J. Phys. Chem. 1985,89, 711-713
711
Structure and Dynamics in Three-Component Microemulsions F. D. Blum,+ S. Pickup: B. Ninham,I S. J. Chen,! and D. F. Evans*$ Deparment of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104; Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, 2600, Australia; and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis. Minnesota 55455 (Received: June 28, 1984; In Final Form: August 16, 1984)
NMR self-diffusion coefficients are reported for three-component micromulsions formed from didodecyldimethylammonium bromide/water/alkane. For hexane through tetradecane the oil diffusion coefficients are approximately half as large as those of the bulk oils and independent of composition. Therefore, the systems are oil continuous through the entire one-phase region. The diffusion coefficients for the surfactant are small and independent of composition. The water self-diffusion coefficients decrease with added water for hexane through dodecane and change in a manner consistent with the abrupt conducting-nonconducting transition known to occur in this system. The microemulsions are bicontinuous in the conducting regions and disconnected water-in-oil droplets in the nonconducting regions of the phase diagrams. The phenomena reported clearly demonstrate the interplay between forces due to oil penetration and those due to head-group interactions which control curvature and therefore microemulsion structure.
Introduction Properties of three-component ionic microemulsions have been delineated in three recent papers.’-3 These systems are formed from the double-chained surfactant didodecyldimethylammonium bromide (DDAB), alkanes, and water, and differ in two important ways from most microemulsions studied earlier. The absence of cosurfactant has an important consequence; the role of the oil in prescribing curvature becomes dramatically obvious. This prescription can be used to tune up microemulsion structure. Further, unlike most emulsifiers, DDAB is virtually insoluble in both the oil and water phases. As a result, the oil-water interfacial area can be set by the amount of surfactant. From conductance, viscosity, and solubility measurements, the following facts have emerged: The minimum amount of water which must be added to surfactant-oil mixtures to form a single-phase microemulsion increases with increasing hydrocarbon chain length. Thus for hexane, a one-phase microemulsion forms at only 7 vol % water, while for tetradecane -26% water is needed. For the corresponding olefins only ‘/2-’/3 as much water is required. At low water content all the microemulsions (except at extremely low o/w ratios) are conducting and hence water continuous. Upon addition of more water, the conductance of all of the microemulsions (except those containing tetradecane) decreases and passes via a critical percolation transition to a nonconducting state. At high water content the microemulsions become gels which in many cases are birefringent. These systems exhibit viscosities which are surprisingly high in the low water content region. For example, the viscosity of a hexane microemulsion at 7 wt % water is 50 times greater than that of bulk hexane. As the percolation threshold is approached, the viscosities decrease to values close to those of the bulk oil. The origin of the differences in oil penetration which dictate curvature and therefore structure can be understood in terms of work on alkane uptake in bilayers5q6and has been described extensively e l s e ~ h e r e . ~ , ~ In this study, we report the results of N M R self-diffusion measurements on the oil, surfactant, and water in these systems. The N M R pulsed-field gradient technique has previously been applied to a variety of four-component microemulsions.8-10 In such systems the surfactant diffusion coefficient is lower than those of the other components and nearly independent of composition. That for the oil is large and changes with composition consistent with a bicontinuous or dynamic microstructure. Alcohol diffusion is complex. The diffusion coefficient of water is consistent with its being a continuous phase. It behaves in a way which reflects Drexel University. Australian National University. 8 University of Minnesota. f
obstruction and hydration effects”J2 but not overall microstructural transitions. In what follows it will be seen that DDAB/water/hydrocarbn microemulsions exhibit properties which are very different from those of systems containing cosurfactant.
