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Phase behavior and emulsion formation of novel fluoroether amphiphiles in carbon dioxide
RUIDPU[ EQUIUBRIA ELSEVIER
Fluid Phase Equilibria 128 (1997) 199-219
Phase behavior and emulsion formation of novel fluoroether amphiphiles in carbon dioxide Edith J. Singley *, Wei Liu, Eric J. B e c k m a n University of Pittsburgh, Chemical Engineering Department, 1249 Benedum Hall, Pittsburgh PA 15261, USA
Received 6 February 1996; accepted 16 July 1996
Abstract
The limitations of carbon dioxide as a processing fluid are primarily attributed to its insolubility of polar compounds. The development of new CO2-philic amphiphiles could resolve the solubility problem. A new series of fluoroether amphiphiles have been developed to address the solubility problem. The development of novel fluoroether amphiphiles has been completed for sorbitol ester, sulfates and sulfonates. The primary objective was to design and synthesize materials that have strong polar hydrophilic heads and highly CO2-philic tails. All of the amphiphiles are soluble in carbon dioxide at moderate pressures and have the capability of forming WI, WII and WIII type emulsions in CO 2. The degree of solubility is heavily influenced by amphiphilic structure including chain length and branching. An examination of pressure effect on emulsion behavior shows that the high compressibility of carbon dioxide enables WI -~ WIII ~ WII phase transitions which are similar to those found with increased electrolyte concentrations. Keywords: Surfactants; Carbon dioxide; Emulsions; High pressure
1. Introduction
Carbon dioxide is one of the only organic solvents that is not classified as a VOC (volatile organic chemical) by the United States Environmental Protection Agency. Carbon dioxide is inexpensive, exhibits low toxicity and is nonflammable. However, a serious obstacle to employing carbon dioxide is its inability to solvate highly polar compounds [1], which may be due to its low dielectric constant (1.5-1.6 range) and its low polarizability per unit volume. The solubility problem could be alleviated by the addition of surfactants, a method which has been successful in enhancing the solubility of polar compounds in non-polar compressible solvents such as propane, ethane and xenon. However, * Corresponding author. 0378-3812/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0378-3812(96)03167-6
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extending these efforts t o C O 2 has been difficult in that conventional alkyl-functional ionic amphiphiles exhibit poor to negligible solubility in CO 2 at moderate pressures [2]. Although nonionic surfactants can exhibit reasonable solubilities in carbon dioxide at moderate pressures, solvation of water and polar compounds has proven difficult [3]. These results appear anomalous given that the solvating capability of carbon dioxide has been previously compared to that of hexane. However, structure solubility relationships in the literature [4-7] suggest that the solvent properties of carbon are quite different form that of ordinary alkanes. Johnston et al., [8] showed that the solubility parameter for CO 2 is inflated by approximately 20% due to carbon dioxide's strong quadrupole moment. As a result, one would anticipate carbon dioxide to be a good solvent for compounds with low solubility parameters (below (6.5 cal/cm3) °5) which include siloxanes, perfluorinated ethers and alkanes. To date several amphiphiles have been designed and synthesized which are not only functional but highly CO2-philic [8,9]. The design strategy is based on the development of molecules with hydrophilic head groups that readily interact with specific solutes (water, proteins, polar molecules), while the hydrophobic tail interacts favorably with CO 2. There are several factors which contribute to CO2-philicity; low solubility parameter, low polarizability and electron donor capability (considering the fact that CO2 is a weak Lewis acid). A variety of studies indicate that compounds which contain fluoroether, fluoroalkyl, or dimethyl siloxane moieties show strong solubility in carbon dioxide [3,7,9]. For example, Hoefling [10] and colleagues generated surfactants by replacing the CO 2 insoluble hydrocarbon tails with perfluoroalkylpolyether carboxylates. The CO 2 soluble surfactants showed complete miscibility at pressures as low as 16 MPa. Johnston [8] and colleagues have shown that replacement of an alkyl tail with a fluoralkyl moiety in a twin-tail sulfate surfactant allows formation of micelles which absorb significant amounts of water in carbon dioxide as pressure increases. Johnston has recently shown that a fluoroether carboxylate also forms micelles in carbon dioxide, allowing solubilization of a protein [11]. Further, DeSimone et al. [12] found that amphiphiles with perfluorinated alkyl chains form aggregates in supercritical carbon dioxide. Small angle X-ray scattering (SAXS) was used to characterize aggregation. The type and degree of aggregation was a function of amphiphilic structure. The effect of pressure on multiphase microemulsions has not been investigated thoroughly due to the complexity of experiments required. A comprehensive review of microemulsions in compressible fluids by Bartscherer [13] and colleagues identified several studies performed on ionic surfactants which do give an indication of phase behavior trends. In 1948, P.A. Winsor published the first classification of phase behavior which accounted for the various phenomenon exhibited in a ternary Oil-H 20-Amphiphile system [19]. Winsor I, lI and III systems (i.e. WI, WII and WIII) are described as follows: WI - - surfactant resides primarily in the aqueous phase; normal micelles in equilibrium with excess oil WII surfactant resides primarily in the oil phase; reverse micelles in equilibrium with excess oil WIII surfactant resides primarily in a middle phase in equilibrium with the aqueous and oil phases. Fotland's study [14] of the 5-component system (sodium dodecyl sulfate (SDS), 1-butanol, cyclohexane, water and sodium chloride) found that mixtures which display WIII behavior at atmospheric conditions exhibit WI behavior with increased pressures (up to 360 bar). The increase in pressure enabled increased water solubilization which eventually leads to mass transfer across the -
-
-
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201
lower interface. Further pressure increases caused precipitation of the solid phase. The decrease in oil solubilization may be due to the lower compressibility of cyclohexane compared to other oils. Kahlweit and colleagues [15] studied a 4-component mixture at various salt concentrations. Overall, the opposite effects of pressure were predicted for ionic and nonionic amphiphiles. Ionic amphiphiles should move from the oil rich to the aqueous phase with increased pressure, unlike nonionic surfactants which should move from the aqueous to the oil rich phase. Beckman and Smith [16] studied the pressure effect on phase behavior of a 3-component mixture, where propylene was the organic component. An increase in pressure on a 3 0 / 7 0 water/propylene system showed phase transitions from WI ~ WII --* WIII, consistent with the observations of Fotland, whereas a 7 0 / 3 0 water/propylene system showed a WI--* WIII ~ WII phase transition. Therefore, as the pressure increased, either WIII or WII behavior occurred depending on surfactant concentration and propyl e n e / w a t e r ratio. McFann and Johnston [17] assessed the phase behavior of (bis(2-ethylhexyl) sodium sulfo-succinate (AOT))-alkane-brine-water systems for a wide pressure range. A close look at alkanes from ethane to dodecane illustrated the effect of pressure on surfactant hydrophilicity. Water-in-oil (WII) microemulsions were converted to middle phase (WIII) microemulsions and then to oil-in-water (WI) for propane systems by reducing pressure. The results indicate that molecular interactions at the surfactant interface may be controlled by simply adjusting pressure. Peck and Johnston used theoretical modeling to predict the pressure effect on the phase behavior of A O T / P r o p a n e / B r i n e Water-in-Oil microemulsions [18]. Phase transitions occurred by varying pressure. The model was based on two important parameters: micelle-micelle interactions and intramicellar interfacial interactions that determine the natural curvature of the microemulsion interface. The R ratio [20], which compares the balance of the individual cohesive energies of the functional groups present at the interface, can be used to explain why micelles (emulsions) will be formed in oil or water. R=
Aco - Aoo - AlL Acw -- Aww - - Ahh
where Aco is the lipophile-oil interaction, Aoo is the oil-oil interaction, A l l is the lipophile-lipophile interaction, Acw is the hydrophile-water interaction, Aww is the water-water interaction and Ahh is the hydrophile-hydrophile interaction. The cohesive energy Acw promotes the miscibility of the amphiphilic compound with the aqueous region and Aco promotes miscibility with the oil region. Both Ahh and Aww oppose miscibility with water and likewise, both Aoo and A H oppose miscibility with oil. Therefore R < 1 yields WI (normal micelles in water), R > 1 yields WlI (reverse micelles in oil) and R = 1 yields WIII (three phases). According to Schechter, an increase in pressure should theoretically decrease R due to the enhanced oil-oil interactions (an increase in Aoo). So in a conventional liquid system WlI ~ Will ~ WI phase transitions as seen by Fotland should be observed [20]. If the oil is very compressible, for instance C02, the pressure effects on Aco might dominate, which would result in R > 1 and therefore WI ~ WIII ~ WII transitions as seen by Beckman and Smith should occur. In this paper the synthesis, phase behavior and emulsion formation of a series of fluoroether amphiphiles in carbon dioxide is discussed. The effect of structure on surfactant solubility in CO 2 is investigated along with the impact of pressure on emulsion formation in CO2/water mixtures.
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2. Experimental Carboxylic-acid capped oligomers of hexafluoropropylene oxide (Krytox, DuPont) were purchased from Miller-Stephenson Chemical Co. and used as received. The series of Krytox fluids are available in three molecular weights; low MW (2500), medium MW (5000) and high MW (7500). Bone dry carbon dioxide and nitrogen (99.9 + / p u r e ) were purchased from Liquid Carbonic Corp. and used as received. All other materials were obtained from Aldrich; all solvents were dried with molecular sieves prior to use. Characterization of products was performed using an F T - I R spectrophotometer (Mattson Polaris) and a 300 MHz NMR (Bruker).
2.1. Synthesis of acid chloride Acid chlorides were generated for low, medium and high molecular weight poly(perfluoropropylene oxides). Typically, 100 g (0.04 moles) of carboxylic acid terminated poly(perfluoropropylene oxide), low molecular weight, ( M n = 2500) and 100 g of methyl- perfluorocyclohexane were placed in a 250 ml 3-neck round bottom flask equipped with condenser and thermometer. The mixture was placed under a nitrogen blanket and then heated to reflux (75°C) while stirring. Dimethylformamide (3.2 ml, 0.04 moles) and thionyl chloride (6 ml, 0.06 moles) were slowly added. The reaction mixture was stirred for 8 h. The solvent and residual thionyl chloride were removed under vacuum using a rotary evaporator (Brinkman). The product was then placed in a separatory funnel to allow the removal of residual DMF. F-I'-IR results show the shift of the carbonyl peak from 1776 c m - t to 1809 cm-J and I H NMR shows the disappearance of the acid proton IH, (9.17 ppm). The above synthesis was completed for the medium and high molecular weight oligomers using the appropriate amount of reactants.
