J. Phys. Chem. 1980, 84, 1541-1547
1541
Ultrasonic Absorption Study of Microemulsions in Ternary and Pseudoternary Systems J. Lang," A. Djavanbakht, and R. Zana CNRS, Centre de Recherches sur les Macromol6cules, 6, rue 6oussingaulf, 67083 Strasbourg Cedex, France (Received November 9, 1979) Pubirications costs assisted by the Centre National de la Recherche Scientlflque, France
The ultrasonic absorption of two ternary systems (water/sodium caprylate/l-butanol and waterln-hexanel 2-propanol)and of two pseudoternary systems (water/sodium dodecyl sulfate (SDS)/cyclohexane/l-pentanol with the weight ratio water/SDS = 2.5 and water/SDS/toluene/l-butanolwith the weight ratio 1-butanol/SDS = 2) has been measured as a function of composition, in the monophasic domain (microemulsions)where phase transitions are known to occur, or close to demixing lines. Moreover, the ultrasonic relaxation spectra for 15 different compositions of the last pseudoternary system have been determined in the frequency range between 1and 156 MHz. The measurements performed on the ternary systems appear to indicate that the ultrasonic absorption is sensitive to concentration fluctuations occurring close to demixing lines, as for binary systems. However, the excess absorption for this process is smaller with ternary than with binary systems. In pseudoternary systems the ultrasonic absorption was found to be practically insensitive to structural transitions and to the vicinity of a demixing line. This appears to indicate that the critical character of a system becomes less and les; marked as the number of components of the system is increased. The ultrasonic relaxation studies have revlealed that oil-in-watermicroemulsions are characterized by two relaxation processes which correspond very likely to the exchange of the detergent ion and of the alcohol between the interfacial film and the water-rich phase. Water-in-oil microemulsions may be characterized by only one relaxation process associated with the exchange of the alcohol, because the detergent does not dissolve in the oil phase.
I. Introduction Microemulsions have received considerable attention in the last decade, However, only very few results have been obtained on their dynamic properties even though these properties may play an important role in determining the stability of .microemulsions,1-3 In the present work, we have examined the following three aspects of the dynamics of microemulsions. The first one concerns the transitions between different structures as, for instance, the transition between a water-in-oil droplet structure to an oil-in-water droplet structure. Our purpose was to try to determine whether suclh transitions occur in a continuous manner as the composiition is changed or involve large concentration fluctuations similar to those encountered in binary liquid systems close to their critical point. Another aspect examined in this work is the dynamic behavior of microemulsions for compositions close to a demixing or to a miscibility curve of the phase diagram. Our purpose was to compare the behavior of microemulsions to that of binary systems where concentration fluctuations have been shown to occur and of ternary systems investigated in the present work. Finally, we have investigated whether chemical equilibria similar to those present in micellar solutions4 also occur in microemulsions whichever the compositilon range. For the purpose of this work we have used the Liltrasonic absorption technique. Indeed, this technique lhas been extensively used to investigate both one- and two-components syst,ems5-l0near the critical region. The results have shown the extreme sensitivity of ultrasoniic waves, a t frequency between 1 and 20 MHz, to fluctuations in density and Composition occurring in these systems. In some instances an ultrasonic absorption associated with fluctuations is still measured at 30 "C above the critical temperature.1° It must also be recalled that the ultrasonic relaxation method has been successfully used for the dynamic study of micellar e q ~ i l i b r i a .The ~ ~ studies reported below involve the dynamic behavior of ternary and pseudoternary systems for compositions corresponding to 0022-3654/80/2084-1541$01 .OO/O
transparent, macroscopically homogeneous and stable solutions. The pseudoternary systems were made of water, oil, alcohol, and surfactant. The ratio of surfactant to water or to alcohol, depending on the system studied, was kept constant.
