Langmuir 1991, 7, 1097-1102
1097
Phase Behavior of Aqueous Mixtures of Hexaethylene Glycol Monododecyl Ether and Sodium Alkylsulfonates Carl B. Douglas and Eric W. Kaler* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received October 1, 1990. In Final Form: November 27,1990 Addition of low concentrationsof sodium alkylsulfonates markedly changes the phase behavior of aqueous solutionsof hexaethylene glycol monododecyl ether. Longer chain sulfonatesraise the cloud temperature and lead to the formation of new high-temperature phases. One of these phases is colored when illuminated with white light and displays streaming birefringence, while another phase is statically birefringent. Long range order, as demonstrated by light scattering,builds up as the new phases are approached through neighboring micellar phases. It is likely that electrostatic micellar repulsions introduced by anionic headgroups act over longer distances than other relevant interactions and lead to the observed long range order.
Introduction
observed solution turbidities.'* Light and neutron scattering data are also consistent with the alternate view that Aqueous solutions of ethoxylated alcohols (CiEj) display the micelles remain small, but a large temperature-deintriguing phase behavior as a function of temperature pendent micelle interaction is important,16althoughsome and concentration.' The notation CiEj indicates the of the parameter values obtained from the fits are unnumber of carbon atoms in the hydrophobic alkyl chain physically large. This latter explanation is supported by (i) and the number of ethylene oxide units 0') in the headmeasurements from methods unaffected by interparticle group. This molecular arrangement is convenient as the interactions that show the micelle molecular weight is hydrophilic nature of the surfactant can be adjusted by relatively insensitive to temperature.I6 It is likely that changing the ratio of i to j in the surfactant molecule. critical fluctuations dominate the light scatteringbehavior Most phase diagrams of aqueous CiEj solutions contain near T,,and that, depending on the surfactant, micelles two interesting features,2-6 namely an asymmetric mismay or may not grow as temperature increases.17 cibility gap at low surfactant concentration and high temAdditives change the location of the cloud curve. Hyperature and liquid crystalline phases at high surfactant drotropic salts (which include anionic surfactants) raise concentration and low temperature. The lower boundary cloud temperatures and lyotropicsalts (such as NaCl) lower of the miscibility gap is referred to as a lower consolute them.lg20 For example, Cl2E6 in water has a cloud point curve or a cloud curve, and at the minimum temperature of 51 "C. The phase diagram of C12E6 in water with 10% (T,) of the gap is a lower critical or cloud point. If i and of the hydrotopic salt NaC104 resembles that of &E7 and j are large enough, a micellar solution exists at temper(Tc = 62 "C), while the diagram of aqueous ClzEs water atures below the miscibility gap, and interactions between 10% NaCl resembles that of C12E5 and water (T,= with surfactant and solvent can lead to strong intermicellar 32 0C).21 Clearly the presence of a salt changes the hyinteractions.' Lattice models predict the qualitative drophilic nature of the nonionic surfactant. The cloud features of such miscibility gaps as long as the surfactantpoints of mixtures of C4E1 (an ethoxylated alcohol too solvent interaction depends strongly on temperature.618 small to form micelles) and sodium alkyl sulfates increase A disputed explanation of the molecular origin of the as the alkyl chain length increases.22 The effect of the miscibility gap is that micelles grow with temperature and anionic surfactant can be neutralized by the addition of eventually separate into a surfactant-rich phase. This view NaCl to mixtures of nonionic and anionic surfactants, originated from analysis of light scattering experiments which returns Tcto near the value of the original nonionic in which an increase in turbidity with temperature was s0lution.~3 equated with an increase in micellar molecular ~eight.~t~JO Except for the determination of cloud temperatures, Results from light scattering,neutron scattering,and NMR few phase properties of nonionic/anionic surfactant mixmeasurementa are consistent with an interpretation of mitures have been measured. ClzE6/sodium dodecyl sulfate celle but the presence of density fluctuations (SDS)mixtures have served as models, and the miscibility due to the nearness of a critical point can also explain gap has been reported to shrink with the addition of anionic
* To whom correspondence should be addressed.