2.2. Synthesis single-tailed surfactants 2.2.1. Fluoroether sorbitol esters The sorbitol ester of the fluoroether acid was prepared via reaction of the acid chloride with sorbitol (Fig. 1). Typically, 10 g (4 mmoles) of acid chloride (2500 MW) were dissolved in 150 ml of 1,1,3-trichlorotrifluoroethane and placed in an addition funnel. A 250 ml 3-neck flask with condenser and addition funnel was charged with 0.723 g (4 mmoles) of sorbitol and 25 ml of cyclopentanone. The mixture was heated while stirring to 80°C to allow the complete dissolution of sorbitol. Next, polyDMAP, a polymer supported dialkylaminopyridine catalyst, was added in excess to scavenge HCI. The system was placed under a nitrogen blanket and the acid chloride solution was slowly added (dropwise) to the sorbitol mixture. The mixture was stirred at 60°C for 3 days. The polyDMAP was removed by filtration and the solvent removal was completed under vacuum. The product was placed in a separatory funnel for further removal of residual solvents. The final product was washed twice with ethyl ether to ensure total removal of residual cyclopentanone. The fluoroether sorbitol esters are mixtures of analogous molecules due to the large number of hydroxyl groups. However, it is expected that the primary hydroxyls would be the most reactive. The results of the F T - I R show a carbonyl shift from 1809 c m - 1 to 1790 c m - J (ester) and C - H stretching at 2995 c m - ~ and hydroxyls present at 3400 c m - J .
E.I. Singley et al./ Fluid Phase Equilibria 128 (1997) 199-219
O O R--~OH + CdCI
O -~ R---~CI
+
SO 2
+
203
HCI
Synthesisof KrytoxAcidChloride CH2OH
R3CI
+
HO~--OH cP/cFo~cc~ HO'-~--H D, H-~--OH
CH20--~--R HO-@--OH
PolyObMAP)
H--~--OH CH2OH
H--~--OH CH2OH + other isomers
Synthesisof Fluoroether-functionalSorbitolEster Fig. 1. Synthesis of acid chloride and fluoroether sorbitol ester.
2.2.2. Fluoroether sulfate surfactants The fluoroether sulfates were generated via reaction of a hydroxyethyl ester of the fluoroether carboxylic acid followed by reaction of the terminal hydroxy group with chlorosulfonic acid. In a typical reaction, 25 g of the fluoroether acid chloride (0.01) moles were dissolved in 100 ml of l,l,2-trifluorotrichloroethane and placed in an addition funnel. A 3-neck 250 ml flask equipped with condenser and the addition funnel was charged with 1.24 g (0.2 moles) of ethylene glycol (99 + %). An excess of polyDMAP was added and the system was placed under a nitrogen blanket. The acid chloride solution was added dropwise, after which the solution was stirred for 24 h at room temperature. The polyDMAP was removed by filtration and the solvents were removed using a rotary evaporator. The concentrated product was then washed with acetone to remove residual ethylene glycol. The end product was vacuum dried for 24 hours. Characterization of the product yields [FT-IR, -COOR, 1790 cm-1, -OH, 3300 cm-~: ]H NMR, -OCH2-C, 4.29 ppm, 2H; -C-CH2-O, 3.69 ppm, 2H; -OH, 5.02 ppm, 1H]. The hydroxyethyl fluoroether ester is reacted with chlorosulfonic acid to yield the fluoroether sulfate. A 3-neck flask equipped condenser was charged with 25 g (0.01 mole) of the hydroxyethl ester and 30 ml of 1,1,2-trifluorotrichloroethane. The system is placed under a nitrogen blanket and stirred vigorously at room temperature. Chlorosulfonic acid (1.17 g (0.01 mole) was added very slowly to the ester solution. Throughout the reaction small HC1 bubbles formed and were swept away by a steady flow of nitrogen. After 1.5 h the solvent and residual HC1 were removed under vacuum at 50°C. The sulfonic acid was neutralized via reaction with an excess of sodium trimethylsilanolate NaOSi(CH3) 3, (1.0 M in THF). The final product was washed several times with THF to remove residual reactant.
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Characterization of product yields [IH NMR, 10.59 ppm, 1H, sulfonic acid proton, -C(O)OCH 2, 4.5 ppm, 2H; and -CH2OSO 3, 4.58 ppm, 2HI.
2.2.3. Fluoroether sulfonate surfactants The sulfonate was formed by charging a 3-neck 250 ml flask with 0.244 g (0.002 moles) of phenethyl alcohol (99%) and 100 ml of a mixture of THF and 1,1,2-trifluorotrichloroethane. A mixture of 15 g (0.002 moles) of acid chloride and 100 ml of 1,1,2-trifluorotrichloroethane was placed in an addition funnel. The reaction system was placed under a nitrogen blanket with an excess of polyDMAP added. The acid chloride mixture was slowly added to flask and the solution was stirred for 24 h at 50°C. The intermediate product was vacuum evaporated to remove solvent. Characterization shows [ FT-IR; -COOR, 1786 cm-1; NMR, C6H5-,7.26 ppm, 5H; -CH2-benzene ring 3.03 ppm, 2H; -CH2-O, 4.55 ppm, 2H]. The purified product (15 g) was placed in a 3-neck flask equipped with condenser and addition funnel. A total of 100 ml of trifluorotrichloroethane was added to the flask to dissolve the ester. Chlorosulfonic acid, 0.233 g (0.002 moles) was then added slowly to the mixture. The solution was stirred for 2 h. The final product was neutralized by sodium trimethylsilanolate. Characterization shows [ F F - I R , - C O O R , 1786 cm - t , IH N M R : - S O 3 H , 10.26 ppm, 1H; C6H4-, 7.14 ppm, 4H; -CH20-, 4.43 ppm, 2H;-CH2-benzene, 2.91 ppm, 2H].
3. Synthesis of multi-tailed surfactants The multi-tailed fluoroether surfactants were generated using the synthesis outlined above, albeit with different molar ratios. The acid chloride ratios were increased two and three fold to yield the twin and the triple tailed surfactants, The characterization of products were performed similarly to the single tailed products. The fluoroether sulfate and fluoroether sulfonate products are shown in Fig. 2.
3.1. Phase behavior of surfactants in CO 2 Phase behavior studies of fluoroether surfactant were conducted using a variable volume high pressure view cell (D.B. Robinson and Associates) as shown in Fig. 3. In the series of experiments conducted, a known amount of fluoroether sorbitol ester was loaded into the quartz tube sample cell along with 5 steel ball bearings. The tube is then sealed inside the steel housing. A known volume of CO 2 at its vapor pressure is injected into the cell using one of the two Ruska syringe pumps. A floating piston located in the quartz cell separates the sample from the pressure transmitting fluid (silicone oil). The piston was raised by injecting additional oil via the second Ruska pump. This action was used to increase pressure to a point where a clear single phase exists indicating surfactant solubility. Complete mixing was achieved by rocking the entire cell/steel housing unit thus allowing free movement of the ball bearings. The cell was heated to 306 K and the system was allowed to reach equilibrium. The pressure was then lowered via silicone oil removal until the first signs of turbidity appeared, which indicates phase transition. This procedure was repeated several times to establish the cloud point. The system pressure was then reduced to vapor pressure to enable the addition of a fixed amount of carbon dioxide. A new cloud point was measured and the data collection continued until a cloud point curve could be generated.
EJ. Singley et d/Fluid
Phase Equilihricc 128 (1997) 199-219
CFfCFd,p-LH
fluoroethcr Acid
Single Tail Fluoroether Sulfate
Twin Tail Fluorocther Sulfate
CF~-CF241F-O-CF2CF
4CH~CH+-+SO~N~
c:F3 Single Tail Fluorocther Sulfonate
Fig. 2. Structures of sulfate and sulfonate fluoroether
surfactants.
Oil Pump
Fig. 3. High pressure volume cell (D.B. Robinson).
205
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EJ. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
Surge Tank
C02 Booster
CO2 Cylinder
High Pressure Cell
Metering Pump Fig. 4. High pressure vessel for emulsion testing.
3.2. E m u l s i o n f o r m a t i o n
Emulsions were formed in either the aforementioned D.B. Robinson cell or in a smaller high pressure device equipped with a three blade stirrer, as shown in Fig. 4. Emulsification in the D.B. Robinson cell allows the pressure to be varied at fixed mass compositions. For experiments in the Robinson apparatus, a known amount of sorbitol ester was loaded into the quartz cell and 65 ml (constant volume) of deionized water was added followed by a fixed amount of CO 2. The pressure was increased to 2000 psi and the cell was rocked for 60 min to obtain adequate mixing. After mixing, the emulsion phase formed in either the aqueous phase, CO 2 phase or at the interface depending on pressure and surfactant concentration. The pressure was varied from t 3.8 MPa to 41.4 MPa for each CO 2 volume change. The CO 2 volume was varied from 15 ml to 65 ml by reducing pressure and injecting CO2 via the Ruska syringe pump. The phase volumes were measured using a cathetometer. The sulfate and sulfonate emulsions were formed in buffered solutions at a fixed pressure. The buffered solution was made from 0.025 moles of sodium phosphate monobasic NaH2PO 4 (from Sigma, reagent grade), and 0.025 moles of sodium phosphate dibasic, Na2HPO 4 (from Sigma, ACS reagent) dissolved in 4.4 moles of distilled water. A known amount of surfactant and buffer were loaded into the 36 ml cell, then CO 2 was slowly introduced into the cell until the specific pressure (21 MPa) was achieved. The solution was agitated vigorously for 5 rain, then allowed to settle.
