ARTICLE pubs.acs.org/Langmuir
Low Fluorine Content CO2-philic Surfactants Azmi Mohamed,†,# Masanobu Sagisaka,‡ Frederic Guittard,§ Stephen Cummings,† Alison Paul,|| Sarah E. Rogers,^ Richard K. Heenan,^ Robert Dyer,z and Julian Eastoe*,† †
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan § Institut de Chimie de Nice FR 3037 CNRS, Laboratoire de Chimie des Materiaux Organiques et Metalliques, CMOM, UFR Sciences, Universite de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 02, France School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom ^ ISIS-STFC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, United Kingdom # Department of Chemistry, Faculty of Science and Mathematics, Universiti Pendidikan Sultan Idris, Tanjong Malim, Perak 35900, Malaysia z Kr€uss Surface Science Centre, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom
)
‡
bS Supporting Information ABSTRACT: The article addresses an important, and still unresolved question in the field of CO2 science and technology: what is the minimum fluorine content necessary to obtain a CO2-philic surfactant? A previous publication (Langmuir 2002, 18, 3014) suggested there should be an ideal fluorination level: for optimization of possible process applications in CO2, it is important to establish just how little F is needed to render a surfactant CO2-philic. Here, optimum chemical structures for water-in-CO2 (w/c) microemulsion stabilization are identified through a systematic study of CO2-philic surfactant design based on dichain sulfosuccinates. High pressure small-angle neutron scattering (HP-SANS) measurements of reversed micelle formation in CO2 show a clear relationship between F content and CO2 compatibility of any given surfactant. Interestingly, high F content surfactants, having lower limiting aqueous surface tensions, γcmc, also have better performance in CO2, as indicated by lower cloud point pressures, Ptrans. The results have important implications for the rational design of CO2-philic surfactants helping to identify the most economic and efficient compounds for emerging CO2 based fluid technologies.
’ INTRODUCTION Carbon dioxide is a promising green solvent as reflected by the increasing number of papers published on this topic over recent years.119 Significant efforts have been made to expand the capabilities of dense CO2 for use in applications due to beneficial properties such as nontoxicity and nonflammability. However, unlike conventional organic solvents, CO2 is a weak solvent, unable to dissolve most high molecular weight polymers or polar compounds. A major advance7,20,21 has been the development of CO2-philic surfactants to stabilize reverse micelles or water-in-CO2 (w/c) microemulsions. These self-assembly structures provide polar nanodomains in the low dielectric CO2 solvent medium, thereby facilitating the dispersion of components of otherwise very low CO2 solubility. The significance is that surfactant micelles are one of the few successful approaches for improving CO2 “solvent quality”, which is essential for the viability of many applications envisaged for CO2. It is recognized that fluorination is a key aspect for designing CO2-philic surfactants.7,9,10,20,2224 Surfactant fluorination r 2011 American Chemical Society
lowers limiting aqueous surface tensions γcmc, and as a result provides better performance and stability in CO2 as indicated by lower cloud point pressures Ptrans.9,2224 In spite of the success of fluorinated surfactants as CO2 amphiphiles, an important open question is “what is the minimum fluorine content necessary to obtain an effective CO2-philic surfactant?” This article answers the question, by describing a systematic study, using model dichain surfactants with controlled variations of fluorination. The surfactants are based on the Aerosol-OT template, an amphiphile well-known for its ability to stabilize reversed micelles in organic solvents: Tables 1 and 2 show structures of surfactants used in this work. As can be seen, all the surfactants bear pentyl carbon chains, the only difference is the controlled variation of fluorination, which can be achieved by employing different F-alcohol precursors in the synthetic diesterification Received: June 10, 2011 Revised: July 20, 2011 Published: July 22, 2011 10562
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Table 1. Surfactants Used in This Work
Table 2. Surfactants Used in This Work
