4974
J . Phys. Chem. 1988, 92, 4914-4919
Temperature-Induced Phase Transitions in Fluorinated Microemulsions: Correlations between Kinetics and Structural Observations C. Burger-Guerrisi, C. Tondre,* and D. Canet Laboratoire d'Etude des Solutions Organiques et Colloidales (LESOC), UA CNRS No. 406, UniversitP de Nancy I , B.P. 239, 54506 Vandoeuvre-12s-Nancy Cedex, France (Received: December 16, 1987)
The phase behavior of microemulsions formulated with a nonionic surfactant is very sensitive to temperature changes. We report here on the kinetics of phase transitions induced by fast temperature jumps in a microemulsion system including salted water (NaCl), a fluorocarbon (C8F17CH=CH2),and a fluorinated nonionic surfactant (C6F,3CH2(E0)s).Three kinds of transitions have been investigated: (i) from an isotropic to a liquid crystalline phase; (ii) between two different isotropic phases; (iii) from an isotropic to a biphasic system. Either birefringence or turbidity changes have been used to monitor the kinetics. It is shown that the response of the system studied to the external perturbation can be very fast. Some experiments have been carried out in which the salted water has been replaced by heavy water. This substitution has permitted us to characterize the liquid crystal by the deuterium NMR quadrupolar splitting and to measure the deuterium relaxation time. Hypotheses concerning the structural changes involved in transition ii are discussed on the basis of the temperature-jump and NMR results in addition to conductivity measurements.
Introduction Very few studies have been devoted so far to the kinetics of phase transformations in binary (water/surfactant) or ternary (water/oil/surfactant) systems. Pressure or temperature changes can be used to induce phase transformations in a system of well-defined composition, but the perturbation must be faster than the response of the system if valuable kinetic information is to be obtained. Chemical relaxation techniques' like pressure-jump or temperature-jump offer a convenient means to meet this requirement. The temperature-jump technique has been previously used by Knight et aL2 to investigate the kinetics of different phase transitions in C12(EO)6/waterbinary systems, whereas pressure-jump experiments have been performed by Schneider et al. to study the kinetics of phase separation in fluid mixture^.^ Different means of detection can be considered to monitor the kinetics of phase transformations: an optical detection can be used with either birefringence or turbidity measurements.2 Electric conductivity measurements have been preferred for studying the micellar-nematic phase transition of the system ammonium perfl~orononanoate/water.~Another very promising means of detection is time-resolved X-ray diffraction, which is presently made possible due to the extreme brilliance of synchrotron sources. Indeed, the kinetics of lyotropic liquid crystal phase transitions has very recently been monitored that way.5 It can be considered that our knowledge of the kinetics and mechanisms of formation of lyotropic liquid crystals is quite poor. Depending on the systems investigated, the rates of formation can be very slow, requiring weeks to achieve equilibrium,6 or on the contrary very fast (a few second^^,^ or even less'). The rates of liquid crystal nucleation and growth may be of interest in some applications. For instance, Millers mentions that the formation of liquid crystal is an important stabilizing factor in emulsions and that it is also one means of dirt removal in detergency. It is thus important to determine the parameters that govern the rate of these processes. One can also expect that the kinetics of transformation of an isotropic phase will depend on the structural organization in this phase (dispersed or bicontinuous, for instance). The results may (1) Eigen, M.; De Maeyer, L.In Techniques of Organic Chemistry; Wiley-Interscience: New York, 1963; Vol. VIII, Part 2. ( 2 ) Knight, P.; Wyn-Jones, E.; Tiddy, G. J. T. J . Phys. Chem. 1985, 89, 3447. (3) Schneider, G. M.; Dittmann, M.; Metz, U.; Wenzel, J. Pure Appl. Chem. 1987, 59, 19. (4) Photinos, P. J.; Saupe, A. J . Chem. Phys. 1986,85, 7467. ( 5 ) Lis, L. .I.Quinn, ; P. J. Mol. Cryst. Liq. Crysr. 1987, 146, 35. (6) Tiddy, G. J. T.; Wheeler, P. A. J . Phys. Colloq. 1975, 36, C1, 167. (7) Tondre, C.; Burger-Guerrisi, C. J . Phys. Chem. 1987, 91, 4055. (8) Miller, C. A. In Lyotropic Liquid Crystals; Advances in Chemistry Series 152; Friberg, S., Ed.; American Chemical Society: Washington, DC, 1976; Q 96.