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207
4. Results and discussion 4.1. Phase behavior
The phase behavior of amphiphiles in organic solvents are derived from the interactions between head groups, tail groups and solvent as described by Bourrel and Schechter [20]. Therefore a shift in the cloud point curve may be a result of chain length, number of tails, surfactant geometry (branched vs. linear) a n d / o r the polarity of the head group. Since the dissolving process is strongly influenced by the interaction between the hydrophobic tails of the surfactants and carbon dioxide, one can surmise that the tail length and number will affect surfactant solubility more so than other variables. Figs. 5 - 7 illustrate the effect of chain length on the cloud points for the fluoroether functional sorbitol ester. The fluoroether sorbitol surfactant shows relatively high solubility in carbon dioxide at moderate pressures, unlike analogous alkyl sorbitan surfactants as shown by Consani and Smith [2]. Complete miscibility for the single tail esters occurs at pressures below 30 MPa. Increased chain length (from a molecular weight of 2500 to that of 7500) shifts the cloud point curve to higher pressures. Similar results were observed for the twin tail ester. The triple tail ester showed a surprising result in that the longer chained ester (7500 MW) exhibited lower cloud point pressures than the shorter chained esters. Previous studies by Newman [9] and co-workers showed a depression of the cloud point curve as a result of branching for fluoroalkyl sulfonates. This was a result of the increase in the number of lyophilic tails which increased carbon dioxide solubility. The effect of branching is illustrated by comparing esters with similar molecular weights over similar concentration ranges. Fig. 8 compares a single tail 5000 M W ester to a twin tail 2500 MW ester with maximum concentration levels near 10 mmole/1 CO 2. Since both esters have a total molecular weight of 5000, the twin tail's increased
30
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MWgt
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4 6 8 10 Ester Concentration (mmole/liter CO 2)
12
Fig. 5. C l o u d point c u r v e s for f l u o r o e t h e r functional single tail sorbitol ester in carbon dioxide at 306 K.
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E J . Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
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2500 MWgt 5000 MWgt 7500 MWgt
i 8
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Ester Concentration (mmole/literCO2) Fig. 6. Cloud point curves for fluoroether functional twin tail sorbitol ester in carbon dioxide at 306 K.
solubility may be attributed exclusively to branching, an observation which is consistent with those by Newman et al. However, as we increase the total molecular weight of the molecule, the effect of increased branching appears to change. For a 7500 MW material (Fig. 9; single-tail 7500 vs. triple tail 2500), the cloud point curves coincide, while for a 15 000 MW material (Fig. 10; twin-tailed 7500 vs triple-tail 5000), the more branched molecule exhibits higher cloud point pressures. In general, Schechter [15] found that branching increases the lipophile-solvent interaction, thus producing better solubility. In our system, the change in branching effect as molecular weight increases may be due to
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Fig. 7. Cloud point curves for fluoroether functional triple tail sorbitol ester in carbon dioxide at 306 K.
E.I. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219 35
~
,
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4
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Fig. 8. Cloud point curves for twin tail 2500 M W g t sorbitol ester vs. single tail 5000 M W g t sorbitol ester.
a decreased ability of the amphiphiles to aggregate as the lyophilic portion of the molecule becomes very bulky. Fig. 1 l(a) and (b) show the cloud points for the fluoroether sulfates, where the effects of both structure and molecular weight can be evaluated. A comparison of single tail 2500 MW sulfate to the twin tail 2500 MW sulfate shows that branching depresses the cloud point curve, although the cloud points converge at higher concentrations. Ordinarily, one would assume that the higher molecular weight material would exhibit higher cloud point pressures, however the higher molecule weight 35
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Fig. 9. Cloud point curves for triple tail 2500 M W g t sorbitol ester vs. single tail 7500 M W g t sorbitol ester.
2 ]0
EJ. Singley et al, / Fluid Phase Equilibria 128 (1997) 199-219
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Ester Concentration (mmole/liter of C02) Fig. 10. Cloud point curves for twin tail 7500 M W g t sorbitol ester vs. triple tail 5000 M W g t sorbito] ester.
compound in Fig. 11 (a) has a higher fraction of CO2-philic fluoroether, as well as perhaps a higher probability of forming aggregates in carbon dioxide. These factors lead to a lower cloud point pressure (at low concentrations), despite the higher molecular weight of the twin-tailed material. In Fig. 1 l(b), we expect that similar factors are operative, and that the increased CO2-philicity of the branched material appears to counterbalance the effect of higher molecular weight, leading to coincident cloud point curves for the single and twin-tailed materials. The cloud point curves for analogous sulfate and sulfonate molecules are nearly identical, suggesting that the high fraction of CO2-philic fluoroether in each masks any effects due to differences in head group polarity (see Fig. 12). A comparison of sulfates to sorbitol esters of similar molecular weights show that nonionic sorbitol esters have lower cloud point pressures than the anionic sulfates, which is not surprising, given the higher polarity of the sulfate head groups (see Fig. 13). 4.2. Effect of pressure on phase behavior in CO 2 / w a t e r / a m p h i p h i l e mixtures
By examining the phase behavior of surfactant/CO2/water mixtures versus pressure and C O J w a t e r ratio, approximate phase maps have been constructed. Fig. 14(a) and (b) contain the phase maps for the triple tail 7500 MW sorbitol fluoroether ester, showing phase transitions as a function of pressure at a specific surfactant mass composition in a s u r f a c t a n t / C O J w a t e r system (note that the maps merely give approximate not exact phase boundaries). As the pressure increases from 13.8 MPa to 41.4 MPa, WI --* WII! ~ WII or WIII ~ WII transitions are observed, depending on the C O 2 / w a t e r ratio, C W. These results are consistent with those by Beckman, Kahlweit and McFann yet Fotland found that increasing pressure improves water solubilization, yielding WIII ~ WI phase transitions. Fotland showed that for commercial anionic surfactants, an increase in pressure
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B
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2
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4
5
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Sulfate Surfactant Concentration (mmole/IC02)
Fig. 11. Cloud points for fluoroether functional sulfate surfactants for (a) 2500 MWgt single and twin tails and (b) 7500 MWgt single and twin tails.
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25
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Single Tail 7500 MWgt Sulfate Single Tail 7500 MWgt Sulfonate
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2
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6
Concentration of Surfactant, (mmole/liter of CO2) Fig. 12. Cloud point curves for fluoroether functional sulfonates vs. fluoroether functional sulfates.
increased water solubilization and decreased oil solubilization, consistent with the phase transitions observed in this system. The effect of pressure on the phase behavior of a m p h i p h i l e / o i l / w a t e r temaries is strongly related to the compressibility of the oil phase versus that of the water. Peck and Johnston analysis of the AOT-propane-brine system showed that as pressure decreases, the AOT abruptly leaves the propane phase and forms a middle phase, i.e. WIII. The formation of the middle phase may be attributed to the compressibility of propane. Since hydrocarbons are generally more compressible than water, an increase in pressure should increase the cohesive energy, Aoo (oil-oil interaction) and decrease R. However, in our ternary mixture, where the compressibility of the oil phase (carbon dioxide) is much greater than that of the aqueous phase, and where increases in pressure can lead to large increases in surfactant solubility in CO2, we would then expect that increasing pressure would lead to WI ~ WIII WII transitions as shown in Fig. 14(a). An increase in surfactant concentration decreases the size of the 3-phase region while also increasing both the pressure and oil/water range in which one can generate Winsor II mixtures (Fig. 14(b)). In our C02/HzO/surfactant systems, an increase in CO2/water ratio yields either Will ~ WI, WII ~ Will -o WI or WII -o WI phase transitions. The sorbitol ester phase maps (note that concentrations are based on a total volume of 130 ml and surfactant weight as listed) show these general trends of emulsion behavior as the C O 2 / H 2 0 ratio increases at fixed pressures. For example, Fig. 14(a) shows that the Will to WI change is observed as the ratio increases from 15:85 to 50:50 at low to moderate pressures. However at pressures above 35 MPa, a W I I - o Will type transition is observed. These transitions are consistent with studies by Bourrel and Schechter [20] who found that
E.I. Singley et a l . / Fluid Phase Equilibria 128 (1997) 199-219
213
30 28 26
24 13.
22
g a_ 20 18 16 14 0
I
I
I
I
I
2
4
6
8
10
12
Surfactant Concentration (mmole/I CO2) Legend •
Single Tail Sulfate
®
Single Tail Sorbitol
Fig. 13. Cloud point curves for 2500 MWgt surfactants sulfatesvs. sorbitol esters.
an increase in o i l / w a t e r ratio promotes a WII ~ WlII ~ WI transition. They concluded that the change could not fully be explained by the R ratio but was rather a result of entropic changes in the system. The ternary diagram in Fig. 15 helps to illustrate the effect of increased water or CO 2 concentrations on phase transitions and is based on the following assumptions: 1. The surfactant is very soluble in carbon dioxide. 2. The surfactant is poorly soluble in water. 3. Carbon dioxide and water are immiscible. 4. A 3-phase region exists. A high CO 2 concentration (right side of the ternary diagram) yields two phases; essentially excess CO 2 plus emulsion, i.e, WI. High water concentrations, on the other hand, (left side) lead to two phases; excess water plus emulsion, i.e. WII. Therefore, a high C O 2 / H 2 0 ratio, C W will produce WI behavior and a low C w will give WlI behavior. As the percent water increases phase transitions follow a WI --->WIII --->WlI path or WIII --* WlI path. In typical nonionic surfactant/water/oil systems, temperature-surfactant concentration diagrams exhibit a " f i s h " shape, where the head of the fish encompasses the 3-phase region, the tail, the single phase region and the WI and WlI regions surround the " f i s h " . If we assume that increasing pressure
214
E.1. Singley et al. / Fluid Phase Equilibria 128 (1997) 199-219
(a)
45 40 35
Pressure (MPa)
30 25 20 15 10 15:85 20:80 25:75 30:70 35:65 40:60 45:55
50:50
CO2/H20 Ratio, Cw
(b)
45 40 35
Pressure (MPa)
30 25 20 15 I0 10:90 15:85 2 0 : 8 0 2 5 : 7 5 30:70 35:65 40:60 45:55 50:50 COJH20 Ratio, Cw
Fig. 14. Emulsion behavior as a function of pressure for triple tail 7500 MWgt: (a) fluoroether sorbitol ester, (0.7 g) 0.5 mmol 1j H20; (b) fluoroether sorbitol ester, (1.0 g) 0.7 mmol 1-~ H20.
affects p h a s e b e h a v i o r similarly to either d e c r e a s i n g t e m p e r a t u r e or increasing salt c o n c e n t r a t i o n , then w e w o u l d also expect to o b s e r v e a f i s h - s h a p e d d i a g r a m , as in the w o r k by B e c k m a n and S m i t h [16]. A s s h o w n in Fig. 16, w e do indeed o b s e r v e a 3 - p h a s e region at various c o n c e n t r a t i o n s levels, w h i c h t r a n s f o r m s to a 2 phase ( W I I ) region w h e n pressures e x c e e d 25 MPa.