11. Experimental Section The ultrasonic absorption a / f " (a= ultrasonic absorption coefficient in cm-l; f = ultrasonic frequency in Hz) of the different solutions were measured by means of the same equipment as in previous studies.11J2 Water was freshly deionized and distilled. Sodium dodecyl sulfate (SDS) from Serva and l-butanol and l-pentanol from Merck (pro analysi grade) were used without further purification. Toluene, n-hexane, cyclohexane, and 2-propanol were purchased from Roth Sochiel and twice distilled before use. Sodium caprylate was prepared by neutralization of caprylic acid (Fluka, purissimum) with NaOH (Merck, Titrisol). The solutions of composition falling on the demixing curves on the phase diagram were prepared in a thermostated glass cell as follows. For the pseudoternary system water/cyclohexane/SDS/l-pentanol (water to SDS ratio of 2.5 in weight throughout this work) the composition was chosen close to the demixing curve so that a turbid solution was obtained when freshly prepared or agitated. 1-Pentanol was next added dropwise until the solution became transparent within one drop. The same procedure was used for the ternary system water/n-hexane/2-propanol where transparent solutions were prepared by dropwise addition of 2-propanol to turbid solutions. 111. Results Ternary System WaterlSodium Caprylatell -Butanol. A simplified representation of the phase diagram, which has been determined by Ekwall,13is given in Figure 1. The ultrasonic absorption was measured as a function of the weight percent of added 1-butanol at constant weight ratio of sodium caprylate to water, equal to 0.098,0.194, and 0.27 0 1980 American Chemical Society
1542
The Journal of Physical Chemistry, Vol. 84,
No. 12, 1980
Lang, Djavanbakht, and Zana
1-BUTANOL
2-PROPANOL
(:.'
WATER
Figure 1. Phase diagram of the ternary system waterhodium caprylate/l-butanol at 20 OC. Compositions are in weight percent.
"
0.4
0.2
Y
"
0.8
0.6
n-HEXANE
Flgure 3. Phase diagram of the ternary system waterln-hexane/2propanol. Compositions are in mole fraction.
4400
I 0
1-BUTANOL
25
50
75
-\.\I 100
Figure 2. Ultrasonic absorption at 25 OC and 3.65 MHz as a function of the weight percent of I-butanol in ternary mixtures made of water, sodium caprylate, and I-butanol. The curves D1, D2, and D3 are associated with the corresponding lines of Figure 1.
Figure 4. Ultrasonic absorption at 25 O C and 6.51 MHz as a function of the mole fraction of water in ternary mixtures made of water, n-hexane, and 2-propanol. The curves D4, D5, and D6 are associated with the corresponding ones of Figure 3.
9
I
1- PE NTPNOL
1-PENTANOL
for lines D1, D2, and D3, respectively. These three lines are located in the L region where either normal or reverse micelles exist depending on the composition of the solution. The R1 region is a biphasic one while the R2 region corresponds to liquid crystalline phases. Figure 2 represents the variation of a/? along the D1, D2, and D3 lines as a function of the weight percent of 1-butanol in the ternary mixture. Each of these curves shows a maximum which appears for a different content of 1-butanol. Ternary System Waterln-Hexanel2-Propanol.Figure 3 shows the phase diagram of this system which has been recently given in some details.l* Region A corresponds to turbid solutions when freshly prepared or agitated. Region B corresponds to stable microemulsions made of water-rich droplets dispersed in a hexane-rich continuous phase while region C corresponds to solutions containing no microemulsions but H-bonded aggregates of water and 2propanol. Finally region D refers to "true" ternary solutions containing no microheterogeneity. Thus contrary to the B/C boundary, the C/D boundary is not a phase boundary. Ultrasonic absorption measurements were performed for several compositions chosen along the straight lines D4 and
,
i
\
T'('f! .=0.284)
1S D S + ~ W A T E R 3.5
3.5
40
60
8 0 CYCLOHEx ANE
Figure 5. Phase diagram of the pseudoternary system water/SDS/ cyclohexane/l-pentanol at 20 OC. Compositions are in volume percent. Weight ratio of water to SDS equal 2.5. p,. represents the water volume fraction in the solution. At T' and T" the volume percents of cyclohexane are 40.35 and 9.83, respectively, and the volume percents of (water I-pentanol) are 48.85 and 75.32, respectively.
+
D5 as well as along the demixing curve D6 of Figure 3. Figure 4 shows the corresponding variations of alf2 as a function of the mole fraction of water. Pseudoternary S y s t e m WaterlSDSlCyclohexanellPentanol. An extensive study of this system has been made15J6for various compositions along the demixing curve C given on Figure 5. It has been shown that a phase
The Journal of Physical Chemistty, Vol. 84, No. 12, 1980 1543
Ultrasonic Absorption Study of Microemulsions 0.3
0.2
1SDS + 2 I-BUTANOL
05
04
3 A
,000~
-1 650
T'
550 19
20
21
22
23
24
25
26
27
Flgure 0. Curve C: ultrasonic absorption at 20 OC and 2.8 MHz as a function of the water volume fraction in pseudoternary mixtures made of water, SDS, cyclohexane, and 1-pentanol. The composition of the mixtures are given by curve C in Figure 5. The symbols ( 0 )and (A) correspond to two series of experiments. Curves T' (+) and T" (0): ultrasonic absorption (at 3.94 MHz vs. temperature for compositlons represented by point 'T' and T" in Figure 5.
