philic Surfactants with Low Fluorine Content - American Chemical

Article pubs.acs.org/Langmuir

Hybrid CO2-philic Surfactants with Low Fluorine Content Azmi Mohamed,†,‡ Masanobu Sagisaka,§ Martin Hollamby,†,∥ Sarah E. Rogers,⊥ Richard K. Heenan,⊥ Robert Dyer,# and Julian Eastoe*,† †

School of Chemistry and #Krüss Surface Science Centre, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K. ‡ Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tanjong Malim Perak 35900, Malaysia § Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan ∥ National Institute of Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, 305-0047, Japan ⊥ ISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K. S Supporting Information *

ABSTRACT: The relationships between molecular architecture, aggregation, and interfacial activity of a new class of CO2-philic hybrid surfactants are investigated. The new hybrid surfactant CF2/AOT4 [sodium (4H,4H,5H,5H,5H-pentafluoropentyl-3,5,5-trimethyl-1-hexyl)-2-sulfosuccinate] was synthesized, having one hydrocarbon chain and one separate fluorocarbon chain. This hybrid H−F chain structure strikes a fine balance of properties, on one hand minimizing the fluorine content, while on the other maintaining a sufficient level of CO2-philicity. The surfactant has been investigated by a range of techniques including high-pressure phase behavior, UV−visible spectroscopy, small-angle neutron scattering (SANS), and air−water (a/w) surface tension measurements. The results advance the understanding of structure−function relationships for generating CO2-philic surfactants and are therefore beneficial for expanding applications of CO2 to realize its potential using the most economic and efficient surfactants.

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INTRODUCTION The potential for environmentally benign solvent media with supercritical CO2 has been extensively reported.1−6 However, owing to various experimental and chemical difficulties, this field has developed in a stop−start manner, impeding understanding of how to improve physicochemical properties and solvency of CO2 for various applications. It is recognized that dense CO2 is a very weak solvent, which can be improved by incorporating CO2-philic surfactants, for example, by stabilizing water-in-CO2 (w/c) emulsions and microemulsions. However, very few commercially available surfactants are sufficiently CO2-philic, requiring design and synthesis of specialized amphiphiles tailored for CO2. Then, if such surfactants are to be commercialized, raw material costs and working pressures need to be minimized to reduce engineering and processing demands. At current market prices and using scientific (not bulk) suppliers, the raw costs for making typical known efficient CO2-philes such as the twin-tailed fluorocarbon di-CF4 (structures in Table 1), can be estimated at ∼$200 per gram.7−10 On the other hand, the lower fluorine content diCF2 (Table 1),9−11 which was designed to minimize fluorine content (50% less fluorine than di-CF4), is also much cheaper at around $15 per gram. Knowledge of the chemical factors governing CO2-philicity is essential for designing cheaper CO2© 2012 American Chemical Society

compatible surfactants, so such information is vital in the development practical applications of dense CO2. However, there is still an active debate regarding the true nature of CO2philic surfactants.9,10,12−22 To fully account for the structural and molecular factors that affect CO2-philicity, a combination of theoretical and experimental approaches is required. Numerous papers have attempted to rationalize CO2-philic surfactant design using simulation and computational approaches.23−29 However, there have been only few systematic investigations on CO2-philes,30−32 and the wildly different molecular structures have led to fairly system-specific conclusions on what constitutes “CO2-philicity”, making it hard to generalize the concept. To date, different models have been proposed as predictive tools for CO2-philicity, including the fractional free volume (FFV) argument by Johnston et al.26 On the basis of this framework, the most effective CO2-philes are expected to have a low FFV value. The FFV index is defined as FFV = 1 −

V tAcmc

(1)

Received: February 5, 2012 Revised: March 23, 2012 Published: March 28, 2012 6299

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Recently, a new predictive model for CO2-philic surfactant design has been introduced, discussed, and evaluated.11 In this model, a correlation between aqueous surface tension properties and the performance of the surfactant in w/c microemulsions is considered. This relative surfactant coverage is expressed as

