J . Phys. Chem. 1990, 94, 5641-5649
5647
Electron Microscopic Study of a Glass-Forming WaterIOil Pseudo-Threecomponent Microemulsion System Jenny L. Green
Chemistry Department, University of Sydney, NSW 2006, Australia (Received: April 3, 1990)
The development of microemulsion systems that do not break down during cooling and in which neither dispersed nor matrix phases crystallize during the cooling process opens the way to direct studies of the microemulsion structure and also the investigation of the dispersed liquid in unusual states. We report the first “water-in-oil” example of this type of system. It was obtained by partial replacement of water by glycerol and total replacement of normal paraffin by ethylcyclohexane, in the water/oil/didodecyldimethylammonium bromide three-component system. The phase diagram at 25 “C indicates a remarkably wide range of clear-phase compositions. A dispersed droplet structure for the “water”-rich range is unequivocally established by direct electron microscope imaging of the vitrified microemulsion, using the freeze-fracture technique. Droplet sizes are -200 8, and show a range of frozen-in shape distortions which are not subject to solidification artifacts because the droplet phase vitrifies while the matrix remains fluid.
Introduction Water-in-oil (w/o) microemulsion systems (MEs) provide a fascinating medium for the study of water in unusual states. The discovery of three-component microemulsionsl over a decade ago somewhat reduced the complexity of the better known four-component systems,24 making the delineation of various structural features, e.g., the disposition of water molecules in the dispersed phase, more straightforward. These thermodynamically stable microdispersed states have prompted much discussion pertaining to their structure and dynamics with relevance not only to their fundamental interest but also to practical applications, for example, the cleanup of oil polluted water. In these systems, either a nonpolar liquid (oil) or an aqueous phase (hydrogen-bonded polar liquid) may be the dispersed phase depending on the surfactant characteristics. One particularly interesting case is the three-component water-in-oil ME discovered by Angel et al.lb using the Ytwo-armed”cationic surfactant didodecyldimethylammonium bromide (DDAB),5-6 which is all but insoluble in both the oil and aqueous phases. This ME apparently exhibits, at large surfactant/water (s/w) ratios, a bicontinuous structure in which the water occurs in long randomly connected tubules?” As the s/w ratio decreases, the tubules disconnect to form isolated water microdroplets. This picture has been supported by conductivity and diffusion measurements7 as well as small-angle X-ray scattering (SAXS) and fluorescence Several years ago, pseudo-three-component oil-in-water glass-forming microemulsions were preparedI0Jl with a noncrystallizing aqueous matrix obtained by adding propane-l,2-diol (propylene glycol, PG) to the water in the ratio 1:3. PG may also have served the role of cosurfactant since the usual addition of (1) (a) Zulauf, M.; Eicke, H. F. J . Phys. Chem. 1979,83,480. (b) Angel, L. R.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1983, 87, 538. (2) Meunier, J.; Langevin, D.; Boccara, N., Eds. Physics of Amphiphilic Luyers; Springer: New York, 1987. (3) Scriven, L. E. In Micellization, Solubilization and Microemulsions; Mittal, K., Ed.; Plenum: New York, 1977. (4) Rosano, H. L.; Clausse, M., Eds. Microemulsion Systems; Dekker: New York, 1987. (5) Chen, S.J.; Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1984, 88, 1631. (6) Ninham, B. W.; Chen, S.J.; Evans, D. F. J . Phys. Chem. 1984, 88, 5855. (7) Blum, F. D.; Pickup, S.; Ninham, B.; Chen, S. J.; Evans, D. F. J . Chem. Phys. 1984, 89, 71 I . (8) Puji, J. E.; Rodriguez-Siordia, 1.; Rangel-Zamudio, L. I.; Billman, J. F.; Kaler, E. W. tangmuir 1988, 4, 806. (9) . ”2 T. N.: Hvde. S.T.: Derian. P.-J.: Barnes. 1. S.: Ninham. B. W. J . Phys. Chem. 1987, 41,3814. Barnes,’I. S.;’Hyde, S. T.;Ninham,’B.W.; Derian, P.-J.; Drifford, M.; Warr, G . G.; Zemb, T. N . Prog. Colloid Polym. Sci. 1988. 76. 90. (10) MacFarlane, D. R.; Angell, C. A. J . Phys. Chem. 1982,86, 1927. ( I I ) Angell, C. A.; Kadiyala, R. K.; MacFarlane, D. R. J . Phys. Chem. 1984, 88. 4593.
