J . Phys. Chem. 1991, 95, 4889-4896
Figure 9. Effect of the deuterium substitution on the relaxation frequencies of the low- and high-dielectric dispersions. The full line rep resents, in a linear plot, a straight line of slope d?.The slopes of the straight lines that fit the low- and high-frequency dispersion separately are 1.5 f 0.3 and 1.42 0.06, respectively. These figure are consistent with a unique process for the relaxation frequencies of the two dielectric dispersions.
Deuterium Substitution Effect. Further support to the involvement of protons in the observed dielectric behavior of the polymer investigated arises from the effect of deuterium substitution. The isotope effect has been measured on two different samples (monomers and a fraction of intermediate molecular weight) dispersed in heavy water at different pD values in the
4889
range from 5.5 to 8.5, under similar experimental conditions. The overall phenomenology observed is very close to that found on similar samples dissolved in normal water. The effect of deuterium substitution is equivalent to a shift of the relaxation time, which should be higher by a factor of d? in D 2 0 than in H20. This is in fact the expected ~ h a n g e ~ in ~ Ja6process involving protons. Figure 9 shows the relaxation frequencies of the low- and high-frequency dispersion process for the polymer solution of chitosan in normal water and heavy water, respectively, the comparison being made a t equal pH and pD values. As can be seen, the relaxation frequencies of both the relaxation processes fall with reasonable agreement on a straight line of slope d?. The fact that the two separate dispersion regions have been observed to shift toward lower frequencies when the deuterium substitution is present agrees with the view that observed dielectric dispersions are due to proton fluctuation along the macromolecular chain. These results represent a good indication that the transport process involving protons through different charge sites on the polyion in solution is the dominating contribution to the dielectric relaxations observed on the samples studied here. However, the details of these mechanisms remain unclear and must be investigated more accurately. (35) Pethig, R. Dielecrric and Electronic Properiies of Biological Marerials; Wiley: Chichester. 1979. (36) Cared, G.;Geraci, M.; Giansanti, A.; Rupley, . . J. A. Proc. Nail. Acad. Sci: U.S.A. 19& 82, 5342.
Phase Behavlor of AOT Microemulsions In Compressible LlquMs Greg J. McFann and Keith P. Johnston* Department of Chemical Engineering, The University of Texas, Austin, Texas 78712 (Received: September 17. 1990; In Final Form: December 1 1 , 1990}
The phase behavior of bis(2-ethylhexyl) sodium sulfosuccinate (A0T)-alkanebrine systems is described over a wide range of pressure, temperature, and salinity for alkanes from ethane to dodecane. The partitioning of AOT between the oil, middle, and brine phases is reported for propane in order to determine the natural curvature. This is important for understanding separation processes with water-in-oil microemulsions. For the lighter, more compressible alkanes, the pressure effect on the hydrophilicity of the surfactant is much larger and in the opposite direction as for the heavier, less compressible ones. In propane at constant temperature and salinity, water-in-oil (w/o) microemulsions have been converted to middle phase microemulsions and then to oil-in-water (o/w) microemulsions by decreasing the pressure. These phase inversions are described in terms of the immiscibilities in the binary systems, and the molecular interactions at the surfactant interface. Although temperature and salinity are used commonly to manipulate interactions primarily on the water side of the interface, these results show it is possible to control interactions on the oil side by adjusting the pressure. The well-established trends in the phase behavior and size of microemulsion drops for dodecane through hexane are not observed for the lighter alkanes. For butane through ethane, a new unusual behavior is identified and attributed to a significant decrease in the strength of the attractive interactions between the surfactant tails and the alkane.
Introduction
The effect of pressure on microemulsion phase behavior for systems composed of surfactant, oil, and brine has been studied previously for liquid solvents,'J but not for highly compressible liquids such as ethane, propane, and butane. For relatively incompressible alkanes, pressure has little effect on the phase be( I ) Turkevich, L. A.; Mann, J. A. Lmgmuir 1990, 6,457. Kim, J. D.: OConnell, J. P.J . fhys. Chem. 1988,92, 3492. Fotland, P.; Skauge, A. J . Dispersion Sci. Technol. 1986, 7. 563. Kim, M. W.; Gallagher, W.; Bock, J. J . Phys. Chem. 1988, 92, 1226. (2) Kahlweit, M.:Strey, R.;Schomacker. R.;Haase, D. Longmuir 1989, 5. 305.