0022-3654/88/2092-4974$01.50/0
thus be valuable to confirm or reject the structure obtained from scattering techniques data, the interpretation of which are always model dependent. We have previously reported some preliminary results obtained with a system including a fluorinated nonionic surfactant, a fluorinated oil, and salted water.7 In the present paper we use the Joule heating temperature-jump technique to study in more detail the kinetics of different kinds of phase transformations (isotropic-liquid crystal, isotropic-isotropic, and isotropic-biphasic), and we try to correlate these data with the structural information that can be deduced from deuterium N M R spectroscopy and conductivity measurements.
Experimental Section The chemicals were of the same origin as in a preceding paper: in which a temperature-composition phase diagram for the ternary which system C6F13CH2(0CH2CH2)s0H/C8F17CH=CH2/water, is the object of the present experiments, was determined. The phase limits were shifted down by about 3 OC by the addition of 0.1 M of sodium chloride. The substitution of H 2 0by D,O has approximately the same effect on the phase limits as the addition of 0.1 M NaC1. An automated setup has been used for the determination of the isotropic domains.1° The liquid crystalline phases were identified through crossed polarizers by visual as well as microscopic observations. The Joule heating temperature-jump technique' used for the kinetic measurements was purchased from Messanlagen Studienges mbH (Gottingen, FRG). The resistance R of the solution was in the range 500-600 Q , and the capacitance C of the highvoltage discharge capacitor was 50 nF. The energy stored in this capacitor can be suddenly dissipated in the system. The time constant of the temperature rise, equal to R C / 2 , was of the order of 12-15 bus in the present investigations. The temperature change AT can be approximated by the relation
where Vo is the capacitor voltage, u is the cell volume, p is the density of the system, and C, is the specific heat a t constant pressure. AT is thus proportional to the square of the capacitor voltage. The apparatus was connected on-line to a Biomation 805 transient recorder interfaced to an N M 4/30 Computerautomation computer (Yrel France). The data can be transferred to a Tektronix 4662 digital plotter. A nonlinear least-squares procedure was available to fit the data with exponential functions. (9) Robert, A,; Tondre, C. J . Colloid Interface Sci. 1984, 98, 515. ( I 0) Tondre, C.; Robert, A,; Burger, C. J. Dispersion Sci. Technol. 1986, 7, 581.
0 1988 American Chemical Society
Phase Transitions in Fluorinated Microemulsions
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4975
90
80 70
-
60-
50 -
40 30 -
0
IO
20
30
40
50
Figure 1. Temperature-composition phase diagram obtained for the
when water is added to system water/C8F17CH==CH2/C6F13CH2(E0)5, a solution of 24.7%surfactant in fluorocarbon. Experiments have been performed for the compositions marked by lines a-d. The arrows indicate where the temperature-jump experiments have been carried out. A calibration curve has been determined to know as precisely as possible the increase of temperature of the solution AT as a function of the capacitor voltage. Since the heating of the solution depends on its specific heat, which is not the same for water and for fluorocarbons, the calibration was done with the microemulsion itself knowing precisely the temperature range in which the solution is isotropic and the temperature at which it becomes cloudy, we fix the initial temperature in the clear region and we determine the capacitor voltage just necessary to observe a cloudiness. Repeating the experiment for different initial temperatures in the clear region allows the determination of a calibration curve. Measurements of the N M R longitudinal relaxation times T I of deuterium have been carried out at 13.8 MHz with a highly modified" Bruker HX90 spectrometer, using the fast inversionrecovery technique 180°-r900.'2 All samples were run in 10-mm probes, and thermostating was ensured by a warm air circulation. The temperature was measured with a thermocouple placed directly in a sample of composition similar to that of the samples under investigation. Only discrete changes of temperature by about 1 "C at a time were possible, with a stability of fO.l "C. Conductivity measurements were performed with a Wayne Kerr B 331 autobalance precision bridge (w = IO4 rad/s) equipped with a microelectrode Tacussel C M 05.556. The cell constant was determined with standard 0.1 M KCI solutions.
