2942
Langmuir 1993,9, 2942-2948
Phase Behavior of Nonionic Surfactant/Oil/Water Systems Containing Light Alkanes Gregory J. McFannt and Keith P. Johnston* Department of Chemical Engineering, University of Texas, Austin, Texas 78712 Received May 14,1993. In Final Form: August 2 , 1 9 9 9
The phaae behavior of nonionic ethoxylate surfactantllight alkanelwater systems is reported in detail. In compressible liquids such as propane, phase transitions which are normally induced by changing temperature or salinity can also be accomplished with pressure A complete transition from a lower to middle to upper phase microemulsion with pressure is reported for the first time in propane. Widely accepted trends in surfactant phase behavior as a function of the alkane carbon number (ACN) of the oil component reverse themselves in the light alkanes butane, propane, and ethane. This pattern occurs in both reverse micelle systems (small waterto-oil ratio) and Winsor microemulsion systems (water-boil ratio near unity). The observed pressure and ACN effects can be explained qualitatively in terms of the miscibiilty gaps of the binary phase diagrams and quantitatively in terms of enthalpic and entropic interactions between the surfactant tails and the compressible solvent.
Introduction A ternary system composed of oil, water, and surfactant can form a wide variety of aggregated structures.14 T w o characteristic compositions are frequently studied reverse micelle systems in which the amount of oil greatly exceeds the amount of water, and Winsor systems which contain equal amounts of oil and water (or brine). It should be noted, however, that these compositions are simply two discrete points on the continuum of possible ternary oil/ water/surfactant compositions, and both are subject to the same fundamental thermodynamic consideration^.^ Most studies of reverse micelle and Winsor systemshave been carried out in essentially incompressible liquid oils such as isooctane. Recently, these systems have ala0 been studied in compressible liquid and supercritical solvents such as ethane, propane, and butane.8 It has been established by a combination of phase behavior:JO light11J2 and neutron13J4 scattering, spectroscopic probe,11J2J618 + Present address: Unilever Research U.S., 45 River Rd.,Edgewater, NJ 07020. e Abstract published in Advance ACS Abstracts, October 1,1993. (1) McBain, J. W.; Salmon, C. S. J. Am. Chem. Soc. 1920, 42, 426. McBain, J. W.; Field, M. C.'J. Phys. Chem. 1926, 30, 1545. (2) Hartley, G. S. Kolloid-Z. 1939,88, 22. (3) Ekwall, P. Adu. Liq. Cryst. 1975, 1, 1. (4) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface. Sci. 1970, 33, 215. (5) Zulauf, M.; Eicke, H. F. J. Phys. Chem. 1979,83,480. (6) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface. Sei. 1984, 97, 9. (7) Bowel, M.; Schechter, R. S. MicroemulsionsandRelclted Systems, Marcel Dekker: New York, 1988. (8) Gale, R. W.; Fulton, J. L.; Smith, R. D. J.Am. Chem. SOC. 1987, 109, 920. (9) Fulton, J. L.; Smith, R. D. J. Phys. Chem. 1988, 92, 2903. (10) Eastoe, J.;Robinson,B. H.; Steytler,D. C.J. Chem. Soc., Faraday Trans. 1990,86, 511. (11) Fulton, J. L.; Blitz, J. P.; Tmgey, J. M.; Smith, R. D. J. Phys. Chem. 1989,93,4198. (12) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J. Phys. Chem. 1990,94, 781. (13) Eastoe, J.; Young,W. K, Robinson, B. H.; Steytler, D. C. J.Chem. Soc., Faraday Trans. 1990,86, 2883. (14) Kaler, E. W.; Billman, J. F.; Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1991,95,458. (15) Johnston, K. P.; McFann, G. J.; Lemert, R. M. In Supercritical Fluid Science and Technology; Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406, American Chemical Society: Washington, DC.,-l989. (16) Smith, R. D.; Blitz, J. L.; Fulton, R. D. In Supercritical Fluid Science and Technology;Johnston, K. P., Penninger, J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989.