a normal alcohol proved unnecessary. The fact that neither phase in these microemulsions crystallizes upon cooling below 273 K means that possible modifications of ME structure by crystal growth are bypassed on the way to formation of a vitreous solid at lower temperatures. Consequently, direct observation of the microemulsion structure using freeze-fracture electron microscopic techniques was p o s ~ i b l e . ~ It ~ -was ~ ~ established in these studies, and similarly confirmed in the present study, that the freezefracture technique did not produce artifactual surface features. The possibility of extending this approach to the direct observation of the tubule droplet transition in the w/o MEs discussed above is obviously of interest. Some related studies have recently been reported by Jahn and Strey,I6 who used a fast-quenching technique to either bypass or minimize crystallization and microemulsion breakdown in several w/o MEs. They observed dispersed droplet structures (and bicontinuous structures of surprisingly large dimensions -2000 A in some cases) in their electron micrographs. While these results are in most respects convincing, it would obviously be better for the purpose of observing subtleties in structure and structural transitions, if the instabilities, and the drastic quenching measures needed to deal with them, could be avoided, i.e., if the structure of a microemulsion which is stable as it vitrifies could be studied. As we will show, this is possible by use of a variant of the DDAB-based system discussed above. Partial vitrification of bicontinuous w/o microemulsions was observed in the water/DDAB/octane system,17 but the relation of the vitreous component to the original ME structure was obscure. This study was also compromised by the facility with which these microemulsions separate and then crystallize on cooling below 273 K. According to R~binson,l*-~~ the instability of aqueous microemulsions is due to desorption of the surfactant molecules from the oil/water interface. By contrast, Robinsonz0 found an extended temperature stability in the alternative three-component w/o system using the anionic Aerosol-OT (sodium bis(2-ethylhexy1)sulfosuccinate (AOT)) as surfactant and replacing the water
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(12) Dubochet, J.; Adrian, M.; Teixeira, J.; Albot, C. M.; Kadiyala, R. K.; MacFarlane, D. R.; Angell, C. A. J . Phys. Chem. 1984,88, 6727. (13) Hildebrand, E. A.; McKinnon, I. R.; MacFarlane, D. R. J . Phys. Chem. 1986, 90, 2784. (14) MacFarlane, D. R.; McKinnon, I. R.; Hildebrand, E. A.; Angell, C. A. In Microemulsion Systems; Rosano, H. L., Clauue, M., Eds.; Dekker: New York, 1987; p 311. ( I 5) Alba-Simionesco, C.; Teixeira, J.; Angell, C. A. J . Chem. Phys. 1989, 91, 395. (16) Jahn, W.; Strey, R. J. J . Phys. Chem. 1988, 92, 2294. (17) Angell, C. A.; Choi, Y. J. Microscopy 1985, 141, 251. (18) Toprakcioglu, C.; Dore, J. C.; Robinson, B. H.; Howe, A. M.; Chieux. P. J . Chem. Soc., Faraday Trans. 1 1984,80, 413. (19) Fletcher, P. D. I.; Howe, A. M.; Perrins, N . M.; Robinson, 8. H.; Toprakcioglu, C.; Dore, J . C. In Sufjacrants in Solution; Mittal, K., Ed.; Plenum Press: New York, 1983; Vol. 3, p 1745. (20) Fletcher, P. D. 1.; Galal, M. F.; Robinson, B. H. J . Chem. SOC., Faraday Trans. I 1984, 80, 3307.