0022-3654/91/2095-4889$02.50/0
havior, unless the system is already near a phase transition a t ambient pressure. However, pressure may have a significantly larger effect for propane even at ambient temperature, because propane's density and solubility parameter are much more adjustable due to its relatively low critical temperature, 96.8 O C . Our objective is to demonstrate and explain large effects on the droplet size and phase behavior of microemulsions in compressible liquids with changes in four principal variables: pressure, temperature, salinity, and molecular volume of the alkane solvent. Because both oil and brine phases are present, the surfactant partitions between the phases, and in some cases, forms a new middle phase. A knowledge of partitioning between the phases is important for understanding separation processes such as the 0 1991 American Chemical Society
4890 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
extraction of biomolecules from aqueous solution into reverse micelles in organic solvent^.^ Recently, reverse micelles have been studied in supercritical fluids and compressed liquid solvents including alkanes from methane to butane and the noble gases krypton and xenon.e8 Reverse micelles are thermodynamically stable surfactant aggregates in which hydrophilic head groups point inwards and lipophilic tails extend outwards into the oil continuous phase. When the reverse micelle core contains solubilized water the system is often referred to as a water-in-oil microemulsion, although there is not a clear distinction between reverse micelles and microemulsions. In most previous studies, a known amount of water was added to a surfactant, bis(2-ethylhexyl) sodium sulfosuccinate (AOT), to form a single oil continuous phase, so that the water-to-surfactant ratio, W,,is fixed. This also fixes the radius of the reverse micelle, since it is correlated directly to W,.The microstructure and interior polarity have been investigated by means of light scattering," UV-~isible,"*~*'and fluorescence7probes, and FTIR spectroscopy! The results of each of these techniques suggest the same conclusions. At a given Wo for a one-phase system, the microscopic properties of reverse micelles are similar in both compressed fluid solvents and relatively incompressible liquids such as isooctane and are pressure independent. In contrast, there are significant changes in the size of reverse micelles in the oil phase with pressure when two phases are present, as AOT and water partition between the phases. This was demonstrated with UV-visible and fluorescence probes7 and in a study of amino acid solubilization6 Also, the maximum amount of water which may be titrated into AOT reverse micelles is strongly pressure dependent for propane.4~~*~ The composition of the newly formed phase was determined in only one previous study to gain insight into the partitioning of the surfactant between the phases.'I In the present study, the compositions of each phase in the twophase and three-phase regions have been determined, both with and without NaCl, in order to understand the mechanism of pressure effects on microemulsions. In addition, visual observations of phase boundaries are reported over a wide range in temperature, pressure, and salinity to obtain a global understanding of the phase behavior. The analytical and visual experiments offer a means to determine the natural curvature of the surfactant interface, i.e., the curvature due to interfacial forces independently of interactions between the micelles. By study of the phase behavior in alkanes from ethane to dodecane, and by examination of pressure effects, new insight may be gained into the mechanism of microemulsion formation. In particular, these variables influence the interactions between the surfactant tail and the alkane on the oil side of the surfactant interface. This information provides a basis for testing improved theories of microemulsion phase behavior in both conventional and highly compressible
Summary of Microemulsion Phase Behavior The anionic surfactant AOT readily forms reverse micelles in a wide variety of nonpolar solvents. When a bulk water phase is present along with bulk oil, AOT partitions into the water phase.IoJ1 The addition of salt is required to force the AOT into ~~
~
(3) Lecdidis, E. B.; Hatton, T. A. J . Phys. Chem. 1990, 94, 6400. (4) Beckman, E. J.; Smith, R. D. J . Phys. Chem. 1990, 94, 345. Kaler, E. W.; Billman, J. F.; Fulton, J.; Smith, R. D. J . Phys. Chem. 1991, 95,458. Tingey, J. M.; Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1990, 94, 1997. Smith. R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J . Phys. Chem. 1990, 94, 781. (5) Eastoe, J.; Robinson, B. H.; Steytler, D. C. J . Chem. Soc., Faraday Trans. 1990, 86, 511. Eastoe, J.; Young, W. K.; Robinson, B.H.; Steytler, D. C. J . Chem. Soc., Faraday Trans. 1990.86, 2883. (6) Johnston, K. P.; Lemert, R. M.; McFann G . ACS Symp. Ser. 1989, No.406, 140-164. (7) Yazdi, P.;McFann, G . J.; Fox, M. A,; Johnston, K. P. J . Phys. Chem. 1990, 94, 7224. (8) Lemert, R. M.; Fuller, R. A,; Johnston, K. P. J . Phys. Chem. 1990, 94, 6021. Lemert, R. M. Ph.D. Dissertation, University of Texas, Austin, 1990. (9) Peck, D.; Schechter, R. S.;Johnston, K. P, submitted to J . Phys. Chem.
McFann and Johnston
I
[ A m
Figure 1. Effect of pressure, temperature, and salinity on the phase behavior of AOT-brine-hydrocarbon microemulsions. For incompressible fluids such as decane increasing pressure is in the upwards direction? while for compressible fluids it is downwards (this study) (2,lower phase o/w microemulsion; 3, middle phase microemulsion; 2, upper phase w/o microemulsion; 1; one-phase microemulsion).
A 0 0
."
A
'
.370C
propane
C 37OC 250
I
3
12-
0
I
"0
ethane
100
200
300
400
P (bar)
Figure 2. Solubility parameter of alkanes versus pressure.
the oil phase where it forms w/o microemulsions. The addition of salt causes phase transitions from o/w microemulsions with excess oil (designated 2 meaning two phases with the microemulsion in the lower phase) to a middle phase microemulsion with excess oil and excess water phases (three phases) to a w/o microemulsion with excess water (designated 2), Le., a 2-3-2 transition. In addition, the surfactant becomes more hydrophobic as temperature decreases. For each variable, the phase behavior may be represented on a characteristic plot vs AOT concentration, in the shape of a 'fish", as shown in Figure 1.**I2 The effect of pressure on the fish has been reported for the AOT-decaneD,O system, but not for more compressible alkanes.* The cohesive energy density of propane, as described by the 6, is moderately adjustable with pressure solubility parameter,'f' a t 25-37 O C as shown in Figure 2. For decane, 6 is much less variable over this pressure range. At 37 "C, ethane is supercritical and 6 is much more adjustable than ropane; however, it is too small to form large reverse micelles.+ Therefore, it seems likely that propane may be used to adjust the phase behavior of AOTbrine-propane microemulsions over a wide range.
.p
Experimental Section The surfactant AOT was purchased from Fluka and purified according to a well-accepted procedure." The purified solid was stored in a desiccator, and its dryness was verified by Karl Fischer titration. Ethane (Big 3,99.0%) was purified through activated carbon before use. Propane (Liquid Carbonic, 99.5%), butane (Big 3, 99.0%),and the liquid alkanes from pentane through dodecane were used as received. Purified deionized water was obtained from a Technic Central Systems Lab Five system. (10) Fletcher. P. D. I. J . Chem. Soc.. Faradav Trans. I 1987.83. 1493. ( 1 l j Aveyard; R., Blinks, E. P., Mead, J. J . Chem. Soc., Faraday'Trans.
I 1986,82, 1755. (12) Kahlweit, M.; Strey, R.; Busse, G . J . Phys. Chem. 1990, 94, 3881. (13) Kotlarchyk, M.; Chen, S. W.; Huang, J. S.;Kim, M. W. Phys. Reo. A 1984, 29, 2054.