0143-1463/93/2409-2942$O4.OOJO
and FTIR1s21 studies that reverse micelles of the ionic surfactant AOT (bis(2-ethylhexyl)sodium sulfosuccinate) or of nonionic poly(ethy1ene oxide) ether surfactants in compressible light alkanes are structurally similar to those that are found in heavier incompressible alkanes. The aggregation number and interior polarity of AOT in supercritical ethane and compressed liquid propane were found to be insensitive to pressure in the one-phaseregion except in the immediate vicinity of phase transition In contrast, the phase behavior and microstructure of AOT systemswhich have a bulk aqueous phase in addition to a bulk oil phase (and in some cases a surfactant-rich middle phase) may be manipulated over a wide range with Compressed liquid or supercritical alkanes are especially useful for the study of solvent effects on surfactant behavior because their solvent strength can be varied continuously by changing pressure. An interesting new result from studies of AOT systems in light alkanes is that trends in surfactant phase behavior as a function of solvent alkane carbon number (ACN) are the opposite of those previously established for higher alkanes.%n This change in behavior is present in both reverse micelle systems and Winsor system^.^^^^^ The phase behavior of AOT systems is observed to be symmetricabout an axis located between pentane and hexane. Additional light can be shed on this symhetric behavior by examining nonionic surfactants, whose structures may be varied systematically, furthermore it is not necessary to add any salt. The objective of the present work is to examine the effects of both pressure and alkane carbon number on the phase behavior of nonionic surfactants in ethane through (17) Yazdi, P.; McFann, G. J.; Fox, M. A.; Johnston, K. P. J.Phys. Chem. 1990,94, 7224. (18) Zhang, J.; Bright, F. V. J.Phys. Chem. 1992, 96,5633. (19) Fulton, J. L.; Yee, G.G.; Smith, R. D. J.Supercrit. Fluids 1990, 3, 169. (20) Yee, G. G.; Fulton, J. L.; Blitz, J. P; Smith, R. D. J.Phvs. Chem. 1991,95, 1403. (21) Yee, G. G.; Fulton, J. L.; Smith, R. D. Langmuir 1992,8, 377. (22) McFann, G. J.; Johnston, K. P. J. Phys. Chem. 1991,96,4889. (23) Lemert, R. M.; Fuller, R. A,: Johnston, K. P. J.Phvs. Chem. 1990. 94,6021. (24) Johnston, K. P.; Peck, D. G.; Kim, S. Ind. Eng. Chem. Res. 1989, 28,1115. (25) Hou, M. J.; Shah, D. 0. Langmuir 1987,3, 1086. (26) Tingey, J. M.; Fulton, J. L.; Smith, R. D. J.Phys. Chem. 1990,94, 1997. (27) Aveyard, R.; Binks, B. P.; Mead, J. J.Chem. SOC.,Faraday Trans. 1 1986,82, 1755.
0 1993 American Chemical Society
Langmuir, Vol. 9, No. 11, 1993 2943
Phase Behavior of SurfactantlOillWater Systems
dodecane. It is expected that the presence of a compressible component will lead to significant effects of pressure on phase transitions. Results are presented for both reverse micelle systems and Winsor systems. The results are interpreted in terms of the miscibilities of the binary pairs and a rigorous molecular theory for microemulsions in compressed solvents.
[C12E5]
-
5.0 wt. %
0 OL-
Summary of Phase Behavior The structures of aggregates formed in ternary water/ oil/surfactant systems depend on a large number of variables. These include (but are not limited to) the size o b 20 40 ' 60 80 and hydrophile-lipophile balance of the surfactant,28the wo proportions of the three c ~ m p o n e n t stemperature,30 ,~~ the Figure 1. Phase diagram for the heptane/water/Cl2Es system. presence of c ~ s u r f a c t a n tor s ~of ~ and the molecular volume of the oil c o m p ~ n e n t . ~ ~ . ~ ' lgepal CO-520 Changes in the variables cause changes in the interactions . oil:water 1:l by volume at the surfactant interface, which affect its curvature. This has been shown by both qualitative argumenta32935936 and quantitative thermodynamic model^.^^-^^ 30 Reverse Micelle Systems. Ternary systems which consist of surfactant, oil, and relatively small amounts of water may be called reverse micelles systems. The phase behavior of both AOT and nonionic surfactant reverse 2 micelle systems have been well ~ t u d i e d .For ~ ~ the ~~~ surfactants of interest in this study, the general trend is 2o that as ACN increases from hexane to dodecane,the alkane becomes a poorer solvent for the surfactant, and the aggregation number of thereverse micellesand their ability 'Od 2 4 6 8 '1'0 1 9 2 ' 1 4 ~ 1 ' 6 ' 1 ' 8 ' 2 b to solubilize water decrease. [surfactant], wt. % Nonionic surfactant reverse micelles are sensitive to Figure 2. Phase behavior for the octane/water/IgepalCO-620 temperature and solubilize large amounts of water only (nonylphenol ethoxylate) system. A characteristic "fiih"shape over a narrow temperature range. An example of such is observed with a head at lower concentrations and a tail at behavior is shown in Figure 1for a C12Edwaterlheptane higher concentrations. system. Water solubilization is reported in terms of W O (molar water-to-surfactant ratio) as is often done for and water form a surfactant-rich second phase due to reverse micelle studies. The upper phase boundary of the micelle-micelle interaction~.a*4~>~3 one-phase region, which is shown by open circles, is called Winsor Systems. Winsor systems aretemary oil/water the solubilization Above this temperature (or brine)/surfactant systems which contain equal amounta the reverse micelles expel excess water into a second phase. of water and These classic systems are designated The location of the solubilizationcurve is governed by the as type I (normal micelles in equilibrium with excess oil, natural curvature of the surfactant interface.25 The lower often designated 2 which means two phases with the boundary of the one-phase region, shown by solid circles, surfactant in the lower phase), type I1 (reverse micelles in is the haze point curve. Below this temperature, surfactant equilibrium with excess water, 2), type I11 (middle phase microemulsion with excess water and excess oil phases, 3), and type IV (one-phasemicroemulsion,1).The boundaries (28) Shinoda,K.; Friberg, S. E. Emulsions and Solubilization; Wiley New York, 1986. between the Winsor types on a plot of temperature vs (29) Shinoda, K.; Kunieda, H.;Arai, T.;Saijo,H. J.Phys. Chem. 1984, surfactant concentration have a characteristic"fieh" shape, 88,5126. as shown in Figure 2 for a nonylphenol ethoxylate/odane/ (30) Lindman, B.;Wennerstrom, H. J. Phys. Chem. 1991, 95,6053. Bjorling, M.; Karlstrom, G.; Linse, P. J. Phys. Chem. 1991,95, 6706. water system. As temperature is increased, e.g. at 6 wt % (31) Strey, R.; Jonstromer, M. J. Phys. Chem. 1992,96,4637. (32) Kahlweit,M.;Strey,R.;Firman,P.;Haase,D.;Jen,J.;Schomacker, surfactant, there is a transition from 2 to 3 to 3 as the surfactant moves from water to middle phase to oil. R. Langmuir 1988,4,499. (33) Sassen, C. L.; Gonzalez Casiellas, A,; de Loos, T. W.; de Swam The effect of the ACN of the oil component on Winsor Arons, J. Fluid Phase Equilib. 1992, 72, 173. systems has often been studied for liquid alkanes. For (34) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.Langmuir 1986,1, example, Wormuth and Zushma&showed that in the series 281. (36)Kahlweit, M.; Strey, R.; Schomacker, R.; Haase, D.Langmuir of alkanes from octane to tetradecane the "fish" for both 1989, 5, 306. straight-chain and branched nonionic poly(ethy1ene oxide) (36) Kahlweit, M.; Strey, R.; Busse, G. J. Phya. Chem. 1990,94,3881. ether surfactants grow larger and move to higher tem(37) Overbeek, J. Th. G.; Verhoeckx, G. J.; de Bruyn, P. L.; Lekkerkerker, H. N. W. J . Colloid Interface Sci. 1987,119, 422. perature ranges as ACN increases. They also found that I I
-
t
'
~~
~
(38) Peck, D. G.; Schechter, R. S.; Johnston, K. P. J. Phys. Chem. 1991,95, 9641. Peck, D. G.; Johnston, K. P. J. Phys. Chem. 1993,97, 5661. (39) Peck, D. G.; Johnston, K. P. J. Phys Chem. 1991,95,9549. (40) Shinoda,K.; Ogawa, J. J. Colloid Interface Sci. 1967,24, 56. (41) Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1970,34,221. (42) Kon-no, K.; Kitahara, A.; El Seoud, 0. H. In Nonionic Surfactants: Physical Chemistry;Schick,M.J.,Ed., MarcelDekker: New York, 1987.
(43) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1989,6, 1209. (44)Winsor, P. A. Solvent Properties of Amphiphilic Compounds; Butterworths: London, 1954. (45) McFann, G. J. Ph.D. Dissertation, University of Texas,Austin, TX,1993. (46) Wormuth, K. R.; Zuahma, S. Langmuir 1991, 7,2048.
McFann and Johnston
2944 Langmuir, Vol. 9,No.11,1993 the optimum point of the system, which is the lowest surfactant concentration at which it is possible to emulsify oil and water completely (the point where the body of the "fish" meets the tail), moves to higher temperatures and higher surfactant concentrations as ACN increases. Studies of ACN effects on other nonionic surfactant systems have shown the same trend^.^^^^^^ Pressure effects on Winsor systems for conventional alkane solventsare normally quite small unless the system is already close to a phase transition point. This is because both the oil and water components are almost incompressible. Different ionic surfactant systems show different responses to pressure.351In nonionic poly(ethyleneoxide) ether systems the work of Sassen et al.=v51 indicates that the effect of increasing pressure is to bring about a 2 - 3 - 2 transition.