reaction. In this way, it is possible to adjust the fluorine level from the maximum at di-CF4 to the minimum at di-CF1, and eliminate fluorine altogether at di-C5SS. In addition, it is possible to change the CF3 chain tips for CF2H between di-CF4 and di-HCF4 (Table 2). This has the effect of introducing a dipole moment in the chain tips, which is also expected to affect surfactant CO2 interactions.13,14 Hence, with this series it becomes possible to identify for the first time the minimum fluorine content needed to render a surfactant CO2-philic. This approach opens up the discussion as to why fluorine is so important for making a surfactant CO2-philic? Up until now, CO2-philicity has been explained by various kinds of “special interactions” between fluorinated surfactants and CO2. Since the carbon atom on CO2 is electron deficient, CO2 is therefore considered a Lewis acid. Owing to the high electronegativity of fluorine, acidbase interactions between fluorine and the carbon in CO2 might be envisaged, which has been claimed based on spectroscopic and theoretical studies.12 On the other hand, due to the high quadrupole moment and charge-separation, CO2 can act both as an electron acceptor (Lewis acid) as well as an electron donor (Lewis base). Consider the potential interactions
with carbonyl functionalities such as an acetate group: the carbon atom in CO2 can serve as a Lewis acid to the acetate carbonyl group, and oxygens in CO2 can behave as Lewis bases with electron-deficient R-hydrogens through a cooperative CHO hydrogen bond.12,17 There are other factors to consider, for example, fluorocarbons have lower cohesive energy densities than hydrocarbons, resulting in weaker chainchain interactions. Recent work also reveals that fluorinated tails, being more bulky than equivalent hydrocarbons, provide greater interfacial packing density, and allow less penetration by CO2.13,14 Despite academic research on CO2 surfactants being dominated by fluorocarbon systems, any practical applications are restricted due to cost and environmental considerations. Indeed, alternative fluoro-free CO2-philic surfactants are sorely needed in order to unlock the vast potential of CO2-based technologies. Importantly, although hydrocarbons are less expensive and less harmful to the environment, until now, only few CO2 compatible hydrocarbon surfactants1519,25 and polymers26 have been reported. For hydrocarbon surfactant design, the rationale is to increase the density of CO2-philic terminal methyl groups in the precise regions of the molecules which come into direct contact 10563
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Langmuir with CO2. Experimental efforts with this aim have been achieved by designing highly methylated branched16 and oxygenated surfactants.15 More recently, the introduction of a third highly methylated branched chain18 was shown to greatly improve CO2 compatibility over equivalent dichain surfactant analogues. Such molecular structures serve to lower surface energy and boost compatibility in CO2, which is consistent with the fractional free volume27 and cohesive density arguments26a introduced to account for CO2-philicity. Significantly, addition of a third chain also improves the space filling efficiency of the surfactant tails in the interface, which is expected to favor formation of reversed micelles by providing better separation between CO2 and water across the interface.11,27,28 Although advances have been made with H-carbon CO2 compatible surfactants, there is still a pressing need to optimize further still these systems. Of particular interest, and a point addressed here, is the effect of interfacial “space filling” as suggested by the theoretical work of Johnston27 and Rossky.13,14 Here, it has been possible to address this from a fresh experimental angle, by controlled variation of the surfactant fractional free volume caused by systematic changes in the ratio of more bulky CF2 groups to the less voluminousCH2 moieties in the surfactant chains (Tables 1 and 2). Owing to increased fragment volumes, the fractional free volume decreases with the CF2/CH2 ratio. Hence, it is possible to follow the response of CO2 compatibility and other relevant physicochemical parameters, to systematic variations in surfactant chain structures. The surfactants have been studied in CO2 by visual cloud point measurements (Ptrans) and micelle formation has been investigated using high-pressure small-angle neutron scattering (HP-SANS). With these approaches, a clear correlation between F-content and performance in CO2 has been established. In addition, to help understand further this correlation, surface tension studies have been performed at a reference airwater interface, analyses of these data provide all important adsorption parameters for assessing the effect of chemical structure on interfacial packing. The purpose of this study is to provide new insight into the molecular structure demands for efficient CO2-philic surfactants. It is now possible to identify the most F-atom efficient molecule from this series, and resolve the long-standing compromise between the expense and problems associated with high fluorination against the limits of low CO2 compatibility characteristic of low-F or fluorinefree surfactants. These findings will help guide new theoretical and experimental efforts to optimize F-free hydrocarbon analogue surfactants with high CO2 compatibility.
