J . Phys. Chem. 1984, 88, 1631-1634
1631
Properties and Structure of Three-Component Ionic Microemulsions S. J. Chen,+ D. Fennel1 Evans,*t and B. W. Ninhamt* Department of Chemical Engineering, and Institute for Mathematics and Its Applications, University of Minnesota, Minneapolis, Minnesota 55455 (Received: April 21, 1983; In Final Form: September 12, 1983)
Partial-phase diagrams and conductivity and viscosity behavior of cationic microemulsions, formed from didodecyldimethylammoniumbromide in various alkanes with water are reported. In the one-phase region the microemulsions are conducting at low water content and exhibit a percolation-type transition to a nonconducting state with increasing water content. Parallel behavior is exhibited by the viscosity of these systems. At very high water content the one-phase system becomes a rigid gel. The behavior is virtually the obverse of that usually observed for microemulsions which require cosurfactant and shows that the distinction between microemulsions and emulsions is tenuous. A high and systematic degree of oil specificity is observed. The data are analyzed and are consistent with simple pictures of microemulsion structure based on mainly geometric considerations. Characteristic sizes ranging from 50 to 2000-5000 A can be achieved.
Introduction In a previous note,' we reported the existence and some of the qualitative features of three-component ionic microemulsions. Most work on microemulsions has been hampered by the (apparent) necessity to use cosurfactant. TheoryZpredicts that cosurfactant is not a necessary requirement for microemulsion stability. We have worked with the cationic surfactant didoceyldimethylammonium bromide, which is readily purified and satisfies the principal theoretical requirement2 v/aolc= 1, where v is the hydrocarbon chain volume, a. the head-group area in a bilayer configuration, and 1, an optimal chain length (for fully flexible chains). Partial-phase data and conductivity and viscosity data as a function of alkane chain length throw considerable light on microemulsion formation and structure. The data admit or appear to admit a suggestive interpretation of the structure of these (one-phase) systems. Our point of departure from earlier studies has been to attempt to inject some systematics into the problem. By eliminating cosurfactant and by retaining a constant head group where interactions are dominated by electrostatic^^,^ rather than hard-core repulsion, one can focus directly on the role of the oil, which turns out to be crucial in dictating phase behavior. Methods and procedures are described in the following section. Results and interpretations are given subsequently. Before launching into detail, we remark that the conductivity does exhibit a critical transition to a nonconducting phase reminiscent of a perocolation threshold. But the situation is reversed from that usually observed. For our system the one-phase region is conducting at lowest water content and nonconducting at higher water content. At first sight this is anomalous. We shall advance a tentative interpretation which suggests that the result is more likely the rule rather than the exception. Experimental Materials Didodecyldimethylammonium bromide (Eastman Kodak) was recrystallized first from an acetone-ether mixture and then from ethyl acetate and dried in a vacuum oven. Hexane, octane, decane, dodecane, and tetradecane (Aldrich, Gold Label of Fischer) were used as received. Doubly distilled water was used in all solutions. The regions of the phase diagrams where homogeneous oilwatersurfactant solutions exist were determined by adding known amounts of water to a surfactant-oil mixture contained in a 25-mL-stoppered Erlenmeyer flask. The minimum and maximum amounts of water which resulted in a homogeneous solution were noted. Solutions within the homogeneous-phase boundaries were observed for several months with no apparent change. The experiments were carried out at room temperatures. The solubility Department of Chemical Engineering. *Institute for Mathematics and its Applications. Permanent address: Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra ACT 2600 Australia.