215
EJ. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
Surfactant, 100 %
H20, 100%
COz, 100% Fig. 15. Ternary phase diagram.
Fig. 16 shows a phase m a p for the 7500 M W triple tail ester at a fixed C O 2 / w a t e r ratio o f 40:60 and variable surfactant concentrations. At all surfactant concentrations, the WI---> WlII---> W I I type phase transition occurs as the pressure increases. Finally, if we e m p l o y a high concentration (2 m m o l e s ) of a highly CO2-soluble material [triple-tail, 5000 MW], we observe W i n s o r II b e h a v i o r o v e r
45 40 35 30 Pressure (MPa) 25 20 15 I0 0.7
0.8
0.9
1.0
1.l
1.2
1.3
Surfactant Weight %, Ws Fig. 16. Emulsion behavior as a function of pressure for triple tail 7500 MWgt. Fluoroether sorbitol ester, C w = 40:60.
216
EJ. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
4540-
[]
I'
•
nn
•
35:65
40:60
45:55
35_ 30
Pressure (MPa)
25 20
15 I0 10:90
15:85
20:80
25:75
30:70
50:50
CO2/H20 Ratio, Cw Fig. 17. Emulsionbehavior as a function of pressure for single tail 5000 MWgt fluoroethersorbitol ester, (0.7 g) 2.1 mmol 1-t H20. the entire pressure range 15 to 42 MPa, and CO2/water ratios of 15/85 to 5 0 / 5 0 . Interestingly, in this mixture we have observed that as the pressure increases, the density of the two phases (emulsion and excess water) approach one another, ultimately leading to the point where the CO2-rich emulsion phase is slightly higher in density than the CO2-saturated water. Primarily WI ~ WIII type transformations are formed for the less CO2-philic single tail ester (Fig. 17). The fact that no WlI regions exist is consistent with its reduced CO 2 solubility. Figs. 18 and 19 show examples of the phase behavior for fluoroether functional sulfate surfactants. The mass of surfactant used in each experiment was 0.25 g (0.8 MW%). The single and twin tail 2500 sulfate exhibited WI ~ WlII behavior as the ratio of CO 2 to buffer decreased. The twin tail 7500 MW sulfate showed WI ~ WIII --->WII behavior with decreasing CO 2 to buffer ratio, C B. The ability to form only WI emulsions at high C B may be attributed to the strong affinity between the hydrophilic head and the buffer. The instability of micelles formation occur as the CO2/buffer begins to decrease and WlII emulsions are formed. A further decrease in CO2/buffer ratio results in a WlI type emulsion. The sulfate results are similar to those found with the sorbitoi ester in that the higher CO2/water ratio also results in WI behavior.
5. C o n c l u s i o n s
The development of novel fluoroether amphiphiles has been completed for sorbitol ester, sulfates and sulfonates. All of the amphiphiles are soluble in carbon dioxide at moderate pressures and have the capability of forming WI, WII and Will type emulsions in CO 2. An examination of pressure
EJ. Singley et cd./ Fluid
Phuse Equilihriu 128 (1997) 199-219
T
120ml
217
9.2 ml
2.2 ml
25.6 ml
t CB=13/24 ml
t&=14/23 ml
Single Tail
2500 MWgt., 0.25 g
13.1 ml
-I=
11.4
2.3 ml
Buffer
21.7 ml
f
13.1 ml
Twin Tail
7.0 ml
co2
I
2500 MWgt., 0.25 g
Cg= 14/23 ml
23.4 ml
Buffer
Cg= 13/24 ml
Cg= 14/23 ml
23.9 ml
(Ce=ratio of COzIBuffer)
02 A $g, :;:‘;J@$& 2.2 ml ..........I.......,,~.:.~.~.~.~.~.~.~.~, .... ....
13.9 ml
Twin Tail
Cg= 8/29 ml
Buffer
Cg= 8/29 ml (Cu=ratio of C02/Buffer)
11.5ml
9.3 ml
i.6 ml
1.7 ml
23.9 ml
C&,13/24 ml 7500 MWgt., 0.25 g
Fig. 18. Emulsion phase volumes at constant pressure,
Buffer
3.1 ml
27.7 ml
R= 9/28 ml
28.9 ml
R=8/29 ml
(CB=ratio of COz/Buffer) P = 20 MPa, for fluoroether
sulfate surfactants.
EJ. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
218
Fluoroether Sulfate Surfaetant 13.1 ml
COz
CO2
9.3 ml
1
CO2
2.7 ml
5.5 ml 2.7 ml
1
illlli~i~ZillliZil
-.,-.-.-.-.-.-.-...-.~-.-.-.-.-.-.....-,
!!!!!}!~!!!!!!!!~:~!{!}~!!!!!!!~! 23.9 ml
.-.-.-.-.-.._-.-.-.-.-.--.-.-....-.-.-. ..................... .:-:-:-:.:-:-:+:-:-:-:-:-:-:.:-:-:-:-:.
Buffer
25.0 ml
7-8.8 ml
Buffer
..................... .....................
"5"5"~'g'?'?'5:"5"5"5"5"5"5"5"~:"5:"~: ,.-.-.-...-.~..-...-.-.-.-.-.-.-.-.-.....
i.~.~.~.;~-~.;~.~.i.~.~.i~.~.~. .1.:~.:.:.:.:,:.:.:.:-:.:.:.:.:.:,:.~:.:
C~=14/23 ml
CB=13/24 ml
Single Tail 7500 MWgt., 0.25 g
CB= 8/29 ml
(CB=ratio of CO2/Buffer)
Fluoroether Sulfonate Surfactant
A
14.1ml
C02 .---....--------..-..--..-..------.-..-.. .---.--.--------..---
C02
13.1 ml 1.1ml
1
4.9 ml
CO2
-- "~.7 ml .........
)T"': ........
'.:.:.:.:.:.:-?:.:.:.:.:.::':':'.?::':': .....................
................................. ..................... ..-.-.-.-.-...-.-.--.--.-..-.._-.-..... .---.--.---.--.-..... ..................... ".'.'.'.'.'.'.'.'.'.'.'.'.'.'.-.'.'L .. .-----..-------.--... .....................
22.9 ml
Buffer
22.8 ml
Buffer
29.4 ml
..................... .................................... ........................................ ..........,.-.-.-.-.....,...............,. :.:.:33333L:3 J 3.:.:33333.:. "':":-:"'"'"":'"":':
CB=18/19 ml
CB=17/20 ml
Single Tail 7500 MWgt., 0.25 g
CB= 8/29 ml (CB=ratio of COz/Buffer)
19. E m u l s i o n phase v o l u m e s at constant pressure, P = 2 0 M P a , for sulfate and sulfonate fluoroether-functional surfactants.
Fig.
effect on emulsion behavior shows that the high compressibility of carbon dioxide enables WI --->WIII WII phase transitions which are similar to those found with increased electrolyte concentrations. Additional studies are required to ascertain the impact of temperature and supercritical conditions on emulsion behavior in carbon dioxide.
E J . Singley et a l . / Fluid Phase Equilibria 128 (1997) 199-219
219
Acknowledgements T h e authors t h a n k the N a t i o n a l S c i e n c e F o u n d a t i o n f o r their s u p p o r t o f this p r o j e c t u n d e r grant # C T S - 9 2 5 8 5 0 and EJS t h a n k s the N a t i o n a l S c i e n c e F o u n d a t i o n for f e l l o w s h i p support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
McHugh, M.A. and Krukonis, V.J., Supercritical Fluid Extraction, Butterworths, Stoneham, MA, 1986. Consani, K.A. and Smith, R.D., J. Supercritical Fluids, 3 (1990) 51. Hoefling, T.A., Stofesky, D., Reid, M., Beckman, E.J. and Enick, R.M., J. Supercritical Fluids, 5 (1992) 237. Dandee, D.K., Heller, J.P. and Wilson, K.V., IEC Prod. Res. Dev., 24 (1985) 162. Francis, A.W., J. Phys. Chem., 58 (1954) 1099. Harris, T.V., Irani, C.A. and Pretzer, R., 1990 U.S. Patent No. 4913235 assigned to Chevron Research Company. Iezzi, A., Bendale, P., Enick, R.M., Turberg M. and Brady, J., Fluid Phase Equil., 52 (1989) 307. Harrison, K., Govea, J., Johnston, K. and O'Rear, E., Langmuir, 10 (1994) 3536. Newman, D.A., Hoefling, T.A., Beitle, R.R., Beckman, E.J. and Enick, R.M., J. Supercritical Fluids, 6 (1993) 205. Hoefling, T.A., Stofesky, D., Reid, M., Enick, R.M. and Beckman, E.J., J. Supercritical Fluids, 5 (1992) 237. Johnston, K., Harrison, M., Clarke, J., Howdle, S., Heitz, M., Bright, F., Carlier, C. and Randolph, T., Science, 271 (1996) 624. Fulton, J., Pfund, D., McClain, J., Romack, Maury, E., Combes, J., Samulski, E., DeSimone, J. and Capel, M., Langmuir, 11 (1995) 4241. Bartscherer, K., Renon, J. and Minier, M., Fluid Phase Equilibria, 107 (1995) 93. Fotland, P., J. Phys. Chem., 91 (1987) 6396. Kahlweit, M., Strey M., Schomacke, R. and Haase, D., Langmuir, 5 (1989) 305. Beckman, E.J. and Smith, R.D., J. Phys. Chem., 95 (1991) 3253. McFann, G.J. and Johnston, K.P., J. Phys. Chem., 95 (1991) 4889. Peck D.G. and Johnston, K.P., J. Phys. Chem., 95 (1991) 9549. McFann, G.J. and Johnston, K.P., Langmuir, 9 (1993) 2942. Bourrel, M. and Schechter, R.S., Microemulsions and Related Systems; Surfactant Series, Marcel Dekker, New York, 1988, Vol. 30.