’ EXPERIMENTAL SECTION Chemicals. Surfactants were synthesized following the same method as described previously22,29 using the appropriate alcohol precursor (Fluorochem and Sigma Aldrich). Details can be found in Supporting Information and elsewhere.22,29 Water (resistivity 18.2MΩ cm) was taken from an Elga Pure Lab Classic system. D2O (98%D atom, Goss Scientific) was used without further purification and liquid CO2 (BOC) was used as received. Surface Tension Measurements. Surface tension measurements were made using Wilhelmy plate method on a Kr€uss K11 or K100 instrument, in the presence of chelating agent EDTA (99.5% tetrasodium salt hydrate, Sigma) to remove trace divalent metal ion impurities, as described elsewhere.2931 All glassware was cleaned using 50% nitric acid solution, then rinsed thoroughly with distilled water and dried with compressed air. The Pt plate was cleaned with pure water,
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followed by heating for few seconds in a blue Bunsen flame before each measurement. The cleanliness of the plate and glassware was monitored by measuring the surface tension of pure water. All measurements were carried out at 25 C. Recent surface tension data for the di-CF2, di-CF1 surfactant were obtained and compared to results which have been obtained over a number of years by the Bristol research group for di-CF4 and di-HCF49,22,24 and di-C5SS32 surfactants. In addition, a repeat isotherm for a newly synthesized batch of di-HCF4 was obtained in order to check reproducibility. The data analyses in terms of the Gibbs equation (see Molecular Packing at the Air/Water Interface) show good agreement with previously derived adsorption parameters. The surface excess is Γ, m = 2 for a 1:1 dissociated surfactant. Certain previous experiments9,22,24,29,32 were performed using different methods/ equipment: the Du Nouy ring (Kr€uss K11) and drop volume (Lauda TVT1) instruments. Small-Angle Neutron Scattering (SANS). Experiments were carried out using the LOQ diffractometer23 at the Rutherford Appleton Laboratory, ISIS, U.K. Protocols on LOQ were as described before.23,30 The incident wavelength range of 2.210 Å, resulting an accessible Q range of 0.0070.25 Å1, was used. The measurements gave the absolute scattering cross section I(Q) (cm1) as a function of momentum transfer Q (Å1). High pressure experiments were conducted formulating the w/c microemulsions in a high pressure cell (Thar, USA) as described elsewhere.28 Constant conditions were used for all samples in liquid CO2: P = 380 bar, T = 25 C, surfactant concentration = 0.05 mol dm3, and water-to-surfactant molar ratio w = 10. The w parameter is generally used to express the water uptake or number of water molecules solubilized by surfactant in CO2. In this case, the small background solubility of D2O in CO223 was not taken into account, as it was constant since all experiments were at constant T and P conditions. w¼
½D2 O ½surfactant
ð1Þ
Data were normalized for transmission, empty cell, solvent background, and pressure induced changes in cell volume.21,23 Experiments have been repeated on numerous occasions, and with different surfactant batches, yielding consistent results over a decade. Neutrons are scattered by short-range interactions with sample nuclei, the “scattering power” of different components being defined by a scattering length density (SLD), F (cm2). For liquid CO2, FCO2 ∼2.50 mass density 1010 cm2.33 At the experimental pressure of 380 bar, the CO2 density is ∼1.0 g cm3 so that FCO2 is approximately 2.5 1010 cm2. The structure of dispersed D2O domains can be elucidated through model fitting analysis owing to the contrast step between D2O (FD2O = 6.33 1010 cm2) surfactant shells (assuming density 1.7 g cm3 gives Fdi-CF4 = 3.23 1010 cm2, Fdi-HCF4 = 3.11 1010 cm2 and 1.4 g cm3 Fdi-CF2 = 2.19 1010 cm2) and the continuous CO2 solvent. Data have been fitted using the FISH interactive fitting program.34 On the basis of many literature reports2124 and extensive trials from a number of different possible models, it was found that a Schultz distribution of spherical particles gave the best fits and most physically reasonable parameters. The scattering law is 3 2 2 FCO2 Þ 7 2 6ϕðF Vi PðQ , Ri ÞXðRi Þ ð2Þ IðQ Þ ¼ 4 D2 O 5 Vi XðRi Þ i
∑i
∑
where ϕ, R, and V are the particle volume fraction, radius, and volume, respectively. P(Q) is the spherical form factor, and X(Ri) is the Schultz function, which is characterized by an average radius (Rav) and RMS deviation (σ) of Rav/(Z + 1)0.5 with Z being a width parameter. Since the sample composition and scattering length densities are all known, these were initially set at physically reasonable values with radii obtained by 10564
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Table 3. Properties of Surfactants in CO2 and Watera radius (2 Å surfactant b
fluorine wt %
cmc/(103 mol dm3) (0.03
γcmc / mN m1 (1
Acmc / Å2 (2
Φsurf
c
model fit
Guinier plot
di-CF4
49.9
2.0
17.7
62
0.97
70
20
19
di-CF2
35.2
19.0
22.4
65
0.79
198
17
15
di-CF1
24.3
48.8
27.0
67
0.69
compatible
-
-
di-C5SSc
0.0
50.9
31.1
68
0.51
incompatible
-
-
di-HCF4
46.9
0.74
di-HCF4b a
Ptrans / bar w = 10 w/c
12.0
26.8
70
12.5
26.4
72
185
21
18
193
19
18
Parameters derived from surface tension measurements. Ptrans is the observed reversible cloud point pressure in CO2 at 25 C. b Data collected by Paul.9 Data collected by Nave.32
preliminary Guinier analyses, R; the parameters Rc and σ/Rav were adjusted in the model fitting process.