0022-3654/84/2088-1631$01.50/0
of the surfactant at room temperature in all the oils is less than 1 wt W ) . Experiments at higher temperatures will be M (I reported later. No experiments with added salt are reported here, but we remark that middle phases are easily formed with excess salt. The conductance measurements were carried out using the cell shown in Figure 1. Since the resistance of our microemulsions changes by a factor of lo4 with increasing water content, two sets of electrodes with widely different cell constants were required. For high resistances, a set of dipping electrodes (cell constant 0.12 cm-') constructed from two concentric platinum cylinders were employed. The electrodes were contained in a glass sleeve which was open at the bottom and closed at the top except for three small holes which permitted the stirred solution to circulate through the electrode assembly. In order to protect the electrodes from being knocked about by the Teflon-covered magnet, a 3-mm glass rod was fused across the bottom of the electrode compartment. The second set of electrodes (cell constant 7.73 cm-I) were contained in the glass loop attached to the cell compartment. Usually such a geometric arrangement is not satisfactory because the cell constant depends upon the liquid level in the cell. This difficulty is avoided by careful design of the glass loop. At the beginning of the experiment, a homogeneous microemulsion containing the minimum amount of water is added to the cell (dipping electrode in place) so that the liquid level is at level A (Figure 1). After the resistance of the initial solution is determined, a small quantity of water (0.01-0.50 mL) is added through the 10/15 ground glass joint (Figure 1, smaller ground glass joint) by using an Eppendorf pipet. The solutions in the cell compartment and loop are mixed by stirring the solution sufficiently rapidly SO that the liquid level rises in the loop until it flows back through the top of the cell. When the stirrer is stopped, the liquid level falls. Provided that the unstirred liquid is at a level below B, the measured resistance is determined only by the solution between the electrodes. At low water content the electrodes in the glass loop are used. At higher water content the dipping electrodes are employed and can be used up to the liquid level C (Figure 1). The conductance cell was thermostated in a 25 & 0.005 O C oil bath. The resistances were measured with a Jomes-Dole conductance bridge. The viscosities of the microemulsions were determined with Cannon Ubbelohde suspended level viscometers. We have not (1) L. R. Angel, D.F. Evans, and B. W. Ninham, J . Phys. Chem., 87, 538 (1983). (2) D. J. Mitchell and B. W. Ninham, J. Chem. Soc., Faraday Trans. 2, 77, 609 (1981).
( 3 ) D. J. Mitchell and B. W. Ninham, submitted for publication in J. Phys. Chem. (4) D. F. Evans and B. W. Ninham, submitted for publication in J . Phys. Chem.
0 1984 American Chemical Society
1632 The Journal of Physical Chemistry, Vol. 88, No. 8, 984
Chen et al.
1-
Il------
Figure 1. Conductance cell containing two sets of electrodes. 50,
A
/
.
90
,
.
70
L
,
50
Yo W A T E R
30
,
,
\ 10
30
10
\
E
90
70
50
% WATER
Figure 2. Triangular phase diagram showing one-phase microemulsion regions for (1) hexane, (2) octane, (3) decane, (4) dodecane, and (5) tetradecane, respectively. Upper portions (>50% surfactant) have beem omitted. Percentages of each component are weight percent.
yet explored these systems for non-Newtonian behavior.
Results and Discussion Partial-phase diagrams are given in Figure 2 I-V. In each the region above the line AE has not been studied. The diagrams are plotted as weight % of each component. Beyond the lines EX toward the water corner the system forms clear viscous gels which become solid with increasing water content. In each diagram inside the boundary AEXB the system is a clear, apparently stable, one-phase microemulsion. The lines CD, C’D’, and C”D” correspond to conducting regions, explored and plotted in Figures 3 and 4, with the points D, D’, and D” representing the limit of the conducting region along lines of constant surfactant-to-oil ratios. With addition of water the boundary AB marks the onset of a single phase. Along this line all systems are conducting. This observation is an important clue to structure, which we can now proceed to explore. There is a suggestive pattern if the data in Figure 2 are converted to volume fractions.5*6In making this conversion, we have taken the density of surfactant to be equal to that of water (-1 g/cm3). This approximation introduces some uncertainties, although they are probably less than experimental error. Consider now the line AB along which the system first forms a one-phase region. Data points are compared in Tables I and 11. Our first observation is that since all systems studied are conducting along the line AB, the single-phase microemulsions must be water continuous. Because of the low solubility of the
25 35 W t % of Water i n Microemulsion Figure 4. As for Figure 3 but for octane, decane, dodecane, and tetradecane. (Tetradecane microemulsions (closed circles) are always conducting and comprise normal oil-in-water spheres; s/o = 0.343.)