Fluid Phase Equilibria 128 (1997) 199-219
Phase behavior and emulsion formation of novel fluoroether amphiphiles in carbon dioxide Edith J. Singley *, Wei Liu, Eric J. B e c k m a n University of Pittsburgh, Chemical Engineering Department, 1249 Benedum Hall, Pittsburgh PA 15261, USA
Received 6 February 1996; accepted 16 July 1996
Abstract
The limitations of carbon dioxide as a processing fluid are primarily attributed to its insolubility of polar compounds. The development of new CO2-philic amphiphiles could resolve the solubility problem. A new series of fluoroether amphiphiles have been developed to address the solubility problem. The development of novel fluoroether amphiphiles has been completed for sorbitol ester, sulfates and sulfonates. The primary objective was to design and synthesize materials that have strong polar hydrophilic heads and highly CO2-philic tails. All of the amphiphiles are soluble in carbon dioxide at moderate pressures and have the capability of forming WI, WII and WIII type emulsions in CO 2. The degree of solubility is heavily influenced by amphiphilic structure including chain length and branching. An examination of pressure effect on emulsion behavior shows that the high compressibility of carbon dioxide enables WI -~ WIII ~ WII phase transitions which are similar to those found with increased electrolyte concentrations. Keywords: Surfactants; Carbon dioxide; Emulsions; High pressure
1. Introduction
Carbon dioxide is one of the only organic solvents that is not classified as a VOC (volatile organic chemical) by the United States Environmental Protection Agency. Carbon dioxide is inexpensive, exhibits low toxicity and is nonflammable. However, a serious obstacle to employing carbon dioxide is its inability to solvate highly polar compounds [1], which may be due to its low dielectric constant (1.5-1.6 range) and its low polarizability per unit volume. The solubility problem could be alleviated by the addition of surfactants, a method which has been successful in enhancing the solubility of polar compounds in non-polar compressible solvents such as propane, ethane and xenon. However, * Corresponding author. 0378-3812/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0378-3812(96)03167-6
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extending these efforts t o C O 2 has been difficult in that conventional alkyl-functional ionic amphiphiles exhibit poor to negligible solubility in CO 2 at moderate pressures [2]. Although nonionic surfactants can exhibit reasonable solubilities in carbon dioxide at moderate pressures, solvation of water and polar compounds has proven difficult [3]. These results appear anomalous given that the solvating capability of carbon dioxide has been previously compared to that of hexane. However, structure solubility relationships in the literature [4-7] suggest that the solvent properties of carbon are quite different form that of ordinary alkanes. Johnston et al., [8] showed that the solubility parameter for CO 2 is inflated by approximately 20% due to carbon dioxide's strong quadrupole moment. As a result, one would anticipate carbon dioxide to be a good solvent for compounds with low solubility parameters (below (6.5 cal/cm3) °5) which include siloxanes, perfluorinated ethers and alkanes. To date several amphiphiles have been designed and synthesized which are not only functional but highly CO2-philic [8,9]. The design strategy is based on the development of molecules with hydrophilic head groups that readily interact with specific solutes (water, proteins, polar molecules), while the hydrophobic tail interacts favorably with CO 2. There are several factors which contribute to CO2-philicity; low solubility parameter, low polarizability and electron donor capability (considering the fact that CO2 is a weak Lewis acid). A variety of studies indicate that compounds which contain fluoroether, fluoroalkyl, or dimethyl siloxane moieties show strong solubility in carbon dioxide [3,7,9]. For example, Hoefling [10] and colleagues generated surfactants by replacing the CO 2 insoluble hydrocarbon tails with perfluoroalkylpolyether carboxylates. The CO 2 soluble surfactants showed complete miscibility at pressures as low as 16 MPa. Johnston [8] and colleagues have shown that replacement of an alkyl tail with a fluoralkyl moiety in a twin-tail sulfate surfactant allows formation of micelles which absorb significant amounts of water in carbon dioxide as pressure increases. Johnston has recently shown that a fluoroether carboxylate also forms micelles in carbon dioxide, allowing solubilization of a protein [11]. Further, DeSimone et al. [12] found that amphiphiles with perfluorinated alkyl chains form aggregates in supercritical carbon dioxide. Small angle X-ray scattering (SAXS) was used to characterize aggregation. The type and degree of aggregation was a function of amphiphilic structure. The effect of pressure on multiphase microemulsions has not been investigated thoroughly due to the complexity of experiments required. A comprehensive review of microemulsions in compressible fluids by Bartscherer [13] and colleagues identified several studies performed on ionic surfactants which do give an indication of phase behavior trends. In 1948, P.A. Winsor published the first classification of phase behavior which accounted for the various phenomenon exhibited in a ternary Oil-H 20-Amphiphile system [19]. Winsor I, lI and III systems (i.e. WI, WII and WIII) are described as follows: WI - - surfactant resides primarily in the aqueous phase; normal micelles in equilibrium with excess oil WII surfactant resides primarily in the oil phase; reverse micelles in equilibrium with excess oil WIII surfactant resides primarily in a middle phase in equilibrium with the aqueous and oil phases. Fotland's study [14] of the 5-component system (sodium dodecyl sulfate (SDS), 1-butanol, cyclohexane, water and sodium chloride) found that mixtures which display WIII behavior at atmospheric conditions exhibit WI behavior with increased pressures (up to 360 bar). The increase in pressure enabled increased water solubilization which eventually leads to mass transfer across the -
-
-
-
E.J. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
201
lower interface. Further pressure increases caused precipitation of the solid phase. The decrease in oil solubilization may be due to the lower compressibility of cyclohexane compared to other oils. Kahlweit and colleagues [15] studied a 4-component mixture at various salt concentrations. Overall, the opposite effects of pressure were predicted for ionic and nonionic amphiphiles. Ionic amphiphiles should move from the oil rich to the aqueous phase with increased pressure, unlike nonionic surfactants which should move from the aqueous to the oil rich phase. Beckman and Smith [16] studied the pressure effect on phase behavior of a 3-component mixture, where propylene was the organic component. An increase in pressure on a 3 0 / 7 0 water/propylene system showed phase transitions from WI ~ WII --* WIII, consistent with the observations of Fotland, whereas a 7 0 / 3 0 water/propylene system showed a WI--* WIII ~ WII phase transition. Therefore, as the pressure increased, either WIII or WII behavior occurred depending on surfactant concentration and propyl e n e / w a t e r ratio. McFann and Johnston [17] assessed the phase behavior of (bis(2-ethylhexyl) sodium sulfo-succinate (AOT))-alkane-brine-water systems for a wide pressure range. A close look at alkanes from ethane to dodecane illustrated the effect of pressure on surfactant hydrophilicity. Water-in-oil (WII) microemulsions were converted to middle phase (WIII) microemulsions and then to oil-in-water (WI) for propane systems by reducing pressure. The results indicate that molecular interactions at the surfactant interface may be controlled by simply adjusting pressure. Peck and Johnston used theoretical modeling to predict the pressure effect on the phase behavior of A O T / P r o p a n e / B r i n e Water-in-Oil microemulsions [18]. Phase transitions occurred by varying pressure. The model was based on two important parameters: micelle-micelle interactions and intramicellar interfacial interactions that determine the natural curvature of the microemulsion interface. The R ratio [20], which compares the balance of the individual cohesive energies of the functional groups present at the interface, can be used to explain why micelles (emulsions) will be formed in oil or water. R=
Aco - Aoo - AlL Acw -- Aww - - Ahh
where Aco is the lipophile-oil interaction, Aoo is the oil-oil interaction, A l l is the lipophile-lipophile interaction, Acw is the hydrophile-water interaction, Aww is the water-water interaction and Ahh is the hydrophile-hydrophile interaction. The cohesive energy Acw promotes the miscibility of the amphiphilic compound with the aqueous region and Aco promotes miscibility with the oil region. Both Ahh and Aww oppose miscibility with water and likewise, both Aoo and A H oppose miscibility with oil. Therefore R < 1 yields WI (normal micelles in water), R > 1 yields WlI (reverse micelles in oil) and R = 1 yields WIII (three phases). According to Schechter, an increase in pressure should theoretically decrease R due to the enhanced oil-oil interactions (an increase in Aoo). So in a conventional liquid system WlI ~ Will ~ WI phase transitions as seen by Fotland should be observed [20]. If the oil is very compressible, for instance C02, the pressure effects on Aco might dominate, which would result in R > 1 and therefore WI ~ WIII ~ WII transitions as seen by Beckman and Smith should occur. In this paper the synthesis, phase behavior and emulsion formation of a series of fluoroether amphiphiles in carbon dioxide is discussed. The effect of structure on surfactant solubility in CO 2 is investigated along with the impact of pressure on emulsion formation in CO2/water mixtures.
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2. Experimental Carboxylic-acid capped oligomers of hexafluoropropylene oxide (Krytox, DuPont) were purchased from Miller-Stephenson Chemical Co. and used as received. The series of Krytox fluids are available in three molecular weights; low MW (2500), medium MW (5000) and high MW (7500). Bone dry carbon dioxide and nitrogen (99.9 + / p u r e ) were purchased from Liquid Carbonic Corp. and used as received. All other materials were obtained from Aldrich; all solvents were dried with molecular sieves prior to use. Characterization of products was performed using an F T - I R spectrophotometer (Mattson Polaris) and a 300 MHz NMR (Bruker).
2.1. Synthesis of acid chloride Acid chlorides were generated for low, medium and high molecular weight poly(perfluoropropylene oxides). Typically, 100 g (0.04 moles) of carboxylic acid terminated poly(perfluoropropylene oxide), low molecular weight, ( M n = 2500) and 100 g of methyl- perfluorocyclohexane were placed in a 250 ml 3-neck round bottom flask equipped with condenser and thermometer. The mixture was placed under a nitrogen blanket and then heated to reflux (75°C) while stirring. Dimethylformamide (3.2 ml, 0.04 moles) and thionyl chloride (6 ml, 0.06 moles) were slowly added. The reaction mixture was stirred for 8 h. The solvent and residual thionyl chloride were removed under vacuum using a rotary evaporator (Brinkman). The product was then placed in a separatory funnel to allow the removal of residual DMF. F-I'-IR results show the shift of the carbonyl peak from 1776 c m - t to 1809 cm-J and I H NMR shows the disappearance of the acid proton IH, (9.17 ppm). The above synthesis was completed for the medium and high molecular weight oligomers using the appropriate amount of reactants.
