1898
J . Phys. Chem. 1988, 92, 1898-1902
Fluid Microstructure Transition from Globular to Bicontinuous in Mdrange Microemulsion J.-F. Bodet,+J. R. Bellare, H. T. Davis,* L. E. Scriven, and W. G. Milled Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Minnesota. Minneapolis, Minnesota 55455 (Received: June 19, 1987; In Final Form: October 7 , 1987)
Pulsed field gradient spin-echo nuclear magnetic resonance (PFGSE NMR), quasi-elastic light scattering (QLS), and freeze-fracture transmission electron microscopy (FFTEM) were used to study surfactant fluid microstructure and dynamics of microemulsions of pentaethylene glycol dodecyl ether (C12ES),water, and octane (C,) having compositions in the plane a = c,/(cg + H20) = 40 wt % of the phase diagram. NMR-determined translational self-diffusion coefficients of oil, water, and surfactant, QLS translational diffusion coefficients, and FFTEM micrographs are reported along a one-phase corridor around the lamellar region in a temperature-surfactant composition phase diagram. Just below the lamellar region, water-continuous microemulsions consisting of oil-rich globules (swollen micelles) in water exhibit small, time-dependent self-diffusion coefficients (over times of the order of 0.1 s) and a biexponential QLS correlation function characteristic of concentrated, polydisperse suspensions of repulsive globules. At temperatures just above the lamellar region, bicontinuous microemulsions exhibit relatively high self-diffusion and a monoexponential QLS correlation function. A rather abrupt transition from discontinuous oil-in-water to bicontinuous microstructure, visualized for the first time by electron microscopy, occurs at low surfactant concentration, close to a three-phase region.
Introduction Microemulsions are thermodynamically stable isotropic phases containing surfactant, water, and oil, and sometimes another amphiphile (often called cosurfactant). These multicomponent solutions are characterized by water-rich domains and oil-rich domains separated by surfactant-rich sheetlike regions. For small amounts of oil in water or water in oil the fluid microstructure is often that of swollen micellar g1obules.l However, when the volume fractions of water and oil are comparable, there is compelling evidence that some microemulsions are bicontinuous, having both oil-continuous and water-continuous domains separated by surfactant-rich The idea of bicontinuity, introduced8 in 1976, led to various models of bicontinuous s t r ~ c t u r e . ~ - ' ~ We presently lack a complete understanding of the nature of the microstructures which occur, their lifetimes, and the transitions among them as temperature, salinity, and composition are varied. Recently, a temperature-composition path along with pentaethylene glycol dodecyl ether (CI2E5),water, and octane form a bicontinuous microemulsion has been reported.' The bicontinuity was inferred from the facts that the self-diffusion coefficients of all components of the solution are relatively large even though neutron scattering indicates relatively large microstructures (several hundred angstroms). As more direct evidence of bicontinuity, electron micrographs were presented of replicas of surfaces exposed by fracturing frozen microemulsion samples. In a similar study, Olsson et aL6 have observed that a small temperature variation can cause a large change in the self-diffusion coefficient of the surfactant in ethoxylated alcohol, water, and hydrocarbon solutions. This behavior is consistent with a temperature-induced transition from bicontinuous to water-continuous or oil-continuous microemulsions. In the research reported here we examine the microstructure along a one-phase temperaturesurfactant composition path for a microemulsion of CI2Es,water, and octane containing water and octane in nearly equal volumes. Spin-echo pulsed-field-gradient NMR, quasi-elastic light scattering, and freeze fracture transmission electron microscopy are used to probe fluid microstructure in the solution. Experimental Section Materials and Sample Selection. The surfactant, pentaethylene glycol dodecyl ether (CI2E5),was obtained from Nikko Chemical Co. Ltd., Tokyo. Octane (c,) was purchased from Aldrich and was of 99% purity. Doubly distilled and deionized water was used. Samples were prepared by weight. 'On leave from Centre de Recherche Paul Pascal, Centre National de la Recherche Scientifique, 33405 Talence, France. *Department of Chemistry. 0022-3654/88/2092-1898$01.50/0
TABLE I: Microemulsion Compositions" mixtureb Y . ~wt % temp, OC mixtureb A
B C D E F G H 1 J
24.3 22.1 20.2 18.1 15.5 14.4 12.4 9.8 8.1 6.0
20.0 21.8 22.0 25.5 26.5 27.3 28.3 30.4 31.2 32.4
I' H' G'
F' E' D' C' B' A
Y . wt ~
% tar"
8.1 9.8 12.4 14.4 15.5 18.1 20.2 22.1 24.3
'a = C8/(C8 + H20)= 40 wt 5%. 'See Figure 1. (C,,ES + HZO + GI.