80
70
4.7
7.0
60
G
WO = 11.8 50
c
40
8 .,6
30
C12E5
1
12.8,
Experimental Section Materials. The nonionicsurfactants C I A , C12E5, and C I A , I which consist of a 12-unit hydrocarbon tail attached to four, five, 2? 1 250 350 and six ethylene oxide groups, respectively, are highly purified single component surfactants obtained from Nikko. The nonP (bar) ylphenol ethoxylates Igepal CO-520and CO-530 are commercial surfactants manufactured by Rhone-Poulenc which contain Figure 3. Solubility loops for c1& and C1& in propane for several closely related components. AOT (bis(2-ethylhexyl) The system are various values of water to surfactant ratio (WO). sodium sulfosuccinate) was obtained from Fluka and purified one phase inside the loops and two phase outside. according to a standard procedure.15 Purified, deionized water was prepared as needed in a Technic Central Systems Lab Five of an oil-in-water Winsor type I system. The W m r type 111 system. The liquid alkanes from pentane through dodecane system is marked by a middle phase in which both methylene (various vendors) and the compressed alkanes ethane (Big 3, blue and Sudan 111are solubilized. Control systems without dye 99%), propane (Liquid Carbonic, 99.9%), and butane (Big 3, were also run to make sure that the presence of the dyes was not 99.0%) were used as received. The polar dye methylene blue changing the phase boundaries. Similar experiments were run and the nonpolar dye Sudan I11 were from Aldrich. with compressed solvents in the variable volume view cell. Methods. Surfactant solutionsin various n-alkaneswere made Methylene blue was added to all the compressed systems to dietinguish between the various Winsor types. up in vials and placed in a controlled temperature water bath. The temperature was then scanned slowly in 5 O increments from 5 to 70 OC, with shaking between temperature increments. The Results boundaries between one-phase and two-phase regions were quite Reverse Micelle Systems. As was stated in the sharp, with phase changes occurring over a temperature range Introduction, we define reverse micelle systems as those of less than 1 OC. Then water was added to the vials, and the in which the amount of oil greatly exceeds the amount of temperature scans were repeated. The entire one-phase region water. Figure 3 shows the maximum (or saturated) water of a plot such as Figure 1can be mapped out by a series of such to surfactant ratio Womtvalues for C12E5 and C~aEsreverse experiments. For compressed solvents a variable volume view cell was used. The entire cell was placed in a watar bath, and micelles in propane as a function of temperature and phase boundaries could be determined as a function of pressure pressure. Since the density and solubility parameter of at constant temperature or temperature at constant pressure. propane are relatively sensitive to pressure,22the phase Measured amounts of water were added to the cell by fiiing a boundaries close in on themselves to form loops, one for sample loop attached to a Valco six-port HPLC valve and then each Womt. The systems are one-phase inside the loops forcing the water into the cell with a motor-driven injector. The and two-phase outaide the loops. The loops for C12& lie procedures for water injection and phase boundary determination at a higher temperature than those for C12& because the in compressed solvents are the same as those that were used in ethylene oxide group makes C12& more hyour earlier studies on surfactanta in compressed s 0 1 v e n t a . ~ ~ ~ additional ~~ drophilic. Thus a higher temperature is required to c a w Winsor systems were also made up in vials and placed in the to dissolve and aggregate in a nonpolar solvent such water bath. They were shaken and allowed to equilibrate at a number of temperatures until the temperature boundaries as propane. The Woeatvalues are much smaller than those between the Winsor types were located. Then the vials were that are seen in a liquid solvent like which opened, more surfactant was added, and the boundaries between suggests that propane is a relatively poor solvent. the types were again located. A series of experimenta of this sort An example of the effect of ACN is shown in Figure 4 results in a 'fish" plot such as Figure 2. Dyes were added to the for the pure surfactant C12E5. For nonane through pentane systems to verify the type of phase behavior present. A twothe trends in phase behavior agree with the l i t e r a t ~ e . ~ ~ ~ ~ ~ phase system with the blue color of the water-soluble dye That is, the maximum W Oof the one-phaseregion increases, methylene blue in the oil phase marks the water-in-oil structures and the one-phase region itself is shifted to lower temof a Winsor type I1 system. In the same way, the orange color peratures, as shown in Figure 4a. For solvents lighter than of the oil-soluble dye Sudan I11 in the water phase is indicative (47) Kilpatrick, P. K.; Gorman, C. A.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J.Phys. Chem. 1986,90, 5292. (48) OConnell, J. P.; Kim, J. D.; Coram, P. T.; B r a g " , R. J. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1981,26, 123. (49) Fotland, P.; Skauge, A. J Dispersion Sci. Technol. 1986, 7,563. (50) Kim, M. W.; Gallagher, W.; Bock, J. J. Phys. Chem. 1988, 92, 1226. (51) Sassen, C. L.;Filemon, L.M.; de Loos, T W.; de Swaan Arons, J. J.Phys. Chem. 1989,93,6511. Sassen, C. L.;de Loos, T. W.; de Swaan Arons, J. J. Phys. Chem. 1991,95, 10760.
pentane, however, these trends reverse themselves, as shown in Figure 4b. The maximum W Odecreases and the one-phase region returns to higher temperature intervals as ACN decreases. Thus the trends in phase behavior are symmetric about pentane. Since the behavior shown in Figure 4b has not been reported before, it is important to determine whether the (52) Kizliig, J.; Stenius, P. J. Colloid Interface Sci. 1987, 118, 482.