Experimental Section Didodecyldimethylammonium bromide (DDAB) was purchased from Kodak and recrystallized from ethyl acetate or acetone-ether mixtures and dried in a vacuum oven before use. Hexane, octane, decane, dodecane, and tetradecane were of high purity and used as received from Aldrich. Distilled deionized water was used. Samples for N M R measurements were prepared by diluting a stock solution directly in preweighed 5-mm N M R tubes which were then sealed. The results obtained were not dependent on the order of mixing of the components. Samples of phase diagram studies were prepared as previously described.2 The volume fractions calculated were based on the densities of the pure components at 25 “C, and the density of the surfactant was taken to be 1 g/mL. Self-diffusion coefficient measurements were performed on a JEOL FX-90Q N M R spectrometer operating a t 90 MHz for protons. The pulsed-field gradient spin-echo technique used has previously been described and used in the study of microemuls i o n ~ . * .The ~ field gradient was calibrated by using neat cyclohexane for which the diffusion coefficient is known. The gradient was calculated to be about 0.05 T/m. The time between the first 90” pulse and the echo was usually about 70 ms. This time is long enough so that even the slowest diffusing species travels a (1) Angel, L. R.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1983,87, 538. (2) Chen, S.J.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1984,88, 1631. (3) Ninham, B. W.; Chen, S. J.; Evans, D. F. J . Phys. Chem. 1984, 88, 1631. (4) “Microemulsions”;Robb, I. D., Ed.; Plenum Press: New York, 1980. (5) Gruen, D. W. R. Biophys. J . 1981, 33, 149. (6) Gruen, D. W. R.; Haydon, D. A. Biophys. J . 1981, 33, 167. (7) Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Tram. 2 1981, 77, 609. (8) Lindman, B.; Stilbs, P.; Mosely, M. E. J . Colloid Interface Sci. 1981, 83, 569. (9) Cheever, E.; Blum, F. D.; Foster, K. A.; Mackay, R. A. J. Colloid Interface Sci., in press. (10) Foster, K. R.; Cheever, E.; Leonard, J. B.; Blum, F. D.; Mackay, R. A. A.C.S. Symp. Ser., in press. (11) Foster, K. R.; Cheever, E.; Leonard, J. B.; Blum, F. D. Biophys J . 1984, 45, 975. (12) Douglas, D. C.; McCall, D. W. J . Phys. Chem. 1958, 62, 1102. (13) Lissant, K. J. J. Colloid Interface Sci. 1966, 22, 462. (14) “Emulsions and Emulsion Technology”; Part 1; Lissant, K. J., Ed.; Marcel Dekker: New York, 1974.
0022-3654/85/2089-0711%01.50/0 0 1985 American Chemical Society
712 The Journal of Physical Chemistry, Vol. 89, No. 4, 1985
Blum et al. TABLE I: Self-Diffusion Coefficients in Three-Component Microemulsions at 25 OC volume fractions DDAB oil water
~
0 0.273 0.263 0.257 0.246 0.236 0.230 0.211
1 0.612 0.588 0.575 0.551 0.530 0.515 0.473
Hexane (s/o = 4.447) 0 0.115 0.56 0.149 0.44 0.168 0.41 0.203 0.29 0.234 0.25 0.256 0.26 0.316 0.26
50.3 21 24 25 27 19 19 27
4.9 3.4 3.6 2.0 1.o 0.60 0.20
0.212 0.200 0.186 0.178
0.717 0.678 0.613 0.604
Hexane ( s / o = 0.296) 0.070 0.48 0.122 0.35 0.182 0.217 0.12
24 35 24 23
4.2 3.1 0.96 0.22
0 0.189 0.180 0.168 0.159 0.152
1 0.662 0.632 0.591 0.559 0.530
Octane (s/o = 0.285) 0 0.148 0.37 0.186 0.35 0.240 0.21 0.282 0.24 0.318 0.26
22.5 17.5 15 14 17 16
3.2 2.0 0.40 0.54 0.24
0 0.280 0.257 0.244 0.225 0.210 0.189
1 0.576 0.529 0.502 0.462 0.431 0.387
Decane (s/o = 0.486) 0 0.144 0.48 0.213 0.37 0.255 0.41 0.313 0.36 0.359 0.31 0.452
16.5 7.8 9.1 8.1 9.6 8.0 10.0
4.8 4.9 3.8 2.6 1.3 0.30
0 0.145 0.134 0.121 0.115 0.109 0.096
1 0.671 0.607 0.557 0.515 0.503 0.444
Dodecane ( s / o = 0.216) 0 10.0 0.184 5.6 0.262 5.4 0.322 5.6 0.374 6.6 0.388 5.8 0.459 5.5
3.6 1.7 0.51 0.38 0.39 0.27
0 0.248 0.234 0.218 0.209 0.199
Tetradecane ( s / o = 0.556) 1 0 0.446 0.305 0.38 0.445 0.342 0.40 0.392 0.398 0.377 0.412 0.359 0.442
--_--I~-~
100
HZ.
Figure 1. Normal NMR proton spectrum for the didodecyldimethylammonium bromide, hexane, and water microemulsion with composition by weight of 0.197, 0.665 and 0.138, respectively.