Results A temperature-composition phase diagram for the systems salted water (0.1 M NaCl over the total solution volume)/ CaF17CH=CH2/C6F13CH2(EO)5 or D20/CaF17CH=CH2/ C6F13CH2(EO)5is represented in Figure 1. Owing to the accuracy of the demixing temperature ( f l "C), the small differences observed between the two systems do not justify showing two distinct diagrams. The displacement of the phase limits resulting on either the addition of salt or the substitution of light water by heavy water is consistent with previous observation^.'^^'^ For reasons of convenience in the determination of these phase diagrams,1° increasing amounts of water were added to an initial solution containing 24.7% surfactant in CaFI7CH=CH2. The surfactant concentration thus varies with the addition of water. The one-phase region consists of two interconnecting channels, which look very much like those reported for the system ~ater/tetradecane/C~~(EO)~.l~ The one-phase region at low water content and high temperature is the existence domain of reverse micelles which have solubilized water, whereas at high (1 1) (12) (13) (14) 9, 137. (15)
Brondeau, J.; Diter, B.; Canet, D. Rev. Sci. Insrrum. 1981, 52, 542. Canet, D.; Levy, G. C.; Peat, I. R. J . Magn. Reson. 1975, 18, 199. Shinoda, K.; Takeda, H. J . Colloid Interfuce Sci. 1970, 32, 642. Weckstrom, K.; Zulauf, M. J . Phys. Colloq. 1984, 45, C7, Suppl. No. Olsson, U.; Shinoda, K.; Lindman, B. J . Phys. Chem. 1986,90,4083.
20
-
10
-
Figure 2. Change of transmitted light when increasing temperature along line a in Figure 1 (right scale) and corresponding temperature (left scale). The initial state in temperature-jump experiments is indicated by Tinit, and the final state is shown for different capacitor voltages. E p i c
E p i c
/ I ' ' ' ' ' 1 Figure 3. Change of transmitted light when increasing temperature along line b in Figure 1 (right scale) and corresponding temperature (left scale). See legend of Figure 2. 1
1
I
1
I
I
3
8
3
0 ' 8
water content and low temperature normal micelles are expected in which the fluorocarbon is dissolved. Around the HLB temperature the system is rich in both water and oil and lamellar liquid crystalline phase may f ~ r m . ' ~ .The ' ~ exact limits of the liquid crystalline phase are usually very difficult to ascertain.I7 They appear to be bordered by narrow regions of coexistence with the neighboring isotropic phase. In addition to the difficulty of detecting the presence of very small amounts of another phase, we can never completely exclude the existence of weak thermal gradients in the bulk of the sample under investigation. Temperature-jump experiments have been performed along two vertical lines represented in Figure 1. These lines correspond respectively to the following compositions (water/fluorocarbon/surfactant): 25.1/56.4/18.5 (line a) and 31.9/51.3/16.8 (line b). In Figures 2 and 3 are shown the changes of transmitted light when temperature is increased along either line a or line b. Kinetics of Liquid Crystal Formation (Path 1 2). Starting from the conditions represented by point 1 in Figure 1, the system is brought in a few microseconds to final temperatures between points 1 and 2. This is achieved in successive experiments between which the system is allowed to return to the initial conditions. The initial state and the final state of the solution, depending on the voltage of the discharge capacitor, are presented in Figure 2, and the corresponding changes of the birefringence signal with time are shown in Figure 4. On this figure the light received by the photomultiplier increases downward. For the lower voltages (9-12
-
(16) Harusawa, F.; Nakamura, S.;Mitsui, T. Colloid Polym. Sci. 1974, 252, 613. (17) Lichterfeld, F.; Schmelting, T., Strey, R. J . Phys. Chem. 1986, 90, 5762.
Burger-Guerrisi et al.
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
4976
Birefringence ' Signal ( a u.)
Turbidity Signal (a.u.)
Capaci lor
Capacitor Vol tage(k V )
1
Time (ms)
0
100
200
300
i00
Figure 4. Change of birefringence versus time along path 1
-
2, depending on capacitor voltage. The light received by the photomultiplier is increasing downward. Broken lines have been used for voltages 18-20 kV for the only reason of making easier the distinction between the curves.
-1
4
-
40
20
60
Figure 6. Change of turbidity versus time along path 1 2 depending on capacitor voltage. The light received by the photomultiplier is in-
creasing downward.
