J . Phys. Chem. 1991, 95, 1445-1448
1445
Micellar and Blcontlnuous Mlcroemulsions Formed In both Near-Crltlcal and Supercrltlcal Propane with Didodecyldlmethylammonlum Bromide and Water Joel M. Tingey, John L. Fulton, Dean W. Matson, and Richard D. Smith* Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratories, Battelle Memorial Institute, Richland. Washington 99352 (Received: February 28, 1990)
Bicontinuous microemulsions readily form in liquid propane at 25 "C and pressures from IO to 500 bar with the addition of the surfactant didodecyldimethylammonium bromide (DDAB) and water. The phase behavior of this system is much like that of the normal liquid alkanes, c6+0, but with unusual and dramatic effects due to pressure. When the pressure of the solution is increased from 80 to 400 bar with the addition of pure propane, the conductivity is observed to decrease by 3 orders of magnitude. In accord with existing structural models for conventional liquid microemulsion systems, these changes in the conductivity are ascribed to changes in the interface region as the propane solvent penetrates and solvates the hydrocarbon tails of the surfactant. The corresponding supercritical propane system studied at 100 OC is best explained as a micellar microemulsion with an oil-continuous phase in which the structure is also affected by the amount of water or the pressure of the system.
Introduction Bicontinuous microemulsions formed from the addition of surfactant to nearly equal proportions of water and oil often have structures that consist of interconnected microdomains of both water and oil. Although the exact structural details of these microemulsions remain uncertain, the microdomains are known to have characteristic structural dimensions between 5 and 100 nm. Structures of this size are poor scatterers of visible light, and hence these solutions are optically clear. Determination of the precise structure and properties of the oil and water regions is currently an area of active study. Certain surfactants of the class of double alkyl chain, quaternary ammonium salts readily form bicontinuous microemulsions in liquid alkanes and water. Evans et al.'-' have conducted various studies that have shown the existence of both bicontinuous and reverse micelle structures in microemulsions formed with these surfactants in liquid alkanes and alkenes. It has been shown, for example, that the nature of the microemulsion phase is strongly dependent upon the mixture composition as well as the length of the alkane chain. We have previously reported on the properties and structure of reverse micelles and microemulsions in near-critical and supercritical fluid continuous In this paper we report the finding of bicontinuous microemulsions in both liquid and supercritical propane whose structures are especially sensitive to the pressure or density of the propane continuous phase. Liquid propane at 25 OC is a fluid that is not far below its critical point ( T , = 96.7 OC, P, = 42.5 bar). Hence, moderate changes in the system pressure (10-500 bar) cause appreciable changes in the properties of the propane such as density, dielectric constant, and diffusion coefficient. In contrast, these properties of the aqueous regions of such a microemulsion in propane are expected to change very little over these pressure ranges. Thus near-critical fluids, and especially supercritical fluids, provide this uniquely adjustable parameter for changing the properties of only the oil-continuous solvent without having to change its chemical composition. The simplest way of determining the extent of interconnectivity of the water conduits is with a conductivity measurement6 Reverse micelles and water-in-oil (w/o) microemulsion systems have low conductivity, since the water is encapsulated in the core of the water-in-oil droplets and the conductivity of the continuous oil phase is very low. Bicontinuous structures have substantially higher conductivities than reverse micellar structures, since the water resides in interconnected conduits.6 In this paper we report on the large changes in the conductivity of bicontinuous microemulsions formed in liquid and supercritical propane as the pressure is increased and speculate on the possible mechanisms of these changes. Corresponding author.