Table 1. Surfactants Used in This Work

Φsurf =

Vcal Vmeas

(2)

where Vcal is the total volume of surfactant molecular fragments (values taken from elsewhere34−36) and Vmeas is that for the total fragment volume at the air/water interface. It seems that CO2-philicity can be increased by promoting higher surfactant coverage as defined by Φsurf, i.e., by providing better separation between CO2 and water at the interface. Gratifyingly, this is also in line with computer simulation studies.23−26 When considered in this way, again di-CF4 (Φsurf = 0.97) appears to be the most efficient surfactant, since it gives a larger relative interfacial packing efficiency. Interestingly, an optimum amount of fluorine is required to give a practically useful Ptrans.8,31 Replacing hydrogen in the surfactant tails of the H-only diC5SS (two n-pentyl chains) with 35 wt % fluorine, in the partially fluorinated di-CF2 (Φsurf = 0.79, Ptrans = 198 bar, w = 10, [surf.] = 0.05 mol dm−3 at 25 °C), turns the inactive diC5SS into a CO2-phile with moderate microemulsifying properties.11 The purpose of this new paper is to investigate this relative coverage index Φsurf as a predictive tool for CO2-philic surfactant design in which chain structure and type of surfactant are explicitly taken into account. While the importance of fluorination of both surfactant18,19,31,37 and polymer38−41 chains has been stressed above, and in the past by groups in the field, this study focuses on low molecular weight surfactants. With polymers it is more challenging to gain fundamental understanding, as compared with low molecular weight surfactants. For example, polymers can be made soluble by incorporation of more of the key solvophilic groups.38 Also, unlike surfactants, polymers can be more structurally complex, requiring additional factors to be considered (e.g., structural isomers, polydispersity, and chemical control) that may mask the underlying chemical reasons for CO2-philicity. Here, the behavior of surfactants with fully fluorinated, partially fluorinated, and hybrid F-chain + H-chain structures are compared systematically (Table 1). Since previous investigations were almost exclusively with fluorinated compounds, it is also interesting to investigate Φsurf for surfactants with F + H hybrid chain structures. Due to their nature, hybrid compounds combine interesting physicochemical properties, suggesting improved CO2 compatibility over hydrocarbon-only surfactants. Despite the previous promising results for the F-chain +

where V is the volume, t is the length of the surfactant chains, and Acmc is the area of the headgroup of the surfactant molecule at a defined reference state, being the aqueous phase critical micelle concentration (cmc). The calculated FFV values were tested for different surfactants and compared in terms of literature values with their cloud points (Ptrans), which represent an important measure of CO2-philic surfactant efficiency. As such, Ptrans is the minimum pressure at which a given dispersion at constant composition and temperature remains a stable single transparent phase; therefore, a lower Ptrans, indicates a more efficient surfactant for CO2. Another parameter used to scale the efficiency of CO2-philes for stabilizing micro/emulsion domains in CO2 is the molar water-to-surfactant ratio w = [water]dispersed/[surf.]. On the basis of previous studies,7,8,31 di-CF4 has the lowest FFV and the lowest phase-transition pressure of any known CO2-philic surfactant (FFV= 0.42, Ptrans = 70 bar, w = 10, [surf.] = 0.05 mol dm−3 at 25 °C), suggesting that chain fluorination improves the microemulsifying properties. Conversely, the FFV value of the fluorine-free hydrocarbon-only analogue AOT4 (Table 1), with highly branched chains,33 is higher than for diCF4 (FFV = 0.46, Ptrans 500 bar, w = 0, [surf.] = 0.10 mol dm−3 at 33 °C). However, the small change in FFV, yet greatly differing CO2 compatibility as indicated by Ptrans (Table 2), suggests limitations of the free volume approach in distinguishing between hydrocarbon and fluorocarbon surfactants and therefore as a general guide rule for the design of CO2-philic surfactants.

Table 2. Parameters Derived from Surface Tension and Phase Behaviour Measurementsa surfactant

fluorine/wt %

cmc/ mmol dm‑3 (±0.03)

γcmc/mN m‑1 (±1)b

Acmc/Å2 (±2)

FFV

Φsurf

Ptrans/barc

di-CF2 hybrid CF2/AOT4 equimolar di-CF2:AOT4 AOT4d AOTe F7H7f

35.2 18.8 18.8 0.0 0.0 48.8

19.0 3.30 2.20 1.10 2.49 0.63

22.4 23.5 24.7 28.0 31.8 22.1

65 71 71 70 75 60

0.51 0.50

0.79 0.70

0.46 0.51 0.46

0.68 0.63 0.86

224 340 translucent incompatible incompatible 130

Ptrans is the observed reversible cloud-point pressure in CO2 or observations on surfactant solubility. bAt 25 °C. cw = 10, w/c, 40 °C. dData collected by Nave.46 eData collected by Mohamed.33 fData collected by Dupont.42 a