0022-3654/90/2094-5647$02.50/00 1990 American Chemical Society
Letters
5648 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 DDAB
A
1911% OOA8 19'
O
'
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100
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TEHPERATUREIK
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\
, ____
WATER
-
,
,
ETHYLCYCLOHEXANE CYCLOHEXAHE
Figure 1. Phase boundary for the ethylcyclohexane/water/DDAB system at 25 OC shown together with the cyclohexane/water/DDAB system (ref
22) for comparison.
totally with glycerol. In view of this observation we decided to explore the idea of forming glassy w/o microemulsions by partially replacing the water in DDAB w/o MEs with glycerol and, in addition, utilizing a glass-forming oil to avoid matrix crystallization. These are the conditions needed to optimize the potential of the freeze-etch electron microscopic structural probe.
Experimental Section Microemulsion Preparation. A good glass-forming oil, viz., ethylcyclohexane (glass transition temperature ( Tg = 98 K) was used along with the glycerol/water aqueous phase and DDAB as surfactant. The DDAB, from Eastman Kodak, was recrystallized from ethyl acetate and dried in a vacuum oven. The ethylcyclohexane was gold label, reagent grade from Aldrich Chemical Co., the water was distilled and deionized?' and glycerol was either reagent grade from Sigma Chemical Co. or commercial drum glycerol which contained 10 wt % H 2 0 . Several different water and glycerol ratios were tested, but most experiments were performed using the solution glycerol.2.1 H 2 0 . Microemulsions containing both pure water and water/glycerol as dispersed phases were made by we8hing all components in glass vials that were sealed, gently shaken, and left overnight to stabilize. Glass transition temperatures were determined with a PerkinElmer Model DSC-7 differential scanning calorimeter at a scan rate of 10 OC/min. Electron Microscopy. In this work a freeze-fracture or perhaps more aptly a freeze-etch technique was used. A small droplet (-0.01 cm3) of sample was placed on a gold disk which was immediately quenched into liquid Freon surrounded by liquid nitrogen. The sample was then placed in an evacuated chamber (at -90 K) in preparation for the etching procedure. A knife was passed across the sample, lowering it a few microns at each pass. Eventually, half the sample having been etched away, the fractured surface was coated with a platinum-carbon (Pt-C) film, followed by another coat of carbon, while being held just above liquid nitrogen temperatures. The Pt-C replica was removed from the chamber and subsequently from the sample by using a dilute chromic acid solution. At this stage difficulties with the replica breaking up were often encountered. After complete removal from the sample, the replica was examined by transmission electron microscopy (TEM). Results and Discussion Figure 1 compares the phase boundaries for the H20/ DDAB/cyclohexane22 and H20/DDAB/ethylcyclohexane mi(21)
Lacey. A. R.; McPhail, A. K.; Trafalski, Z . J. J . Phys. E :
Instrum. 1985, 18, 5 3 2 .
Sci.
GLYCEROL: 1.7 H20
0-
GLYCEROL: 2.1 H20
0
ETHYLCYCLOHEXANE
Pseudo-three-component phase diagram for the system ethylcyclohexane/glycerol.2.1 H20/DDAB. The point marked within the phase boundary corresponds to the sample used for the electron microFigure 2.