0022-3654/91/2095-1445$02.50/0
Experimental Section Didodecyldimethylammonium bromide (DDAB) was obtained from Kodak (>99%) and was further purified by the recrystallization method described by Chen et a1.I The propane and propylene were C P grade from Linde and were used as received. The water was distilled and filtered through a Millipore Milli-Q deionization system. The conductivities of the propane/DDAB/water systems were measured by using a Yellow Springs Instrument conductivity meter (YSIModel 34) with a specially constructed high-pressure cell, which has been described in detail elsewhere.'* Briefly, two 1.68 X 0.95 cm rectangular stainless steel electrodes spaced 0.1 1 cm apart were mounted inside a high-pressure view cell having a volume of 50 mL.9 The solutions for measurement were prepared by loading measured amounts of surfactant and water directly into the stirred, temperature-regulated high-pressure cell. The cell was flushed with propane gas at 1 bar to remove air prior to pressurization. A series of conductivity measurements was made either by increasing the pressure of the system with the addition of pure fluid or by reducing the pressure by discharging small amounts of the microemulsion solution. The former method increases the pressure at nearly constant surfactant and water volume fractions, while in the latter depressurization method, the mole fractions of surfactant and water are held constant. With a sudden increase or decrease in the pressure of the system the conductivity was observed to reach a new equilibrium value within minutes. Selected systems which were allowed to stand 24 h showed no significant ~~
( I ) Chen, S. J.; Evans, D.F.; Ninham, B. W. J . Phys. Chem. 1984, 88, 163 1-1634. (2) Ninham, B. W.;Chen, S. J.; Evans, D. F. J . Phys. Chem. 1984,88. 5855-5857. . ... ... (3) Blum, F. D.;Pickup, S.; Ninham, 8.;Chen, S. J.; Evans, D.F. J. Phys. Chem. 1985,89, 71 1-713. (4) Evans, D.F.; Mitchell, D.J.; Ninham, B. W. J . Phys. Chem. 1986, 90. 281 7-2825. (5) Allen, M.; Evans, D.F.; Mitchell, D.J.; Ninham, B. W. J. Phys. Chem. 1987, 91, 2320-2324. 92.7 (6)74-7 Warr, 83. G . G.; Sen, R.; Evans, D.F.; Trend, J. E. J . Phys. Chem. 1988,
(7) Blum, F. D.; Evans, D.F.; Nanagara, B.; Warr, G. G. Langmuir 1988, 4, 1257-1261. (8) Gale, R. W.; Fulton, J. L.; Smith, R. D.J . Am. Chem. Soc. 1987, 109,
920-921. (9) Fulton, J. L.; Smith, R. D. J . Phys. Chem. 1988, 92, 2903-2907. (IO) Fulton, J. L.; Blitz, J. P.;Tingey, J. M.;Smith, R. D.J.Phys. Chem. 1989, 93, 4198-4204. ( I I ) Smith, R. D.; Fulton, J. L.; Blitz, J. P.; Tingey, J. M. J . Phys. Chem. 1990, 94, 781-787. (12) Tingey, J. M.;Fulton, J. L.;Smith, R. D. J. Phys. Chem. 1990, 94. 1997-2004.
0 1991 American Chemical Society
Tingey et al.
1446 The Journal of Physical Chemistry, Vol. 95, No. 3, 2991 I
DDAB
Propane
/
T = 25°C P = 350 bar
-3
50 %
A A
6 .
n
-4
v)
r E
Y
100% 100%
50%
0%
Water
Figure 1. Ternary phase diagram of the propane/DDAB/water system at 25 OC and 350 bar. Only the alkane-rich corner from 50 to 100% propane was invcstigatcd. The two heavy lines bound the single-phase region; the lighter dashed line within the single-phase region shows the approximate location of the conductivity percolation transition from high conductivity above the line to low conductivity below the line.
change in the conductivity. Typically, the solutions were allowed to equilibrate for 15 min following a change in the conditions. For the propane systems, there is very little hysteresis with increasing or decreasing pressure; conductivity readings taken as the pressure of the system is incrementally increased are the same as readings taken with decreasing pressure at the same conditions. The techniques used in the determination of the surfactant solubility and phase behavior of the system have been previously d e ~ c r i b e d .An ~ upper limit on the solubility of the DDAB in propane was determined in a simple visual experiment. DDAB (1 0 mg) was introduced into the 50-mL view cell, and the system was filled with liquid propane to 350 bar at 25 "C. Most of the purified surfactant that was introduced into the view cell did not dissolve after several hours of stirring. Hence, the solubility of DDAB in propane is less than 0.5 mM. The solubility of the hydrated surfactant for W = 5 (where W is defined as the molar water-to-surfactant ratio) is likewise less than 0.5 mM.
*
WJ
A
-5 SI0
J
-4
A A
. .=
'A
d o . . SI0
= 0.09
9. SI0
-7 -I
0
= 0.37
= 0.21
I
20
10
30
wt % H,O
Figure 2. Conductivity of propane/DDAB/water microemulsions at 25 O C and 350 bar as a function of the amount of added water for three different surfactant-to-oil ratios: ( 0 )s/o = 0.09; (B) s/o = 0.21; (A) s/o = 0.37.