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4891
Phase Behavior of AOT Microemulsions
TABLE I: P b w Behavior of AOT-PropaneBrine (1.7%NaCI) Microemulsions at 37 OC Based on Measured AOT and Water Concentrations in tbe Propane and Aqueous wlseso DroDane phase aqueous phase middle phase exms P. W of total % of total W of total vol % vol W vol W
+
. . .
bar
vol, om3
[AOT], M
AOT
310 241 172 138 103 86 69
12.8 13.1 13.3 13.5 12.8 12.6 12.5
0.080 0.076 0.076 0.074 0.033 0.009 0.010
90 88 90 89 38
10 11
Wo vol, cm3 24 24 26 25 37 58 0
10.1 10.0 10.2 10.2 9.7 9.9 10.4
[AOT]. M
AOT
0.0036 0.0033 0.0033 0.0028 0.0030 0.0030 0.0038
3 3 3 3 3 3 4
vol, cm3
AOT
AOT
1.4 1.5 1.2
7 9 7 8 59 87 85
19 26 31
H20 propane
37 33 9
44 41 60
aOverall cell contents are listed in Figure 3. Variable-volume view cells (1.75 cm. i.d., 35 mL maximum volume) equipped with Valco six-port switching valves containing 30-105-pL calibrated sample loops were used to equilibrate the phases and withdraw samples for analysis. Because of the small volume of the sample loops, pressure drops were minimal during sampling. Each variable-volume cell was used to sample either the upper (oil) phase as done previously6.*or the lower (brine) phase by the same method. The phases were equilibrated for 2-5 days for the first sample and typically for 24 h for subsequent samples. The use of a piston allows for multiple increases and decreases in the pressure and/or temperature without changing the composition in the cell. The position of the piston was determined in order to know the system volume. This was accomplished by fixing a sight gauge to the back of the cell. The gauge consisted of a piece of fused silica capillary tubing attached to the back side of the piston, which was contained in a 6 mm 0.d. X 0.5 mm i.d. Pyrex tube. The tube was fire polished and stress relieved in order to withstand pressures up to 310 bar, and it was shielded thoroughly with polycarbonate for safety. The height of the meniscus between the phases was determined with a cathetometer to within 1%. The accuracy of the sampling technique for the lower phase was verified for a test system of water, propane, and hydroquinone. The hydroquinone partitions only in the water phase, and its concentration was determined by UV absorption. The variable-volume view cell has also been used to determine cloud points over a bulk water phase. In the present study the cloud points correspond to a 2-3 transition. They were determined by reducing P at constant T until the propane phase first becomes cloudy. This synthetic experimental technique is much more rapid than the analytical technique, since it is necessary for only a small amount of AOT to leave the oil phases a t the cloud point. The pressure could be increased to bring this AOT back into the oil phase and a new temperature could be studied. The horizontal variqble-volume cell is usually but not always convenient for the observation of middle phases. The large interface area along the centerline of a horizontal cylinder means that the middle phase which forms can be quite thin. Therefore, a fixed volume 1.27 cm i.d. X 3.18 cm 0.d. X 10 cm long vertical sapphire tube was used in a few cases. It was pressurized by adding propane and depressurized by venting a small amount of propane. Dew points in the sapphire tube were found to be very close to those measured in the variable-volume cell. Dura Analysis. Data analysis was done in the same manner as described previously? except that the volume of the cell was variable instead of fixed. Even with the uncertainty in measuring the location of the piston, the volumes of the phases could still be measured to within f2%. AOT concentrations in the propane phase were measured to f3%. The AOT concentrations in the brine phase were typically much smaller than in the propane phase, so that the error was typically *14%. The aqueous-phase AOT concentrations reported in this paper were determined by direct sampling rather than by mass balance. The determination of W, by Karl Fischer titration from a high-pressure system is difficult, and so the typical error in W,was f5%. The volumes and compositions of middle phases were determined by difference given the compositions of the aqueous and propane phases, along with overall composition of the system. Typical uncertainties are as
sot
"0
I
i
/
100
200
300
400
P (bar) Figure 3. Degree of partitioning of Aerosol-OT into the propane phase as a function of pressure at 37 OC. Overall cell contents: 0.5006 g of AOT, 10 mL of brine, 6.558 g of propane. (m) 5.9 wt W NaCI in water, (0) 1.7% NaC1, (A)0.59% NaCI, (0) 0.29% NaCI.
0'
100
200
300
400
P (bar)
Figure 4. Water-to-surfactant molar ratio, Wo, in the propane phase versus pressure for the data in Figure 3.
follows: middle phase volume, f 14%; AOT content f 1% propane content f20%, water content f40%. The low uncertainty in the AOT content is due to the typically low compositions of AOT in the aqueous and propane phases when the middle phase is present.
Results Pressure Effect. The effects of pressure on the AOTpropane-brine system are shown in Figures 3, 4, and 5 and in Table I. At 310 bar at the highest salinity, the AOT is in the propane phase. As pressure is decreased, the AOT suddenly leaves the propane phase and forms a middle phase between the propane and aqueous phases. The pressure at which this transformation occurs depends on the salinity. As salinity increases, AOT has a lower affinity for water and so a larger pressure decrease is required to cause the AOT to drop out of the propane phase. For a salinity of 1.7 wt %, the amount of surfactant in the propane phase varies from 10 to 89% as the pressure is increased by only 32 bar.
4892 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991
' I-
60
P (bar) Fipn 5. Percentage of total AOT in each phase as a function of pressure at 37 O C and 1.7 wt % NaCl: ( 0 )oil phase, (0)water phase, (A)middle phase.