Langmuir, Vol. 9, No.11, 1993 2945
Phase Behavior of SurfactantlOillWater Systems 50
2 100
c
~~
-
ti
I
hexane
c5
-
c6
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c7
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.
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.
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.
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Figure 5. WO-* for 0.07 M Aerosol OT as a function of density for various hydrocarbons. propane, 276 bar
phase boundaries have the same physical significance as those for heavier alkanes in Figure 4a. In the case of the pentane and butane systems, the upper phase boundary is marked by a gradual fogging of the solution due to precipitation of water as temperature increases, which is consistent with a solubilization mechanism. The lower haze point boundary is marked by the sudden precipitation of surfactant and water out of solution, which is wellknown in liquid s y ~ t e m s . ~In ~ 9the ~ ~propane system the solubilization of water is so low that it is difficult to distinguish between the two types of phase transitions. Despite this exception, the relative position of the haze point and solubilization curves appears to be the same for both light and heavy alkanes. Water solubilization in ionic AOT reverse micelle systems has been studied previously by measurements of WOat the phase boundary, WoBat,at a fixed temperat ~ r e . ~ ~The , ~data ~ , are ~ ~in, reasonable ~ ~ agreement for the heavy solvents octane, nonane, and decane, and for the light solvents ethane, propane, and butane, but the results are not in agreement for the intermediate solvents pentane, hexane, and heptane.17 The Wosatexperiments have been conducted in the past by titrating an AOT solution with water up to the first appearance of a phase
change. It is difficult to observe this phase boundary, however, and temperature has to be controlled precisely.6*u The best way to find WoSatis to map out the entire onephase region, just as was done for C12E5 in Figure 1. One can then draw a constant temperature line across the temperature vs WOplot to determine WoSat. Such an experiment has been carried out for 0.07 M AOT solutions in pentane, hexane, heptane, and 0ctane.4~ When the new WoSatdata for pentane through octane are combined with the results reported earlier," all at 25 "C, a smooth trend in WoSatis obtained as a function of solvent density for solvents from ethane through decane (Figure 5). The Womtresulta are symmetric about n-pentane, as was the case for the nonionic surfactant in Figure 4. The other significant point about Figure 5 is that all of the WoSatdata points are located on the haze point curves in their respective solvents. This was shown for pentane through octane by phase behavior and also for propane.22 Therefore, intermicellar interactions determine this type of phase boundary in both light and heavy alkanes. Winsor Systems. Initial experiments on propane/ brinelnonionic surfactant systems were carried out using C12E4 and C12E6 as the surfactants. These systems were found to be surprisingly lipophilic in propane. That is, the "fish" for C12E4 and appeared to lie well below room temperature, if they existed at all. Finally the hydrophilic surfactant C12Ea was used, with 5.0 w t % NaCl added to the water phase to bring the 'fish" down to a reasonable temperature range. The results are shown in Figure 6. As expected, the system is in the2 configuration at low temperatures and in the 9 configuration at high temperatures. The "tail" region of the "fish" appears to consist of a one-phasemicroemulsion (the 1configuration) in equilibrium with a liquid crystal precipitate, which is similar to nonionic systems observed by Kahlweitet al.3234 The "fish" shift to higher temperatures as pressure decreases from 345 bar to 276 bar to 207 bar. The upper boundary of the "fish" moves up in temperature much faster than the lower boundary, so the net effect is a large increase in the size of the "fish". Since the upper boundary is the one associated with the compressible oil phase, its
(53) Middleton, M. A.; Schechter, R. S.; Johnston, K. P Langmuir 1990, 6, 920.
(54) Kenez, P. H.;Carlstrom, G.; Furo, I.; Halle, B.J. Phye. Chem. 1992,96,9524.
fl butane, 276 bar
penta
I
01 A I)
,
, 20
,
,
,
40
. 60
80
100
wo
Figure 4. One-phase region for 5.0 wt % ClzEs in various alkanes: (a, top) high alkane carbon number (ACN);(b, bottom) low ACN. Open symbols are the solubilization boundaries;closed symbols are the haze points.
McFann and Johnston
2946 Langmuir, Vol. 9,No. 11, 1993 0
P-207bar
0
P=276bar
60
b
30~
5
’
10
15
Figure 6. “Fish”for the Cl2E$propane/brinesystem as a function
of temperature.
2
Od
’
4
’
6
’
8
’
1’0
’
1’2
’
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oil phase ACN
Figure 8. Microemulsion phase behavior for 5.0 wt % Igepal CO-520 in an oil/water/surfactantsystem. Butane and propane data are at 276 bar; others are at ambient pressure.