’ RESULTS AND DISCUSSION High Pressure Small-Angle Neutron Scattering (HP-SANS). Reverse Micelle Formation. High pressure small-angle neutron
scattering (HP-SANS) is an ideal method to provide information about the relative stability, size, and shape of reverse micelles in CO2. Prior to HP-SANS experiments, preliminary phase P-T behavior was conducted, and the cloud point pressures Ptrans as a function of temperature are given in Table 3 and Supporting Information. For direct comparison of HP-SANS experiments, all systems were studied under the same conditions P = 380 bar and temperature 25 C. The transition pressure Ptrans is where the single phase (Φ) gives way to an unstable (2Φ) system is a characteristic measure of the efficiency of any CO2-philic surfactant. The lower Ptrans is at a given temperature, the more efficient is a given surfactant. It is instructive to compare data that have been collected over a number of years by different researchers. The results show good agreement with our previous measurements9,22,24 and those by Erkey and Liu.35 For example, reproducibility of samples using a newly made batch of di-HCF4 with previous data gave Ptrans values within about (8 bar. Interestingly, increasing the fluorination level in these new F-AOT analogues leads to a notable reduction of Ptrans (Table 3). It is clear that w/c microemulsion stability is extremely sensitive to subtle changes in surfactant structure and fluorination. The effect of fluorination level on microemulsifying performance of these surfactants in CO2 is demonstrated in Figure 1. Scattering data give clear evidence for the presence of water droplets in CO2 for di-CF4, di-HCF4, and di-CF2. Parameters derived from model fits are summarized in Table 3, with radii for di-CF4, di-CF2, and di-HCF4 being 20 Å, 17 Å, and 21 Å, respectively. Remarkably, the decrease in fluorination down to di-CF2 still leads to the formation of a transparent microemulsion under these experimental conditions. However, changing to di-CF1, with only one fluorinated carbon at the CF3 chain tips, does not stabilize the w = 10 w/c system, as evidenced by the flat background scattering. In fact, di-CF1 has only moderate CO2 compatibility with Ptrans = 209 bar at 35 C w = 0, i.e., it will not stabilize any microemulsified water. (see Figure S5 in Supporting Information). In order to estimate affective headgroup areas, Ah, of surfactants at the different the w/c microemulsion interfaces, Porod analysis plots and Rc vs w swelling law gradient analyses were generated (see Supporting Information). The Ah values are in a
Figure 1. High-pressure SANS from D2O droplets with different surfactants at 0.05 mol dm3 and w = 10, T = 25 C, P = 380 bar. Lines through data points are model fits, and parameters are listed in Table 3.
similar range 95120 Å2, consistent with other fluorinated sulfosuccinate surfactants in w/c microemulsions.22 These molecular areas on CO2 microemulsions are notably higher as compared the extensive data collected for various hydrocarbon AOT analogues (e.g., di-C5SS) in water-in-oil (w/o) microemulsions, and also at the airwater interface, suggesting a lower packing density of fluorinated surfactants in these CO2 systems.29,30,32,36 This has been hinted at in the past21,24,37 and a larger effective area per molecule of F-surfactants is considered one of the distinguishing features explaining why they are better in CO2 as compared to hydrocarbon analogues. This average molecular area is consistent with an effective surface excess at the CO2water interface of ΓCO2water ∼ 1.7 106 mol m2, a value that will be compared below with the same surfactants at the clean reference waterair interface. Indeed, the result for di-CF2 represents the lowest reported fluorination level for any CO2-philic surfactant, indicating that diCF2 is a critical structure, being on the borderline between CO2philic and CO2-phobic. Clearly, the surfactant chain structure and especially the fluorination level are important factors for CO2 compatibility. Taken together, the phase behavior (cloud points) and HP-SANS data analyses show a clear link between fluorination of the surfactant and ability to stabilize water-in-carbon dioxide microemulsions. This is an interesting and potentially significant observation which deserves further explanation. Ideally, it would be natural to interrogate surfactant adsorption as a 10565
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Figure 2. Airwater surface tension γ vs ln (concentration) plots for different surfactants at 25 C. Lines are quadratic fits to the pre-cmc data and linear fits to post-cmc data.