The effect of pressure on Wo is even more dramatic than on the partitioning of AOT, as shown in Figure 4. At pressures well above the phase transition point, Wochanges only slightly with pressure. As pressure is decreased beyond the phase boundary where a middle phase forms, Woincreases dramatically. It keeps on increasing until the middle phase reaches its maximum extent and then decreases back to zero as the last of the AOT leaves the propane phase. In the region where Wo increases, the AOT concentration in the propane phase decreases. This means that the micelles that remain are growing larger, due to a reduction in the interfacial curvature. Changes in interfacial curvature are characteristic of surfactant systems near phase inversion points." This interesting effect of pressure on Wo, as well as on the formation of the middle phase, which has not been reported previously, arises from the compressible nature of liquid propane. In propane, as pressure increases, the system changes from 3 to 2. This behavior is the opposite of the previously reported behavior for an incompressible liquid solvent such as decane. In the decane/AOT/brine system, the sequence is j-3-2. as was shown in Figure the same result has been observed for other ionic and nonionic systems.I*l5 Another kind of sequence was observed in the system Tergitol 7 (3,9-diethyl-6-tridecanol,sodium sulfate salt)/water/propylene.16 The surfactant was primarily in a middle phase, and the volume of the middle phase increased at the expense of the oil phase as pressure was increased. Extensive data have been collected for the 1.7 wt 3'% salt case as shown in Figures 3-5 and Table I. The same phase behavior was Seen both in the variable-volume cell and in the sapphire tube. At the highest pressure, 3 10 bar, a clear propane upper phase and a clear aqueous lower phase are present. However, a small amount of surfactant was visible at the propane-brine interface and on the walls of the sapphire tube. The system reaches this stage within the first 24 h of mixing and does not change further after 5 days of equilibration. Based on our observation of a range of salinities, this is believed to be the equilibrium situation. That is, both the brine and propane phases are saturated, with slight excess surfactant at the interface and on the walls of the view cell. The results of experiments in the variable-volume view cell in Table I show this excess amount of surfactant to be about 7 wt % between 310 and 138 bar, based on a total AOT loading of 0.5006 g. Because this is such a small amount and could be caused by impurities in the AOT, this excess phase is not considered when the number of phases in the system is counted. The system initially (14)
Winsor, P. A. Chem. Reo. 1968, 68, 1. Bourrel, M.; Schechter, R.
S. MIcrocmnlslons and Related Sysrems; Surfactant Science Series No 30; Marcel Dekker: New York, 1988. (IS) Sassen. C. L.;Filemon, L.M.; de Loos, Th.W.; de Swaan Arons, J. 1. Phys. Chem. 1989, 93,651 1. (16) Beckman, E. J.; Smith, R. D. J . Phys. Chem., in press.
McFann and Johnston changes very little in appearance as pressure is reduced. Further reductions in pressure cause the propane phase to become increasingly straw colored. The same color was seen at high WO'S in our previous study.6 At 117 bar, a slight cloudiness appears in the propane phase, and liquid flows downwards toward the aqueous phase. This is followed at 114 bar by a more distinct cloud point with formation of a middle phase. When the AOT finished precipitating, three clear phases were revealed. Between 114 and 86 bar the middle phase appears to grow, based on the data of Table I. Below 86 bar the middle phase appears to shrink, but only slightly. In fact, the middle phase persists with little further change all the way to the vapor pressure of propane, 13 bar. Both Table I and Figure 5 indicate that the middle phase begins to form at the same time as the AOT is leaving the propane phase, with little or no change in the AOT concentration in the brine phase. The data of Table I suggest that the middle phase contains significant amounts of all three components: propane, water, and AOT. We could not check the middle phase for anisotropy with polarizing filters because of the anisotropy of the sapphire tube. We observed that middle phases in liquid alkanes at ambient pressure are isotropic for similar concentrations and salinities. Since the middle phase appears to contain a larger amount of propane than water, propane is probably the continuous phase. However, the large amounts of AOT and water that are also present would suggest there are large size fluctuations and frequent collisions among the droplets. Such phases can have very large conductivities,caused by percolation, rapid collisions with exchange of contents, or charge hopping.l7J* Salinity Effect. At high pressures where Wois relatively constant (see Figure 4),the size of the reverse micelles (as reflected in W,)decreases as salinity increases. The electrolyte decreases the electrical double layer thickness and thus decreases the repulsion between the surfactant head groups. This increases the interfacial curvature; i.e., the drops shrink. At very low salinity, AOT prefers the aqueous phase and W,goes to zero. The influence of salt on the partitioning of surfactant between phases is well-known. Increasing salinity both salts out the surfactant from the aqueous phase and assists the bending of the interface about water. For an ionic surfactant like AOT, increasing salinity leads to a 2-3-2 transition.%"J9 The propane system conforms to this classical behavior. As shown in Figure 3 at 310 bar, increasing amounts of salt increase the amount of AOT in the propane phase. The amount of salt required to effect phase transitions is higher in propane than in a liquid solvent like heptane. For propane at 3 IO bar and 37 OC about 1.1 wt % salt is required to drive AOT into the propane phase. For the h e p tancbrine-AOT system at the same AOT concentration, only 0.5 wt % salt is required to achieve the same effect." Clearly the propane system is more hydrophilic than the heptane one, meaning that the AOT has less affinity for the oil phase and more for the water phase. AOT Concentration in the Brine Phase. The AOT concentration in the brine tends to be slightly greater at high pressures than at low pressures. At very low pressure the AOT concentration becomes larger, as AOT drops out of the middle phase. The effect of salinity is very small, probably because at the salinities for which AOT concentrations were measured the salting-out effect is essentially complete. At 310 bar and 37 OC, the AOT concentration is 0.0033 M at 0.6 wt % NaCI, 0.0036 M at 1.7wt % NaCI, and 0.0030 M at 5.5 wt % NaCI. In pure water at 25 OC, we measured a solubility of 0.027 M for AOT. The solubility drops to 0.0016 M at 0.1 wt % NaCl and 0.0004 M at 0.4 wt % NaCl.20 For a salinity of 0.6 wt %, Aveyard et al.'l reported a concentration of 0.0002 M AOT in the aqueous phase in equilibrium with (17) Billman, J. F.;Kaler, E. W. Langmuir 1990, 6, 61 I. (18) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.;Candau, S. J. J . Phys. Chem. 1990, 91, 387. Middleton, M. A,; Schcchter, R. S.; Johnston, K. P.Langmuir 1990, 6, 920. (19)Kuneida, H.;Shinoda, K. J . Colloid Interface Sci. 1980, 75, 601. (20) f i g , J. E.;Mares, M. T.; Miller, W. G.; Franses, E.1. Colloids Surf. 1985, 16, f 39.