(1)Degiorgio, V. In Physics of Amphiphiles: Micelles, Vesicles and Microemulsions;Degiorgio, V., Corti, M., Fds.; North-Holland: Amsterdam, 1985; p 303. (2) Balmbra, R. R.; Clunie, J. S.;Corkill, J. M.; Goodman, J. F. Trans. Faraday SOC.1962,58,1661. (3) Clunie, J. S.; Corkill, J. M.; Goodman, P. C.; Symons, P. C.; Tate, J. R. Trans. Faraday SOC.1967,63, 2839. (4) Shinoda, K. J. Colloid Interface Sci. 1970, 34, 278. (5) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, P. M. J . Chem. SOC.,Faraday Trans. 1, 1983, 79,975. (6) Lang, J. C. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; p 35. (7) Strey, R.; Pakuech,, A. In Surfactants in Solution; Mittal, K. L. Bothorel, P., E&.; Plenum: New York, 1986; p 465. (8) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (9) Corkill, J. M.; Walker, T. J . Colloid Interface Sci. 1972, 39,621. (10) Ottewill, R. H.; Storer, C. C.; Walker, T. Trans. Faraday SOC. 1967,63,2796.
(11) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman B. J. Phys. Chem. 1983,87,4548. (12) Cebula, D. J.; Ottewill, R. H. Colloid Polym. Sci. 1982,260,1118. (13) Kato, T.; Seimya, T. J . Phys. Chem. 1986,90, 3159. (14) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981,85,1442; 1984,&?, 309. (15) Hayter, J. B.; Zulauf, M. Colloid Polym. Sci. 1982,260, 1023. (16) Herrington, T. H.; Sahi, S. S. J.Colloid Interface Sci. 1988,121,
107.
(17) Wilcoxon, J. P.; Kaler, E. W. J. Chem. Phys. 1987,86,4684. (18)Maclay, W. N. J. Colloid Sci. 1966, 11, 272. (19) Kuriyama,K.; Inoue, H.; Nakagawa,T. Kolloid Z. Z.Polym. 1962, 183, 68. (20) Valaulikar, B. S.; Manohar, C. J. Colloid Interface Sci. 1986,108, 403. (21) Kahlweit, M.; Strey, R.; Haase, D. J.Phys. Chem. 1986,89,163. (22) Firman, P.; Haase, D.; Jen, J.; Kahlweit, M.; Strey, R. Langmuir, 1985, 1, 718. (23) Marzall, L. Langmuir 1988,4, SO.
0743-7463/91/2407-1097$02.50/0 0 - 1991 American Chemical Societv
Douglas and Kaler
1098 Langmuir, Vol. 7,No. 6, 1991
s u r f a ~ t a n t . In ~ ~ this mixed system, a lamellar phase appears at temperatures above the miscibility gap,25in contrast to the micellar phase that exists above the miscibility gap in the nonionic diagram. A new but uncharacterized phase has been reported at elevated pressures in mixtures of &E4 and SDS.26 Here we show that the phase behavior of C12E6 as a function of added sodium alkylsulfonate (CnS03Na) is more complicated than previously reported. The phase diagrams do not evolve toward that of a homologous nonionic surfactant, so anionic surfactant does not act like a simple salt.