function of chemical structure at the waterCO2 interface. However, studies of this kind are fraught with experimental difficulties, and so instead, extensive studies have been performed with these surfactants to determine adsorption parameters using the easier-to-study airwater interface as a reference state. Surface Tension Measurements. Critical Micelle Concentrations (cmc). Figure 2 shows equilibrium surface tension plots expressed in terms of ln concentration (mol dm3). The equilibrium curves display clean breaks at the critical micelle concentrations (cmc) with no minima or shoulders indicating the absence of surface active impurities. A slight decay was observed above the cmc, commonly encountered with anionic surfactants due to ionic strength effects at higher concentrations. Table 3 compiles the cmc values from surface tension analyses of these CO2surfactants at the reference airwater interface: there are obvious trends in cmc as a function of fluorination. Clearly, as expected, the hydrophocitiy increases with extra CF2 groups, leading to lower cmcs. For example, di-CF4 (8CF2’s) has a cmc 10-fold lower than that of di-CF2 (4CF2), indicating that each additional CF2 decreases the cmc by a factor of ∼2 showing a weaker hydrophobic effect than seen with single chain surfactants.38 There is also a notable effect on replacing the chain terminal CF3 group for the dipolar HCF2 unit (compare cmcs of di-CF4 and diHCF4, Table 3). The trend is consistent with increased hydrophilicity after the introduction of a dipolar group.39 Absolute Surface Tensions. At low concentrations, surface tension (γ) values did not always reach γ of pure water (72.2 mN m1) due to adsorption of trace impurities at ppm levels. These impurities such as polyvalent cationic species (Mn+) are inevitably present in raw materials and glassware (see below). However, this effect was counteracted by addition of EDTA to sequester polyvalent cationic species present as shown in previous studies29,31 (see Supporting Information). The M2+ surfactant species preferentially adsorb to form more condensed monolayers than the Na+ form, giving rise to a decrease in the pre-cmc tension values; this in turn affects the Gibbs isotherm analysis and the resulting limiting headgroup area Acmc.29,31 Surfactant effectiveness can be expressed in terms of the limiting surface tension (γcmc). Fluorinated surfactants typically possess an ability to lower the surface tension of water to around
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Figure 3. Adsorption isotherms for various surfactants derived from the data given in Figure 2.
1525 mN m1; levels that are rarely achieved with hydrocarbon surfactants. Among the tested surfactants, di-CF4 has the lowest γcmc = 17.7 mN m1. This effectiveness may be attributed to the presence of ∼50 wt % fluorine atoms in the surfactant chains. However, γcmc shows an increasing trend with decreasing fluorine content as shown in Table 3. Interestingly, for di-CF2 and di-CF1, the decrease of 30% in fluorine content feeds through to a change in γcmc of approximately +5 mN m1. Moving now to the fluorinefree hydrocarbon di-C5SS results in further increases in γcmc, reflecting higher surface energies of CH2 compared to CF2 groups.40 However, there are more significant differences in γcmc between di-CF4 and the di-HCF4 as discussed previously.39 The introduction of a single terminal H at the fluorochain tip is expected to introduce permanent a dipole moment, for example, in CF3CF2H μ = 1.54.41 Molecular Packing at the AirWater Interface. To study structurally related surfactants, a useful way to compare surface properties is by considering packing efficiencies in monolayers, by assessing surface excesses, Γ, and limiting headgroup areas at the cmc Acmc. These parameters have been derived from surface tension measurements by fitting the pre-cmc γln c curve to quadratics.28,39,42 By use of the Gibbs equation, adsorption isotherms were determined using the accepted prefactor of m = 2 for a 1:1 ionic surfactant.22,29,43 Γ¼
1 dγ mRT d ln c
ð3Þ
The obtained parameters are listed in Table 3 and isotherms are plotted in Figure 3. The trends in Γ broadly follow expectations based on increased surface activity as a function of fluorination (Figure 3). The trend in increasing surfactant adsorption at the airwater interface follows the increase in fluorination and hence hydrophobicity. The obtained Acmc values clearly agree well with existing neutron reflectivity literature data.39 Comparing molecular areas derived by HP-SANS at the waterCO2 interface with values for identical surfactants at the reference airwater interface points to differences between the adsorption scenarios. For the same surfactant, molecular areas are larger (∼100 Å2), and so adsorptions lower (ΓCO2water ∼1.7 106 mol m2), compared to at the airwater interface (∼67 Å2, Γcmc ∼2.5 106 mol m2). 10566
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Figure 4. Visual representation of fluorosurfactants at an airwater interface, depicting the different fragment and interfacial volumes used in the calculation of the surface coverage Φsurf. The measured surfactant molecular volume is Vmeas, and the calculated volume is Vcal based on the summation of fragments and the free space in the interface Vfree.