The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 4893
Phase Behavior of AOT Microemulsions
100 r . . . .
TABLE II: Effect of Temperature d Preswre on AOT-RopuK-Briw (1.7%NiCI) MicroemulPioaPO
. . . .
8
.
.
.
. . .. I
.
propane phase aqueous middle phase phase 96 of total bar vol. cm3 [AOT], M AOT W, vol,cm3 vol, cm3 T = 25 " C
P.
276 241 207 172 138 103 69 34
12.6 12.7 12.8 13.0 13.2 13.3 13.4 12.8
0.082 0.083 0.079 0.076 0.079 0.073 0.073 0.014
91 94 90 88 92 a6 87 16
31 30 29 27 28 28 107
9.8 9.8 10.0 9.8 9.8 10.0 10.1 9.6
35 30 36 29 35
10.2 9.9 10.0 9.8 9.7
1.5
'b
1 0 0 200 300 400
T = 47 "C 310 241 172 155 138
13.3 13.8 14.2 13.0 13.2
0.072 0.076 0.060 0.029 0.014
85 93 76 33 8
P (bar)
1.4 1.5
OOverall cell contents are listed in Figure 3. heptane, which is much lower than for propane. This is another indication of the greater hydrophilicity for the propane system. It is likely that propane aids the dissolution of AOT into the brine and vice versa. For a system containing 1.7 wt 7%salt, the solubility of propane in the aqueous phase is 0.2-0.3 M,which is 20 times higher than the reported solubility of propane in pure water.2' Temperature Effect. For AOT-watersalt-alkane systems, Kunieda and Shinodatgconstructed temperature versus salinity diagrams. As temperature increases at a given salinity, 2-3 and 3-2 transitions occur at well-defined phase inversion temperatures. These inversion temperatures vary according to the oil/water ratio, AOT concentration, salinity, and the molecular volume of the alkane. In propane, however, temperature effects are always accompanied by significant changes in density, so it is necessary to attempt to distinguish between direct and indirect temperature effects. In Table I1 and Figure 6, the effect of temperature on the partitioning of AOT and W,is reported for 1.7 wt % NaCI. At the highest pressures, the partitioning of AOT into the oil phase is nearly complete, i.e., about 90%, and changes little with temperature. However, there is a large temperature effect on the cloud point pressure. It is 60 bar at 25 OC, 114 bar a t 37 OC, and 165 bar at 47 OC. Since density changes with temperature, it is more instructive to plot the data as a function of density as shown in Figure 6b. Here the three isotherms are quite similar, which implies that a certain density is required to cause the 3-2 transition. The similar behavior for the three isotherms suggests that the temperature effect on the AOT-propane and AOT-water interactions are either small or similar. All of the results in Figures 3-6 reflect a single 2-3 transition from the oil phase to the middle phase as pressure (or likewise density) is reduced. It is likely that the salt and AOT concentrations are too high to allow 3-2 transition in the above experiments. The capacity of the water phase for AOT is too low, even a t elevated temperatures, for the formation of an o/w microemulsion. Thus, experiments were conducted for a lower salinity of 0.76wt % and an AOT loading of 1 wt %, as shown in Figure 7. Although, the 2-3 transition can be observed easily at the cloud point, the 3-2 transition is much more difficult to decipher. It is easier to observe the 3-2 transition isobarically, by raising T, although another approach is to observe the 2-3 transition isothermally by increasing pressure. These are the first results that h y e been reported in a compressible alkane that show the entire 2-3-2 microemulsion phase behavior. This global view affords an opportunity to understand microemulsions in compressible liquids based on the concept of Kahlweit's fish. As pressure and temperature increase, the thickness of the three-phase region diminishes. It is conceivable (21) Kobayashi, R.;Katz,
D.L. I d . Eng. Chem. 1953,45,440.
0.51
#.49
0.53
0.55
density (glcc) Figure 6. Degree of partitioning of AOT into the propane phase at
various temperatures versus (a, top) pressure and (b, bottom) propane density. Overall cell contents are the same as in Figure 3. (0) 25 OC (0)37 "c (A) 47 OC.
t u
3
50 -
v
c
40
P
i
-
30 -
20d.
.
, 1 0 0 . ' 2 0 0 ' ' 3 0 0 . '4AO
P (bar) Figure 7. Upper-to-middle phase (2-3) and lower-to-middle (2-3) phase transitions for AOT-brine-propane microemulsions for 0.76 wt % NaCI. Overall cell contents: 0.0768 g of AOT (1 wt %), 5 mL of brine, 2.682 g of propane. ( 0 )Cloud point observed isothermally by reducing pressure, (0)cloud point determined isobarically, and (A)cloud point de-
termined isothermally by increasing pressure. that the two transitions merge at a higher pressure, which could not be reached with our apparatus. This would be the optimal point where the head and the body of the fish meet. The "optimal salinity" is that amount of salt required to form a middle phase having equal amounts of oil and water. Since propane is a compressible solvent, it would be expected to have a range of optimal salinities, one for each pressure. At other salinities and AOT concentrations, this point may be at a more accessible pressure. This technique could be used to generate a series of fish plotted as temperature versus AOT concentration, each one a t a given
4894 The Journal of Physical Chemistry, Vol. 95, No. 12, 1991 300
200
McFann and Johnston
7
-i
1
2
sallnlty (wt %) Figure 8. Upper-to-middle phase (2-3) cloud point transitions for
AOT-brine-propane microemulsions as a function of pressure, salinity, and temperature: (W) 25 OC, (0)37 OC, (A)47 OC, ( 0 )60 OC. Overall cell contents same as Figure 3. AOT
Figure 9. Schematic phase diagram for the AOT-water-propane system without added NaCl at 25 OC. A, overall composition in the cell: line bcginning at B, path followed in the visual determination of W,at saturation.