transition occurs for compositions around the representative point 'T on curve C. The systems on curve C at the right of T ciorrespond to water-rich droplets dispersed in a cyclohexane-richcontinuous phase, whereas those at the left of T correspond to cyclohexanerich droplets dispersed in a water-rich continuous phase. The variation of alf" with compociition between the representative points P1 and P2 along curve C on Figure 5 is given on Figure 6. The absorption goes through a maximum as the water volume fraction is increased. The thermal ultrasonic absorption coefficient is known to be negative for water and positive for cyclohexane. It might have been hoped that the temperature dependence of alp for the two systems represented by points T' and T" on Figure 5, where the continuous phases are cyclohexane-rich and water-rich, respectively, would show opposite variations. Figure 6 showei that, in fact, in both instances a decrease of absorption was found upon increasing temperature, as for pure water and 1-pentanol. Since both T' and T" contained a volume of water + 1pentanol larger than the volume of cyclohexane, the thermal absorption coefficient appears to be mainly sensitive to the overall composition of the solution and not to the nature of the continuous phase. This is confirmed by the fact that T' which contains more cyclohexane than T" has indeed a thermal absorption coefficient less negative than T". Pseudoternary System WaterlSDSlToluene11-Butanol. The phase diagram of this system (weight ratio of 1-butanol to SDS equal to 2), was determined at the laboratory of A.RTEP17and is represented on Figure 7 . The compositions relative to the R region correspond to stable and transparent solutions which have been the subject of several s t ~ c l i e s . ~ ~In - ' ~region R various structures have been shown to exist. In the water-rich region toluene-rich droplets are dispersed in a water-rich continuous phase. In the toluene-rich region water-rich droplets are dispersed in a toluene-rich continuous phase. Lamellar structures seem to be present in the (SDS + 1-butanol)-richregion. Curves 1antl 2 represent a demixing and a solubility curve, respectively. Ultrasonic absorption measurements have been performed at 25 O C on solutions of compositions chosen along the straight lines AB, CD, EF, antl GH shown in Figure
WATER
40
20
80
60
TOLUENE
Figure 7. Phase diagram of the pseudoternary system water/SDS/ toluene/l-butanol with the I-butanol to SDS weight ratio equal to 2. Compositions are in weight percent.
L
0
fi
I
1
I
I
.
,
. .
klO0
X Figure 8. Ultrasonic absorption at 25 OC and 2.8 MHz as a function of the welght fractlon X a s defined in the text (see Results).
0.5
7 . The variations of alp with composition are given in Figure 8 where the abscissa is in arbitrary units. For instance, for the line AB, X = 0 corresponds to the composition of the representative point A in the phase diagram, X = l is for the representative point B and any X is the weight fraction of B in a mixture of solutions represented by points A and B. For the other straight lines the value X = 0 corresponds to the composition of the representative point C, E, G. Figure 8 shows that, at the exception of curve EF, the absorption vs. composition curves show no maximum. Note that points I and H on the phase diagram, which are on the extension of curve AB, correspond to systems with alp values which are in good agreement with the extrapolated part of curve AB of Figure 8, on B side. It must be pointed out that the measured absorptions are relatively high compared to the values for water (22 X 1O-l' cm-l s2), toluene (84 X cm-' s2)and 1-butanol (104 X cm-l s2) at 25 "C. The highest values are found for solutions corresponding to the left-hand side of the phase diagram, i.e., to the region of low toluene content. In this region the absorption is higher for the water-rich region than for the (SDS + 1-butanol)-richregion. In order to try to identify the mechanism responsible for these large absorptions, spectroscopic measurements were undertaken between 1 and 156 MHz for the compositions referred to as S1 to S11 in Figure 7 . The toluene
1544
The Journal of Physical Chemistry, VoL 84, No. 12, 1980
Lang, Djavanbakht, and Zana ,
1600 1
,
,
, , , ,, ,:
,
,
,
,
,
, , , , ,;
4nl.
1400
350
1200
300
1000
256
1200 .'
800
200
'.
6W
150
400
100
200
5C
1600
t 1200
800
400 ..
0
0
1
1.5
2
3
7
5
10
F
15 20 (W)
30
50
1,s 2
1
150
100
70
Figure 9. Ultrasonic relaxatlon spectra of the pseudoternary mixtures S1 and S5 made of water, SDS, toluene, and 1-butanol at 25 OC. The composition of these mixtures are given by points S1 and S5 in Figure 7.