the amount of added water for three surfactant-to-oil (s/o) ratios. Measurements were made through the entire single-phase region at 350 bar and 25 "C.At low surfactant-to-oil ratios, s/o < 0.30, the systems have low conductivity, indicative of an oil-continuous microemulsion structure. However, at higher s/o ratios, a highly conducting solution is observed at low water content that becomes nonconducting on the addition of water. This is the same type of "antipercolation" behavior observed in the liquid alkane systems. Results and Discussion This behavior has been attributed to a structural transition from a bicontinuous to a water-droplet-in-oil type of microemulsion upon A portion of the ternary phase diagram for the propane/ the addition of water. In Figure 1, the microemulsion structure DDAB/water system representing the alkane-rich corner is shown in Figure 1. The central region bounded by the solid lines is the in the region near the upper phase boundary can be described by one-phase region. For this study, the phase behavior was only (and is consistent with) a system of interconnected rods or cylinvestigated in the propane-rich corner of the ternary phase diinders, where the degree of interconnectivity decreases as one agram. In this region, the observed phase behavior is very much moves toward the oil-rich corner of the phase diagram. In the like that reported by Evans and co-workers for liquid alkanes, region near the lower phase boundary, the microemulsion conespecially h e ~ a n e .The ~ surfactant is practically insoluble in either ductivity is consistent with a system composed of dispersed drooil or water phases so that in the microemulsion it must reside plets. at the interface between the oil and water domains. The charThe conductivity of the propane/DDAB/water systems at s/o acteristic low solubility of DDAB in liquid alkanes6 was found of 0.42 and 0.24 is strongly affected by the pressure of the system. to hold for propane as well. To obtain a single-phase system, As shown in Figure 3, the conductivity of a single-phase system enough water must be added to an oil/surfactant mixture to create at s/o = 0.42 is reduced by almost 3 orders of magnitude when sufficient interfacial area, having the appropriate c u r ~ a t u r e to , ~ ~ ~ the pressure is increased from 100 to 450 bar. The systems become dissolve the surfactant. Because of this, we see that for the two-phase below 100 bar, so that reducing the pressure has the propane/DDAB/water system, the single-phase region resides in effect of bringing the system closer to a phase boundary. The an area that contains nearly equal proportions of water and system remains single-phase up to at least 450 bar, the limit of surfactant. This behavior is characteristic of a variety of alour experimental apparatus. These changes were observed to occur kylammonium salts.6 We found that neither the liquid ethat both constant volume fraction of surfactant and water (by ane/DDAB/water system nor the liquid ethylene/DDAB/water increasing the pressure with addition of pure propane) and at system formed single-phase solutions at pressures up to 400 bar constant overall mole fraction of surfactant and water (by disin the region of the phase diagram investigated for propane shown charging small amounts of the propane solution). Over the in Figure 1. pressure range from 100 to 400 bar, the propane density increases I n liquid alkane microemulsions, a structural change from a by about 10% (from 0.51 to(0.56 g/cm3), while that of the water high-conductivity, bicontinuous solution to a low-conductivity, increases only 1 %. Such a large change in the conductivity with oil-continuous microemulsion can be induced by the addition of the system pressure was unexpected, since the volume fractions water.' This effect has been explained by a structural change in of the water, surfactant, and propane remain essentially constant. which an interconnected rodlike structure transforms to a dispersed Clearly, variation of the density of the oil-continuous portion of droplet phase.6 Our studies of the propane microemulsions have the microemulsion is sufficient to induce a structural transition. shown similar conductivity behavior. Figure 2 shows the conAs shown in Figure 3, the microemulsion at s/o = 0.24 has a less ductivity of the propane/DDAB/water system as a function of pronounced pressure effect. An initial decrease in conductivity
Micellar and Bicontinuous Microemulsions in Propane
The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
T 25%
-4.0
I
T = 100°C
I
P s 350 bar
I
r-
I
SI0
IO.09
I
Propsne 8lO 3 0.42 WI oh H P = 21.0
-
I
I
IA I
I I I
I
I A
E
P
E
Y
-5.0-
3
A
-6.0-
A
wt X H$
I
IA I
I
m
I
A
I
0
I
A
I I
A A
16.0
\I
2o
Y
A
Propane do a 0.24
r v
A A A
Y
I
E
A A
A
E
P
iA
8
r
1447
A A
A
-,."0-
100
200
300
400
500