I -
I
J - ,
Experimental Section Anionic surfactants were purchased from Lancaster Synthesis (99% pure) and from Nikko Chemicals (>98% pure). All surfactants were used as received except for sodium dodecylsulfonate, which was twice recrystallized from a 9/1 ethanol/ water mixture. Distilled, deionized water was used and was deoxygenated by contact with bubbling argon. Samples were prepared by weight with low weight percent samples being made by dilution of a stock solution. Glass cells containingthe sampleswere purged with argon and sealed. Tefloncoated stirring bars were sealed inside of samples to be used for phase behavior analysis. Samples for light scattering were prepared from twice filtered (0.2 pm) stock solutions of water, anionic surfactant, and nonionic surfactant. Phase behavior was observed visually in a water bath with 10.01 "C temperature stability. Temperatures were changed in increments of 0.1 "C in the vicinity of phase transitions. After each temperature change, samples were stirred and allowed to equilibrate. Both heating and cooling cycles were used to establish phase transition temperatures. A microscope illuminator was used to aid in identifying phase transitions, and crossed polarized films were used to test for birefringence. Static light scattering measurements were made with 488-nm light from an Ar+ laser. Samples were held in a temperaturecontrolled cell and surrounded by dodecane as an index matching fluid. Scattered light was measured at angles of 30-130' with a Thorn EM1 photomultiplier tube and reported as a function of q, the magnitude of the scattering vector. q = (4?m/X) sin (8/2), where n is the solvent refractive index, X is the light wavelength, and 0 is the scattering angle.
Results The cloud curve in aqueous mixtures of C12E6 rises in temperature as anionic surfactant is added, and this rise is larger as the alkyl chain of the anionic surfactant tail is made longer (Figure 1). The number of moles of anionic surfactant per mole of nonionic surfactant in solution (the molar mixing ratio) is defined as R. In Figure 1,R for the decyl- and octylsulfonates is 0.005, while the butyl- and hexylsulfonates have R = 0.050. Clearly the larger anionic surfactants move the cloud curve up in temperature more effectively, and the minimum of the cloud curve also progresses to lower nonionic surfactant concentrations as the anionic surfactant alkyl chain grows. The surfactant concentration at the minimum of the cloud curve (c,) for C12E6 alone in water is 2.0 w t % ,but c, moves lower as n increases and finally appears as an apparent cusp at a nonionic surfactant concentration less than 0.2 wt % when CloSO3Na is present at R = 0.005. Cloud temperatures were determined by placing the illuminator behind the sample and recording the temperature when the filament was no longer visible. Samples with low surfactant concentrations can become highly tinted upon approaching (24) De Salvo Souza, L.; Corti, M.; Cantu, L.; Degiorgio, V. Chem. Phys. Lett. 1986, 131,160. (25) Carvell, M.; Leng, C. A.; Leng, F. J.; Tiddy, G. J. T. Chem. Phys. Lett. 1987, 137,188. (26) Nishikido, N. J . Colloid Interface Sci. 1989, 136,401.
0
n = 10
.
I
1
.
I
2
.
I
3
.
I
4
.
I
5
.
I
6
.
I
7
Figure 1. Effect of sodium alkylsulfonates (C,SOaNa) on the cloud curve of aqueous C12E&. For longer anionic tail lengths, the curve moves higher in temperature and the minimum of the curve shifts to lower nonionic surfactant concentration. The amount of anionic surfactant added is determined by the molar mixing ratio, R, which is equal to 0.050 for n = 4 and 6 and 0.005 for n = 8 and 10. 80
60
Figure 2. Phase behavior of C12S03Na/C12& with R = 0.005 showing the appearance of the new 16 regions. The l®ion is colored upon illumination and exhibits streaming birefringence. The 166 region is clear and statically birefringent. a phase boundary and appear turbid, but the filament
remains easily visible through the solution. Attempts to observe complete separation into two phases in samples with surfactant concentrations in the region of the cusp (Figure 1)failed as the samples remained cloudy for over a day without separating. Clouded samples containing such low surfactant concentrations often failed to phase separate even after long equilibration times, and holding samples at high temperature for long periods of time resulted in surfactant degradation as discussed below. Substituting C12S03Na for CloS03Na while keeping R = 0.005 radically changes the phase diagram (Figure 2). In the C&03Na/C12E6 diagram, the two-phase gap is pushed out in concentration and bounded at high temperatures by new one-phase regions that are distinct from the usual single phase micellar region present at lower temperatures. At low nonionic surfactant concentrations, the new single-phase region displays streaming birefringence and bright colors when illuminated with white light and is denoted as l&. The color of the sample depends upon the angle of observation with respect to the incident white light, and at a constant observation angle, the color depends on surfactant concentration. When viewed in reflected light, the color of samples changes from green to blue to violet as surfactant concentration increases. The one-phase region lying above the miscibility gap in temperature (14s)shows static birefringence and is clear under
Langmuir, Vol. 7, No. 6, 1991 1099
Phase Behavior of Mixtures of C1&6 and C,SOaa
F 0
R = 0.0025 20
10
30
C12E6
0
20
40
I 50
(wt%)
40
60
80
C12E6 (W%)
Figure 3. Phase diagrams for C1,$303Na/C12Egas R increases leading to the appearance of the 14 regionswithin the miscibility gap: (a) R = 0.0025;(b) R = 0.005. For R = 0.005,the dashed line indicates the transition between 1& and 148. The shaded areas at high concentration show the location of the lamellar (La) and hexagonal (H) liquid crystalline phases.