11
15
surfactant in both water and oil, it must be located at the oil-water interface. Oil Specifcity. An important clue to the nature of these systems is provided by the systematic variation of packing fraction from hexane (93%) to tetradecane (73%). Work on alkane uptake in bilayers7,*shows that oil penetration is larger for small alkanes. ( 5 ) K. J. Lissant, J . Colloid Interface sei. 22, 462 (1966). (6) “Emulsions and Emulsion Technology”, Part 1, K. J. Lissant, ed., Marcel Dekker, New York, 1974).
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
Three-Component Ionic Microemulsions 30 1
Wt % o f Water 34 36 38
32 1
261
1
1
1
1
1
1
1
TABLE 11: End Points of Conducting and Nonconducting Regions along Lines of Constant Surfactant-to-Oil Ratios Given in Figure 21-Va
40 1
1
1633
l
/446
vol fraction
factant
surfactant water
0.287 0.213 0.186 0.163 0.132 0.124
0.36 0.53 0.28 0.37 0.21 0.26
0.163 0.146 0.119 0.091
0.26 0.34 0.20 0.26
0.280 0.167 0.135 0.110
0.43 0.65 0.26 0.40
0.096 0.086
0.25 0.34
0.152 0.132
0.42 0.49
surwater
oil
+
Hexane
C D C’ D’ C” D”
0.073 (7) 0.313 (38) 0.090 (12) 0.202 (32) 0.081 (16) 0.137 (29)
C D
0.101 (16) 0.192 (34) 0.084 (18) 0.171 (48)
0.640 0.474 0.724 0.635 0.788 0.739 Octane
C’ D’
decane s/o=0.250 ctane s/o=o. 293
tet radecane s/o=0.343 I
10
I
I
I
I
I
I
Decane 22
I
I
C
I
18 22 26 30 W t % of Water Figure 5. Viscosity as a function of weight fraction of water at fixed surfactant/oil ratios for octane, decane, and tetradecane. 14
D C’ D’
TABLE I: Volume Fractions of Components in Various Oils along ABa vol fraction
suroil
factant
0.375 0.287 0.186 0.132 0.065
0.94 0.93 0.91 0.92 0.95
0.230 0.163 0.119 0.042
0.91 0.90 0.92 0.96
0.367 0.280 0.135 0.081
0.87 0.85 0.87 0.89
0.233 0.096 0.033
0.86 0.85 0.89
+
oil
C” B A C C’ B
0.093 (10) 0.101 (16) 0.084 (18) 0.036 (22)
A B
0.131 (9) 0.146 (13) 0.127 (24) 0.113 (36)
A C B
0.142 (16) 0.152 (40) 0.115 (89)
C C’
0.566 0.640 0.724 0.788 0.880 Octane
0.677 0.736 0.797 0.922 Decane
c
C’
0.502 0.574 0.738 0.867 Dodecane
0.625 0.752 0.852 Tetradecane
0.306 0.70 0.152 0.73 0.094 0.74 Data points are labeled as per Figure 21-V. (Assume density A C B
a
0.299 (25) 0.268 (45) 0.259 (71)
0.152 (40) 0.249 (75)
0.752 0.665
0.268 (45) 0.362 (70)
0.579 0.505
a Numbers in brackets show number of water molecules per surfactant molecule. psurfactant = pwater = 1 g/cm3.
Now, in the absence of oil penetration the surfactant parameter2
Hexane
0.059 (4) 0.073 (7) 0.090 (12) 0.081 (16) 0.055 (24)
A
0.574 0.343 0.738 0.604
Tetradecane
C D
surfactant
0.146 (13) 0.490 (75) 0.127 (24) 0.289 (35)
Dodecane
C D
water
0.736 0.662 0.797 0.738
0.398 0.579 0.647
of surfactant = density of water 0 1 g/cm3.) These correlate qualitatively with observed viscosity behavior.