O C
33.5 34.1 35.1 36.3 36.9 31.3 38.1 39.1 40.1
= C,,E5/
At constant pressure, a ternary system has three independent variables. We describe the CI2E5,H20, C, system by the variables temperature, weight percent of oil in the mixture of oil and water a = C8/(C8 + H 2 0 ) , and weight percent of the surfactant in the system y = C12ES/(C12ES + H 2 0 + C8). Recently, Kahlweit et al.' explored this system by various techniques. Most of their studies were in the plane y = 7 wt % at variable a and temperature. Our work has been done in the plane a = 40 wt % at variable y and temperature. The experimental points (see Figure 1 and Table I), marked A, ..., I, J, 1', ,.., A', map a one-phase corridor that bends back around the lamellar region as temperature and y are varied. Pulsed Field Gradient Spin-Echo N M R (PFGSE N M R ) . (1) Shulman, J. H.; Stoeckenius, W.; Prince, L. M. J. P h p Chem. 1959, 63, 167. (2) Kaler, E. W.; Davis, H. T.; Scriven, L. E. J . Chem. Phys. 1983, 7 5 ,
5685. (3) Auvray, L.; Cotton, J.; Ober, R.; Taupin, C. J . Phys. Chem. 1984, 88, 4586. (4) Guering, P.; Lindman, B. Langmuir 1985, I , 464. Lindman, B.; Kamenka, N.; Kathopoulis, T.-M.; Brun, B.; Nilsson, P.-G. J . Phys. Chem. 1980, 8 4 , 2485. (5) Clarkson, M. T.; Beaglehole, D.; Callaghan, P. T. Phys. Reo. Let?. 1985, 54, 1722. (6) Olsson, U.; Shinoda, K.; Lindman, B. J . Phys. Chem. 1986, 50,4083. (7) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; Schemling, T.; Faulhaber, B.; Borkovec, M. J.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funck, Th.; Richmann, H.; Magid, L. J.; Soderman, 0.;Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn, W., submitted for publication in J . Colloid Interface Sci. (8) Scriven, L. E. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977, p 877. (9) Talmon, Y.; Prager, S.J . Phys. Chem. 1978, 69, 2984. (10) De Gennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86, 2294. ( 1 1 ) Widom, B. J . Chem. Phys. 1984, 81, 1030. (12) Safran, S.A,; Roux, D.; Cates. M. E.; Andelman, D. Phys. Reu. Letr. 1986, 57, 491
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988 1899
Microstructure Transition in Microemulsion
40
T
["CI
t
b
30 20 a
b
C
7
d
CH3-(CH,),,-CH2-(O-CH2-CH~)~-OH/HOH 10
0
10
20
-
a
30
y(wt % C,,E,) waterloctane 60:40 Figure 1. Plane a = 40 wt 7%through the phase diagram of the system pentaethylene glycol dodecyl ether, water, n-octane. Samples studied are as indicated by filled circles (e). r3G11~U11iU31UllbUOIIIblGII1 IIIGLIJUIGIIIGLILJ W G I G ~ G I I U I I I I G U U11
a
Nicolet N M C 1180 N M R spectrometer operating at 300 MHz for protons. The spin-echo pulsed field gradient technique used has been previously describedI3-l5 and used in the study of surfactant sy~tems."'*'~'~ The experiments were performed by varying the field gradient pulse duration, 6 , with a constant field gradient pulse interval, A. The self-diffusion coefficients, D, were determined by fitting the decay of the echo amplitude, A , with the equation A = A. exp(-y,G2D6*(A
- 6/3))
b
a
CH3-(CH2)6-CH3
Figure 2. Proton NMR spectrum of a Cl2ES,HzO, C8 microemulsion with composition by weight of 0.181, 0.492, 0.328, respectively, at 25.0
"C.