scopic study (see Figure 3). The inset shows the glass transition for the dispersed phase of the same sample. The dotted lines show the change in phase boundary for a cut with aqueous-phase composition glycerol. 1.7H20. croemulsion systems at 25 OC. The differences are qualitatively not unexpected for the longer chain oil. Figure 2 shows the pseudo-ternary-phase diagram for the system glycerol.2.1 H20/DDAB/ethylcyclohexane at 25 OC. Another isotropic phase was confirmed lying in the water-rich corner of the phase diagram. The phase boundary has an extra lobe extending to the water-rich corner of the phase diagram compared with that of the H,O/DDAB/ethylcyclohexane system shown in Figure 1. This suggests that the glycerol is structurally very different from or severely disrupts the water network in this system. It should also be noted from Figure 2 that a slight change in the glycerol to water ratio (viz., glycerol-2.1H 2 0 glycerol.1 .7H20) causes a shift in the phase boundary. The structural effect of glycerol in this system will be the subject of a future investigation. We find that a wide range of clear-phase compositions can be vitrified, Le., no increase in turbidity, upon immersion of the sample vial in liquid nitrogen. However, some microemulsion samples exhibited breakdown upon rewarming. The inset in Figure 2 shows the glass transition for the aqueous phase of a microemulsion sample (62 wt % ethylcyclohexane/l9 wt % DDAB/19 wt % glycerol.2.1H20). The Tgvalue, 172 K, agrees with the bulk binary solution data of Rasmussen and Ma~Kenzie,~ which implies that the microdispersed phase exhibits the same relaxation properties as the bulk liquid. The matrix phase Tgof 98 K is too low to be observed with our instrument. To our knowledge, this system provides the first example of a glassforming w/o microemulsion and hence the first chance to accurately observe the microstructure in these systems. In Figure 3 we show a transmission electron micrograph obtained from the 69 wt % ethylcyclohexane/l9 wt % DDAB/19 wt % glycerol-2.1 H 2 0 microemulsion. The location of this composition in the phase diagram is given by the solid circle. The structural definition obtained is of rather good quality. It is clear
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(22) Chen, V.;Warr, G.G.; Fennel Evans, D.; Prendergast, F. G. J . Phys. Chem. 1988, 92, 768. ( 2 3 ) Rasmussen, D. H.; MacKenzie, A. P. J . Phys. Chem. 1971, 75,967.
Letters
Figure 3. Transition electron micrograph of the 19 wt '7 DDAR/ 19 wt % glycerol*2.1H20/62wt o/o ethylcyclohexane microemulsion obtained by the freeze-etch technique at 56500X magnification. All micrographs were recorded on a Philips EM400.
from Figure 3 that at this composition the structure is one of isolated droplets rather than bicontinuous structures. This is confirmed by electrical conductivity results we have recently obtained. I n fact, the conductivity data show that there is no tubule droplet transition across the entire single-phase region of this system at 25 "C. Consequently, the dispersed phase in this system only exhibits droplet structure, the dimensions of which
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The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5649 will vary depending on the percentage of aqueous phase present. The droplets are well separated and have diameters of -200 A, somewhat smaller than that found by Jahn and StreyI6 for quenched pure water MEs of larger weight percent aqueous phase. Attempts to observe the structure of low water content samples were frustrated by difficulties in retaining Pt-C replica integrity when separating it from the substrate. The polydispersity seems greater than the 20% which is ptedicted by Safran2*as a maximum value compatible with stability in a three-component microemulsion at ambient temperature. In the work of Jahn and Strey16in the system H20/ndodecane/13.9 wt 96 DDAR using equal volumes of water and oil, the polydispersity appears to be even greater than in our case. This, together with the surprisingly large dimensions of some of their structures, raises the question of whether their quenching procedure was fully adequate to preserve the ambient-temperature structure. The droplet dimensions we observe are comparable with the oil droplets observed with equal resolution by Hildebrand et aL13 in an o/w ME. We should emphasize, however, that for the purpose of using the results of such electron microscopic investigations to discuss size distribution and particularly shape distortions in relation to theoretical predictions, the present system has a clear advantage over the o/w cases of refs 10-15. This is due to the fact that the matrix vitrifies at a lower temperature than the droplet phase in the present system. A concern with the previous work, in which o/w glassy microemulsions were probed by using the freeze-fracture TEM technique, pertained to the shape variations of the droplet phase which could have been partly due to stress exerted by the aqueous matrix vitrifying around them while they remained liquid. In conclusion, the formation of aqueous glass-forming microemulsions with the ethylcyclohexane/DDAB/glycerol~H20system has been demonstrated and the droplet structure unequivocally established by direct electron microscope imaging using the freeze-etch technique.
Acknowledgment. We thank Maria Guzman and Greg Warr for clarification of the pseudo-ternary-phasediagram, C. Austen Angel1 for many helpful discussions, and Dennis Dwarte and the University of Sydney Electron Microscope Unit for use of their facilities. (24) Safran, S. A. In Surfucrunrs in Solurion: Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York. 1984; Vol. 3.