I pressure because of the enhanced ability of the oil
“0
5
10
15
20
[C12E6], wt. Yo
Figure 7. “Fish”for the Cl2E$propane/brinesystem as a function of pressure. movement with pressure is quite understandable. Figure 6 also shows clearly that the optimum point where the “head” and “tail” of the “fish” meet moves to higher surfactant concentrations and higher temperatures as pressure decreases. The changes in size and position of the “fish” with pressure are in accordance with the studies of Wormuth and Z ~ s h m a .They ~ ~ reported that the optimum point shifts to higher temperatures and higher surfactant concentrations as the oil becomes a poorer solvent for the surfactant. Since the solubility parameter its solvent of propane decreases as pressure strength is reduced, and so the “fish” shift as expected. Experiments were performed on the propane/brine/ C12E6 system to determine whether a 2 - 3 - 2 transition can be created by means of pressure alone. The results are shown in Figure 7. It was found necessary to reduce the NaCl concentration in the brine component of the propane/brine/ClzEa system to 3.0 wt 5% in order to make the 2 3 - 2 transitions lie within the pressure limits of the variable volume view cell apparatus. The work of Kahlweit et al.= on the decane/brine/AOT system provided an example of a pressure versus composition “fish”, but Figure 7 is the first experimental confirmation of this behavior for a nonionic surfactant and a compressible alkane. Both the propane/brine/ClzE6 and propane/brinel AOT systems show 2 - 3 - 2 transitions with increasing
-
0
20
[ C 1 2 E 6 ] (wt. %)
400
0
lot
component to solubilize the surfactant as pressure increases. To examine the effect of ACN on nonionic Winsor systems, the commercial surfactant Igepal CO-520 was used. This surfactant has as ita main component a nonylphenol tail attached to five ethylene oxide units. Ita three-phase region lies in a convenient temperature range. Furthermore, multicomponent surfactants have been found to be much more stable in their phase behavior with respect to the presence of impurities than are singlecomponent surfactants.’ Figure 8 shows the domains of the Winsor types for alkane/water/Igepal CO-520 systems as a function of temperature and ACN for a surfactant concentration of 5.0 w t % . The right half of Figure 8, from hexane through dodecane, shows the same trend in phase behavior that has been seen in previous That is, the threephase region grows steadily wider and moves to higher temperature ranges as ACN increases. However, as ACN decreases below six the trend reverses itself, and the threephase region moves back up in temperature and again grows wider. The same experiment was conducted using 5.0 wt 5% Igepal CO-530, which has one more ethylene oxide unit than Igepal CO-520. Again the same pattern in the three-phase region is seen, but it is shifted upward in temperature 15 “C by the increased hydrophilicity of the surfactant. Thus the nonionic Winsor systems and the nonionic reverse micelle systems show a symmetry in their phase behavior as a function of ACN. However, the Winsor systems are centered around hexane, whereas the reverse micelle systems are symmetric about pentane. Figure 9 shows “fish” for Igepal (20-520 in a aeries of alkane solvents. Compared to the relatively symmetrical “fish” of Kahlweit et al.,32f34 and even the “fish” of Figures 6 and 7, the Igepal CO-520 “fish” appear distorted, particularly at higher surfactant concentrations. These distortions are caused by differential partitioning of surfactant components between water and oil phases.& The nominal structure of Igepal CO-520 is a nonylphenol tail coupled to a headgroup of five ethylene oxide units, but the surfactant is actually a mixture of closely related components, especially nonylphenols with fewer than five ethylene oxide units. These more lipophilic components (55) Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93,474.
Phase Behavior of SurfactantlOillWater Systems 601’ ” .
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.
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.