2.2. Synthesis single-tailed surfactants 2.2.1. Fluoroether sorbitol esters The sorbitol ester of the fluoroether acid was prepared via reaction of the acid chloride with sorbitol (Fig. 1). Typically, 10 g (4 mmoles) of acid chloride (2500 MW) were dissolved in 150 ml of 1,1,3-trichlorotrifluoroethane and placed in an addition funnel. A 250 ml 3-neck flask with condenser and addition funnel was charged with 0.723 g (4 mmoles) of sorbitol and 25 ml of cyclopentanone. The mixture was heated while stirring to 80°C to allow the complete dissolution of sorbitol. Next, polyDMAP, a polymer supported dialkylaminopyridine catalyst, was added in excess to scavenge HCI. The system was placed under a nitrogen blanket and the acid chloride solution was slowly added (dropwise) to the sorbitol mixture. The mixture was stirred at 60°C for 3 days. The polyDMAP was removed by filtration and the solvent removal was completed under vacuum. The product was placed in a separatory funnel for further removal of residual solvents. The final product was washed twice with ethyl ether to ensure total removal of residual cyclopentanone. The fluoroether sorbitol esters are mixtures of analogous molecules due to the large number of hydroxyl groups. However, it is expected that the primary hydroxyls would be the most reactive. The results of the F T - I R show a carbonyl shift from 1809 c m - 1 to 1790 c m - J (ester) and C - H stretching at 2995 c m - ~ and hydroxyls present at 3400 c m - J .
E.I. Singley et al./ Fluid Phase Equilibria 128 (1997) 199-219
O O R--~OH + CdCI
O -~ R---~CI
+
SO 2
+
203
HCI
Synthesisof KrytoxAcidChloride CH2OH
R3CI
+
HO~--OH cP/cFo~cc~ HO'-~--H D, H-~--OH
CH20--~--R HO-@--OH
PolyObMAP)
H--~--OH CH2OH
H--~--OH CH2OH + other isomers
Synthesisof Fluoroether-functionalSorbitolEster Fig. 1. Synthesis of acid chloride and fluoroether sorbitol ester.
2.2.2. Fluoroether sulfate surfactants The fluoroether sulfates were generated via reaction of a hydroxyethyl ester of the fluoroether carboxylic acid followed by reaction of the terminal hydroxy group with chlorosulfonic acid. In a typical reaction, 25 g of the fluoroether acid chloride (0.01) moles were dissolved in 100 ml of l,l,2-trifluorotrichloroethane and placed in an addition funnel. A 3-neck 250 ml flask equipped with condenser and the addition funnel was charged with 1.24 g (0.2 moles) of ethylene glycol (99 + %). An excess of polyDMAP was added and the system was placed under a nitrogen blanket. The acid chloride solution was added dropwise, after which the solution was stirred for 24 h at room temperature. The polyDMAP was removed by filtration and the solvents were removed using a rotary evaporator. The concentrated product was then washed with acetone to remove residual ethylene glycol. The end product was vacuum dried for 24 hours. Characterization of the product yields [FT-IR, -COOR, 1790 cm-1, -OH, 3300 cm-~: ]H NMR, -OCH2-C, 4.29 ppm, 2H; -C-CH2-O, 3.69 ppm, 2H; -OH, 5.02 ppm, 1H]. The hydroxyethyl fluoroether ester is reacted with chlorosulfonic acid to yield the fluoroether sulfate. A 3-neck flask equipped condenser was charged with 25 g (0.01 mole) of the hydroxyethl ester and 30 ml of 1,1,2-trifluorotrichloroethane. The system is placed under a nitrogen blanket and stirred vigorously at room temperature. Chlorosulfonic acid (1.17 g (0.01 mole) was added very slowly to the ester solution. Throughout the reaction small HC1 bubbles formed and were swept away by a steady flow of nitrogen. After 1.5 h the solvent and residual HC1 were removed under vacuum at 50°C. The sulfonic acid was neutralized via reaction with an excess of sodium trimethylsilanolate NaOSi(CH3) 3, (1.0 M in THF). The final product was washed several times with THF to remove residual reactant.
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E.J. Singley et al. / Fluid Phase Equilibria 128 (1997) 199-219
Characterization of product yields [IH NMR, 10.59 ppm, 1H, sulfonic acid proton, -C(O)OCH 2, 4.5 ppm, 2H; and -CH2OSO 3, 4.58 ppm, 2HI.
2.2.3. Fluoroether sulfonate surfactants The sulfonate was formed by charging a 3-neck 250 ml flask with 0.244 g (0.002 moles) of phenethyl alcohol (99%) and 100 ml of a mixture of THF and 1,1,2-trifluorotrichloroethane. A mixture of 15 g (0.002 moles) of acid chloride and 100 ml of 1,1,2-trifluorotrichloroethane was placed in an addition funnel. The reaction system was placed under a nitrogen blanket with an excess of polyDMAP added. The acid chloride mixture was slowly added to flask and the solution was stirred for 24 h at 50°C. The intermediate product was vacuum evaporated to remove solvent. Characterization shows [ FT-IR; -COOR, 1786 cm-1; NMR, C6H5-,7.26 ppm, 5H; -CH2-benzene ring 3.03 ppm, 2H; -CH2-O, 4.55 ppm, 2H]. The purified product (15 g) was placed in a 3-neck flask equipped with condenser and addition funnel. A total of 100 ml of trifluorotrichloroethane was added to the flask to dissolve the ester. Chlorosulfonic acid, 0.233 g (0.002 moles) was then added slowly to the mixture. The solution was stirred for 2 h. The final product was neutralized by sodium trimethylsilanolate. Characterization shows [ F F - I R , - C O O R , 1786 cm - t , IH N M R : - S O 3 H , 10.26 ppm, 1H; C6H4-, 7.14 ppm, 4H; -CH20-, 4.43 ppm, 2H;-CH2-benzene, 2.91 ppm, 2H].
3. Synthesis of multi-tailed surfactants The multi-tailed fluoroether surfactants were generated using the synthesis outlined above, albeit with different molar ratios. The acid chloride ratios were increased two and three fold to yield the twin and the triple tailed surfactants, The characterization of products were performed similarly to the single tailed products. The fluoroether sulfate and fluoroether sulfonate products are shown in Fig. 2.
3.1. Phase behavior of surfactants in CO 2 Phase behavior studies of fluoroether surfactant were conducted using a variable volume high pressure view cell (D.B. Robinson and Associates) as shown in Fig. 3. In the series of experiments conducted, a known amount of fluoroether sorbitol ester was loaded into the quartz tube sample cell along with 5 steel ball bearings. The tube is then sealed inside the steel housing. A known volume of CO 2 at its vapor pressure is injected into the cell using one of the two Ruska syringe pumps. A floating piston located in the quartz cell separates the sample from the pressure transmitting fluid (silicone oil). The piston was raised by injecting additional oil via the second Ruska pump. This action was used to increase pressure to a point where a clear single phase exists indicating surfactant solubility. Complete mixing was achieved by rocking the entire cell/steel housing unit thus allowing free movement of the ball bearings. The cell was heated to 306 K and the system was allowed to reach equilibrium. The pressure was then lowered via silicone oil removal until the first signs of turbidity appeared, which indicates phase transition. This procedure was repeated several times to establish the cloud point. The system pressure was then reduced to vapor pressure to enable the addition of a fixed amount of carbon dioxide. A new cloud point was measured and the data collection continued until a cloud point curve could be generated.
EJ. Singley et d/Fluid
Phase Equilihricc 128 (1997) 199-219
CFfCFd,p-LH
fluoroethcr Acid
Single Tail Fluoroether Sulfate
Twin Tail Fluorocther Sulfate
CF~-CF241F-O-CF2CF
4CH~CH+-+SO~N~
c:F3 Single Tail Fluorocther Sulfonate
Fig. 2. Structures of sulfate and sulfonate fluoroether
surfactants.
Oil Pump
Fig. 3. High pressure volume cell (D.B. Robinson).
205
206
EJ. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
Surge Tank
C02 Booster
CO2 Cylinder
High Pressure Cell
Metering Pump Fig. 4. High pressure vessel for emulsion testing.
3.2. E m u l s i o n f o r m a t i o n
Emulsions were formed in either the aforementioned D.B. Robinson cell or in a smaller high pressure device equipped with a three blade stirrer, as shown in Fig. 4. Emulsification in the D.B. Robinson cell allows the pressure to be varied at fixed mass compositions. For experiments in the Robinson apparatus, a known amount of sorbitol ester was loaded into the quartz cell and 65 ml (constant volume) of deionized water was added followed by a fixed amount of CO 2. The pressure was increased to 2000 psi and the cell was rocked for 60 min to obtain adequate mixing. After mixing, the emulsion phase formed in either the aqueous phase, CO 2 phase or at the interface depending on pressure and surfactant concentration. The pressure was varied from t 3.8 MPa to 41.4 MPa for each CO 2 volume change. The CO 2 volume was varied from 15 ml to 65 ml by reducing pressure and injecting CO2 via the Ruska syringe pump. The phase volumes were measured using a cathetometer. The sulfate and sulfonate emulsions were formed in buffered solutions at a fixed pressure. The buffered solution was made from 0.025 moles of sodium phosphate monobasic NaH2PO 4 (from Sigma, reagent grade), and 0.025 moles of sodium phosphate dibasic, Na2HPO 4 (from Sigma, ACS reagent) dissolved in 4.4 moles of distilled water. A known amount of surfactant and buffer were loaded into the 36 ml cell, then CO 2 was slowly introduced into the cell until the specific pressure (21 MPa) was achieved. The solution was agitated vigorously for 5 rain, then allowed to settle.