5
3
7
10
15 20
50
30
f (mz)
100
70
150
Flgure 11. Ultrasonic relaxation spectra of the pseudoternary mixtures S3,S4, and 55 made of water, SDS, toluene, and 1-butanol at 25 OC. The composition of these mixtures are given by points S3,S4, and S5 In Flgure 7. 1400.1
I
, , ,
I
, , , ,I
,
,
,
,
, , , ,,
f
,
+
1200h
1000
150
1
I
1
I
1,5
2
3
I
,
,
1
5
I
t
7
I
l
10
1
15 20
F (w
.i
800
+
70
SIC
0 I
30
1
1
1
50
1
1
70
1
1
I
100
150
I
1.5
3
2
7
5
10-
1520
30
50
7C
113
W
F (Nil)
Figure 10. Ultrasonic relaxation spectra of the pseudoternary mixtures s2 and S7 made of water, SDS, toluene, and I-butanol at 25 OC. The composltlon of these mixtures are given by points S2 and S7 in Figure 7.
content remains constant in systems S2 to S5 whereas the (SDS 1-butanol) content remains constant in S11, S4, S6, and S7. Also, spectra S5, S1, and S6 correspond to compositions on line AB, and S9 is the intercept of AB with the water-(SDS 1-butanol) axis. Finally, S8 to S l l correspond to aqueous solutions of (SDS 1-butanol) at constant weight ratio 1-butanol/SDS. The corresponding ultrasonic spectra are given in Figures 9-12. The spectra for 5 2 and S7 (Figure 10) could be fitted within experimental error to a relaxation equation with one relaxation term (eq 1 with A2 = 0) while all other spectra required two relaxation terms as in the equation
Figure 12. Ultrasonic relaxation spectra of the pseudoternary mixtures S8 to S11 made of water, SDS, toluene, and 1-butanol at 25 OC. The composition of these mlxtures are given by points S8 to S11 in Figure 7.
+
+
a
AI
+
+
A2
+B
170
140 tv\
110
80
(1)
5C
For each spectrum, the relaxation amplitudes (Al, A2),the relaxation frequencies (fR1,f R 2 ) and the constant B were obtained by a weighted least-squares procedure. The different values of the relaxation parameters are given in Figures 9 to 12. The full lines in these figures represent the best fit to the data. Some spectra of aqueous SDS solutions, S12 to 515, have also been determined and are shown in Figure 13. The full lines S13, S14, and S15 represent the best fit to the
20
1+
(f/fRd2
1 -k
1.
cf/fRd2 1
1,s 2
3
5
7
10
F 15
20
30
50
70
100
150
(&)
Figure 13. Ultrasonic relaxation spectra of aqueous solutions of SDS at 25 OC.
data obtained by the same weighted least-squares procedure, using eq 1with A2 = 0. In case of S12 no convergence of the least-squares calculation could be obtained. However, the values of the parameters in the Table in Figure
Ultrasonic Absorption Study of Microemulslons
13 give a good fit to the data and correspond to the smooth curve S12 going through the experimental results. The values of fR1 given in Figure 13 are in good agreement with the results obtained by other authors25at lower SDS concentrations. IV. Discussion It has already been emphasized that the ultrasonic absorption is very sensitive to fluctuations in concentration which are present near the critical temperature or composition of binary liquids. The value of a/f2 goes through a maximum of large amplitude as the composition is varied around the critical composition.5i6 Besides, the absorption increases on both sides of the demixing curve as the composition comes closer and closer to the critical point and diverges at this point. Maximum or divergence of the absorption has also been obtained near transitions occurring in liquid ~ r y s t a l s . ~ ~ - ~ O Such large excess absorptions (maximum or divergence) were thus expected as the composition of the systems under investigation was changed either close to or along a demixing curve or in a transition region of the phase diagram if long-ramge concentration fluctuations were present. Ternary Systems. In the case of the water/sodium caprylate/ 1-butanol system a structural transition has been induced along the lines D1, D2, and D3 in Figure 1 since, as the concentration of l-butanol is increased, one goes from normal to reverse micelles. However, the observed maxima of absorption do not seem to be due to this transition but rather to the vicinity of the demixing curve which separates the R1 from the 1, region. Indeed, the maximum is observed only for D2 and D3, and not for D1, even though the micelle structure transition occurs when changing the composition along the three lines. Moreover the amplitude of the maximum decreases and finally vanishes as the distance between the demixing line and lines D1, D2, and D3 increases on the phase diagram. The small maximum which appears for D l at concentration of about 90% butanol is probably due to some other processes, perhaps similar to those observed upon addition of water to solutions of ionic amphiphiles in organic solv e n t ~ It . ~must ~ be noted that the values of a/f measured along the line D1 do not show any increase in the neighborhood of the liquid crystal region of the phase diagram. In the water/n-hexane/2-propanolsystem a structural change is iniduced as the composition of the solution is varied along the dernixing curve D6 (see Figure 3). At the right of point F of the phase diagrani the solution contains water-rich droplets dispersed in a l-hexane-rich continuous phase, while at the left of point F the droplets no longer exist.14 Point F has been represented on the absorption vs. composition curve shown in Figure 4, which corresponds to the demixing curve D6. A large increase of absorption occurs before and after F, probably associated with the change of structure, but no absorption maximum is observed. A large increase of absorption also occurs as the water content is increased along the lines D4 and D5 and brings the composition of the solutions closer and closer to the demixing curve. Thus for the two investigated ternary systems the ultrasonic absorption appears to be sensitive to the concentration fluctuations which exist close to demixing curves, as in the case of binary systems. It must be pointed out, however, that the excess absorption associated with these fluctuations is smaller with ternary systems than with binary ones,6,6in the frequency range 1-20 MHz. There are two possible explanations to this behavior. Either the spectrum of frequencies of these fluctuations is much lower