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40
3 I-
Discussion The effects of ACN and pressure may be understood with a variety of thermodynamic descriptions of phase behavior and interactions at the surfactant interface.w2 We focus here on two of them: (1)a qualitative description of binary and ternary phase behavior as formulated by and (2) a quantitative unifiedclassical Kalhweit et and molecular thermodynamic theory of Peck et al.38*Se QualitativeDescription Based on Phase Behavior. In the qualitative description the key parameters that determine the phase behavior of a ternary oil/water/ surfactant system are the miscibility gaps of the oilsurfactant and water-surfactant binary subsystems. Their relative locations are indicated by the upper critical solution temperature (UCST), T,, of the oil-surfactant binary and the critical solution temperatures of the watersurfactant binary. Nonionic surfactants in water normally have a lower critical solution temperature (LCST), To,for the typical temperature range encountered in surfactant studies. A change in the ACN of the oil component of a ternary oil/water/surfactant system normally has no discernible effect on the oil-water binary subsystem, and the watersurfactant binary is by definition unaffected, so ACN effects can be described in terms of the oil-surfactant binary alone. Kahlweit et al. have shown3’ that nonionic surfactants become less soluble in the oil phase as ACN increases, so T,rises. The further T,increases above Tb, the larger the body of the “fish”becomes in a temperature versus composition plot. Also, the “fish” shifts to higher temperatures. The above explanations of ACN effects were devised originally on the basis of experimental results for alkanes heavier than hexane. However, the same arguments can be used to interpret the light alkane results reported in this paper. As ACN decreases below six, the surfactant becomes less soluble in the oil phase. Therefore the oilsurfactant miscibility gap gets _larger and T, increases. The increase in T, produces a 2 - 3 - 2 transition from hexane to ethane, as shown for nonionic surfactant systems in Figures 8 and 9 and for previously studied AOT systems.22 Pressure effects can be explained by much the same reasoning as was used for ACN effects. For typical alkanes, there is a competition between pressure effects on the oil-surfactant and water-surfactant binaries. This competition is the origin of the different pressure effects observed in the literature which were mentioned in the phase behavior section above. An experimental demonstration of these competitive pressure effects is given in the work of Sassen et al.33on the dodecane/water/C7Es system. They plotted the miscibility gaps of the dodecaneC7E5 and water-C7Es subsystemsas a function of pressure. Both T,and Tb increased with pressure, but T,increased faster than Tb,so the net effect of increasing pressure on the ternary dodecane/water/C,E5 system was to increase the size of the “fish” and move it to higher temperatures. If the oil component of the ternary system is a compressible one such as propane, the pressure effect is al.32*35136
w
ow
Langmuir, Vol. 9, No. 11,1993 2941
’
30-
20 -
m 5 10 15 20
Oi,
[surfactant], wt.
k
Figure 9. “Fish”for Igepal CO-520 in a series of alkanes at (a, top) high ACN and (b, bottom) low ACN. partition into the oil phase ahead of the main component as temperature increases. At higher surfactant concentrations there are enough of these components to form reverse micelles in the oil phase, so the boundary between the 3 and 3 configurations is forced to alower temperature than would be the case for a pure component nonylphenol surfactant. The result is the characteristic narrowing of the “fish”at higher surfactant concentrations seen in Figure 9. The smallest “fish” in Figure 9a is the one for hexane. Its optimum point (where “head” meets “tail”) lies at the lowest temperature and surfactant concentration. For solvents heavier than hexane the “fish” get larger and the optimum point moves to higher temperature and higher surfactantconcentration,just as was observed by Wormuth and Zushma.& For solvents lighter than hexane, the same trend can be seen in Figure 9b. That is, the “fish” for pentane and for propane at 345 bar are larger than the hexane “fish” and are shifted to higher temperature and surfactant concentration ranges. The propane “fish” in particular is very large and extensively intercepted by liquid crystal regions, which suggests that the size and solvent strength of propane are approaching the lower bound of that which is necessary to solubilize the large Igepal CO-520 surfactant molecules. The symmetry in the phase behavior about hexane is similar to that in AOT Winsor systems,22except that temperature effects are the opposite for ionic versus nonionic surfactants.
(56) Ruckenstein, E.; Nagarajan, R. J. Phys.Chem. 1980, 84,1349. (57) Larson, R. G.; Scriven, L. E.; Davis, H. T. J. Chem. Phys. 1985, 83, 2411. (58) Woods,M. C.; Haile, J M.; O’Connell, J. P. J. Phys.Chem. 1986, 90, 1875. (59) Blankschtein, D.; Thurston, G. M.; Benedek, G. B.J.Chem. Phys 1986,85, 7268. (60) Dawson, K. A.; Lipkin, M. D.; Widom, B. J. Chem. Phys. 1988, 88, 5149. (61) Brown, D.; Clarke, J. H. R. J.Phys. Chem. 1988,92,2881. (62) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934.
