreement between these results and those of Erkey and Liu.9 From Figure 1 of ref 9 at 25 °C and w ) 10, Ptrans ≈ 190 bar, while we find 193 ( 3 bar. (At 15 °C the comparison is 140 bar here versus ≈130 bar in ref 9.) As noted by Erkey 9 Ptrans showed no obvious dependence on overall surfactant concentration in the range 0.025-0.10 mol dm-3. To determine the influence of surfactant structure on w/c phase stability, w ) 10 systems at 0.05 mol dm-3 were evaluated. Correlation between Cloud Points and Aqueous Surface Tensions. While there is no apparent link between aqueous phase cmc and Ptrans (Table 1), Figure 2 demonstrates a clear correlation between the limiting surface tension γcmc and the corresponding w/c phase boundary pressure (for w ) 10 systems at 0.05 mol dm-3 and 15 °C). Figure 2 contains data for eight different related surfactants. These results also explain apparent anomalies in the effect of chain length on phase behavior:

The aim of this study was to identify key chemical groups necessary for highly efficient surfactants to stabilize waterin-CO2 microemulsions. This has been achieved by investigating a range of homologous anionic surfactants, which can be readily synthesized from commercially available reagents. A clear correlation was observed between limiting surface tension of a given surfactant at its cmc, γcmc, and its performance in water-CO2 microemulsions, as measured by the phase transition pressure Ptrans. These results have important implications for the rational design of CO2-philic surfactants. The advantage is that studies of aqueous solutions are relatively easy to carry out, and surface tension measurements can be used to screen target compounds expected to exhibit enhanced activity in CO2. Therefore, potential surfactant candidates can be identified before making time-consuming phase stability measurements in high-pressure CO2. The results presented here suggest an optimum hydrophobic chain structure of CF3-(CF2)4-(CH2)n-, where n ) 1. These particular molecules bare headgroup carbonyl groups (Figure 1), and there may be specific interactions with CO2 in this region of the interface. It is known that CO2 acts as a Lewis acid in the presence of electron-donor groups,17-20 leading to enhanced solubility of certain polymers. It is apparent that w/c systems are sensitive to extremely subtle changes in surfactant structure and that the hydrophobic chain-tip dominates interfacial behavior. In all cases the F-terminated surfactants are more effective giving significantly lower phase boundaries. It is illuminating that aqueous phase (γcmc) and CO2 phase properties (Ptrans) are correlated. Given the correlation between Ptrans and γcmc, it is conceivable that branched fluorocarbon chain analogues would also be effective, owing to an increased density of CO2-compatible terminal CF3- groups. Drawing parallels with hydrocarbon compounds in air-water (a/w) and water-in-oil (w/o) systems, branching the chains would lower γcmc and the phase boundary.15,16 Johnston and daRocha, who introduced a hydrophilic-CO2-philic balance (HCB) scale based on results for PFPE ammonium carboxylates,21 have made some inroads into a more quantitative approach. Although the range of compounds studied was limited, it was possible to predict whether a surfactant would preferentially form a w/c or c/w emulsion.21 One frequently asked, and valid, question concerns the economics of fluorocarbon surfactants for applications in CO2. At current market prices and using scientific (not bulk) suppliers, the raw costs can be estimated at $0.6/g, $2.4/g, and $50/g for di-HCF4, di-CF4H, and di-CF4, respectively (not including human resource costs). For any practical purposes di-CF4H would be the preferred option, owing to cost, despite the ∼20 bar increase in (17) Meredith, J. C.; Johnston, K. P.; Seminaro, J. M.; Kazarian, S. G.; Eckert, C. A. J. Phys. Chem. 1996, 100, 10837. (18) Kazarian, S. G.; Vincent, M. F.; Bright, F. V.; Liotta, C. L.; Eckert, C. A. J. Am. Chem. Soc. 1996, 118, 1729. (19) Rindfleisch, F.; DiNoia, T. P.; McHugh, M. A. J. Phys. Chem. 1996, 100, 15581. (20) Sarbu, T.; Styranec, T.; Beckman, E. J. Nature 2000, 405, 165. (21) daRocha, S. R. P.; Johnston, K. P. Langmuir 2000, 16, 3690.

Structure Variation of CO2 Surfactants

Ptrans over the more expensive di-CF4. In summary, aqueous surface tension properties can provide a useful guide to the CO2-philicity of surfactants. In fact this idea of examining trends in γcmc was used recently to design analogues of hydrocarbon Aerosol-OT, bearing tert-butyl chain termini, which were shown to micellize in CO2.22 (22) 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.

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Acknowledgment. This work was funded by EPSRC as part of programmes into surfactants for CO2 (GR/ L05532 and GR/L25653) and dynamic surface tension (GR/ M83780). A.P. and E.R. acknowledge the support of EPSRC in terms of studentships. Drs. Martin Murray and Ken MacNeil from the School of Chemistry, Bristol, U.K., are thanked for assistance with NMR and mass spectroscopy, respectively. LA015576W