illumination. Recrystallizing the anionic surfactant did not change the phase boundaries. Samples containing low concentrations of mixed surfactants are slow to cloud and phase separate. These samples often had to be held at high temperatures for extended periods of time and this resulted in degradation, presumably due to hydrolysis of the surfactant. Oxygen is known to react with the headgroups of polyoxyethylene surfactant^,^' and the cloud temperatures of aqueous C12Es solutions decrease steadily over time in an oxygen environment.28 Oxygen was carefully excluded from these samples, but nonetheless, the temperature history of a sample was a direct factor in its lifetime. Reexamination of samples stored at room temperature for over a week gave cloud temperatures within 0.5 "C of the original value, but a similar sample left at 70 "C for 3 days clouded 15 "C lower. Thus, except for determination of cloud temperatures, the phase behavior of mixtures with less than 1wt % concentration could not be determined in the presence of anionic surfactant. The expanded phase diagrams of mixtures of CloSO3Na and C12E6 as R increases from 0.0025 to 0.005 (Figure 3) show the origin of the new one-phase regions. When R is small, the miscibility gap is uninterrupted but it is found at higher temperatures and its lower boundary turns down at low surfactant concentration (Figure 3a). The miscibility gap moves further up in temperature at higher (27) Schick, M.J. In Nonionic Surfactants; Schick, M. J., Ed.; Dekker: New York, 1967; p 971. (28) Wilcoxon, J. P. Submitted for publication.
R, and there are now small one-phase regions within the miscibilitygap (Figure3b). As before, the one-phaseregion at lower surfactant concentrations contains solutions that are colored and show streaming birefringence (l&), while at higher surfactant concentrations the solutions are clear and statically birefringent (14~).In addition to the new one-phase regions, clouded samples within the miscibility gap show birefringence. Below -75 "C, clouded samples within the miscibility gap will separate into two clear nonbirefringent phases. Between 75 and 85 "C, samples at concentrations higher than those in the 148 region will phase separate and one of the two phases is birefringent. Above -85 "C, the separated phases are both nonbirefringent. Thus the "miscibility gap" contains more than a single two-phase region. Anionic surfactant added at low R values does not affect the location in the phase diagram of the liquid crystalline phases at high surfactant concentration, as shown for CloS03Na/C&6 (R = 0.005) (Figure 3b). The phase transition temperatures are near those for C12Ee/H20.5 This is in contrast to the effect of NaC1, which, when added at concentrations of 10 wt % to C&6/&0, results in phase behavior similar to C12E5/H2024and involves significant change in the extension and location of the lamellar liquid crystalline phase. Substitution of C12S03Na for CloSO3Na in mixtureswith C12& leads to more dramatic changes in the phase behavior (Figure 4). At R = 0.0025, the phase diagram is influenced much the same as by the decylsulfonate, but the phase transition boundary is slightly different (cf. Figures 3a and 4a). There is a gradual rise in transition temperature as surfactant concentration decreaseswith a rapid downturn at low surfactant concentration. When R = 0.005, exand 146)appear within tensive one-phaseregions (both l&! the miscibility gap, and although the one-phase regions appear to have pushed the miscibility gap out to higher concentrations, this is not the case. Samples at extremely low surfactant concentrations (0.5 and 0.75 wt % ) clouded at a temperature of 69.5 "C, so the one phase regions are embedded within a miscibility gap. Like CloSOaNa mixtures, birefringent clouded samples are observed when the new one-phase regions exist. The growth of the onephase regions