The interfacial tension of the pure waterCO2 (w-c) interface is typically 2030 N m1, being much lower than the pure a-w interface. Hence, it may take less surfactant at the w-c interface to lower interfacial tension, and so a larger effective area per molecule is observed. However, the variations in Acmc for aqueous systems are small enough that results within a series are only just outside the experimental errors. Fluorine Coverage at the AirWater Interface. As discussed in the previous section, it is clear that, upon increasing fluorination from hydrogenated di-C5SS to the fully fluorinated di-CF4 variant, there is a marked increase in effectiveness for stabilizing the waterCO2 interface. Despite this, the correlation between surfactant chemical structure and CO2-philicity remains controversial and is not completely understood. Hence, molecular modeling has been employed to explore the fundamental nature of fluorocarbon and hydrocarbon interactions with CO2.11,44 The most recent molecular simulation studies13,14 suggest that the introduction of in-chain fluorine provides stronger interactions with CO2 via quadrupolar and dispersion interactions. Furthermore, fluorinated chains also have weaker chainchain interactions than hydrocarbon counterparts, down to a weak repulsion electrostatic in origin. These properties conspire together to give fluorinated surfactant reversed micelles better solvation by CO2 preventing intermicellar aggregation, a lower surfactant packing interfacial density, and weaker attractive intermicellar interactions.11,13,14,26,27,4448 The differences in surfactant chemical structure are also expected to give rise to effects on interfacial packing density. This is obvious when it is recognized that pure fluorocarbon chains are themselves more voluminous (take up more space) than an equivalent hydrocarbon counterpart. For example, the limiting close-packed cross-sectional area in a condensed monolayer is 26 Å249,50 for linear chain fluorocarbons, but 18 Å243 for the hydrocarbon analogue. One way to characterize the packing density of these adsorbed layers is to consider the fractional coverage of the interface by fluorocarbon chains; here, a reference airwater interface is considered rather than (the more experimentally challenging) CO2water interface for reasons explained above. In fact, rough approximations of interfacial coverage have been previously reported;24,39 however, in those previous studies the surfactant tails were assumed in all cases to be fully fluorinated chains. However, here (Tables 1 and 2) it can be seen that chains are only even partially fluorinated, meaning in the hydrophobic (CO2-philic) part of any monolayer only a fraction of the volume is occupied by F-carbon fragments, the remainder being H-carbon. Hence, to account for the strong effect of F/H ratio on the CO2 philicity it becomes necessary to consider how the fluorination level affects the fractional coverage of the interface.