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positioned so as to provide a perpendicular 10-mm optical path. The temperature of the cell was controlled by circulating water with a thermostat bath. Spectroscopic measurements were performed and the resulting absorption spectra of the cell windows were compared with those of a standard quartz cell for an aqueous MO solution at ambient pressure. Both the spectra were observed to be in good agreement with each other. For UV-spectral measurements, surfactant concentration, temperature, and pressure were fixed at 0.05 mol dm−3, 45 °C, and 400 bar, respectively. Concentration of MO was 0.1 wt % in water. High-Pressure Small-Angle Neutron Scattering (HP-SANS). The newly commissioned SANS2d instrument at the Rutherford Appleton Laboratory at ISIS UK was used in conjunction with a stirred high-pressure cell as described previously.33 The measurements gave the absolute scattering cross section I(Q) (cm−1) as function of momentum transfer Q (Å−1). The accessible Q range was 0.005−0.7 Å−1, arising from incident neutron wavelength of 2.2−14 Å. For all experiments the path length was 10 mm, P = 380 bar, and T = 40 °C. The data were normalized for transmission, empty cell, solvent background, on the basis of many reports in the literature.10,50 Neutrons are scattered by short-range interactions with sample nuclei, the “scattering power” of different components being defined by a scattering-length density (SLD), ρ (cm−2). For liquid CO2, ρCO2 ∼ 2.50 × mass density × 1010 cm−2.51 At an experimental pressure of 380 bar, the CO2 density is ∼1.0 g cm−3 so that ρCO2 is approximately 2.50 × 1010 cm−2. SANS arises principally from the contrast step between D2O (ρD2O = 6.33 × 1010 cm−2) and surfactant shells/CO2 solvent, so the structure of D2O domains can be elucidated from subsequent analyses. Assuming surfactant densities of 1.45, 1.25, and 1.10 g cm−3 for di-CF2, hybrid CF2/AOT4 and AOT4 respectively gives ρdi‑CF2 = 2.19 × 1010 cm−2, ρhybrid CF2/AOT4 = 1.34 × 1010 cm−2, and ρAOT4 = 0.486 × 1010 cm−2. For SANS experiments using mixtures of di-CF2 and AOT4, the SLD of the mixed surfactant ρequimolar diCF2:AOT4 was calculated as a first-order summation, weighted by the different volume fractions. The data have been fitted using the FISH interactive fitting program.52 After extensive trials on the basis of the literature for many AOT analogue-stabilized microemulsions,7,8,10,31,43,50,53,54 a model incorporating a Schultz distribution of polydisperse spheres was found to give the best fits and most physically reasonable results. Further information on SANS modeling can be found in the Supporting Information. Surface Tension Measurements. Tensiometric measurements were carried out using the Wilhelmy plate Krüss K11 or K100 instrument. As explained elsewhere,11,33,46,55 a trace level of background EDTA (99.5% tetrasodium salt hydrate, Sigma) was included in the solutions to chelate any polyvalent cation contaminants. Further details are in the Supporting Information.

H-chain hybrid surfactants, for example, sodium 1-pentadecafluoroheptyl-1-octanesulfate (F7H7; Ptrans 130 bar, w = 10, [surf.] = 0.05 mol dm−3 at 40 °C),42 multiple steps and complexity of the synthetic routes do impose limitations on their applications.19,32,43−45 The new AOT-analogue double chain hybrid surfactant CF2/AOT4 (Table 1) is studied here, having significant benefits of a simpler and more efficient synthesis compared to compounds like F7H7. The presence of an optimum level of fluorine is expected to enhance the microemulsifying properties.32 Such compounds may offer environmental and economical benefits, since they contain only the bare minimum fluorine needed to achieve CO2-philicity. To reveal any specific effect of using the true hybrid surfactant CF2/AOT4, where the separate H- and F-carbon chains are tethered at the headgroup in the same molecule, comparisons were made with the properties of equimolar mixtures of the related twin-tailed fluorinated di-CF2 and hydrocarbon AOT4 (Table 1). The work advances the understanding of how CO2-philic surfactants may be tuned by modifying their tail architecture. In particular, it has been shown here that control over fluorination levels can be achieved using the principles of surfactant relative packing coverage Φsurf.11 In addition, significant steps forward have been made in the design of economic CO2-philic surfactants. It is believed that the hybrid CF2/AOT4 is the lowest fluorine content CO2-philic dichain surfactant reported to date. Given that this hybrid surfactant needs only relatively moderate T, P conditions to stabilize w/c microemulsions, it is conceivable that such low F-content stabilizers could find application, for example, in enhanced oil recovery, nanoparticle stabilization, and dry cleaning.