was obtained after correction for the contribution from surfactant hydroxyl protonsla on the assumption of fast exchange. Quasi-Elastic Light Scattering (QLS).Diffusion coefficients were determined by using a 2-W argon ion laser (Spectra Physics 164) with an output powder of 0.5 W at a wavelength of 5145 A. The cylindrical sample cells were immersed in a bath of refractive index matching fluid and temperature controlled to f0.5 OC. The scattered intensities were received at 90' by a Pacific 126 photometer and analyzed by a Langley-Ford Model 1096 correlator. The homodyne correlation functions, C ( 7 ) ,were analyzed by the cumulant method. In this analysis, the quantity In [ C ( T ) ]is fitted as a quadratic function of time 7, Le.,
(1)
(3)
Here A. is the echo amplitude in the absence of any gradient pulse, G is the field gradient strength, and ygis the gyromagnetic ratio. The field gradient was calibrated by using neat benzene for which the self-diffusion coefficient is known [D(25 "C) = 2.28 X m2/s]. The field gradient strength, G, could be varied from 0.01 to 0.25 T/m. The value used was 0.15 T/m. The diffusion time, defined by the pulse interval, A (A >> 6/3), could be varied from 1 to 4000 ms. This led to a sensitivity to diffusion over distances from 0.1 to 10 pm for typical values of D. The temperature of the measurements was controlled within f0.2 OC. Samples were flame sealed in 5-mm N M R tubes. In the proton N M R spectrum of a typical microemulsion sample, Figure 2, the main methylene peaks of CI2E5and octane overlap. As the field gradient pulse duration is increased, the peak amplitude follows6 equation
The parameter r is the inverse of the characteristic correlation time of the scattered light. Assuming that scattered light is due to concentration fluctuations of scattering objects, r is related to the z average translational diffusion coefficient of the scattering objects through D, = r/k2
where k is the scattering wave vector, Le., (4?m/X) sin (0/2), n being the refractive index of the solvent, X the wavelength of the laser light, and B the scattering angle. The parameter p 2 / r 2 characterizes the deviation of exponentiality of the correlation function due, e.g., to polydispersity. Systems exhibiting two well-separated correlation times were analyzed according to the equation
C(7) =
b(T)
- Y(m)11'2
= C(O)[Aexp(-rFT) because there is no exchange between the methylene protons. Here x and (1 - x) denote the fractional contributions of CI2E5and Ca, respectively, to the peak amplitude. From the known C12E5 self-diffusion coefficient, DC,2EJ, as determined from the protons in the group denoted by c in Figure 2, the octane self-diffusion coefficient, Dc,, was determined by fitting the peak decay by means of a nonlinear least-squares procedure in which x , A,,, and Dc, are the adjustable parameters. The water self-diffusion coefficient ~
~~~
~
(13) Callaghan, P. T. Aust. J . P h p . 1984, 37, 359. (14) Blum, F. D. Spectroscopy 1986, 1(5), 32. (1 5) Stilbs, P. Prog. N M R Spectrosc. 1987, 19, 1. (16) Lindman, B.; Stilbs, P.; Moseley, M. E. J . Colloid Interface Sci. 1981, 83, 569. (17) Blum, F.; Pickup, S.;Ninham, B.; Chen, S. J.; Evans, D. F. J . Phys. Chem. 1985, 89, 71 1. (18) Nilsson, P . G.; Lindman, B. J. Phys. Chem. 1983, 87, 4756.
(4)
+ (1 - A ) exp(-rs7)]
(5)
Here y ( 7 ) is the experimental value of the correlation function, A and 1 - A are the weights of the fast and slow modes, and rp and rSare the inverses of the correlation times of these modes. Electron Microscopy by Vitrification-Fracture-Replication. The freeze-fracture transmission electron microscopy (FFTEM) technique used in the work reported here to image microemulsions differed from conventional sample preparation techniques in the use of the controlled environment vitrification system (CEVS). By rapid cooling of samples from a temperature and solvent saturated controlled environment, the CEVS reduces or prevents artifacts during sample preparation prior to vitrification. The CEVS, described by Bellare et al.,'9-22 was used with custom (19) Bellare, J. R. Proc. 44th Annu. Meeting of the EMSA 1986, 236-231.