Indeed, as a rough rule of thumb, it appears that chains of length less than the (double-chain) surfactant length penetrate completely, whereas for larger oils, alkane uptake drops dramatically. These results are in accord with what is known from elementary consideration of dispersion self-energies of the various o i k 9 (7) D. W. R. Gruen and D. A. Hayden, Pure Appl. Chem., 52, 1229 (1980); Biophys. J., 33, 149 (1981). ( 8 ) D. W. Gruen, Biocbim. Biophys. Acta, 595, 161 (1980); Chem. Phys. Lipids, 30, 105 (1982).
v/aolc.l1. At very low water content elementary theory4 predicts that oil penetration will change the volume per surfactant molecule by a factor2 of up to 2 to give small inverse (water) microemulsion drops in oil. However, the favorable free energy associated with such a configuration is opposed by the requirement of the surfactant for “bound water (compare Tables I and 11, column 1) and a natural tendency due to attractive forces to collapse t o a lamellar state. The trade-off between these two competing processes leads to the onset of the one-phase region which is water continuous as shown by conductance measurements. This is a surprising result. An understanding of the phenomena will evidently require a more complete characterization of these systems by a variety of techniques. Conducting-Nonconducting Transition. The transition from a conducting to nonconducting phase can now be understood at least qualitatively. It is clear that there is a critical cutoff at which oil penetration takes over, and the system makes a transition to disconnected water-in-oil droplets. Figures 3 and 4 show specific conductance, K (Q-I/crn-’), as a function of water content for hexane through tetradecane. The data fits the empirical form K = KOI(@- cP,)I-*, where 9 and aCare the volume fractions of water, typically as follows: KO = 19.2, a = 3.44, @ < aC;KO = 5.9 X a = -0.193, @ > aC(hexane, C’D’, s/o = 0.389). KO = 29.22, a = 4.40, < aC;KO = 4.15 X lo-’, a = -0.10, 9 > aC(octane, C’D’, s/o = 0.176). KO = 44.7, a = 4.0, 9 < aC;KO = 8.25 X a = -0.42, 9 > @, (decane, C’D’, s/o = 0.251). 9:s are determined graphically as indicated in Figures 3 and 4. The values of KO and aCchange with surfactant/oil ratios. (9) J. Mahanty and B. W. Ninham, “Dispersion Forces”, Academic Press, New York, 1976. (10) See articles by M. Sahimi et al. in “Mathematics and Physics of Random Media”, B. Hughes, B. W. Ninham, G. Sell, and H. Weinberger, eds., Springer-Verlag, New York, 1983. (11) M. Lagiies, R. Ober, and C. Taupin, J . Phys. Lett. (Orsay, Fr,), 39, L-487 (1978). (12) M. Lagues, J . Phys., Lett. (Orsay, Fr.), 40, L-33 (1979). (1 3) “Microemulsions”, I. D. Robb, ed., Plenum Press, New York, 198 1 .
1634 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
a.
0.
b.
d.
Transition from sphere t o RDH
Figure 6. Rhombododecahedral packing of microemulsions taken from
Li~sant.~.~ Shaded regions indicate flattened areas with facing surfactant head groups separated by thin water sheets. Curves of the RDH contain structures reminiscent of reverse phase (water in oil) droplets. With increasing water content flattened regions decrease in area. This functional form is characteristic of the behavior expected for Q percolation threshold. However, it is difficult to relate the observed critical phenomena to theoretical models at this time. The critical exponents (a)are notoriously sensitive to the choice of the critical point aCand show some variation. The range of that variation is less than the spread of theoretical estimates given by a large number of authors.’, The behavior observed is qualitatively different from that observed in the more complicated These systems are nonconmicroemulsions studied so ducting at low water content and become conducting at high water content. Viscosity. Parallel viscosity data have been obtained for hexane, octane, and decane microemulsions along some of the same lines in the corresponding phase diagrams. The viscosity is highest at lower water content and drops to a minimum at the onset of the conducting phase, increasing thereafter. Some of these data are exhibited in Figure 5 . The viscosity of hexane microemulsions a t the onset of the one-phase region is 50 times that of hexane. The viscosity behavior of tetradecane microemulsions is as expected from its conductance behavior. At high water content a (sometimes extremely rigid) gel region is observed as indicated on the phase diagrams. These regions have not yet been extensively explored and remain an open problem. Microemulsion Structure. We first consider the oil plus the surfactant hydrocarbon tails to comprise an “internal phase”. Perusal of column 5 of Table I shows that the volume fraction of this internal phase ranges from about 93% for hexane, systematically decreasing to =74% for tetradecane. As shown by Lissant5*6in his remarkable