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EXPERIMENTAL SECTION

Chemicals. Synthesis, purification, and chemical characterization of the di-CF2 and AOT4 have been described previously.11,46 The hybrid CF2/AOT4 was synthesized following the same method and alcohol precursors with an additional step for the diester purification. Further information concerning surfactant synthesis and characterization can be found in the Supporting Information. H2O of resistivity 18.2 MΩ cm was taken from an Elga Pure Lab Classic system. D2O (98% Datom Goss scientific) and CO2 (BOC) were used as received. High-Pressure Phase Behavior and UV−Visible Absorption Measurements. The phase behavior and formation of w/c microemulsions were examined by visual observation and UV−visible measurements at temperatures of 35−75 °C and pressures up to 400 bar. A detailed description of the experimental apparatus and procedures for the measurements can be found elsewhere.47,48 The CO2 densities were calculated using the Span−Wagner equation of state (EOS).49 Predetermined amounts of hybrid CF2/AOT4 (surfactant concentration = 0.05 mol dm−3) and CO2 were loaded into a variable-volume high-pressure optical cell. Then, water was added into the optical cell through a six-port valve until a clear WinsorIV w/c microemulsion (1Φ) became a turbid macroemulsion (2Φ). Repeat phase measurements were conducted, with the same batch of surfactant, in different cells (two in Bristol, U.K. and the other in Hirosaki, Japan) and by different operators. These repeats indicate reproducibilities in Ptrans of typically ±15 bar. In order to gain evidence for the presence of aqueous cores in the w/c microemulsions, UV−visible absorption spectroscopy measurements were performed using methyl orange (MO) as a trace marker dye, on a double-beam spectrometer (Hitachi High-Technologies, U2810). A quartz window pressure cell (volume: 1.6 cm3) was employed, connected to the six-port valve with a sample loop (5 μL) and high-pressure pump. The cell was made of stainless steel (SUS316) and had three quartz windows with a thickness of 8 mm. Each window had an inner diameter of 10 mm; the windows were

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RESULTS AND DISCUSSION High-Pressure Phase Behavior and UV−Visible Absorption Measurements of CO2 Systems. As an initial step in this study, surfactant phase behavior was examined at fixed surfactant concentration of 0.05 mol dm−3 and w ([water]/ [surf.]) = 10. The phase stability of a w/c microemulsion is driven by both solvent density and compatibility with surfactant tails.10,31,50 Phase diagrams and values for Ptrans, are presented in Figure S4 (Supporting Information) and Table 2, respectively; the behavior for the newly synthesized di-CF2 was in good agreement with other results.11 Exchanging one of the di-CF2 tails with a hydrocarbon 3,5,5-trimethylhexyl (AOT4) tail in the CF2/AOT4 hybrid had a significant effect on the instability boundary, increasing Ptrans by approximately 116 bar at 40 °C. Evidence for microemulsion formation is provided by the MO-probe UV−visible adsorption spectra, which clearly demonstrate water + MO partitioning into the CO2 continuous medium. The observed broad peaks are centered around λ ∼ 430 nm and display increasing intensities with water content. 6301

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Equation 3 implies that there is a minimum interfacial tension, γint, needed for a microemulsion to favorably form. The surfactant-free interfacial tension at water/CO2 (w/c) is approximately 20 mN m−1,20,21,58 typically lower than that found for a water/oil (w/o) interface (∼50 mN m−1). Hence, a larger area per surfactant headgroup, Ah, is observed in w/c microemulsions as a result of a lower demand for surfactant at the w/c interface as compared to standard w/o interfaces. To confirm this picture, Porod analyses have been applied to the set of data in Figure 1. The effective area per surfactant headgroup, Ah, at the water/CO2 interface was obtained from the high Q portion of the SANS data by applying the Porod equation59 (see Figure S8, Supporting Information). The limitation of using this Porod analysis is direct dependence of the scattering intensity on the amount of water uptake (w ratio) into w/c microemulsions. This is only possible for surfactants that can provide higher w microemulsions. Despite that, the effective molecular headgroup area Ah of di-CF2 surfactant can be calculated as ∼116 Å2, consistent with values for similar sulfosuccinate surfactants at the w/c interface.7,8,11,31 On the other hand Ah values in w/c systems are typically larger (90− 120 Å2)7,8,11,20,21,31 compared to those for water/fluorinated oil (∼55 Å2),60 which is attributed to the lower w/c interfacial tension and the entropy contribution from surfactant tail/oil interactions, which are likely to be enhanced in w/c systems by the small molecular volume of CO2. Conversely, the hybrid CF2/AOT4 at the w/c interface displays quite different behavior, with Ah ∼ 93 Å2. This lower value suggests a lower packing density of fully fluorinated surfactants compared to hybrid surfactants at the w/c interface, as has been found before.8,11,23,32 The phase behavior and HP-SANS data analyses strongly point to the influence of fluorination levels in CO2-philicity (see Table 2). In addition, the nature of interactions between hydrocarbons and CO2, and fluorocarbons and CO2 is fundamentally different, with fluorination having an important effect on molecular packing. This can explain why the fully fluorinated di-CF2 is significantly better at stabilizing w/c microemulsions than the hydrocarbon-only chain AOT4 or even the mixed F + H chain hybrid CF2/AOT4. Owing to of the poor performance of hydrocarbon surfactants in CO2,53,61,62 expensive and environmentally persistent fluorine unfortunately still remains a key element in CO2-phillic surfactants. However, the promising results above suggest that hydrocarbon−fluorocarbon hybrids may be an option for the stabilization of w/c microemulsions, allowing a reduction in fluorine levels but still retaining efficiency. In the case of the hybrid CF2/AOT4 surfactant, further explanation is needed to understand the stabilization of the w/c microemulsions. As noted above, recently an interface coverage index Φsurf was established11,31 to predict CO2-philicity and optimize fluorination levels. The following sections will investigate structure− function relationships for the hybrid CF2/AOT4 surfactant, in order to account for its remarkable efficiency, despite the low fluorination. This necessitates careful measurements of surface tensions at the air/water interface to generate adsorption parameters for computing Φsurf. Surface Tension Measurements. Figure 2 shows the γ− ln(concentration) curves for aqueous solutions of hybrid-CF2/ AOT4 and equimolar di-CF2:AOT4 mixtures in the presence of EDTA, alongside data for the individual compounds diCF211 and AOT4 (see Supporting Information).33 The plots show clean breaks at the cmc with no indications of minima or