( 2 0 ) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y . Proc. XIrh Int. Congr. Electron Microsc., Kyoto 1986, 1, 361-368.
Bodet et al.
1900 The Journal of Physical Chemistry, Vol. 92, No. 7 , 1988 I
loab
lo "
2
D PUREH20 D PURECe O 0 D D CH20 8
1
+ x
D
t
C12E5
Mixture #
Figure 3. N M R self-diffusion coefficients for the studied microemulsions: DCllES (--); DH20(-n-);Dc8 (-+). Pure water self-diffusion coefficients (+) and pure octane self-diffusion coefficients ( X ) are shown at the same temperature as the microemulsions. designed tweezers23 to which are attached two thin (0.1 mm) copper plates about 4 mm square. A drop (about 2 pL) of the sample, previously equilibrated in the water vapor and octane vapor saturated environmental chamber, held to within 0.1 "C, was placed on one of the plates. The tweezers were then closed to squeeze the liquid specimen between the copper plates until only a thin layer of sample remained between them. The resulting sandwich was rapidly quenched by abruptly forcing the tweezers through a synchronously opened shutter in the environmental chamber and into a brass cup of liquid ethane which is freezing from the edges of the cup. This preparation technique ensured that the sample components were kept at fixed chemical potentials until vitrification. Specimen cooling rates, estimated to be 2 X lo4 K/s (Costello et al. 1983), are sufficient to vitrify water samples.23 Vitrified samples were transferred from liquid ethane into liquid nitrogen and mounted in a Balzers freeze-etch device. The samples, held at a temperature below -150 "C and a pressure lower than Torr, were fractured by separating the two copper specimen support plates. The fracture surfaces were replicated by shadowing with platinum (by electron-beam evaporation) and reinforcing with carbon (by thermal evaporation). Fractured samples will etch (sublime) if the partial pressure of water vapor in the replication system is below the partial pressure exerted by ice. Water vapor will recondense on the specimen surface and obliterate specimen detail or add artifactual microstructure unless a cold trapping surface is kept near the sample. Therefore, in this work, etching was reduced or avoided by fracturing the sample after the platinum evaporation was started. Furthermore, the knife mount of the freeze-etch device was cooled with liquid nitrogen to act as a cold trap to prevent condensation of volatiles on the fracture surface. The replicas were floated into nitric acid, washed with distilled water, air-dried, and examined in the conventional transmission mode of a JEOL lOOCX analytical electron microscope. Micrographs were recorded on Kodak 4489 film and developed for 4 min in Kodak D19 (1:2) developer.
Results Echo Pulsed Field Gradient Spin-Echo N M R (PFGSE N M R ) . Self-diffusion coefficients were measured in the microemulsions A-J and A'-I' along the one-phase corridor shown in the temperature-composition diagram (Figure 1). The self-diffusion coefficients are presented in Figure 3. In the microemulsions A-G lying below the lamellar liquid phase, the self-diffusion coefficients of water are almost as high as molecular diffusion (21) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y . Proc. XIth Int. Congr. Electron Microsc., Kyoto 1986, 1 , 369-370. (22) Bellare, J. R.; Davis, H . T.; Scriven, L. E.; Talmon, Y . J. Electron Microsc. Tech., in press. (23) Bellare, J. R.; Sheehan, J.; Davis, H. T.; Scriven, L. E. In Proceedings of the 1987 Annual Meeting of the Materials Research Society, Symposium W, Boston, in press.
'
10-14;
1I
'
2I
I
'
'
3 I [sec]
'
'
'
4
5
Figure 4. Dependence of CBand C,,E, N M R self-diffusion coefficients on the field gradient pulse interval ( A ) for the microemulsion A (y = 24.3%. 20.0 "C). 0
-. ... t 1
-4
0
8
1
'
1
'
7
1
1
/
1
[sec]
0.04
Figure 5.