1
diffusion coefficients DDAB oil water
Yo WATER
Yo WATER Yo WATER Figure 2. Partial-phase diagrams showing the one-phase microemulsion region and the water dilution paths studied for hexane, octane, decane, dodecane, and tetradecane, respectively. Percentages of each component are weight percent. The dashed line passing toward the oil corner is drawn from the oil corner through the end points of the conducting regions (see text). The shaded region to the left of the single-phase region marks the gradual transition to a viscous gel-like system.
distance of about a micron, which is greater than the dimensions of any possible structure in these systems. The accuracy of the method is about f 10% for species with diffusion coefficients greater than 10-lo m2/s but somewhat lower for more slowly diffusing species. The spectra were taken at the ambient probe temperature which was typically 25-28 "C. In the NMR spectrum the water resonance is always resolved enough that it can easily be used to measure the water self-diffusion coefficient. The situation is also simplified because there are no exchangeable protons on the oil or surfactant. However, the surfactant and oil resonances completely overlap, except for the methyls and methylenes of the surfactant which are next to the nitrogen. Since the diffusion coefficients of the surfactant and oil usually differ by an order of magnitude or more, both can be determined from overlapping resonances. At very small gradient times the effects of the oil (fast) diffusion dominates, while at long gradient times the oil contribution is zero and only the surfactant contributes to the observed peak. The results for the surfactant diffusion are independent of whether the head-group or tail resonances are used. This verifies the appropriateness of using the overlapping resonances for the measurement of the diffusion coefficient of either oil or surfactant. Conductance measurements were carried out as previously described.
Results The normal proton spectrum of a DDAB/hexane/water sample with compositions by weight of 0.197/0.665/0.138, respectively, is shown in Figure 1. The assignments are given in the figure. As water is added to the samples, the head-group resonances of the surfactant become difficult to use for diffusion measurements. The paths chosen for diffusion coefficient measurements for DDAB/oil/water systems are shown in Figure 2 as solid straight
6.85 3.2 3.0 2.6 3.0 1.9
6.4 5.9 5.2 6.2 6.6
a Diffusion coefficients in units of lo-'' m2/s; compositions given in volume fractions (assuming surfactant density is 1 g/cm3).
lines which point toward the water corner of the diagram. Some of these paths were chosen to correspond to the paths previously studied by using conductance measurements.2 The partial-phase diagrams define the single-phase microemulsion region which for all oils except tetradecane extend down into the oil corner. The shaded regions mark the transition from fluid microemulsions to viscous gels. Diffusion coefficients for the three-component microemulsions and for the neat oils at 25 "C are shown in Table I as a function of composition. The compositions correspond to a water dilution path (constant oilsurfactant ratio). As can be seen, the diffusion coefficient of the surfactant is always ca. 3 X lo-' m2/s regardless of the oil used. For this reason it was not always measured. The dotted lines are drawn through the approximate end points of the conducting region.2 The independence of the surfactant selfdiffusion coefficient and its relatively small value are consistent with previous microemulsion s t ~ d i e s ~and . ~ suggest that the properties of the interface are nearly constant (as far as molecular transport is concerned). In fact, the transport of surfactant, which is located at the interface, is not sensitive to structural changes which greatly diminish transport in the aqueous phase.
Structure and Dynamics in Three-Component Microemulsions
The Journal of Physical Chemistry, Vol. 89, No. 4, 1985 713
TABLE II: Specific Conductance along the A-B Line Microemulsion of Figure 2" SI0 H,O,wt % specific conductance, i2-I r d
by the balance between oil penetration and electrostatic interactions. The extent of oil penetration will depend on both chain length and the surfactant to oil ratio. On addition of water the conduits reorganize in such a way that constant curvature is maintained. The high concentration of counterions present in the 4-50) aqueous phase (moles of water/moles of bromide ion makes ion solvation and ionic interactions crucial variables. While the interplay of all these factors is complex, the evidence obtained to date supports the initial formation along most of the AB line of a highly interconnected chaotic network which can be described in terms of a dynamic porous medium. Addition of water leads to disruption of the interconnected networks. This is reflected in the systematic decrease in conductance and in the N M R diffusion coefficients. This progressive disconnection of conduits ultimately becomes an avalanche process which results in the percolation-like threshold for the formation of water-in-oil droplets. The percolation threshold occurs when enough water is added so that the minimum surface area to volume ratio (spheres) is achievable. As shown in the phase diagrams (Figure 2), the composition a t the conducting-nonconducting transition occurs at roughly constant surfactant/water ratios for a given oil. This is indicated by the dotted lines which connect the percolation thresholds reported previously. Note that this line extends down to the point labeled B which is nonconducting (Table 11). That is, near the oil corner the microemulsion contains reverse droplets as it must.' From hexane to dodecane it takes more water per head group to convert from cylindrical conduits to spheres as the molecular weight of the oil is increased. This is entirely consistent with the notion of oil penetration and increasing curvature forced onto the system. Since hexane penetrates the most, the conversion to inverted spheres is the easiest (least water). We are now in a position to note why the behavior of tetradecane systems appears to be different from that of the other oils studied. If one uses the same s/w ratio for the tetradecane system as for the dodecane system at the percolation threshold, one observes that this threshold lies just below the single-phase microemulsion region. Therefore, the tetradecane system probably always retains a bicontinuous structure. The diffusion coefficients of water with tetradecane as oil are about the same as those for the other oils a t low water content. Upon addition of water, the viscosity of the tetradecane system increases. This contrasts with the decrease observed with the shorter chain oils. We conjecture that this phenomenon is due to increasing conduit diameter as opposed to conduit disconnection in the lower oils.