Pressure (bar) Figure 3. Effect of pressure on the conductivity of propane/DDAB/ water microemulsions at 25 O C for s/o ratios of 0.24 and 0.42 at weight %'s water of 16 and 21, respectively. was observed from 80 to 150 bar, followed by a slowly rising conductivity up to 450 bar. Such behavior suggests an initial structural transition to reverse micellar structures is occurring at low pressures, and the following region of slowly rising conductivity may simply reflect the decreased mobility of micelles in the higher viscosity solvent. The phase behavior of the supercritical propane/DDAB/water system was also investigated at an elevated temperature of 100 OC, where somewhat different behavior was observed. At this temperature, the pure propane continuous phase is above its critical temperature and takes on the highly pressure-dependent properties of a supercritical fluid. Surprisingly, DDAB was found to be quite soluble (>250 mM) in propane at 100 OC and 350 bar without the addition of water. However, the addition of about 1% water induces a phase separation, but the further addition of moderate amounts of water (wt % > 5.5) returns the system to a single phase. At water contents above 16%, the system is again twophase. Thus, the propane/DDAB system exhibits two distinct one-phase regions and two distinct two-phase regions upon variation of water content. In Figure 4, the conductivity of the high-temperature (100 "C) propane/DDAB/water solution is given in the two different, single-phase regions. In the absence of water, the conductivity is only slightly above that of pure propane, and from this point the conductivity increases steadily upon addition of water up to the phase boundary. Without water, the surfactant likely exists as small reverse micelles in solution, rather than as monomers or dimers. This hypothesis is supported by the observation that dodecyltrimethylammonium bromide (DTAB), a surfactant that does not have a molecular packing geometry amenable to the formation of reverse or inverted aggregates! is insoluble in propane at temperatures up to 140 "C. As shown in Figure 4, for the second, single-phase region, which is found at water contents between 5.5 and 15%, the conductivity of the microemulsion was found to decrease rapidly upon addition of water up to about 8%. A more gradual decrease in the conductivity is observed for further additions of water up to the phase boundary at 15% water. In both single-phase regions the conductivity is quite low, and like the liquid propane microemulsion at 25 O C with a low s/o ratio, these solutions likely contain aqueous microdomains separately
5
0
10
15
wt % H,O Figure 4. Conductivity of a propane/DDAB/water microemulsion at 100 OC and 350 bar with s/o = 0.09. Two single-phase regions exist: one at low water contents up to I % H20and the other at water contents between 5.5 and 15%.
T E 25%
A
A
Decane 810 = 0.32 WZ K H P = 18.0
I
Y
m d
]
A
-5.01
A
A
,
,
;p:"! I Hexane
rlo = 0.36
A . A
,A
A -6.0 0
100
200
300
400
,
A 500
Pressure (bar)
Figure 5. Effect of pressure on the conductivity of a hexane/DDAB/ water microemulsion and a decane/DDAB/water microemulsion at 25 OC for s/o ratios of 0.36 and 0.32, respectively.
dispersed in the propane-continuous phase. The higher conductivity region near the phase boundary at about 5.5% water is possibly near a transition to a bicontinuous structure. The effect of pressure on conductivity of microemulsions formed in two liquid alkanes, hexane and decane, was also investigated. Figure 5 shows the conductivity for these two liquid alkane microemulsions, formed at the same oolume fractions of water, surfactant, and oil as the propane microemulsion that exhibits the strongly pressure-dependent conductivity (s/o = 0.42, wt % = 21 .O). With hexane and decane, we also see that the conductivity decreases with increasing pressure, but to a much lesser extent
1448 The Journal of Physical Chemistry, Vol. 95, No. 3, 1991
than for the propane microemulsion. The compressibilities of hexane and decane are much smaller than those of propane, so that the solvent properties of the oil phase change very little with pressure. The decane microemulsion has the highly conducting properties of a bicontinuous microemulsion. Decane is expected to solvate the tail region of the surfactant interface to a lesser extent than the lower alkanes, and this would favor a lower surface curvature of a bicontinuous microemulsion phase rather than that of a dispersed inverted microemulsion droplet phase. The large change in conductivity of the liquid propane/ DDAB/water systems shown in Figure 3 was surprising in light of the fact that liquid propane is only a slightly compressible liquid. Three possible mechanisms for the observed pressure-dependent conductivity were considered: (i) increasing solubility of the surfactant monomer in higher density propane reduces the available interfacial area, (ii) propane solvation of the hydrocarbon tails of the surfactant changes the packing geometry or the elastic curvature energy of the interfacial layer, or (iii) enhanced solvation of the surfactant/water aggregate structures by the continuousphase solvent reduces the extent of connectivity between the water conduits. If the solubility of the surfactant monomer in the propane phase increases appreciably with pressure, then the amount of surfactant at the interface region is reduced, and likewise the total interfacial area is reduced. Such a change in the interfacial surface to aqueous phase volume ratio would lead to an increase in the size of the aggregates and to a