pressure and salinity. An interesting result is that pressure provides an additional degree of fredom to move the location of the fish. The 2-3 transition is indicated over a wide range of salinity, temperature, and pressure in Figure 8. As expected, the transition pressure demases as the salinity increases for a given temperature, since the system becomes more hydrophobic. At a given salinity, the transition pressure increases with temperature in order to maintain a high enough density to attract AOT to the propane phase. If the data are replotted in terms of propane density, the isotherms become much closer, but not the same. In fact, they are reversed. That is, as temperature increases the phase transition occurs at a lower density-an unexpected result. The temperature increase would be expected to cause AOT to become more soluble in water; thus it would seem that this should require an increase in the propane density. Other factors must be present. AOT- Water-Propane System without Added Salt. The purpose of this section is to establish a connection between studies of systems with a bulk water phase and extensive earlier studies of the phase boundary of one-phase propane continuous systems.4J.' An experiment was performed with equal volumes of water and propane, 12 wt % ' AOT, and no added salt. This composition is in the three-phase region, for example, at point A in the schematic diagram in Figure 9. Certain regions in the diagram are exaggerated in order to be more visible. The compositions of both the propane and aqueous phases were measured by withdrawing samples, after equilibration for 10 days. Although the upper phase was clear, the lower phase contained insoluble AOT. An enormous three-phase region occupies the interior of the triangle, with a liquid crystal surfactant phase a t approximately 70 wt % AOT. The propane phase is 0.2 wt k AOT and the water phase is 0.94 wt %. Thus, for low concentrations of AOT below the three-phase region, the AOT prefers the aqueous phase, which is also the case for The base of the three-phase triangle slopes upwards toward the water side. The amount of water in the propane phase was too small to measure, and the AOT concentration was low; therefore, the degree of aggregation was negligible.
alkane carbon number Figure 10. Degree of partitioning of AOT into the alkane phase as a function of the carbon number at 25 O C . Overall cell contents: 0.151 1 g of AOT, 10 mL of brine, IO mL of alkane, 0.40 wt 5% NaCl in the brine. Ethane, propane, and butane at 310 bar; other alkanes at 1 bar. ( 0 )Oil phase, (0)aqueous phase, (A)middle phase.
20
I/
l11i
alkane carbon number
Figure 11. Unusual behavior in Woas a function of the carbon number of the alkane at 25 OC.
For reverse micelles and w/o microemulsions, the twephase region to the right of the three-phase region is of great interest. The boundary of this region is not fully known, but it begins from the two vertices on the three-phase triangle. The branch near the propane vertex has been studied by several investigation^.^^^^^^^^ To do this, a solution of AOT is prepared in propane, e.g., at point B. Water is added along a line toward the pure water vertex until a cloud point is observed at the boundary of the two-phase region. Although this water titration procedure may be used to determine Woat saturation, the composition of the newly formed phase is unknown. However, the analytical data in Figure 9 show quite clearly that this new phase must be surfactant-rich, and not water-rich, which is consistent with conductivity data! In contrast, at high salinities and much lower AOT concentrations, a w/o microemulsion phase is in equilibrium with a brine phase containing little surfactant, as shown in Figure 3. This condition would be desirable for the extraction of hydrophiles from water, since only a small amount of the AOT would be lost to the aqueous phase. It is important to notice that there is a rich variety of types of phase diagrams and regions within these diagrams as a function of pressure, temperature, salinity, AOT concentration, and oil/water ratio. Phase diagrams of the type shown in Figures 7 and 8 are most useful for understanding the relationship between these variables from a global perspective. Eflect of the Molecular Volume of the Alkane. Most previous studies of the effect of the alkane molecular volume on AOT microemulsions did not consider compressed liquid solvents like propane, butane, and pentane. To extend the range to include these light alkanes, we chose the same oil/water ratio and concentration of salt and AOT as in a previous study." For ethane,
The Journal of Physical Chemistry, Vol. 95. No. 12, 1991 4895
Phase Behavior of AOT Microemulsions
the number of phases in the system, the location of the threaphase bodies in the phase prism, and the phase inversion temperatures. As Kahlweit has pointed out, AOT will not form a middle phase unless a fourth component, such as salt, is added? This is because the upper critical solution temperature of the watersurfactant binary, Tdris much too low compared to that of the oil-surfactant binary, Tu. Adding salt raises Tdsufficiently so that an inflection is created in the locus of two-phase critical points, leading to a "break" which opens up a threephase body. The threephase body is shown in the head of the fish in Figure 1. This concept may also be used to explain pressure effects. For the AOT-brinepropane system, pressure has a much larger effect on the density of propane than water. As pressure increases AOT becomes more soluble in propane, leading to a smaller miscibility gap, and therefore a lower T,. The interaction between the solvent and surfactant tail becomes more favorable. At the highest pressure studied, AOT is most soluble in propane. As pressure is lowered Tu increases, which decreases the slope of the critical line until the inflection a p p r s , and with it the formation of a middle phase. Although AOT becomes more hydrophobic in propane with an increase in pressure over the range studied, it becomes more hydrophilic in decane.2 The reason for decane's pressure effect on microemulsion phase behavior is not fully understood.' The important conclusion is that the new effect of pressure may be explained in terms of the immiscibilities in the binary systems, as is done for temperature and salinity. Thermodynamics of the Curvature of Microemulsions in Alkanes from Ethane to Dodecane. The free energy of a miDiscussion croemulsion is a function of the interfacial tension and bending Although AOT reverse micelles and liquid crystals have been moment of the interface, the entropy of mixing of the droplets studied extensively, surprisingly few complete phase diagrams