Hexane 1.492 0.998 0.665 0.427 0.250 0.175 0.114
14.69 5.86 9.97 10.48 9.96 7.83 7.04
0.539 0.315 0.176 0.0523
14.00 12.90 10.95 5.02
0.667 0.251 0.0517
17.26 15.86 6.23
Octane
2.70 X 10-1 6.72 X 1.47 x lo-' 1.14 X 5.19 X 2.36 X 1.58x 10-5 1.71 X lo-' 1.03 X 10-1 5.43 x 10-2 7.64 x 10-5
Decane
a
3.35 x 8.21 X 1.58 X
lo-' lod
Results are given in weight fractions.
The diffusion coefficients for the oil in the microemulsions (Table I) are dependent on the molecular weight of the oil molecule in much the same way as bulk liquids.'* The diffusion coefficient for a given system does not seem to vary much as the composition is changed. This is most obvious for the hexane system. The oil diffusion constant is approximately 0.5-0.75 times that for the neat oil, in agreement with values obtained by Douglas and McCall.12 These large oil microemulsion diffusion coefficients suggest that the oil always remains a continuous phase in the regions studied. By contrast, the water diffusion coefficients (Table I) decrease as water is added to the microemulsion for hexane through dodecane. A transition occurs which mimics the conductance behavior,' the diffusion coefficients decreasing by an order of magnitude. At lowest water content the measured diffusion m2/s) are lower by a factor of 5 than those coefficients (4 X of bulk water. The N M R data confirm the inference drawn from conductance measurements that the aqueous regions become discontinuous upon the addition of water. The extreme sensitivity of the water diffusion coefficient to water content contrasts strongly with the relative insensitivities found in other three- and four-component microemulsions. For example, the water diffusion coefficient for the n-tetradecane ethylene glycol monoether/water/n-hexadecane system increases substantially but not dramatically upon addition of water! Again, over most of the composition range the change in water diffusion in the cetyltrimethylammonium bromide/butanol/water/benzene system has been attributed to "hydration" and "obstruction" effects9J0 and shows no significant structural changes. We have also obtained and list in Table I1 specific conductances along the AB line for hexane, octane, and decane. The specific conductance gradually decreases as one traverses the line from A to B. Near the points labeled B, the specific conductance for all the oils hexane through dodecane shows an abrupt decrease consistent with formation of a water discontinuous microemulsion.
Discussion The patterns which emerge from NMR, conductance, and viscosity data for the single-phase microemulsion region are all consistent. The microemulsions are all bicontinuous over a large region of the phase diagram and water-in-oil droplets at high water volume fraction. We can imagine that the bicontinuous structures are comprised of interconnected fluctuating conduits filled with water. The oil-water interfacial area will then be fixed by the surfactant concentration, and the conduit volume will be determined by the amount of water and counterion in the system. The diameter of the conduits (curvature of the system) is determined
-
Conclusion The picture which emerges from the results can be summarized as follows. The minimum amount of water required to form a single-phase microemulsion shows a strong dependence upon oil chain length. At large s/w ratios, like those typified by the regions near point A (Figure 2), a bicontinuous phase which can be viewed as water-filled interconnected conducting conduits is formed. As the s/w ratio decreases, either by moving down the A-B line or toward the water corner of the triangular phase diagram, the conduits disconnect. At a nearly constant s/w ratio which is specific for each oil, an abrupt conducting-nonconducting transition is observed. This signals the conversion of conduits to water-in-oil spheres. The interplay between oil penetration and ion-solvent interaction is quite delicate but readily discernible at least in our systems. These opposing forces are clearly the key to microemulsion design. Acknowledgment. D.F.E.acknowledges support by U S . Army Contract DAA G29-81-K-0099. F.D.B. acknowledges the support of NSF Grant CPE 82-0491 1, the Drexel University Graduate School, and the donors of the Petroleum Research Fund, administered by the American Chemical Society. Registry No. DDAB, 19959-22-9; hexane, 110-54-3; octane, 11 1-65-9; decane, 124-18-5;dodecane, 1 12-40-3; tetradecane, 629-59-4.