reduction in the interconnectivity of the aqueous microdomains. Some increase in the solubility of the surfactant monomer with increasing propane density is likely, since the higher density, higher dielectric constant fluid would be a better solvent. However, the solubility of the surfactant monomer in liquid propane at pressures from 10 to 400 bar was determined to be less than 0.5 mM (s/o = 2 X at 25 OC,whereas the bicontinuous microemulsions studied in this report were formed at concentrations above 100 mM (s/o = 0.10). It is unlikely that minute reductions of the surfactant at the interface caused by increased solubility of the surfactant monomer would have an appreciable effect on the interfacial structure. Hence, we find it unlikely that a change in the surfactant solubility in the propane phase is responsible for the observed changes in the conductivity with pressure. increased solvation of the hydrocarbon tails of the surfactant in the interface region may alter the curvature of the interface. Such an effect would necessarily have to occur at a constant surface area to aqueous volume ratio for the following reasons. The strong ionic interactions of the head groups of the surfactant molecules define the spacing of the head groups, whereas the amount of water added to the microemulsion determines the total aqueous volume. increasing the interfacial curvature at constant surface to volume ratio would drive a structural change from cylindrical (conduit) aggregates to spherical structures. Hence, the interconnectivity of the aqueous microdomains would decrease with a resulting reduction in the conductivity of the microemulsion. An alternate but related explanation is based upon the concept of elastic curvature e n ~ r g y . ' ~ .Increased '~ solvation of the hy~
~~~~
( I 3) Mukherjee, S.; Miller, C. A.; Fort, T. J . Colloid InrerJace Sci. 1983. 91. 223-243.
Tingey et al. drocarbon tail region by propane decreases the elastic curvature energy on the oil side of the interfacial film, causing the film to bend more toward the aqueous regions. The large decrease in the conductivity of the propane/DDAB/water microemulsion, as shown in Figure 3, can be explained by using either one of these related surface curvature mechanisms. A third explanation for the observed decrease in the conductivity of the propane microemulsion involves the amount of aggregation of the characteristic aqueous microdomains in solution. The solvent strength of the oil-continuous phase may affect the amount of association of the water microdomains. It is possible that the overall structure of the aqueous microdomains, e.g., rodiike structures, remains constant, but the number of connection points between these structures changes as the density of the propane phase changes. Such aqueous structures would be better solvated by the higher density, higher dielectric constant propane solvent and would be less likely to aggregate into interconnected networks. Our recent studies of propane/AOT/water microemulsion systems have shown the extent of droplet-droplet interaction decreases dramatically as propane pressure is i n c r e a ~ e d . ' ~ . In ' ~ a similar fashion, it is possible that structural differences occurring in the DDAB microemulsion might be accounted for by an enhanced connectivity resulting from stronger attractive interactions of the aqueous microdomains at lower propane densities. Spectroscopic or scattering (e.g., neutron or X-ray scattering) studies may help to resolve the questions regarding the type of pressure-induced structural changes in these propane microemulsions.
Conclusions DDAB bicontinuous microemulsions can be formed in a lower molecular weight liquid alkane such as propane. As with the normal liquid alkane microemulsions, propane also exhibits an antipercolation effect, where the conductivity of the solution decreases upon the addition of water. A transition is observed from a highly conducting solution, having a conductivity approaching that of water, to a solution with a low conductivity approaching that of an alkane liquid. An unusual finding was that the conductivity of the solution was extremely sensitive to pressure, decreasing by almost 3 orders of magnitude as the pressure was increased from 100 to 450 bar. This conductivity effect appears to be consistent with a change in the microstructure of the solution brought about by the increased solvation of the hydrocarbon tails of the surfactant. In accord with previously developed geometric models for liquid systems, the increased solvation increases the interfacial curvature toward the water side, and hence a structural change from interconnected conduits to dispersed droplets is induced. However, other possible contributing factors, such as pressure-dependent aggregation interactions, remain to be examined in detail. Acknowledgment. This work was supported by the Chemical Sciences Division of the US. Army Research Office under Contract No. DAAL03-87-K-0137. Registry No. DDAB, 3282-73-3; H20,7732-1 8-5; propane, 74-98-6. (14) Ruckenstein. E. J . Colloid Interface Sci. 1986, 114, 173-179. (15) Kalcr, E. W.; Billman, J. F.; Fulton, J. L.; Smith, R. D. J . fhys. Chem. 1991, 95. 458.