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2948 Langmuir, Vol. 9, No. 11,1993
overwhelmingly larger on the oil-surfactant binary than on the water-surfactant binary. Increasing pressure increases the solubility parameter of the oil, which makes the surfactant and oil more miscible for the systems of this study. Thus T, is reduced and the system is driven toward the 2 configuration. This trend is the cause of the -2 - 3 - 2 transition in the pressure-driven “fish” of Figure 7 and the changes with pressure in the upper temperature boundary of the “fish” of Figure 6. QuantitativeDescription Based on a Unified Classical and Molecular Thermodynamic Theory. A model of Peck et al.38939 is of special interest for this study because it was developed in conjunction with experimental work on the phase behavior of AOT in compressed It*employs a classical thermodynamic de~olvents.l~ ~~ scription of the surfactant interface.63 The interfacial curvature is calculated by considering separately the intermolecular interactions on the oil and water sides of the interface. On the oil side, interactions between the oil and surfactant tails depend upon an enthalpic contribution characterized by a Flory parameters4 and on an entropic contribution based on the ability of the oil molecules and The surfactant tails to fill a lattice space effi~iently.~~ entropic contribution describes the ability of oil molecules to penetrate between surfactant tails, which is influenced by the similarity in chain 1ength.M The oil-tail interaction, which is the one most affected by ACN, decreases from the optimum because of enthalpic and/or entropic restrictions. For AOT, alkanes heavier than hexane are hindered in their ability to penetrate between the surfactant tails. Alkanes lighter than hexane penetrate very well, but their solubility parameters are too small in comparison to those of surfactant tails. Therefore the systems on each side of hexane are less stable than the hexane system and, hence, are effectively more hydrophilic. This results in symmetry in phase behavior of AOT Winsor systems as a function of ACN about hexane.22 The above mechanism for the enthalpic and entropic oil-surfactant tail interactions also applies to Winsor systems containing nonionic surfactants. Thus the nonionic Igepal CO-520 systems of Figures 8 and 9 exhibit symmetrical ACN effects. In addition, this mechanism is also operative in reverse micelle systems. Here an optimum oil-tail interaction leads to the largest water uptake ( Womt). In AOT systems, the optimal interaction and maximum Womt are found at pentane (see Figure 5). For C12E5 systems (Figure 41, the oil-tail interactions are responsible for the maximum in water solubilization ( W P ) at pentane, and the symmetrical progression in size and temperature intervals for W p t in solvents lighter and heavier than pentane. (63)Gibbe, J. W. The Scientific Papers of J. W. Gibbs; Longmana:
The results of this study indicate that surfactant aggregation becomes increasingly difficult in light solvents where the enthalpic contribution to the oil-tail interaction is insufficient. These weak enthalpic interactions can limit formation of surfactant aggregates in C02. The dispersion interactions between hydrocarbon tails and C02 are even weaker than those for ethane, because of COis lower polarizability per volume.67 Therefore, cosurfactants or new types of surfactants are required to form reverse micelles and microemulsions in C02.67-69
Conclusions The trends in phase behavior for nonionic surfactants in a series of n-alkane solvents, which were previously thought to be monotonic, are reversed for the compressible alkanes butane, propane, and ethane. This work confirms the ideas derived from our earlier studies of AOT syst e m ~ Reversals . ~ ~ ~ in~phase ~ behavior with ACN occur both in reverse micelle systems and in Winsor microemulsion systems. In reverse micelle systems (small waterto-oil ratio), this reversal in trends is observed for Womt. In Winsor systems (water-to-oil ratio near unity), this reversal is observed in the location of the “fish”,specifically the temperature at the midpoint of the three-phase region. For surfactants with tail lengths in the range considered in this study, the reversals in both reverse micelle and Winsor systems occur in the vicinity of pentane and hexane. Pressure has a large effect on phase behavior of nonionic surfactants in light alkanes due to their large compressibilities. A complete pressure versus surfactant concentration “fish” diagram has been presented for the first time in propane, with a 2 - 3 - 2 transition with an increase in pressure. Pressure also causes large shifts in temperature-driven 2 - 3 - 2 transitions for nonionic surfactants in propane. Both ACN and pressure effects on surfactant phase behavior can be explained in terms of enthalpic and entropic contributions to the interactions between the surfactant tails and oil. In the lightest alkanes, surfactants are not very soluble due to poor enthalpic solvation, which leads to emulsification failure through micelle-micelle interactions. This phenomenon partially accounts for the difficulty of forming microemulsions in c02. Acknowledgment. We acknowledge support from the National Science Foundation under Grant No. CTS8900819, the Separations Research Program at the University of Texas, the State of Texas Energy Research in Applications Program, and the Camille and Henry Dreyfus Foundation for a Teacher-Scholar Grant (K.P.J.). We wish to thank R. Schechter, B. Robinson, and J. Eastoe for helpful discussions.
New York, 1975. Lam,A. C.; Falk, N. A.; Schechter, R. S. J. Colloid Interface Sci. 1987, 120, 30. (64) Flory, P. J.; Krigbaum, W. R. J. Chem. Phys. 1950,18, 1088. (65) Mukherjee, S.; Miller, C. A.; Fort, T. J. Colloid Interface Sci. 1983, 91, 223.
(66)Baneal, V. K.; Shah,D. 0.;OConnell, J. P. J. Colloid Interface
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(67) McFann, G. J.; Howdle,S. M.; Johnston, K. P. SubmittedtoAIChE
(68) Iezzi, A.; Enick, R.; Brady, J. Am. Chem. SOC.Symp. Ser. 1989, No. 406. Consani, K. A.; Smith, R. D. J. Supercrit. Fluids 1990,3, 51. (69) Hoefling, T. A,; Enick, R. M.; Beckman, E. J. J. Phys. Chem. 1991,95,7127.