The Journal of Physical Chemistry, Vol. 84, No. 12, 1980
1545
and/or the amplitude factor for these fluctuations is much smaller with ternary than with binary systems. The last explanation appears to be more likely. Indeed the frequency spectrum involves transport properties (diffusion) of the species and these should not be drastically affected in going from binary to ternary mixtures of small molecules. On the contrary amplitude factors are related to thermodynamic quantities which may be very sensitive to the presence of an additional component. Pseudoternary Systems. A. Change of Absorption with Composition. At least two explanations can be given to the maximum in the variation of c i / f vs. the water volume fraction in Figure 6. This maximum may result either from concentration fluctuations occurring for compositions around the representative point T in Figure 5, or from chemical equilibria involving the surfactant or the alcohol molecules. The maximum extends in a very large concentration range of added water compared to that where the transition has been shown to occur by means of other techniques as for instance conductivity.la In spite of the fact that absorption maxima associated to critical phenomena extend to concentrations well below and above the critical composition in the case of binary liquid systems, it does not seem reasonable to attribute the maxima in Figure 6 to fluctuations in concentration because it is much more flat that those found near the critical point of binary liquids.@ This maximum is rather due to chemical processes as, for instance, exchange of SDS between the interfacial film and the water-rich continuous phase or of l-pentanol between the interfacial film and the cyclohexane-rich continuous phase. As the concentration of water, SDS, and l-pentanol are increased in the cyclohexane-rich continuous phase, the structure of the droplets is not changed but their concentration is which might well explain the increase of the absorption. Further additions of water SDS + l-pentanol lead to the phase inversion and to changes in the composition of the solution which might be at the origin of the decrease of alp. Indeed the cyclohexane content tends to zero whereas the concentration of the other components increases. Thus several mechanisms could be at the origin of the maximum and cannot be identified with certainty with the present results. The aim of this work, however, was not to identify these processes but to check whether large variations of the absorption with composition occur owing to concentration fluctuations. This does not appear to be the case. For the pseudoternary system made of water/SDS/ toluene/ l-butanol, several structural transitions have been identified when changing the composition of the solution along the representative lines ABH (oil-rich droplets to water-rich droplets), CD (oil-rich droplets to lamellae), and EF (lamellae to water-rich droplets) of Figure 7.18-24 As for the previous system no evidence for the existence of concentration fluctuations in the solutions can be found from the curves in Figure 8. A maximum occurs only for compositions along the line EF and it is again a very flat one. It must be pointed out that lines AB and GH are very close to the demixing and the solubility curves, respectively, and that no maximum appears for the corresponding variations of a/f" shown in Figure 8. Moreover line CD is going across the transparent domain of the pseudoternary system from the demixing to the solubility curve and no peculiar change of absorption is observed as the composition of the solution is changed. Thus our results appear to indicate that the excess absorption, which usually occurs close to a demixing line owing to concentration fluctuations, decreases in going from binary to
+
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The Journal of Physical Chemistty, Vol. 84, No. 12, 1980
ternary and to pseudoternary systems. The contribution of shear viscosity to the absorption of the systems investigated has been evaluated, usin the values of the viscosity reported by Lalanne et a1?3F and the classical equation
(;)
= v
ET2L 3
PV3