An index to assess relative surfactant coverage, Φsurf is introduced: Φsurf ¼
Vcal Vmeas
ð4Þ
where Vcal is the total physical volume of surfactant molecular fragments (values taken from literature5153), and Vmeas is that total volume occupied by a molecule at the reference airwater interface, calculated using experimental values: Vmeas ¼ Acmc τ
ð5Þ
Surfactant headgroup areas, Acmc are given above,9,22,24 and τ is an interfacial thickness. Assuming a uniform layer, the Tanford equation53 was used to provide a measure of τ. Therefore, the free volume Vfree, which is the part of the interfacial layer not occupied by molecular fragments is simply Vfree = Vmeas Vcal. These dimensions and volumes are depicted in Figure 4, and in Supporting Information, the calculations are discussed and justified in more detail. As previously suggested by simulations,13,14 CO2 compatibility is dominated by surfactant chains when all other factors are equal (i.e., as here for headgroup and number of tail groups). The hydrocarbon di-C5SS has relatively low Φsurf (0.51). Introducing the first fluorinated carbon with di-CF1 raises Φsurf from 0.51 (di-C5SS) to 0.69. Next, on further fluorination Φsurf increases sharply from 0.79 for di-CF2 to 0.97 for the di-CF4. This figure means the interface is essentially fully covered by surfactant chains, with only 3% empty space. Comparing di-CF4 (Φsurf = 0.97) and di-HCF4 (Φsurf = 0.74) shows the dramatic effect of introducing permanent dipoles to the chain tips: the surfactants space out at the interface creating more free volume and hence covering the interface less efficiently. The importance of chaintip chemistry on surfactant performance in aqueous and CO2 systems has been highlighted previously.39 Figure 5 shows that the trend in relative surfactant coverage can be correlated with surfactant effectiveness γcmc and phase transition pressures in CO2. Lower Ptrans is consistent with enhanced in CO2-philicity. Apart from di-C5SS, a linear correlation between γcmc and Ptrans with Φsurf was observed. It is also clear that an “optimum” region of CO2-philicity exists, as a function of both Φsurf and fluorination. In these regions, spherical microemulsion droplets were detected, even for the low F-content di-CF2. It is postulated that this is the minimum level of fluorination needed for a surfactant to have sufficient CO2-philicity to stabilize water-in-CO2 microemulsions. A reason for the absence of aggregation of di-CF1 may well be the lack of a CF bond dipole moment to interact with the quadrupole of CO2.13 Comparing the values of Φsurf, it would seem that 10567
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Figure 5. Relationship between relative surfactant coverage, Φsurf, limiting aqueous surface tension γcmc and cloud point pressure, Ptrans, for CO2-philic surfactants. No Ptrans present for di-C5SS. There is a linear correlation linking Φsurf, γcmc, and Ptrans, in that higher Φsurf goes hand in hand with both lower γcmc and Ptrans. For example, for di-CF2 Φsurf = 0.79 is and this compound displays Pc = 198 bar and γcmc = 22.4 mN m1. The line is a guide to the eye.
the higher the interfacial coverage, the greater the CO2-philicity. Interestingly, the interface coverage index introduced in this work allows prediction of CO2-philicity, representing a remarkably simple principle to guide the design of CO2-philic surfactants.
’ CONCLUSIONS With regard to CO2-based processing and handling technologies, research into optimizing CO2-philic surfactant design is especially important. Evidently, fluorination is a crucial factor influencing surfactant compatibility with CO2. For these particular molecules, there may specific interactions between CO2 and the surfactant tails which are likely at the heart of increased stability of w/c systems. This argument was invoked in numerous reports of simulation studies by many groups in the field for over 15 years.11,13,14,27,4448 Most of previous studies from different research groups focused have been piecemeal, limiting any chance to draw quantitative conclusions regarding the effect of surfactant structure on CO2philicty. An early attempt was reported,9 which highlighted a correlation between surface tension of aqueous solutions and the performance of the compound for stabilization of w/c microemulsions. However, until now there have been no systematic “atomby-atom” studies such as reported here. This work represents the first systematic study on optimization fluorination levels for attaining CO2-philic surfactants. A clear relationship between limiting aqueous-phase surface tension of a surfactant at its critical micelle concentration γcmc and its performance in CO2, as measured by cloud point phase transition Ptrans, has been demonstrated providing new insight into the molecular structure requirements for designing CO2-philic surfactants. Although it is only a gross approximation for the w-c interface, these easy to perform studies on aqueous systems provide a scale for comparing CO2-philicity of different structures. The results presented here show surfactant coverage at the interface is the most important factor affecting CO2-philicity. The surfactant coverage, Φsurf, can be readily quantified using