have with the continuous phase, and the attractive intermicellar inbeen constructed. A somewhat unique feature of AOT is that teractions.26 If the interfacial tension is small, the entropy of its twin tails with their short side chains give the molecule the mixing term would cause the formation of many small droplets; shape of a truncated conea which allows it to pack efficiently into however, the bending moment can prevent the drops from bea reverse micelle structure, without the need for a c o s u r f a ~ t a n t . ~ ~ coming too curved. The interfacial tension and bending moment The ternary AOT-water-alkane phase diagram is characterized terms in the free energy have contributions due to interactions by a relatively large isotropic w/o region region, a liquid crystal on the oil and water sides of the interface. The natural curvature region, and a twephase o/w + excess oil r e g i ~ n . ' ~The * ~ central of the microemulsion droplet is defined as the curvature that would region of the triangle can contain various interesting combinations: result from these interactions at the interface, without considering w/o microemulsion + liquid crystal, w/o microemulsion water contributions from the entropy of mixing of the drops or the liquid crystal, and even two microemulsion phases in equilibrium attractive micelle-micelle interactions. with each other.25 In the systems that are most hydrophobic, In alkanes including propane, AOT prefers the water phase if where the affinity of AOT for the oil phase is the largest, the w/o salt is not added;" Le., the natural curvature is about oil, as is microemulsion region reaches its maximum size. This is observed evident from the slope of the base of the three-phase triangle in for hexane. In the decane system which is more hydrophilic, the Figure 9. Therefore, the reverse micelles formed in previous liquid crystal and multiphase regions grow at the expense of the studiese7 in the one-phase region without added salt have a w/o microemulsion regions. To try to avoid the liquid crystal curvature which is opposite that of the natural curvature. This regions, we chose to keep the AOT concentration low and the is due to the entropy which results from dispersing a small volume salinity high. As a result, the phase transition from w/o to middle of water in an oil continuous phase. As pressure and thus the phase microemulsion to o/w may be explained with somewhat cohesive energy density of the solvent are reduced, the interactions idealized phase diagrams described below. between the tails of the surfactant and the fluid can become Microemulsion Phase Behavior in Terms of the Component sufficiently weak to lead to a phase transition.' Here intra-" and Binaries. The phase behavior of microemulsions may be explained intermicellar4v5attractions between the tails cause surfactant and by the strength of the surfactant-water and surfactant-oil inwater to precipitate, as shown in Figure 9. The observation of teractions, as was first noted by Winsor." In Kahlweit's fora high surfactant concentration in the condensed phase below the mulation,2J2 a series of surfactant-oil-water triangular diagrams propane phase complements two recent neutron-scattering studat various temperatures are stacked to form a prism. The prism iesa4v5Near the phase transition pressure, these studies indicate is unfolded to display temperature versus composition diagrams that the micelle-micelle interactions become large. These inof each of the three binaries. The degree of miscibility of the teractions would likely lead to a higher surfactant concentration oil-surfactant and watersurfactant binaries influences the affinity in the condensed phase. In the furture, it would be interesting of the surfactant interface for the oil and water phases, respectively. to measure the size and structure of the microemulsion in this Thus, the ternary phase behavior is shown to be a consequence condensed phase. of the miscibility gaps of the oilsurfactant and brinesurfactant A different type of behavior is observed in the present study binary systems. The relative size of the miscibility gaps determines for systems containing equal amounts of water and oil phases, especially with salt present. For equal amounts of water and oil, (22) Eicke, H.F.; Kvita, P. In Reuerse Micelles; Luisi. P. L., Straub, B. the entropy due to dispersing drops into a continuous phase does E., Eds.; Plenum: New York, 1984. not favor w/o microemulsions versus o/w microemulsions. At propane, and butane, the pressure was 310 bar to keep the density high, w h e m it was 1 bar for the other alkanes. For each solvent, the AOT concentrations were measured in both the oil and aqueous phases as well as Woin the oil phase. As shown in Figures 10 and 11, the results of the two studies are in good agreement for hexane through dodecane, except that the domain of middle phases is smaller in the present study. This may be due to differences in purification techniques. A novel and exciting result is that the phase behavior reverses itself for pentane and lower alkanes. We believe that this is the first experimental demonstration of this unusual behavior. From dodecane to hexane the sequence of phase behavior is 2-3-2, in line with all previous results. However, a seoond 2-3-2 sequence, as shown by AOT partitioning, occurs from hexane to ethane, As will be shown in the Discussion section, this has implications for models used to explain surfactant aggregation. The unusual behavior is indicated quite clearly in the Wodata shown in Figure 11. Wohas a local minimum for pentane or hexane, where AOT is primarily in the organic phase, and maxima at C3 and between CBand Cg. Starting at hexane, Woand thus the micelle size increases with either a decrease or increase in the molecular volume of the alkane. This is due to the decrease in curvature as the surfactant migrates to the middle phase. Eventually, the curvature changes to become concave about water as the surfactant partitions mostly into the aqueous phase for ethane, undecane, and dodecane, where Wois effectively 0.
+
+
(23) Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1981, 77, 601. (24) Tamamushi, B.; Watanabe, N . Colloid Polym. Sci. 1980,258. 174. Ekwall, P.; Mandell, L; Fontell, K. J . Colloid Interface Sci. 1970, 33, 215. Franses, E. 1.; Hart, T.J. J . Colloid Interface Sci. 1983,91, 1. Ghmh, 0.; Miller, C. A. J . Phys. Chem. 1987, 91, 4528. (25) Hou, M. J.; Shah, D. 0. Langmuir 1987,3, 1086.
~~
~
(26) Overbeck, J. Th. G.;Verhoeckx,G. J.; De Bruyn,P. L.; Lekkerkerker, H.N . W. J. ColloidInterfaceSci. 1987, 119, 422. Lam, A. C.; Falk, N . A.; Schechter, R.S. J . Colliod Interface Sci. 1987, 120, 30. Huh, C. Soc. Per. Eng. J . 1983, 829.