where 7 represents the shear viscosity, p the density of the solution, and v the velocity of ultrasound in the solution. The calculation indicates that (a/$), represents a large part of the absorption of curve GH since it amounts to and 22 X cm-l s2 whereas values between 150 X it represents less than one-fifth and one-tenth of the measured values of a/$ for curves EF and AB, respectively. Another mechanism is that shear viscosity is therefore at the origin of the absorption of these solutions. B. Relaxation Spectra of the Water/SDS/Toluene/ 1-Butanol System. As pointed out above, each of the relaxation spectra relative to aqueous solutions of SDS (S12 to S15) is characteristic of a single relaxation process which corresponds in all likelihood to the exchange of the dodecyl sulfate ion between micelles and surrounding sol ~ t i o n .The ~ spectra of SDS solutions in presence of 1butanol at constant weight ratio of 1-butanol/SDS (S8 to S11, with SDS concentrations of 0.069, 0.176, 0.279, and 0.387 M, respectively) are characteristic of two relaxation processes. This behavior can be understood in the light of a recent study where it has been shown that the addition of alcohol to a micellar system gives rise to an ultrasonic relaxation process due to the exchange of the alcohol between the resulting mixed micelles (detergent alcohol) and the surrounding solution which is superimposed on the exchange of the detergent ion.s2 An alcohol such as 1-butanol would exchange more rapidly than the dodecyl sulfate ion. Therefore, it seems reasonable to attribute the faster of the two relaxation processes of spectra 58 to S11 to the alcohol exchange and the slower one to the detergent ion exchange. Thus our results indicate a very fast exchange of the alcohol (exchange rate of about lo8 s-l) between the interfacial film and the continuous phase of the microemulsions. They explain why NMR21which is restricted to processes in the millisecond range could not resolve this exchange. The SDS concentration ranges in solutions S8 to S11 and S12 to S15 largely overlap thereby allowing a comparison between the relaxation frequencies for the detergent exchange. At a given SDS concentration the relaxation frequency for this process in the presence of 1-butanol is always larger than without 1-butanol. This result confirms a recent study where the addition of alcohols ranging from ethanol to hexanol to myristyl- and cetyltrimethylammonium bromides were found to result in a faster rate of exchange of the detergent ion.33 Spectra S1 to S7 correspond to solutions containing the four components. A t constant weight fraction of SDS + 1-butanol (spectra S11, S4, S6, and S7) the relaxation frequencies decrease upon increasing the toluene content. In fact the low frequency relaxation process (detergent exchange) vanishes for system S7 which is characterized by a single relaxation process clearly associated with the alcohol exchange, as can be seen from the plots of fR1 and f R z vs. the toluene content. The vanishing of the relaxation process associated with the detergent exchange suggests that one goes from an oil-in-water to a water-in-oil microemulsion between compositions S6 and S7. Indeed in case of an oil-continuous phase the detergent cannot dissolve in oil, even as contact ion pairs. Therefore the amplitude factor I'34for the detergent exchange process be-
+
Lang, Djavanbakht, and Zana
comes extremely small which results in a relaxation amplitude, which is proportional to I?, close to zero. In agreement with this conclusion, it must be recalled that electrophoretic measurementsz4have shown that the microemulsion inversion occurs somewhere on the toluene side, as is found here. Additional measurements are in progress to determine more accurately the relaxation behavior of microemulsion for compositions between S6 and
s7.
Finally some comments must be made about the spectra at constant toluene content (S2 to S5). In going from S5 (water-rich systems) to S2 ((SDS 1-butanol)-rich systems) the fast relaxation process vanishes. This again can be understood in terms of the microstructures present in the solution. In water-rich systems both the alcohol and detergent can exchange thereby giving rise to two relaxation processes. In (SDS 1-butanol)-rich systems, one has water droplets surrounded by detergent in a continuous phase mostly made of 1-butanol with a certain amount of toluene. Some exchange of the detergent (slow process) remains therefore possible, but the amplitude factor and thus the associated relaxation amplitude are likely to be small, in agreement with the experimental result, as the amount of detergent in the continuous phase 1-butanol + toluene must be small.
+
+
V. Conclusion Two ternary and two pseudoternary systems have been investigated by means of ultrasonic absorption. The measurements performed on ternary systems show the sensitivity of the ultrasonic absorption to concentration fluctuations occur ring^ close to demixing lines. The amplitude of the associated absorption is however comparatively much smaller than that measured with binary mixtures for the same process. In pseudoternary systems the ultrasonic absorption was found to be practically insensitive to structural transitions such as water-in-oil microemulsion to lamellae, or oil-in-water to water-in-oil microemulsion, and to the vicinity of a demixing line. These results support the conclusion that the critical character of a system becomes less and less marked as the number of components in the system is increased. The ultrasonic relaxation studies revealed some interesting features which deserve additional measurements. For instance, water-continuous microemulsions are characterized by two relaxations processes corresponding to the exchange of the detergent and the alcohol. When the continuous phase changes from water to oil, the detergent exchange process apparently vanishes.