Capacitor Voltage ( k V )
nhximum amplitude i f turbidity signal(a.u. 1
2 15
Figure 5. Change of birefringence versus time along path 1 2 when changing the initial temperature so that different voltages lead to the same final temperature. The AT are respectively 1.4, 3.2, and 4.2 "C. -+
kV) the signal detected is slightly increasing during the first milliseconds. In these cases, according to Figure 2, the final temperature falls in the turbid region existing between the isotropic phase and the liquid crystal. The slight increase of the signal can thus be attributed to a turbidity component. This is an indication that the darkness obtained through the crossed polarizers is not total and that a contribution of a turbidity change can always be superimposed to the change of birefringence. The amplitude of the birefringence signal is increasing until the capacitor voltage is about 15-16 kV, which corresponds to a AT of 3.25-3.8 OC. For larger voltages (1 8-20 kV) it is observed that, after a very fast appearance of the birefringence, the signal increases instead of decreasing further. The curves showing the maximum birefringence are characterized by an exponential decay with a relaxation time of 25-30 ms. We have checked that the change of behavior is truly related to the induced temperature change and not to the applied voltage. This is illustrated in Figure 5, where instead of fixing the initial temperature we have fixed the final one. Proceeding that way we bring the solution to the same final state, applying different voltages after having adjusted the initial temperature each time. Figure 5 shows that the recorded signals remain practically the same within the acceptable reproducibility of these experiments. In a parallel manner to the birefringence detection, we have also performed a series of experiments taking advantage of the change of turbidity of the solution when the temperature is varied (Figure 2). The recordings of the change of transmitted light are shown in Figure 6 for capacitor voltages from 9 to 20 kV (an increase of the signal corresponds to less light received by the photomultiplier). Several minutes were allowed between each temperature jump for the system to go back to the initial temperature indicated in Figure 2. We have plotted in Figure 7 the maximum amplitude reached by the turbidity signal in arbitrary units versus the capacitor voltages. It appears that the curve obtained faithfully reproduces the right part of the turbidity curve shown in Figure 2. The kinetics of appearance of the turbidity is much faster than the kinetics obtained for the change of birefringence. If we except the curves having a sigmoidal shape (final temperature in the
voltage(k
10
15
20
Figure 7. Change of the maximum amplitude of the turbidity signal shown in the preceding figure versus capacitor voltage.
turbidity well of Figure 2), the signals can be fitted with exponential functions whose relaxation times are on the order of 2.5 ms . Transition between Two Different Isotropic Phases (Path 3 4 ) . We increase now the water content of the system so that its composition is now represented by line b in Figure 1. Along this line the temperature gap between the points noted 3 and 4, respectively, is now small enough for using the temperature-jump technique to directly induce a transition from the lower isotropic channel to the higher one. This is clearly shown in Figure 3, where we have indicated the initial condition and the final states for different voltages. Since an inversion of the structure could possibly take place between points 3 and 4, a question of interest was does the system have to get through the formation of the liquid crystalline phase? The result obtained with a turbidity detection indicates that the rearrangement of the system is completed within the heating time of the solution. It is thus much faster than the time needed to build a liquid crystalline phase as it was described in the preceding paragraph. This conclusion can be drawn from the fact that there is no signal change at all, because the solution which was clear in the initial state is still clear immediately after the temperature jump and remains clear for the time allowed before the thermal return takes place. In addition, the change of turbidity recorded during the thermal return (when the system goes back
-
The Journal of Physical Chemistry,Vol. 92, No.17, 1988 4977
Phase Transitions in Fluorinated Microemulsions 2\Turbidity signal (a.u.)
-I
'
I
7
D20
-
Figure 8. Change of turbidity versus time during the thermal return
along path 4 3. Note the simulitude with the turbidity change observed in Figure 3 between the two isotropic domains.
Figure 10. 2HNMR relaxation time versus temperature: (0)pure D20 ( 0 )40% D20(line d in Figure 1); (+) 30% D20(line c in Figure 1). The nonhatched areas correspond to the isotropic domains.
5.0
\
z
*\
+\
p, ,
7, 2,
I
1
1
I
Figure 11. Resonance spectrum of deuterium characteristic of a liquid crystal (with a trace of isotropic solution).