and seminal analysis of emulsion structures from geometric considerations alone, in the range of 94-74%, rhombododecahedral (RDH) packing of the internal phase is the preferred state. Hence, one expects structures of the form indicated in Figure 6 and already well described by Lissant. With hexane a t 93% volume (cf. Figure 6d), R D H flattened sheets of reversed bilayers (with facing head groups separated by water) must almost touch a t the edges of the R D H (they touch at 96.6% volume fraction). These flattened sheets provide a connected conducting water path for ions. With increasing oil chain length and with corresponding decreased oil penetration of the surfactant chains, a matter to which we return, these flattened regions decrease in area until at tetradecane the “internal phase” (74%) forms a system of close-packed spheres (cf. Figure 6a). Upon addition of water, a natural tendency of surfactant in the flattened regions is to migrate to the corners and along the edges
Chen et al. of the tessellating structures to achieve a reverse microemulsion droplet configuration. These structures are almost certainly bicontinuous. Due to the high degree of flexibility associated with the oil penetration allowed to the short-chain hydrocarbons, the surfactant tail curvature required at 95% volume fraction at the edges and corners of the basic rhombodecahedron (Figure 6d) can be accommodated by hexane and octane, but only at a lower volume fraction by longer chain alkanes. An alternative interpretation of the data is that both the oil and water phases are continuous. We envision the conducting water paths in this bicontinuous system in terms of chaotic interconnected conduits which are dynamic in nature. For hexane, the large value of u (high oil penetration) and the small value of a, (high bromide concentration in the water phase, Table I) results in conduits of small diameter (u/l,ao < 1). The average conduit diameter can be roughly estimated as follows. If we assume that all of the surfactant is located at the oil-water interface with a, =40 AZ,the total oil-water interfacial area for 1 cm3of the hexane microemulsion (Table I, solution C) is 1.5 X lo6 cm2. The total volume of enclosed water plus bromide is 0.095 cm3 if we assume v(Br-) ii: 2V(H20). This gives an average conduit diameter of =25 A and a total conduit length of 10l2cm. Addition of water along the line AE decreases the bromide concentration and increases a, while the conduits grow in diameter (Table 11, solution D, diameter =40 A2). At a critical water concentration (Table 11), the conducting paths begin to disconnect (percolation threshold) and re-form as water droplets immersed in oil. As the chain length of the oil increases, more water must be added to obtain conduits with the minimum diameter compatible with the curvature dictated by the oil penetration. For tetradecane, the very low degree of oil penetration results in a reversal of curvature and only water continuous microemulsions are formed. Stated in this way, the microemulsion structural problem reduces to a study of a dynamic porous media.I4 We are obtaining data which will differentiate between these two possible models. Conclusion
We have reported partial-phase information and conductivity and viscosity behavior of a class of ionic microemulsions. Because no cosurfactant is involved, it seems a relatively straightforward matter to interpret observations and ascribe structures to the system in a way which has not previously been possible. Extensive use of light scattering and other techniques brought to bear on the problem would be illuminating. It would appear that, bearing in mind the role of oil specificity which has not been widely recognized, it should be possible to lay down some specific principles to design microemulsions and indeed emulsions for specific tasks. The region of the one-phase microemulsion can be vastly expanded by using mixed oils. For example, design of microemulsions and emulsions of prescribed viscosity, temperature properties, size, and/or oil selectivity should be possible. Especially will this be so when complementary studies are undertaken of anion or cation specificity. There are enormous controllable variations possible here due to the interplay between oil penetration and structural forces in water. Applications to systems, which could serve to separate different oils, should also come within reach.
Acknowledgment. D.F.E. acknowledges support of U.S. Army Contract DAA G29-8 1-K-0099. Registry No. Didodecyldimethylammonium bromide, 3282-73-3; hexane, 110-54-3;octane, 111-65-9; decane, 124-18-5; dodecane, 11240-3; tetradecane, 629-59-4. (14) B. D. Hughes and S. Prager, “Random Processes and Random Systems: An Introduction” in “The Mathematics and Physics of Disordered Media”, B. D. Hughes and B. W. Ninham, Ed., Springer-Verlag,New York,
1983.