Acknowledgment. The authors thank the DGRST (France) for its financial support under grant 77-7-1456.
References and Notes Gllberg, G.; Lehtlnen, H.; Frlberg, S. J. Colloid Interface Sci. 1960, 33, 40. Hansen, J. R. J. Phys. Chem. 1974, 78, 256. Skoullos, A.; Guillon, D. J. Phys. Lett. 1977, 38,L-137. (a) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffman, H.; Klelmann, I.; Ulbrlcht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905. (b) Graber, E.; Lang, J.; Zana, R. KolloM 2. 2. Polym. 1970, 237,470. Chynoweth, A. G.; Schneider, W. G. J. Chem. Php. 1951, 79, 1566; 1952, 20,760. Alfrey, G. F.; Schnelder, W. G. Discuss. Faraday SOC. 1953, No. 15, 218. Fixman, M. J. Chem. Phys. 1962, 36, 1961. Breazeale, M. A. J. Chem. Phys. 1962, 36, 2530. Bains, E. M.; Breazeale, M. A. J. Chem. Phys. 1974, 67, 1238; Ibid. 1975, 62, 742. Anantaraman, A. V.; Waiters, A. B.; Edmonds, P. D.; Pings, C. J. J. Chem. Phys. 1966, 4 4 , 2651. D'Arrigo, G.; Sette, D. J. Chem. Phys. 1966, 48, 691. Lang, J.; Zana, R. Trans. Faraday SOC.1970, 66, 957.
J. Phys. Chem. 1980, 8 4 , 1547-1556 (12) Zana, R.; Lang, J. Adv. Mol. Relaxation Processes 1975, 7 , 21. (13) Ekwall, P. “Advances In Liquid Crystals”; Brown, G. H., Ed.; Academic Press: New York, 1975; Vol. 1, p 1. (14) Keiser, B. A.; Varie, D.; Barden, R. E.; Holt, S. L. J . Phys. Chem. 1979, 8 3 , 1276. (15) Dvolaitzky, M.; Guyot, M.; Lagiies, M.; Le Pesant, J. P.; Ober, R.; Sauterey, C.; Taupln, C. J . Chem. Ph,ys. 1978, 69, 3279. (16) Lagiies, kl.; Ober, R.; Taupin, C. J . Ph,ys. Lett. 1978, 39, L-487. (17) ARTEP (Associationde Recherches sur les Technlques d’Exploitation du P6trole) includes the French Companles C.F.P., S.N.E.A. (P), and I.F.P. (18) Graclaa, A; Lachaise, J.; Martinez, A.; Bourrel, M.; Chambu, C. C. R. Hebd. Seances Acad. Scl., Ser. N 1976, 282, 547. (19) Graciaa, 14.;Lachaise, J.; Martinez, A.; Rousset, A. C . R . Hebd. Seances Acad. Sci., Ser. B 1977, 284, 343. (20) Graciaa, 14.;Lachaise, J.; Chabrat, P.; Letamendia, L.; Rouch, J.; Vaucamps, C.; Bourrel, M.; Chambu, C:. J . Phys. Lett. 1977, 38, L-253. (21) Bials, J.; Ciin, B.; Lalanne, P.; Lemanceau, B. J. Chim. Phys. 1977, 74, 1197. (22) Biais, J.; klercier, M.; Lalanne, P.; Clin, 8.; Beilocq, A. M.; Lemanceau,
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B. C . R . Hebd. Seances Acad. Scl., Ser. C 1977, 285, 213. (23) ’Lalanne, P.: Blals, J.: Clln. B.: Beiloca. A.M.: Lemanceau. B. la) J. Chim. Phys. 1978, 75,236. (b) C . k . Hebd. Seances Acad SEI. Ser. C 1978, 286, 55. (24) Goset, 0. DSc. Thesls, Universlt6 de Pau et des Pavs de I’Adour. (25) Yasunaga, T.; Oguri, H.; Miura, M. J. Colloid Interface Scl. 1987, 2 3 , 352. (26) Edmonds, P. D.; Orr, D. A.; Mol. Cryst. 1988, 2 , 135. (27) Dyro, J. F.; Edmonds, P. D. Mol. Cryst. Llq. Cryst. 1969, 8 , 141. (28) Nishikawa, S. J. Collold Interface Scl. 1973, 45, 259, (29) Nagai, S.; Martinoty, P.; Candau, S.; a n a , R. Mol. Cryst. Llq. Cryst. 1975, 31, 243. (30) Kiry, F.; Martinoty, P. J . Phys. 1978, 39, 1019. (31) Zana, R. In “Solution Chemistry of Sufactants”; K.L. MHtal Ed.; Plenum Press: New York, 1979, Vol. I,p 473. (32) Yiv, S.; Zana, R. J. Colloid Interface Scl. 1978, 65, 286. (33) Ylv, S.; Zana, R.; Hoffmann, H. Unpublished results. (34) Eigen, M.; de Maeyer, L. “Investlgatlon of Rates and Mechanlsms of Reactions” in “Technique of Organic Chemistry”; Friess, S., Lewis, E., Welssberger, A,, Ed.; Interscience: New York, 1959; Vol. VIII, Pari 11.