QLS correlation functions (a) characteristic of "low temperature" microemulsions, e g , A (y = 24.356, 20.0 "C), C (20.2%, 22.0 "C), and G (12.4%, 28.3 "C)[points are experimental results, solid lines are biexponential fits], or (b) characteristic of "high temperature" microemulsions, e.g., D' (18,156, 37.3 "C).
in pure water. Oil and surfactant diffusivities are, however, much lower than the water diffusivities. This is a characteristic fingerprint of oil-swollen micelles in a water-continuous microemulsion. From G to J a rather abrupt change in self-diffusion behavior occurs. The oil self-diffusion coefficient increases by 3 orders of magnitude to m2/s) and the surfactant self-diffusion coefficient increases by 2 orders of magnitude (10-" to m2/s). The microemulsions J-A' in the one-phase corridor running above the lamellar region have self-diffusion coefficients that are relatively independent of the composition and temperature. The self-diffusion of both oil and water is fast and typical of free molecular diffusion. The self-diffusion coefficient of surfactant is almost constant (1O-Io m2/s) and is comparable to the diffusion coefficient of an amphiphile along a lamellar layer.24 The diffusivities in microemulsions J and 1'-A' are consistent with the hypothesis of bicontinuous fluid microstr~cture.~-' If the molecular components undergo unrestricted three-dimensional diffusion, their mean square displacement along the field gradient direction is d = 2DA and the self-diffusion coefficient D is independenti3-'5of the diffusion time A. We found this to indeed be the case for oil, water, and surfactant molecules in all the samples above the lamellar region. However, below the la(24) Roeder, S. B. w.; Burnell, E. E.; Kuo, A. L.; Wade, P h i s . 1979, 64, 1848.
c. G.J . Chem.
Microstructure Transition in Microemulsion
Mixture t Figure 6. QLS diffusion coefficients for the studied micoemulsions.
mellar region, from A to G, the oil and surfactant self-diffusion coefficients displayed dependence on diffusion time. A typical result is shown in Figure 4. Quasi-Elastic Lighf Scaffering ( Q L S ) . QLS correlation functions and QLS apparent diffusion coefficients are respectively shown in Figures 5 and 6. Below the lamellar region, the microemulsions A G have a marked nonexponential QLS correlation function; however, the fit to two exponentials is excellent (Figure sa). At low temperature, the slow mode becomes predominant and the fast mode becomes undetectable on the time scale shown. In contrast, the microemulsions H-A' have a monoexponential QLS correlation function decay (Figure 5b) with an apparent diffusion coefficient independent of composition and temperature changes (Figure 6). The solutions do not absorb strongly a t the wavelength 5145 A. Results did not change with modest increases and decreases in laser power (about the 0.5 W used), a fact
The Journal of Physical Chemisfry, Vol. 92, No. 7 , 1988 1901 indicating that laser heating of the solution was not a problem. Electron Microscopy by Vifrificafion-Fracture-Replicafion. Images of samples in the one-phase corridor are shown in Figure 7. Only isolated spherical structures (nominally 30 nm in diameter) appear in micrographs of a replica of a sample of microemulsion F, which has high surfactant concentration at low temperature. In microemulsion H,which has a lower surfactant concentration, some disjoint spherical structures of larger size, about 50 nm in diameter, coexist with larger microstructures that may result from globule coalescence. In microemulsion J, a t the bend in the corridor, nondisjoint structures that span relatively long lengths can be interpreted as bicontinuous in view of the high Octane and water self-diffusivities. In microemulsion H', a t higher surfactant concentration and higher temperature, intertwined water and Octane domains, about 60 nm wide, are evident. The intertwined domains are presumably both continuous in view of the self-diffusion coefficients. Discussion The patterns which emerge from NMR, QLS, and FFTEM measurements of the microemulsion states in the plane o( = 40 wt % (about equal volume fractions of oil and water) are all consistent. We have obtained convincing evidence of a transition from swollen micellar solution to bicontinuous microemulsion at this fixed oil-to-water ratio. This transition differs from the percolation threshold which occurs as the oil-to-water ratio varies. It appears to be driven instead by the effect of temperature on the mean curvature of the surfactant-rich zones that separate the oil-rich from the water-rich regions. For the first time, the transition is visualized by FFTEM (the sequence of states studied by Kahlweit et al.' with FFTEM was apparently entirely bicontinuous). Transmission electron microscopy is the only technique that currently can provide direct,
Figure 7. Freeze-fracture electron micrographs of microemulsion F (y = 14.4%. 27.3 "C) characteristic of "low temperature" microemulsions, of microemulsion H (9.8%. 30.1 'C) near the transition from bicontinuous to oil-in-water microemulsion. showing disjoint globular structures. and of microemulsions J (6.090.32.4 ' C ) and H' (9.8%. 34. I "C). characteristicof 'high temperalure" microemulsions (note intertwined bicontinuous domains).