+ '
30
35 T('
Figure 9. Change of conductivity versus temperature along line b in
Figure 1. The nonhatched areas correspond to the isotropic domains. to its initial temperature) proves that the system has truly reached the state presented by point 4 in Figure 1. This is demonstrated by Figure 8, which, apart from a scaling factor, is the exact image of the turbidity change observed in Figure 3 when the system 3. follows the path 4 It is well-known that the values of the conductivity measured in microemulsion systems can provide informations about the nature of the continuous phase.'* We have thus measured the change of conductivity when the temperature is increased following line b in Figure 1. The measurements, reported in Figure 9, have been done under constant agitation, so that the conductivity was also measured in turbid conditions outside the isotropic phases. The conductivity is only very slightly increasing in the lower isotropic phase, whereas a neat drop is observed in the upper phase. The global change between these two phases remains nevertheless quite small with a factor less than 2 between the extreme values. Since the N M R deuterium relaxation time is known to be affected by the state of water depending on whether it is more or less bound or free,Ig this technique can be employed in order to characterize the structural changes involved in the transitions from the lower isotropic zone to the upper one. We have therefore substituted the salted water by D20to study the dynamics of water molecules by N M R . Relaxation time measurements versus temperature have been performed along the vertical lines noted c and d in Figure 1. As a reference, we have also measured the relaxation time TIof pure D20.The results are represented in Figure 10. As expected, they show an increase of T,with temperature for pure D20, which is found to be approximately linear in the considered range. At a fixed temperature, the value of TIdecreases significantly with
the system at 40% D20and still a little bit more at 30%. For the latter, a linear behavior is observed in the two isotropic regions but there is a discontinuity between the two straight lines. In the intermediate region, where the liquid crystalline phase is found, we have observed a quadrupolar powder spectrum characteristic of an anisotropic medium:20 Figure 1 1. A trace of isotropic phase is visible in the middle of this spectrum, which may be well due to thermal gradients in the NMR probe. For the system at 40% D20, corresponding to the region where the two isotropic channels coalesce, the evolution of TIis no longer linear at temperatures above 298 K. N o quadrupolar splitting could be detected in the region of coalescence.
(18) Peyrelasse, J.; Boned, C.; Heil, J.; Clausse, M. J . Phys. C Solid State Phys. 1982, IS, 7099. (19) Halle, B.; Carlstrorn, G. J . Phys. Chem. 1981, 85, 2142.
(20) Wennerstrom, H.; Persson, N. 0.; Lindman, B. In Colloidal Dispersions and Micellar Behavior; Mittal, K. L., Ed.; ACS Symposium Series 9; American Chemical Society: Washington DC, 1975; p 253.
-
P
D20 2
3.3
VTx103('K
3.4
3.5
Figure 12. Arrhenius plots corresponding to data of Figure 10.
4978
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988
Time
0
,
,
,
-
,
500
,
,
,
(rns) ,
I
1000
Figure 13. Change of turbidity versus time corresponding to a temperature-jump along path 5 6 with a capacitor voltage of 14 kV. The dashed line represents the best fit with an exponential function.
Figure 12 represents the Arrhenius plots as calculated from the data given in Figure 10. The plot is linear, as expected, for pure DzO, but it exhibits a break in the case of the micrcemulsions. Transition from the Upper Isotropic Phase to Biphasic (Path 5 6 ) . Some experiments have been carried out with the temperature-jump technique in order to get an idea of the kinetics of appearance of an excess phase when the temperature is suddenly risen from point 5 to point 6 in Figure 1. A typical result giving the change of turbidity with time is represented in Figure 13. The time scale of this process is at least 10 times greater than in Figure 6; this confirms that the mechanism involved is probably very different. An induction period is observed due to the fact that the nuclei of the new phase have to grow in size or in concentration before they can be detected. When the part of the curves following the induction period is assimilated to an exponential, relaxation times between 0.25 and 0.5 s are obtained whatever the final temperature in the biphasic region.