Roles of ILiquid Crystals and Micelles in Lowering Interfacial Tension E. I. Franses,+J. E. Pulg, Y. Talmon,$ W. G. Miller, L. E. Scriven, and H. 1. Davis” Depiirfments of Chemical Engineering and Materials Science and of Chemistty, University of Mlnnesota, Minneapolis, Minnesota 55455 (Received November 7, 1979) Publication costs assisted by the University of Minnesota
The phases present and the tension behavior of (i) a pure alkyl aryl sulfonate (sodium 8-phenyl-n-hexadecyl-,p-sulfonate),(ii) ik mixtures with sodium dodecyl sulfate, and (iii) a commercial petroleum sulfonate (Witco TRS 10-80) have been studied in aqueous sodium chloride and decane by a battery of techniques, including a newly developed fast-freeze cold-stage transmission electron microscopy technique. The pure sulfonate is only slightly soluble in water or brine in the temperature range 25-90 “C and forms above the solubility limit a lamellar liquid crystalline phase. Lamellar liquid crystalline phases were also identified in aqueous dispersions of the commercial surfactant and in water-decane dispersions of the pure surfactant. A Krafft-like solubility boundary was observed near 50 “C for the pure surfactant in decane. In the presence of small amounts of surfactant, oil-brine interfaces can exhibit ultralow tension. The body of data brought together in this paper lead to the conclusion that for the systems examined ultralow tensions are caused by the presence of a surfactant-rich third phage at the oil-brine interface. Micelles do not appear to induce low tensions. With commercial surfactant, oil-brine tension is known to have a minimum with respect to salt and to surfactant concentration; the same behavior is mimicked qualitatively by a mixture of the pure sulfonate (a liquid crystal former) and sodiium dodecyl sulfate (a micelle former).
Introductiam Ultralow tensions (less than 0.01 dyn/cm) between oil and water are essential to mobilize residual oil in surfactant water flooding proce~ses.l-~ Low tensions are intimately related to phase behavior in the low surfactant concentration range and in the cosurfactant-aided so-called micellar fluid or microemulsion range.4v5 The literature and patents concentrate on commercial sulfonate surfactant systems.2iP8 Few studies of the relation of tension to phase behavior have a ~ p e a r e d Although .~ some tensions correlate with salinity and equivalent alkane carbon number,2J0 the physical imechanisms of ultralow tensions are far from clear. Time effects, order-of-mixing effects, and frequent inconsistency and lack of reproducibility of tension data strongly indicate that basic understanding of the low tension phenomenon is crucial to interpreting correctly School of Chemical Engineering, Purclue University, West Lafayette, Ind. Department of Chemical Engineering, Technion, Haifa, Israel. 0022-3654/80/2084-1547$01 .OO/O
laboratory and field data. Such understanding is a prerequisite for rational design of surfactant flooding processes, that is, processes depending on aqueous surfactant concentrations normally less than 4 wt 5% and free of the alcohols or cosurfactants associated with microemulsion flooding p r ~ c e s s e s . ~ ~ ~ Commercial alkly aryl sulfonate surfactants are complex mixtures that are inadequately characterized for basic physicochemical studies. Using an isomerically pure alkyl aryl sulfonate, sodium 8-phenyl-n-hexadecyl-p-sulfonate, provided by Professor W. H. Wade of the University of Texas and referred to herein as SPHS, we have measured and interrelated phase behavior, microstructure, and interfacial tension. The surfactant is alternatively named sodium p-(I-heptylnony1)benzenesulfonateand is often referred as Texas No. 1. Our goals are to understand why ultralow tensions occur when they do, to measure surfactant solubility in water, brine, and oil, and water- and oil-uptake in surfactant-rich phases, to determine the state of aggregation in isotropic phases and the microstructure 0 1980 American Chemical Society