(Figure S6, Supporting Information). Similar absorption spectra were previously observed for sulfosuccinate surfactantstabilized w/c microemulsions systems.47,56 Comparing the hybrid CF2/AOT4 surfactant with the performance of an equimolar di-CF2:AOT4 mixture in CO2 indicates that the hybrid surfactant provides better CO2 compatibility (Figure S5, Supporting Information). Attempts to add appropriate low levels of water to the mixed surfactant did not result in a stable one-phase system below 400 bar at 40 °C. Previously, AOT4 was reported to have good CO2 compatibility,53 due to the incorporation of tert-butyl tipped chains, but poor w/c microemulsifying properties.33 Here the results reflect this, and for w > 0 over the investigated P, T conditions, the system was biphasic. High-Pressure Small-Angle Neutron Scattering (HPSANS) of CO2 Systems. High-pressure small-angle neutron scattering (HP-SANS) is recognized as a key diagnostic technique for detecting surfactant self-assembly structures in CO2 and, in particular, the presence of dispersed nanosized domains of surfactant and water (D2O).10 The structure can be characterized by contrasting surfactant-stabilized D2O droplets against the CO2 continuous phase. The SANS profiles from the tested samples at w = 10 are plotted in Figure 1. Scattering

Figure 1. HP-SANS profiles with different surfactants at 0.05 mol dm−3 and w = 10, T = 40 °C, P = 380 bar. Lines through data points are model fits (see Supporting Information).

profiles were consistent with the formation of reverse micelles in all cases except for AOT4. The radii were obtained by fitting analyses52 employing a model for polydisperse spheres (see Supporting Information). For di-CF2 and hybrid CF2/AOT4, the fitted radii were 18 and 14 ± 2 Å, respectively. Interestingly, the equimolar di-CF2:AOT4 mixture is shown to have essentially the same micellar size (R = 14 ± 2 Å) as the hybrid surfactant systems. In order to understand and give a simple picture for describing the behavior of surfactants at w/c interfaces, it is helpful to consider thermodynamics (eq 3). The associated free energy change for microemulsification, ΔGmic, can be expressed as a sum of the free energy for creating a new area of interface, γintΔA, and configurational entropy, ΔS, as follows:57 ΔGmic = γintΔA − T ΔS

(3)

Here, ΔA is the change in interfacial area A and γint is the interfacial tension at the w/c interface at temperature, T (K). 6302