1902 The Journal of Physical Chemistry, Vol. 92, No. 7, 1988
high-resolution images of colloidal systems. However, electron microscopy of microemulsions is fraught with artifacts due to radiation damage during imaging and phase separation during sample preparation. Freeze-fracture electron microscopy avoids the radiation damage problem by creating an inverse replica of an internal surface of the frozen sample. The internal surface is exposed by fracturing the sample and then a replica is created as an inert film that can be imaged without danger of radiation damage. This technique has been successfuly applied to lamellar liquid crystalline phase^.^^,^^ However, microemulsions are highly dynamic microstructures, and conventional sample preparation techniques can induce phase change. By providing a sample preparation environment where component activities, Le., chemical potentials, the CEVS can be controlled and maintained, prevents phase separation of the sample during preparation, and vitrifies it with little or no microstructural reorganization. These unique features, previously applied by us to the successful study of concentration-sensitive and high-temperature surfactant aggreg a t e ~ , are ~ ~particularly - ~ ~ ~ ~advantageous ~ for electron microscopy of systems as phase-sensitive as the microemulsions studied here. At temperatures lower than 30 “C, the microemulsions A-G display QLS correlation functions with two well separated correlation times. Such behavior is expected of highly concentrated, slightly polydisperse suspensions of repulsive droplets.29 Similar observations have been reported for concentrated silica dispers i o n ~mixed , ~ ~ micelles of Ci2E5and sodium dodecyl sulfate,31and concentrated hexanol, sodium dodecyl benzenesulfonate, xylene, water micro emulsion^.^^ The dynamical properties of slightly polydisperse permanent spheres have been investigated by Pusey, Fijnaut, and Vrij.33 They showed that, for relatively high volume fractions (4) and fairly narrow size distributions, the light scattering correlation function should be composed of two independent modes with well-separated decay times corresponding to two different diffusion processes. The faster decaying mode describes collective stochastic “compression-dilation” motions of the particle mixture in which the relative concentrations remain unchanged (Le., 6pi/p1 = 6p2/p2 in the case of two species, p, being the number density of species i ) . The slower decaying mode describes “concentration-fluctuation” dynanics in which the different species are exchanged through self diffusion processes that preserve a constant total number density (Le., 6pl + 6p2 = 0 in the case of two species). The second process, which under usual conditions contributes very little, becomes visible at high volume + 0.5) fraction of particles (4 > 0.2; in our case [4C,IEs particularly in systems of repulsive particles where the self-diffusion process is much slower than the fast collective ”compression-dilation” motion of the particles. A general theory for computing the diffusion coefficients D,and D,,related to the two modes (so-called collective or fast mode and slow mode), is not available. However, in the case of ”scattering power polydispersity”, the collective diffusion coefficient, D,,can be expressed as3’
-
p
Here f,is a friction coefficient, P is the osmotic pressure, and is the number density of particles. For strongly (repulsively)
(25) Balmbra, R. R.; Clunie, J. S.; Goodman, J. F. Molecular Crystals; Gordon and Breach: London, 1967; Vol. 3, p 28 1. (26) Zasadzinski, J. A,; Davis, H. T.; Scriven, L. E. Philos. Mug. 1985, A S ] , 287. (27) Adrian, M.; Dubochet, J.; Lepault, J.; McDowall, A. W. Nature 1984, 308, 32. (28) Miller, D. D.; Bellare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J . Phys. Chem. 1987, 91, 614-685. (29) Cazabat, A. M.; Chatenay, D.; Langevin, D.; Meunier, J.; Leger, L. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 1729. (30) Kops-Werkhoven, M. M.; Mos, H. J.; Pusey, P. N.; Fijnaut, H. M. Chem. Phys. Lett. 1981, 81, 365. (31) Guering, P.; Nilsson, P. G.; Lindman, B. J . Colloid Interface Sci. 1985, 105, 41. (32) Cebula, D. J.; Ottewill, R. H.; Raltson, J.; Pusey. P. N. J . Chem. Soc.. Faraday Trans. 1 1981, 77, 2585. (33) Pusey, P. N.; Fijnaut, H. M.; Vrij. A. J . Chem. Phys. 1982, 77, 4270.