48%
The Journal of Physical Chemistry, Vol. 95, No. 12, 199'1
pressures well above that of the Z to 3 transition, the attractive intermicellar interactions would be expected to be small, by analogy with systems at atmospheric pressure.I7 Here, only the interfacial terms remain, so that it is possible to determine the natural curvature. For the case where AOT is primarily in the propane phase, the natural curvature is about water. Here the attractive interactions between the propane and surfactant tails are sufficiently strong compared with the tail-tail and solventsolvent interactions, such that w/o microemulsions are favored. As pressure is d d , we have shown that the system undergoes a 2 to 3 to 2 transition. At presures below the 3-2 transition, the natural curvature is about oil. It seems likely that this 2 to 3 to -2 transition is due to a decrease in the strength of the attractive interactions between the surfactant tails and oil, as the cohesive energy density of the oil is lowered. This leads to intra- and intermicellar attractions between the tails which cause surfactant to partition out of the oil phase. The curvature of w/o microemulsions varies considerably for a series of alkane solvents, as was shown in Figure 11. The combinatorial entropy of the t a i l 4 interaction has been examined previously for alkanes at atmospheric pressure. As the molecular volume of the oil decreases, the solvent penetrates the tails more effectively. This favors bending about water, so that the natural curvature increases (radius decreases) as observed experimentally and t h e o r e t i ~ a l l y . ~The ~ ~ ~combinatorial entropy of the tails increases as the curvature and thus the volume on the tail side of the interface increases. Although this decrease in radius is observed from octane to pentane, the opposite behavior is present for both larger and smaller alkanes. For larger alkanes, it is well-known that the decrease in Wois due to an increase in the attractive interactions between droplets.25 For pentane through dodecane, the enthalpy of mixing the alkane and surfactant tails is very small, since the solubility parameters are similar. This term has little effect on the free energy of the microemulsion. For smaller alkanes, the enthalpy of mixing of the alkanes and surfactant tails becomes unfavorable as discussed above. In order to maximize the AOT partitioning into the oil, the entropic and enthalpic interactions must be balanced. The solvent must penetrate between the tails so that the lattice sites are occupied by a sufficient number of oil molecules, and the attractive interactions between the oil molecules in the lattice sites and the tails must be sufficiently strong to prevent the tails from entangling with each other. In Winsor's terminology this maximizes the tail-solvent interaction and minimizes the tail-tail intera~ti0n.I~ This occurs at hexane in Figure 10. A small solvent like propane penetrates the tails very well but its attractive interactions with the tails are too weak to prevent tail-tail interactions. A large molecule like decane has favorable enthalpic interactions with the tails, but it does not penetrate the tails as well, so that the combinatorial entropy of the tails is too low. In both propane and decane, w/o microemulsions are destabilized with respect to hexane, and thus micelle-micelle interactions and changes in interfacial curvature reduce the partitioning of AOT into the oil phase. The increase in Wo(micellar radius) on each side of hexane indicates that the curvature decreases as the AOT moves towards the middle phase (seeFigure 11). Therefore, each of the observed trends may be described in terms of the combinatorial entropy and the enthalpy for the tail-solvent interactions. (27) Mukherjee, S.;Miller, C. A,; Fort,T.J . Colloid Inrerfuce Sci. 1983,
91, 223.
McFann and Johnston
conclusioas This study of AOT partitioning between the oil, middle, and brine phases provides a basis for understanding practical applications such as separations from aqueous solution with w/o microemulsions. In the AOT-propane-brine system at certain salinities, the AOT migrates almost completely from the middle phase to the propane phase with a pressure increase of only 30 bar. At low salinity, an inversion in curvature may be achieved isothermally by raising pressure, that is a complete 2-3-2 transition. This reduces the propane-AOT miscibility gap, since it strengthens the attractive interactions between the surfactant tails and propane. This 2-3-2 transition is opposite of the common 2-3-2 transition in relatively incompressible liquid solvents like decane. For decane, pressure has a significant effect on phase behavior only if the system is already near a phase transition. Because propane is much more compressible, pressure can cause phase transitions even if the system does not start near a transition. The study of microemulsions in propane and butane provides a better understanding of the mechanism of tailsolvent interactions, which is relevant even for larger alkanes at ambient pressure. An unusual 2-3-2 sequence in phase behavior occurs for the series of alkanes from hexane to pentane, to compressed butane, propane and ethane. This is the opposite of the normal -2-3-2 sequence observed for alkanes from dodecane through hexane. Normally the interface becomes more curved about water as the alkane chain length is decreased, because of the combinatorial entropy of the tail-alkane interaction. However, for the light alkanes the decrease in the enthalpic attractive interaction between the oil and the surfactant tail becomes the limiting factor, and the surfactant becomes more curved about oil. Therefore, microemulsions in propane are more hydrophilic relative to those in hexane, so that more salt or lower temperature is required to form w/o microemulsions. Without added salt, the natural curvature of AOT microemulsions in propane is around oil as in other alkanes. There is good agreement between many of these experimental results and those of a theoretical model in a companion paperag The unique feature of compressed liquids is that the same kind of transitions that are accomplished with a change in alkane chain length may also be achieved with a single liquid, propane, simply by changing pressure. In addition, pressure may be used to manipulate interactions on the oil side of the interface, whereas for temperature and salinity, that is done on the water side. Pressure could be a useful tool for the extraction of hydrophiles such as biomolecules from aqueous solution, where changes in temperature and salinity are sometimes undesirable. Acknowledgmenr. This material is based on work supported by the National Science Foundation under Grant No. CTS8900819. Any opinions, findings, and conclusions or recommendations expressed in this publication do not necessarily reflect the views of the National Science Foundation. Acknowledgment is made to the State of Texas Energy Research in Applications Program, the Camille and Henry Dreyfus Foundation for a Teacher-Scholar (to K.P.J.), and the Separations Research Program at the University of Texas. We thank Bob Schechter and Doug Peck for many useful discussions. Registry No. AOT, 577-1 1-7; NaCI, 7647-14-5; ethane, 74-84-0; propane, 74-98-6; dodecane, I 1 2-40-3; butane, 106-97-8; pentane, 10966-0; hexane, 110-54-3; heptane, 142-82-5; octane, 1 1 1-65-9; nonane, 1 1 1-84-2; decane, 124-18-5; undecane, 1120-21-4.