surface tension measurements of aqueous systems showing a direct link between Φsurf and CO2-philicity. In addition, a minimum fluorination level has been identified for generation of CO2 microemulsions. This represents a chemical structure and F/H composition giving rise to optimum interfacial packing density needed to promote favorable interactions with CO2. The coverage values can then be used to estimate CO2-philicity, which could subsequently be used to predict whether or not a newly designed compound could be sufficiently CO2-philic, prior to embarking on synthesis. Interestingly, high effective coverage is expected to favor formation of reverse micelles and microemulsions by providing better separation between CO2 and water across the interface.11,27 The results also rationalize why fluorosurfactants are better as compared to hydrocarbon surfactant analogues for applications in CO2. It can be seen that fluorinated surfactants still perform best for potential applications in CO2, but of course there remain environmental and economic concerns around fluorinated molecules. Another important issue in surfactant design is the ability to stabilize w/c microemulsions under relatively moderate pressure and temperature conditions. This is important for scale-up of CO2 applications where processes are much simplified when liquid CO2 is used at accessible pressures. Being mindful of these limitations of fluorinated surfactants, endeavors have been made to identify more economic and environmentally friendly surfactants that support w/c microemulsions at low P-T conditions.15,16,18 Here, special attention has been paid to minimizing the F content, while retaining an acceptable level of activity in CO2. The results and ideas presented here should help in the quest for effective fluorinefree CO2-philic surfactants.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional details of surfactant synthesis, characterization, high pressure SANS experiments, SANS data modeling and surface coverage calculations.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT A.M .thanks the Ministry of Higher Education of Malaysia and Universiti Pendidikan Sultan Idris, for the provision of a PhD studentship. M.S. thanks the Japan Society for the Promotion of Science for a 1-year Fellowship and Hirosaki University for support. S.C. thanks the EPSRC, STFC and School of Chemistry at the University of Bristol for the provision of PhD scholarship. We also acknowledge STFC for allocation of beam time, travel and consumables grants at ISIS and the Kr€uss Surface Science Centre for provision of surface tension equipment. The EPSRC is thanked for provision of funding under grants EP/C523105/1, EP/F020686 and EP/I018301/1. ’ REFERENCES (1) Weibel, G. L.; Ober, C. K. Microelectron. Eng. 2003, 65, 145. ^ . A.; Dos Santos Girardi, J.; Mossi, A.; Jacques, (2) Esmelindro, A R. A.; Dariva, C. J. Agric. Food Chem. 2004, 52, 1990. (3) Dewees, T. G.; Knafelc, F. M.; Mitchell, J. D.; Taylor, R. G.; Iliff, R. J.; Carty, D. T.; Latham, J. R.; Lipton, T. M. U. S. Patent 5,267,455, Dec. 7, 1993. (4) Shi, C.; Huang, Z.; Beckman, E. J.; Enick, R. M.; Kim, S.-Y.; Curran, D. P. Ind. Eng. Chem. Res. 2001, 40, 908. (5) Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990, 3, 51. (6) DeSimone, J. M. Science 2002, 297, 799. (7) Hoefling, T. A.; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991, 95, 7127. (8) Sagisaka, M.; Fujii, T.; Ozaki, Y.; Yoda, S.; Takebayashi, Y.; Kondo, Y.; Yoshino, N.; Sakai, H.; Abe, M.; Otake, K. Langmuir 2004, 20, 2560. (9) Eastoe, J.; Paul, A.; Downer, A.; Steytler, D. C.; Rumsey, E. Langmuir 2002, 18, 3014. (10) Heitz, M. P.; Carlier, C.; deGrazia, J.; Harrison, K. L.; Johnston, K. P.; Randolph, T. W.; Bright, F. V. J. Phys. Chem. B 1997, 101, 6707. (11) Stone, M. T.; da Rocha, S. R. P.; Rossky, P. J.; Johnston, K. P. J. Phy. Chem. B 2003, 107, 10185. (12) Raveendran, P.; Wallen, S. L. J. Phys. Chem. B 2003, 107, 1473. (13) Dalvi, V. H.; Srinivasan, V.; Rossky, P. J. J. Phys. Chem. C 2010, 114, 15553. (14) Dalvi, V. H.; Srinivasan, V.; Rossky, P. J. J. Phys. Chem. C 2010, 114, 15562. (15) Eastoe, J.; Gold, S.; Rogers, S.; Wyatt, P.; Steytler, D. C.; Gurgel, A.; Heenan, R. K.; Fan, X.; Beckman, E. J.; Enick, R. M. Angew. Chem., Int. Ed. 2006, 118, 3757. (16) 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. (17) Raveendran, P.; Wallen, S. L. J. Am. Chem. Soc. 2002, 124, 7274. (18) Hollamby, M. J.; Trickett, K.; 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. (19) Liu, J.; Han, B.; Li, G.; Zhang, X.; He, J.; Liu, Z. Langmuir 2001, 17, 8040. (20) Harrison, K.; Goveas, J.; Johnston, K. P.; O’Rear, E. A. Langmuir 1994, 10, 3536. (21) 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.
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