EJ. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
207
4. Results and discussion 4.1. Phase behavior
The phase behavior of amphiphiles in organic solvents are derived from the interactions between head groups, tail groups and solvent as described by Bourrel and Schechter [20]. Therefore a shift in the cloud point curve may be a result of chain length, number of tails, surfactant geometry (branched vs. linear) a n d / o r the polarity of the head group. Since the dissolving process is strongly influenced by the interaction between the hydrophobic tails of the surfactants and carbon dioxide, one can surmise that the tail length and number will affect surfactant solubility more so than other variables. Figs. 5 - 7 illustrate the effect of chain length on the cloud points for the fluoroether functional sorbitol ester. The fluoroether sorbitol surfactant shows relatively high solubility in carbon dioxide at moderate pressures, unlike analogous alkyl sorbitan surfactants as shown by Consani and Smith [2]. Complete miscibility for the single tail esters occurs at pressures below 30 MPa. Increased chain length (from a molecular weight of 2500 to that of 7500) shifts the cloud point curve to higher pressures. Similar results were observed for the twin tail ester. The triple tail ester showed a surprising result in that the longer chained ester (7500 MW) exhibited lower cloud point pressures than the shorter chained esters. Previous studies by Newman [9] and co-workers showed a depression of the cloud point curve as a result of branching for fluoroalkyl sulfonates. This was a result of the increase in the number of lyophilic tails which increased carbon dioxide solubility. The effect of branching is illustrated by comparing esters with similar molecular weights over similar concentration ranges. Fig. 8 compares a single tail 5000 M W ester to a twin tail 2500 MW ester with maximum concentration levels near 10 mmole/1 CO 2. Since both esters have a total molecular weight of 5000, the twin tail's increased
30
i
i
I
l
I
Y 25
20 13..
{
Legend
{
15
[
0
2
I
I
[
•
2500
• •
5oo0MWgt 7500MWgt
MWgt
I
4 6 8 10 Ester Concentration (mmole/liter CO 2)
12
Fig. 5. C l o u d point c u r v e s for f l u o r o e t h e r functional single tail sorbitol ester in carbon dioxide at 306 K.
208
E J . Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
30
25
20 O_
Legend
15
[3 A ~7
10
i 2
J 4
~ 6
2500 MWgt 5000 MWgt 7500 MWgt
i 8
10
Ester Concentration (mmole/literCO2) Fig. 6. Cloud point curves for fluoroether functional twin tail sorbitol ester in carbon dioxide at 306 K.
solubility may be attributed exclusively to branching, an observation which is consistent with those by Newman et al. However, as we increase the total molecular weight of the molecule, the effect of increased branching appears to change. For a 7500 MW material (Fig. 9; single-tail 7500 vs. triple tail 2500), the cloud point curves coincide, while for a 15 000 MW material (Fig. 10; twin-tailed 7500 vs triple-tail 5000), the more branched molecule exhibits higher cloud point pressures. In general, Schechter [15] found that branching increases the lipophile-solvent interaction, thus producing better solubility. In our system, the change in branching effect as molecular weight increases may be due to
35
30
e3 (3_
t~
25 ~Y 20
i
10
Y @ •
i
5
0
t
[
I
1
2
3
Legend 2500MWgt 5000MWgt 7500MWgt I
4
5
Ester Concentration (mmole/liter of COz)
Fig. 7. Cloud point curves for fluoroether functional triple tail sorbitol ester in carbon dioxide at 306 K.
E.I. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219 35
~
,
~
~
,
,
,
209
,
30
t~ 13.
25
20 o~
a.
15
Legend
10
•
Twin Tail 2500 M W g t
•
Single Tail 5000 M W g t
I
t
I
I
t
I
I
I
2
3
4
5
6
7
8
9
10
Ester Concentration (mmole/liter of COa)
Fig. 8. Cloud point curves for twin tail 2500 M W g t sorbitol ester vs. single tail 5000 M W g t sorbitol ester.
a decreased ability of the amphiphiles to aggregate as the lyophilic portion of the molecule becomes very bulky. Fig. 1 l(a) and (b) show the cloud points for the fluoroether sulfates, where the effects of both structure and molecular weight can be evaluated. A comparison of single tail 2500 MW sulfate to the twin tail 2500 MW sulfate shows that branching depresses the cloud point curve, although the cloud points converge at higher concentrations. Ordinarily, one would assume that the higher molecular weight material would exhibit higher cloud point pressures, however the higher molecule weight 35
30
i t~ O_
25 ,Y 20
i
o~
n
15 Legend 10
•
• 5 0
T r i p l e T a i l 2500 M W g t
Single Tail 7500 MWgt
t
I
I
I
1
2
3
4
5
Ester Concentration (mmole/liter of COz)
Fig. 9. Cloud point curves for triple tail 2500 M W g t sorbitol ester vs. single tail 7500 M W g t sorbitol ester.
2 ]0
EJ. Singley et al, / Fluid Phase Equilibria 128 (1997) 199-219
30
# J
25
t
|
13_ 20
|
q.j 15
i
|
Legend 10 5
I
0.0
.5
_
•
T w i n T a i l 7500 M W g t
•
Triple Tail 5000 MWgt
I
I
i
1.0
1.5
2.0
2.5
Ester Concentration (mmole/liter of C02) Fig. 10. Cloud point curves for twin tail 7500 M W g t sorbitol ester vs. triple tail 5000 M W g t sorbito] ester.
compound in Fig. 11 (a) has a higher fraction of CO2-philic fluoroether, as well as perhaps a higher probability of forming aggregates in carbon dioxide. These factors lead to a lower cloud point pressure (at low concentrations), despite the higher molecular weight of the twin-tailed material. In Fig. 1 l(b), we expect that similar factors are operative, and that the increased CO2-philicity of the branched material appears to counterbalance the effect of higher molecular weight, leading to coincident cloud point curves for the single and twin-tailed materials. The cloud point curves for analogous sulfate and sulfonate molecules are nearly identical, suggesting that the high fraction of CO2-philic fluoroether in each masks any effects due to differences in head group polarity (see Fig. 12). A comparison of sulfates to sorbitol esters of similar molecular weights show that nonionic sorbitol esters have lower cloud point pressures than the anionic sulfates, which is not surprising, given the higher polarity of the sulfate head groups (see Fig. 13). 4.2. Effect of pressure on phase behavior in CO 2 / w a t e r / a m p h i p h i l e mixtures
By examining the phase behavior of surfactant/CO2/water mixtures versus pressure and C O J w a t e r ratio, approximate phase maps have been constructed. Fig. 14(a) and (b) contain the phase maps for the triple tail 7500 MW sorbitol fluoroether ester, showing phase transitions as a function of pressure at a specific surfactant mass composition in a s u r f a c t a n t / C O J w a t e r system (note that the maps merely give approximate not exact phase boundaries). As the pressure increases from 13.8 MPa to 41.4 MPa, WI --* WII! ~ WII or WIII ~ WII transitions are observed, depending on the C O 2 / w a t e r ratio, C W. These results are consistent with those by Beckman, Kahlweit and McFann yet Fotland found that increasing pressure improves water solubilization, yielding WIII ~ WI phase transitions. Fotland showed that for commercial anionic surfactants, an increase in pressure
211
EJ. Singley et a l . / Fluid Phase Equilibria 128 (1997) 199-219
30
(a)
28-
26t~
24
g ffl
~ 22 D_
Legend
20
18
16 0
3O
•
Single Tail 2500 MWgt
m
Twin Tail 2500 MWgt
i
I
I
t
2
3
4
5
(b)
28 26 24 13.
22
~ 20 18 Legend 16
•
SingleTail 7500 MWgt
B
TwinTail 7500 MWgt
14 12 0
I
I
I
I
I
1
2
3
4
5
6
Sulfate Surfactant Concentration (mmole/IC02)
Fig. 11. Cloud points for fluoroether functional sulfate surfactants for (a) 2500 MWgt single and twin tails and (b) 7500 MWgt single and twin tails.
212
E.J. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219 30
25
20
15
Legend A
Single Tail 7500 MWgt Sulfate Single Tail 7500 MWgt Sulfonate
10
1
I
I
1
I
2
3
4
5
6
Concentration of Surfactant, (mmole/liter of CO2) Fig. 12. Cloud point curves for fluoroether functional sulfonates vs. fluoroether functional sulfates.
increased water solubilization and decreased oil solubilization, consistent with the phase transitions observed in this system. The effect of pressure on the phase behavior of a m p h i p h i l e / o i l / w a t e r temaries is strongly related to the compressibility of the oil phase versus that of the water. Peck and Johnston analysis of the AOT-propane-brine system showed that as pressure decreases, the AOT abruptly leaves the propane phase and forms a middle phase, i.e. WIII. The formation of the middle phase may be attributed to the compressibility of propane. Since hydrocarbons are generally more compressible than water, an increase in pressure should increase the cohesive energy, Aoo (oil-oil interaction) and decrease R. However, in our ternary mixture, where the compressibility of the oil phase (carbon dioxide) is much greater than that of the aqueous phase, and where increases in pressure can lead to large increases in surfactant solubility in CO2, we would then expect that increasing pressure would lead to WI ~ WIII WII transitions as shown in Fig. 14(a). An increase in surfactant concentration decreases the size of the 3-phase region while also increasing both the pressure and oil/water range in which one can generate Winsor II mixtures (Fig. 14(b)). In our C02/HzO/surfactant systems, an increase in CO2/water ratio yields either Will ~ WI, WII ~ Will -o WI or WII -o WI phase transitions. The sorbitol ester phase maps (note that concentrations are based on a total volume of 130 ml and surfactant weight as listed) show these general trends of emulsion behavior as the C O 2 / H 2 0 ratio increases at fixed pressures. For example, Fig. 14(a) shows that the Will to WI change is observed as the ratio increases from 15:85 to 50:50 at low to moderate pressures. However at pressures above 35 MPa, a W I I - o Will type transition is observed. These transitions are consistent with studies by Bourrel and Schechter [20] who found that
E.I. Singley et a l . / Fluid Phase Equilibria 128 (1997) 199-219
213
30 28 26
24 13.