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together in the same molecule. The intramolecular F-chain and H-chain interaction may open up the hybrid surfactant tails and then increase the contact area of water/hydrophobic tail compared with the equimolar system.45 Pure hydrocarbon AOT4 showed a higher γcmc value compared to pure di-CF2 but a value still lower than those of typical linear chain hydrocarbon surfactants, which are normally 30−40 mN m−1.46 The lower γcmc value for AOT4 is due to methylation on the chain tip that increases the hydrophobicity and lowers the surface energy.9,10,53,62 The incorporation of terminal chain low surface energy methyl groups decreases intermolecular interactions,53 which is an important factor in order to improve CO2 compatibilty.33 Further comparison of the partially fluorinated di-CF2 compound with existing data for F-chain + H-chain hybrids, e.g., F7H7, shows that both surfactant efficiencies are in the same range.19,32,43,44 Interestingly, this result demonstrates that partial fluorination of branched-tail hydrocarbon anionic surfactants, such as AOT4 to make hybrid surfactants, produces a decrease in γcmc, while at the same time maintaining or improving compatibility in CO2 versus the nonfluorinated analogue. The adsorption parameters Γcmc and Acmc (Table 2 and Figure S10, Supporting Information) obtained for all surfactants are consistent with literature data on other sulfosuccinate surfactants.7,8,46 This consistency was observed by different measurements, including surface tension7,8,31,46 and neutron reflectivity,71 showing values ranging between 62 and 72 Å2 and Γcmc ∼ 2.5 × 10−6 mol m−2. A small discrepancy in Acmc can be seen for aqueous systems with decreasing fluorination on the surfactant chains. However, the results are only just outside the experimental errors. Tools for Predicting CO2-philic Surfactant Design. One of the most outstanding properties of fluorinated surfactants, and the reason they are often found to be CO2-philic, is believed to be a “special” interaction between CO2 and F− carbon surfactant tails. Indeed, fluorinated surfactants do tend to have stronger quadrapolar and dispersion interactions with CO2 and weaker chain−chain interactions.23,25 Another consideration is that these effects essentially arise from a higher packing density of fluorosurfactants, which occupy more volume than the corresponding hydrocarbon surfactant.57,72,73 As discussed above, various researchers have used molecular simulations to elucidate the nature of fluorinated surfactants in CO2.23−25,29 The studies suggested that bulkier fluorocarbon chains provide better separation between CO2 and water and would result in lower interfacial tensions and stable w/c microemulsions.23−29,74,75 However, despite these studies, optimization of surfactant architecture to control CO2-philicity still needs to be addressed. This is partly due to the experimentally challenging nature of measurements at the water−CO2 interface. The primary step is therefore to identify suitable parameters that can be used to quantitatively predict CO2-philicity.11,26,33 The relative surfactant coverage index (Φsurf; see eq 2 and Table 2) has been calculated on the basis of chain volume, surfactant headgroup areas from a/w surface tension data, and known molecular fragment volumes (see Supporting Information).11 Significantly, fluorination is already recognized to increase surfactant chain volumes, which appear to be quite densely packed at the w/c interface.7,8,31 This is clearly demonstrated by di-CF2, for which Φsurf is 0.79. The coverage index is shown to decrease significantly with decreasing

Figure 2. Air−water surface tension γcmc vs ln(concentration) for surfactant at 25 °C. Quadratic lines fitted to the pre-cmc data are shown. The inset shows a close up of the cmc region.

shoulders that may arise if impurities are present. Quadratics fitted to the pre-cmc data were used to generate surface excesses, Γ, and limiting headgroup areas at the cmc, Acmc, using the Gibbs equation for 1:1 dissociating compounds33,63,64 shown below Γ=−

dγ 1 mRT d ln c

Acmc =

1 ΓNa

(4)

(5)

Table 2 gives values for the cmc, limiting surface tension (γcmc), and Acmc obtained from these plots. Comparisons between cmc’s are in agreement with increasing hydrophobicity on addition of fluorine into the surfactant chains. Meanwhile, the data also show changes in limiting surface tension (γcmc) with structural variations in the CO2-philic tails respectively. Among the surfactants, di-CF2 exhibited an excellent surface tension lowering ability and the surface tension of water was reduced below 23 mN m−1. This low value of γcmc is attributed to the very low cohesive energy density between fluorinated chains. Decreasing the fluorination level, e.g., in the hybrid CF2/AOT4, brought about an increase in γcmc to 23.5 mN m−1. Interestingly, in the case of the equimolar mix of di-CF2/ AOT4, the cmc and γcmc values were quite different from those found for either surfactant alone, indicating synergism after mixing the surfactants.65,66 It is postulated that at concentrations approaching the cmc, the adsorbed surfactant monolayer is densely packed, and repulsive fluorocarbon− hydrocarbon interactions could occur in the monolayer. In this case, segregation might take place owing to the imbalance of hydrocarbon−hydrocarbon and fluorocarbon−fluorocarbon attractions.67−69 The higher solubility of monomeric di-CF2 in water (cmc = 19 mmol dm−3) is likely to lead to an AOT4 (cmc = 1.10 mmol dm−3) rich monolayer at the surface of the aqueous mixed-surfactant solution.66,70 As a result, γcmc of the mixed surfactant system is not likely to reflect an adsorbed monolayer of equimolar composition. This effect is driven by interactions between surfactant molecules and the known antipathy between hydrocarbons and fluorocarbons.70 On the other hand, interfacial segregation should not occur for the hybrid CF2/AOT4 since the F-chain and H-chain are bound 6303