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Discussion Until now practically nothing was known concerning the rate at which a new structure is able to get organized in ternary mixtures of the type studied here. The rate of formation of the liquid crystalline phase appears to be much faster than in the case of C12E06/waterbinary systems, for which a time of the order of 1 s was required,2 to be compared to less than 100 ms in the present case. Unfortunately, the birefringence signal is affected by the concomitant change of turbidity which complicates the kinetic interpretation. Our apparatus, in its present state, does not allow us to perform separate experiments under identical conditions to monitor either the variation of birefringence or of turbidity. This is due to the necessity of using a higher number of dynode stages of the photomultiplier when monitoring the birefringence than when detecting turbidity changes. For this reason, if the turbidity curves shown in Figure 6 had been performed with the same adjustments as those used for Figure 4,the sensitivity of the photomultiplier would have been so high that a saturating current would have resulted. It can be considered that the curves of Figure 4 are the sum of a pure birefringence change and of a turbidity change whose kinetics is given by Figure 6 . The turbidity change is always completely accomplished in less than 20 ms whatever the capacitor voltage and even less than 10 ms for voltages between 13 and 18 kV. This is thus in some way the time required to obtain Figure 7, which is the exact replica of the turbidity recorded in conditions where the system is left enough time to reach an equilibrium state. This observation indicates that the system is able to get adjusted to the new conditions in a very short time. Note that this situation differs significantly from that encountered when measuring the kinetics of the turbidity change associated with the appearance of an excess phase (path 5 6). The initial rate at which the birefringence changes (Figure 4) increases significantly when the voltage goes from 13 to 20 kV. The reason why the signal reverses itself when voltages of 18-20
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Burger-Guerrisi et al. kV are applied is unclear for the moment. A possible explanation could be that an orientation effect in the electric field becomes possible for the very high voltages. We would then see successively a fast orientation and then a disorientation of the liquid crystal. This explanation appears to be very unlikely although (i) nonionic surfactants can be oriented in an electric field2I and (ii) orientation effects can happen in temperature-jump experiments,22 because we have several arguments to rule out the contribution of the electric field. The arguments are the following: the orientation effect should take place in the very short time during which the electric field is effectively applied to the solution. This time is at most 5 times the time constant for the heating of the solution, . establishment of the birefringence is obi.e., about 75 ~ s The viously not so fast, as can be seen from Figure 4 (even for the capacitor voltages of 18-20 kV the fast birefringence change at the beginning of the curves is characterized by a relaxation time of the order of 1 ms). A second important point is that an orientation effect would be expected to depend on the strength of the electric field in contradistinction to the results presented in Figure 5. An important implication of the absence of electric field effect is that the behaviors observed in Figure 4 can thus be considered to be entirely governed by the final temperature. A simple suggestion can be put forward to try to explain why the initial rate of birefringence change is so much dependent on the final state of the system. It could be related to the already mentioned difficulty to establish the precise limits of the pure liquid crystalline phase. Suppose that the liquid crystalline phase crossed by line a in Figure 1 is in a proportion of 95-99%, depending on temperature. In that case, the pure liquid crystalline phase would not be included in the plane of Figure 1. What is suggested then is that the rate of formation of the liquid crystal could depend on how far the system is from the composition of the pure liquid crystal (distance to be measured on the tie line joining the separate phases). This suggestion should be considered as a working hypothesis, which will have to be verified. Alternatively, the rapid changes observed could be a consequence of the highly labile nature of the system rather than the presence of a minor amount of a second phase, and the very fast changes could indicate spinodal decomposition. Assuming a reasonable value for the diffusion coefficient of the oil/surfaccm2 s-'), one can also notice that tant/water aggregates (the distance moved in 1 ms must be of the order of the structural reorganization required for the formation of an La phase. We come now to the transition between the two isotropic phases, which constitutes the second point of interest. The very short time of rearrangement observed with the temperature-jump technique is in favor of a very small structural difference between the two isotropic channels at position of line b in Figure 1. Considering that the density of C8FI,CH=CH2 is 1.66 at 25 0C23and that the system contains 3 1.9% (w/w) water and 5 1.3% fluorocarbon, we are under conditions where the volumes of water and fluorocarbon are both large and almost identical (around 41-43% in volume). In such situations it is generally assumed that bicontinuous structures prevail. These structures are characterized by a high conductivity and also by a rapid diffusion of both water and oil. Diffusion coefficients have been measured, for instance, for the three components of the system water/Clz(EO),/nC,4H3015 whose phase behavior is very similar to that of the fluorinated system investigated here. Only small changes of the diffusion coefficients have been observed when going from one isotropic channel to the other one near the HLB temperature. It was concluded that both isotropic phases have a close structural similarity, which can be described as a perturbed lamellar phase, with the bilayers being opened up and closed on a very short time s ~ a 1 e . lSuch ~ a structural organization appears to be perfectly consistent with the present kinetic observations. The very fast (21) Neeson, P. G.; Jennings, B. R.; Tiddy, G. J. T.Chem. Phys. Lett. 1983, 95, 533. ( 2 2 ) Dourlent, M.; Hogrel, J. F.; Helene, C. J . Am. Chem. SOC.1974, 96, 7398. ... ..