22
g a_ 20 18 16 14 0
I
I
I
I
I
2
4
6
8
10
12
Surfactant Concentration (mmole/I CO2) Legend •
Single Tail Sulfate
®
Single Tail Sorbitol
Fig. 13. Cloud point curves for 2500 MWgt surfactants sulfatesvs. sorbitol esters.
an increase in o i l / w a t e r ratio promotes a WII ~ WlII ~ WI transition. They concluded that the change could not fully be explained by the R ratio but was rather a result of entropic changes in the system. The ternary diagram in Fig. 15 helps to illustrate the effect of increased water or CO 2 concentrations on phase transitions and is based on the following assumptions: 1. The surfactant is very soluble in carbon dioxide. 2. The surfactant is poorly soluble in water. 3. Carbon dioxide and water are immiscible. 4. A 3-phase region exists. A high CO 2 concentration (right side of the ternary diagram) yields two phases; essentially excess CO 2 plus emulsion, i.e, WI. High water concentrations, on the other hand, (left side) lead to two phases; excess water plus emulsion, i.e. WII. Therefore, a high C O 2 / H 2 0 ratio, C W will produce WI behavior and a low C w will give WlI behavior. As the percent water increases phase transitions follow a WI --->WIII --->WlI path or WIII --* WlI path. In typical nonionic surfactant/water/oil systems, temperature-surfactant concentration diagrams exhibit a " f i s h " shape, where the head of the fish encompasses the 3-phase region, the tail, the single phase region and the WI and WlI regions surround the " f i s h " . If we assume that increasing pressure
214
E.1. Singley et al. / Fluid Phase Equilibria 128 (1997) 199-219
(a)
45 40 35
Pressure (MPa)
30 25 20 15 10 15:85 20:80 25:75 30:70 35:65 40:60 45:55
50:50
CO2/H20 Ratio, Cw
(b)
45 40 35
Pressure (MPa)
30 25 20 15 I0 10:90 15:85 2 0 : 8 0 2 5 : 7 5 30:70 35:65 40:60 45:55 50:50 COJH20 Ratio, Cw
Fig. 14. Emulsion behavior as a function of pressure for triple tail 7500 MWgt: (a) fluoroether sorbitol ester, (0.7 g) 0.5 mmol 1j H20; (b) fluoroether sorbitol ester, (1.0 g) 0.7 mmol 1-~ H20.
affects p h a s e b e h a v i o r similarly to either d e c r e a s i n g t e m p e r a t u r e or increasing salt c o n c e n t r a t i o n , then w e w o u l d also expect to o b s e r v e a f i s h - s h a p e d d i a g r a m , as in the w o r k by B e c k m a n and S m i t h [16]. A s s h o w n in Fig. 16, w e do indeed o b s e r v e a 3 - p h a s e region at various c o n c e n t r a t i o n s levels, w h i c h t r a n s f o r m s to a 2 phase ( W I I ) region w h e n pressures e x c e e d 25 MPa.
215
EJ. Singley et al./Fluid Phase Equilibria 128 (1997) 199-219
Surfactant, 100 %
H20, 100%
COz, 100% Fig. 15. Ternary phase diagram.
Fig. 16 shows a phase m a p for the 7500 M W triple tail ester at a fixed C O 2 / w a t e r ratio o f 40:60 and variable surfactant concentrations. At all surfactant concentrations, the WI---> WlII---> W I I type phase transition occurs as the pressure increases. Finally, if we e m p l o y a high concentration (2 m m o l e s ) of a highly CO2-soluble material [triple-tail, 5000 MW], we observe W i n s o r II b e h a v i o r o v e r
45 40 35 30 Pressure (MPa) 25 20 15 I0 0.7
0.8
0.9
1.0
1.l
1.2
1.3
Surfactant Weight %, Ws Fig. 16. Emulsion behavior as a function of pressure for triple tail 7500 MWgt. Fluoroether sorbitol ester, C w = 40:60.
216
EJ. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
4540-
[]
I'
•
nn
•
35:65
40:60
45:55
35_ 30
Pressure (MPa)
25 20
15 I0 10:90
15:85
20:80
25:75
30:70
50:50
CO2/H20 Ratio, Cw Fig. 17. Emulsionbehavior as a function of pressure for single tail 5000 MWgt fluoroethersorbitol ester, (0.7 g) 2.1 mmol 1-t H20. the entire pressure range 15 to 42 MPa, and CO2/water ratios of 15/85 to 5 0 / 5 0 . Interestingly, in this mixture we have observed that as the pressure increases, the density of the two phases (emulsion and excess water) approach one another, ultimately leading to the point where the CO2-rich emulsion phase is slightly higher in density than the CO2-saturated water. Primarily WI ~ WIII type transformations are formed for the less CO2-philic single tail ester (Fig. 17). The fact that no WlI regions exist is consistent with its reduced CO 2 solubility. Figs. 18 and 19 show examples of the phase behavior for fluoroether functional sulfate surfactants. The mass of surfactant used in each experiment was 0.25 g (0.8 MW%). The single and twin tail 2500 sulfate exhibited WI ~ WlII behavior as the ratio of CO 2 to buffer decreased. The twin tail 7500 MW sulfate showed WI ~ WIII --->WII behavior with decreasing CO 2 to buffer ratio, C B. The ability to form only WI emulsions at high C B may be attributed to the strong affinity between the hydrophilic head and the buffer. The instability of micelles formation occur as the CO2/buffer begins to decrease and WlII emulsions are formed. A further decrease in CO2/buffer ratio results in a WlI type emulsion. The sulfate results are similar to those found with the sorbitoi ester in that the higher CO2/water ratio also results in WI behavior.
5. C o n c l u s i o n s
The development of novel fluoroether amphiphiles has been completed for sorbitol ester, sulfates and sulfonates. All of the amphiphiles are soluble in carbon dioxide at moderate pressures and have the capability of forming WI, WII and Will type emulsions in CO 2. An examination of pressure
EJ. Singley et cd./ Fluid
Phuse Equilihriu 128 (1997) 199-219
T
120ml
217
9.2 ml
2.2 ml
25.6 ml
t CB=13/24 ml
t&=14/23 ml
Single Tail
2500 MWgt., 0.25 g
13.1 ml
-I=
11.4
2.3 ml
Buffer
21.7 ml
f
13.1 ml
Twin Tail
7.0 ml
co2
I
2500 MWgt., 0.25 g
Cg= 14/23 ml
23.4 ml
Buffer
Cg= 13/24 ml
Cg= 14/23 ml
23.9 ml
(Ce=ratio of COzIBuffer)
02 A $g, :;:‘;J@$& 2.2 ml ..........I.......,,~.:.~.~.~.~.~.~.~.~, .... ....
13.9 ml
Twin Tail
Cg= 8/29 ml
Buffer
Cg= 8/29 ml (Cu=ratio of C02/Buffer)
11.5ml
9.3 ml
i.6 ml
1.7 ml
23.9 ml
C&,13/24 ml 7500 MWgt., 0.25 g
Fig. 18. Emulsion phase volumes at constant pressure,
Buffer
3.1 ml
27.7 ml
R= 9/28 ml
28.9 ml
R=8/29 ml
(CB=ratio of COz/Buffer) P = 20 MPa, for fluoroether
sulfate surfactants.
EJ. Singley et a l . / F l u i d Phase Equilibria 128 (1997) 199-219
218
Fluoroether Sulfate Surfaetant 13.1 ml
COz
CO2
9.3 ml
1
CO2
2.7 ml
5.5 ml 2.7 ml
1
illlli~i~ZillliZil
-.,-.-.-.-.-.-.-...-.~-.-.-.-.-.-.....-,
!!!!!}!~!!!!!!!!~:~!{!}~!!!!!!!~! 23.9 ml
.-.-.-.-.-.._-.-.-.-.-.--.-.-....-.-.-. ..................... .:-:-:-:.:-:-:+:-:-:-:-:-:-:.:-:-:-:-:.
Buffer
25.0 ml
7-8.8 ml
Buffer
..................... .....................
"5"5"~'g'?'?'5:"5"5"5"5"5"5"5"~:"5:"~: ,.-.-.-...-.~..-...-.-.-.-.-.-.-.-.-.....
i.~.~.~.;~-~.;~.~.i.~.~.i~.~.~. .1.:~.:.:.:.:,:.:.:.:-:.:.:.:.:.:,:.~:.:
C~=14/23 ml
CB=13/24 ml
Single Tail 7500 MWgt., 0.25 g
CB= 8/29 ml
(CB=ratio of CO2/Buffer)
Fluoroether Sulfonate Surfactant
A
14.1ml
C02 .---....--------..-..--..-..------.-..-.. .---.--.--------..---
C02
13.1 ml 1.1ml
1
4.9 ml
CO2
-- "~.7 ml .........
)T"': ........
'.:.:.:.:.:.:-?:.:.:.:.:.::':':'.?::':': .....................
................................. ..................... ..-.-.-.-.-...-.-.--.--.-..-.._-.-..... .---.--.---.--.-..... ..................... ".'.'.'.'.'.'.'.'.'.'.'.'.'.'.-.'.'L .. .-----..-------.--... .....................
22.9 ml
Buffer
22.8 ml
Buffer
29.4 ml
..................... .................................... ........................................ ..........,.-.-.-.-.....,...............,. :.:.:33333L:3 J 3.:.:33333.:. "':":-:"'"'"":'"":':
CB=18/19 ml
CB=17/20 ml
Single Tail 7500 MWgt., 0.25 g
CB= 8/29 ml (CB=ratio of COz/Buffer)
19. E m u l s i o n phase v o l u m e s at constant pressure, P = 2 0 M P a , for sulfate and sulfonate fluoroether-functional surfactants.
Fig.
effect on emulsion behavior shows that the high compressibility of carbon dioxide enables WI --->WIII WII phase transitions which are similar to those found with increased electrolyte concentrations. Additional studies are required to ascertain the impact of temperature and supercritical conditions on emulsion behavior in carbon dioxide.
E J . Singley et a l . / Fluid Phase Equilibria 128 (1997) 199-219
219
Acknowledgements T h e authors t h a n k the N a t i o n a l S c i e n c e F o u n d a t i o n f o r their s u p p o r t o f this p r o j e c t u n d e r grant # C T S - 9 2 5 8 5 0 and EJS t h a n k s the N a t i o n a l S c i e n c e F o u n d a t i o n for f e l l o w s h i p support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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