Bodet et al. interacting suspensions it is well-known that, whereas & l a p increases rapidly with concentration, fc also increases in roughly similar fashion, and so 0,is only a weak function of concentration. We found this to indeed be the case for samples D-G (Figure 6). The slow-mode QLS diffusion coefficient, which has nearly the same value as the N M R self-diffusion coefficients of octane and pentaethylene glycol dodecyl ether, corresponds to the translational motion of the globules. The random walk diffusion of a single globule is severely hindered by both direct and hydrodynamic interactions, so that D,is a strongly decreasing function of concentration (Figure 6). In such a case the translational diffusion coefficient cannot yield a globule size. At approximately 30 OC, the NMR-determined self-diffusion behavior over the sequence changes dramatically, indicating a transition from oil-rich globules in water to bicontinuous fluid microstructure. The micrograph of microemulsion H (Figure 7b), wlhere globular structures coexist with interwined microstructures, suggests a fluid microstructure progression from swollen micellar solution to bicontinuous microemulsion by globule aggregation and coalescence. At temperatures higher than 30 OC, the surfactant fluid microstructure appears to be bicontinuous. The sizes of the intertwined microdomains of water-rich material and octane-rich material observed by microscopy are in excellent agreement with the results obtained by small-angle neutron scattering7 (about 60 nm for microemulsion H’). The evolution of the surfactant fluid microstructures with temperature and fluid composition can be understood in general terms of topological and geometrical necessity. The particular case can be further rationalized by invoking an apparently abrupt change in surfactant polar head hydration. Recent direct measurements of the solvation force between C12E5monolayers in aqueous solution34 show a dramatic change in short-range interactions from attractive to repulsive at approximately 30 “C, presumably34related to an increase in hydration with decreasing temperature. That the hydration increases with decreasing temperature was inferred by Nilsson and LindmanI8 from N M R self-diffusion data. There might be some relationship between the solvation force and hydration behavior and the curvature changes responsible for the transition observed in this work.
Summary The surfactant fluid microstructure of pentaethylene glycol dodecyl ether, water, and octane microemulsions exhibit, at a water to octane weight ratio of 60/40, a microstructural transition from discontinuous oil-in-water globular to bicontinuous phase as temperature is increased and weight percent of C12E5is varied. These results show the dramatic effect of temperature on the curvature of the pentaethylene glycol dodecyl ether-rich layer that separates oil-rich and water-rich regions, and therefore the temperature effects on all the properties of polyethylene glycol n-alkyl ether, water, alkane mixtures, in particular, their phase behavior. The combination of the FFTEM images, PFGSE N M R selfdiffusion coefficients of surfactant, water, and oil, and QLS apparent diffusion coefficients provides a coherent body of evidence upon which we base our assignment of the globular to bicontinuous fluid microstructure transition. Acknowledgment. We thank Prof. M. Kahlweit and Prof. R. Strey for the phase diagram prior to publication and Dr. S. Philson for assistance with the PFGSE N M R measurements. We also thank Dr. S. J . Salter for stimulating our interest in the C12E,-octane-water system. This work was supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the National Science Foundation and the Department of Energy. J.-F.B. is grateful to his coauthors for their hospitality during his stay in Minnesota. Registry No. C,,E,, 3055-95-6; Cg, 11 1-65-9. (34) Claesson, P.; Christenson, H.; Kjellander, R. J . Chem. Soc., Faraday Trans. 1 1986, 82, 2735. (35) Costello, M. J.; Fetter, R., Corless, J. M . Science of Biological Specimen Preparation; SEM Inc.: Chicago, 1983; pp 105-1 1 5 .