( 2 3 ) Stebe, M. J. DSc. Thesis, University of Nancy I, 1984.
Phase Transitions in Fluorinated Microemulsions
-surfactant water molecules 0,W oil, water
:.
Figure 14. Schematic representation showing the postulated change of curvature of the surfactant film when temperature is increased, and the resulting modification of the interactions between water molecules and the hydrophilic moiety of the surfactant.
structural rearrangement can be imagined to correspond to a simple inversion of the curvature of the surfactant layers from a concavity toward the fluorinated oil to a concavity toward water, according to Figure 14. This model implies a change in the mobility of the water molecules. The way in which this change will occur is not easy to predict: either some water molecules squeezed between the hydrophilic tails will be expelled leading to more free water molecules or, on the contrary, they could be more squeezed leading to less freedom in their movement. The reciprocal relaxation time obtained from deuterium N M R measurements, is proportional to the effective correlation time Teff, which characterizes TI-' a Teff the reorientation of water molecules. Assuming a simple two-state model, the observed relaxation time is an average value, which is the result of a linear combination of the relaxation rates relative to the bound and free water molecules, respectively:z0
The fraction of bound water molecules p can be expressed as a function of the concentrations of surfactant C, and water Cw and of the number n of water molecules bound per molecule of surfactant P = nCs/Cw
The value of T I for the free water is given by the measurements
on pure D20. A decrease of T I at a fxed temperature, as observed in Figure 10 when going from pure D 2 0 to 40% DzO and then to 30% DzO, indicates an increasing proportion of bound water. Concerning the T I evolution with temperature, we notice a drop of T I beyond 298 K, both in the second isotropic zone at 30% D 2 0 and in the upper part of the connecting area at 40% D 2 0 . This drop can be interpreted as a slowing down of the water dynamics or as an increase of the proportion of bound water. We prefer the first interpretation because the solvation of the ethylene oxide units of the surfactant is known to decrease when the temperature
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 4979 increases, thus yielding more free water. There are two explanations for the slowing down of bound water dynamics, which are not necessarily contradictory: (i) the reorientation of each water molecule can be hindered because the ethylene oxide heads become closer to each other, as shown in Figure 14; (ii) the translational diffusion around the curved surface of the aggregates would contribute more efficiently to the relaxation rate in the second isotropic region. The Arrhenius plots represented in Figure 12 provide a further confirmation of the change of the state of water: the slope of the straight lines is proportional to the activation energy characterizing the average rate of reorientation of water molecules. At low temperatures the slope is roughly the same for pure DzO and for the two microemulsions, but the activation energy changes significantly when the temperature increases. The change of conductivity represented in Figure 9 is also consistent with the preceding conclusions. If the solution was a simple electrolyte solution, the conductivity would necessarily increase with the temperature (for most aqueous electrolyte solutions the temperature coefficient brings about an increase of the conductivity of about 2% per degreez4). Consequently, the decrease of conductivity observed in the second isotropic zone can only reflect a change of the structural organization of the microemulsion evolving toward a structure where the mobility of the ionic species is reduced. If a complete inversion of structure from oil in water to water in oil was taking place, we would expect a much larger change of conductivity.
Conclusion We have shown in this paper that the kinetics of formation of a liquid crystalline phase in ternary fluorinated microemulsion systems can be much faster than previously reported for binary nonionic surfactant systems. Even in the case where the liquid crystal is forming in the presence of a coexisting phase, the kinetics is much faster than the kinetics of appearance of the excess phase in the diphasic system. On the other hand, we have shown that the transition between two different isotropic phases, under conditions where the system is rich in both water and fluorocarbon, can be quasi-instantaneous. A simple model, assuming an inversion of the curvature of the surfactant film in a structure globally bicontinuous, is consistent with both N M R deuterium relaxation and conductivity measurements, aiming to probe the dynamics of water or the ionic mobility, respectively. Acknowledgment. We thank the valuable help of B. Diter in the deuterium N M R relaxation measurements. We also acknowledge one of the reviewers for suggesting an alternative explanation for the fast changes in birefringence and for fruitful comments concerning the analysis of the N M R data. Registry No. NaCI, 7647-14-5; C8F17CH=CH2, 21652-58-4; C6F13CH2(E0)5, 82576-80-5. (24) Robinson, R. A,; Stokes, R. H. In Electrolyte Solutions; 2nd ed.; Butterworths: London, 1959; p 162.