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fluorination level, with the hybrid CF2/AOT4 having Φsurf = 0.70. Introduction of terminal chain methyl tips is known to decrease interfacial tension of hydrocarbon AOT4 surfactants at the w/c interface.26,33 However, despite this, branched tails interpenetrate each other less effectively as a result of low CO2 compatibility, leading to an inability of AOT4 to stabilize w/c microemulsions. Therefore, the terminal methyl tip substituents on AOT4 chain give rise to Φsurf = 0.68. In comparison, commercially available AOT has a relatively low value of Φsurf (0.63). Moving to AOT4, there is a large increment in relative coverage index (AOT4 Φsurf = 0.68), presumably because of the incorporation of more voluminous trimethylated chain tips. The calculated values Φsurf and measured values of Ptrans from the above section are compared in Table 2. It is shown that the increase in Φsurf can be associated with an increase in CO2philicity. As Φsurf increases, the surfactant is able to more efficiently separate water and CO2, assisting the surfactant to stabilize w/c microemulsions at a relatively low cloud transition pressure Ptrans. The performance of similar F + H hybrid surfactants in w/c microemulsions has been studied extensively.19,32,42 It is interesting that this coverage index also works well for the F7H7 hybrid surfactant, bearing a very different structure to the sulfosuccinates and having a sulfate rather than a sulfonate headgroup. For F7H7, Φsurf = 0.86 is higher than it is for di-CF2, consistent with the enhanced performance of F7H7, which is capable of stabilizing w/c microemulsions at fairly low pressures (Ptrans = 130 bar, w = 10 at 40 °C).42 The results also show that surfactants known to stabilize w/c microemulsions have a coverage index more than 0.68. It is postulated that this is a critical region, and sufficient coverage is needed for a surfactant to have sufficient CO2-philicity to stabilize w/c microemulsion. Figure 3 shows correlations of relative surfactant coverage with surfactant effectiveness at the reference air/water interface γcmc and cloud point pressure Ptrans in CO2, along with literature data for difluorocarbon chain surfactants di-CF4, di-CF2, and di-CF1 and the related hydrocarbon surfactant di-C5SS (data from ref 11). A linear correlation between γcmc and Ptrans with Φsurf clearly can be seen (except for the CO2 inactive surfactant di-C5SS). Interestingly, the interfacial coverage index of hybrid CF2/AOT4 (Φsurf = 0.70) is consistent with increasing chain volume, as expected with added fluorination. On the other hand, there is also a trend in Φsurf for hydrocarbon surfactants from less branched AOT (Φsurf = 0.63) to highly branched alkyl AOT4 (Φsurf = 0.68). It should be noted that the linear relationship observed in Figure 3 for fluorinated surfactants cannot be directly compared with the hydrocarbon surfactants. For example, the surface tension of liquid perfluoropentane is ∼11 mN m−1,76 whereas the highly branched chain liquid alkane 2,2,4-trimethylpentane shows comparably higher interactions with a surface tension of ∼19 mN m−1.77 It is also recognized that fluorocarbon chains are more voluminous (take up more space) than an equivalent hydrocarbon counterpart.72,73 These variations in surfactant chemical structure are expected to affect interfacial packing density. Hence, the difference between these two was attributed to fluorocarbons having much lower surface energy (and cohesive energy density) than methyl groups, thereby favoring solvation by CO2.78−80 This is the reason why fluorinated surfactants outperform common hydrocarbon analogues in CO2.23 Even though the CO2 microemulsifying efficiencies of hydrocarbon surfactants are notably lower than their fully fluorinated analogues at accessible pressures, an increase in Φsurf suggests

Figure 3. Correlation between relative surfactant coverage (Φsurf), limiting aqueous surface tension (γcmc), and cloud point pressure (Ptrans). Asterisks denote data from ref 11. Circled points represent CO2-philic surfactants with fully fluorinated (red) and partially fluorinated or hydrocarbon (black) surfactants. The uncircled points are for hydrocarbon-only surfactants, which are not soluble in CO2 (non-CO2-philic) but are included for comparison purposes. The correlation is a linear relationship linking Φsurf, γcmc, and Ptrans, with higher Φsurf going hand in hand with both lower γcmc and Ptrans. For example, hybrid CF2/AOT4 Φsurf = 0.70 and this compound displays Ptrans = 289 bar at 25 °C (conditions [surf.] = 0.05 mol dm−3, w = 10, at 25 °C: note the data reported in Table 2 refer to 40 °C) and γcmc = 23.5 mN m−1. The line is a guide to the eye. The error bars represent the uncertainty of Ptrans (±40 bar).

that the introduction of a highly methylated chain into the surfactant lowers the surface energy and increases the volume in the interface more efficiently. Clearly, this newly developed hybrid CF2/AOT4 appears to have some advantages over the hydrocarbon-only AOT453 and TC1461 compounds previously reported, such as the ability to stabilize a higher amount of water (higher than w ∼ 5) at accessible cloud point pressures Ptrans. Therefore, it is now actually possible to control CO2philicity by modifying surfactant chains with fluorination and added terminal methyl groups. Although the hybrid CF2/ AOT4 surfactant is not as CO2-philic as the fully fluorinated diCF2, it is certainly more economical and environmentally friendly. Hopefully, the relative coverage index will be helpful in the future design of other CO2-philic surfactants.

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CONCLUSIONS Here efforts have been made to design more economic and environmentally friendly CO2-philic surfactants, with special attention focused on reduction of fluorine content. Although some light has been shed previously on the development of low-cost hydrocarbon CO2-philic surfactants,53,61,81,82 a remaining challenge is the limited ability to disperse water, being an example polar compound. Furthermore, multiple steps involved in synthesis of other F-chain + H-chain hybrid sulfates, e.g., F7H7,19,32,42 impose limitations to produce sufficient quantities of structural variants. By using sulfosuccinate surfactants a range of different structures can now be reliably and reproducibly synthesized, characterized, and purified, for fundamental surface chemistry (air/water, CO2/water interface) studies. It has been shown that the surfactant coverage at the interface is the most important factor affecting CO2-philicity. This can be quantified 6304

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in terms of an interface coverage index Φsurf, based on an established correlation between aqueous surface tensions of fluorinated surfactants and compatibility with CO2.7,8,11,31 In the case of hybrid CF2/AOT4, the results are consistent with this Φsurf model, where the introduction of fluorination into the chain promotes solubility in CO2 to stabilize w/c microemulsions and reverse micelles (e.g., Ptrans 340 bar, w = 10, [surf.] = 0.05 mol dm−3 at 40 °C). Interestingly, it is also shown that variations in Φsurf for hydrocarbon surfactants in CO2 follow the same trends as observed for fluorinated surfactants. The introduction of surfactant chains with highly methylated groups in AOT4 lowers surface energy and increases the coverage Φsurf at the interface more efficiently. However, the hydrocarbon-only and fluorocarbon-only systems cannot be strictly compared to the fluorinated systems, because fluorocarbons have a much lower surface energy (and cohesive energy density) than H-methyl groups, thereby favoring solvation by CO2.78,79 In particular, compared to the hydrocarbon-only analogue AOT4, the hybrid CF2/AOT4 was effective at lowering the microemulsion cloud point pressure, Ptrans, and moderate amounts of water could be dispersed as hydrated reverse micelles. The CO2-philicity enhancement of these surfactants can be related to their dual nature; on the one hand, they have terminal methyl groups on hydrocarbon moieties (AOT4-part of the molecule) and on the other a fluorocarbon tail (di-CF2 part), and both of these moieties boost CO2-philic interactions. This combination could facilitate stabilization of reverse micelles in which increased interactions between surfactant and the solvent can be achieved. Significantly, the introduction of fluorinated surfactant chains improves the surfactant coverage (increase in Φsurf), agreeing well with simulation studies that show these chains prevent penetration of CO2 molecules into water at the interface to form reverse micelles.11,23,26 Hence, this hybrid CF2/AOT4 surfactant provides an interesting option as compared to hydrocarbon-only surfactants, representing a lower limit to fluorination and a compromise structure minimizing the F content (∼19 wt %), whereas at the same time maintaining CO2-philicity. The results are beneficial for expanding industrial applications and realizing the goal of employing CO2-based surfactant solutions with environmentally friendly and economic surfactants.

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studentship. M.S. and M.H. thank the Japan Society for the Promotion of Science for a 1-year Postdoctoral Fellowship. M.S. also thanks Hirosaki University for support. We acknowledge STFC for allocation of beam time, travel and consumables grants at ISIS, and the Krϋss Surface Science Centre for provision of surface tension equipment.

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on surfactant synthesis, characterization, high-pressure SANS experiments, SANS data modeling, and surface coverage calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +44-117-9289180. Fax:+44-117-925-1295. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The EPSRC funded this work under grants EP/C523105/1, EP/F020686, EP/I018301/1 and EP/I018212/1. A.M. thanks the Ministry of Higher Education of Malaysia and Universiti Pendidikan Sultan Idris, for the provision of a Ph.D. 6305

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