continues when R is increased to 0.0075 (Figure 4c). The miscibility gap becomes narrower in concentration but occurs at essentially the same temperature, and there is yet another one-phase region present in this system. At high temperature, the high concentration side of 14Bborders a nonbirefringent phase (l+*) with an apparent lower viscosity as determined by the effect of stirring. In the phase diagram of C1&03Na/C12E5 withR = 0.005, the anionic surfactant has little influence on the lamellar phase (Figure 5). In the C12E5/H20 phase diagram, the lamellar phase protrudes into the miscibility gap and extends to low surfactant con~entration.~~ With added anionic surfactant, a one-phase region in which samples are streaming birefringent and colored upon illumination replaces the lamellar phase at low nonionic concentration (l&).There is a transition region between l&and the lamellar phase with samples between 3 and 5 w t % Cl2E5 showing areas of both streaming and static birefringence. Once stirring stops, portions of the samples slowly become nonbirefringent while other areas remain brightly birefringent. No interfaces appear in the solution within an equilibration time of several hours, but these samples are likely in a two-phase region. Again with mixed surfac~
(29) Strey,T.;Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. SOC.,Faraday Trans. 1990,86, 2253.
1100 Langmuir, Vol. 7, No. 6, 1991
Douglas and Kaler 90
f
,
1
60/ 1 4 , 1 I
CloS03Na
\ I
R = 0.0025 50 ,
I
20
10
0
40, 0
i
30
C12E6
40
50
.
14
.
,
-
. 20
10
30
(wt%o)
C12E5
8
.
.
50
40
60
(wt%'o)
Figure 5. Phase diagram for Cl&30sNa/C12Eb with R = 0.005. The lower portion of the miscibility gap lies just below the boundary of the lamellar (L,) phase, which is bordered at low concentration by a 14, region. The dashed line indicates the transition between these two regions. 14* is a nonbirefringent phase appearing at high temperature, and 244 marks a two phase region in which one of the separated phases is birefringent.
/ /
80
ClzSOsNa R = 0.005 601
70
I
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R =O
\
=0.020
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60
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Y
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.
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Figure 6. Effect of added NaCl on the miscibility gap of (212S03Na/C&6 with R = 0.005 (R' = 0). The amount of NaCl is given by the molar mixing ratio, R', between salt and nonionic surfactant. The one-phase regions are omitted for clarity. As R' increases to 0.050, the miscibility gap expands back toward the cloud curve present in aqueous C1&.
R = 0.0075
14
0
Y
30
40
50
(wt%'o)
Figure 4. Phase diagrams for C&303Na/C12& as R increases: (a) R = 0.0025;(b) R = 0.005;(c) R = 0.0075. 14, and 148 appear and expand as R increases. The transition between these two phases is marked by the dashed lines. An asterisk (*) identifies a 14 nonbirefringent region, which appears at high temperature for R = 0.0075.
tants, the cloud curve is raised to higher temperatures than that found in binary mixtures of and water. Figure 5 also shows a nonbirefringent phase (1$*) above the lamellar phase, and a two-phase region in which one of the separated phases is statically birefringent ( 2 4 ~ ) . The addition of NaCl counteracts the influence of anionic surfactant for a single nonionic surfactant concentration,21 and for C&03Na/C12E6 with R = 0.005, added NaCl results in phase behavior similar to that of pure nonionic surfactant (Figure 6, and cf. Figure 4b). Each curve represents a constant molar mixing ratio of salt to nonionic surfactant, R'. With increasing values of R', the miscibility gap extends in temperature and concentration toward the gap present in pure C12E6. The one-phaseregions within the miscibility gap are not shown
for clarity, but at the highest value of R', no one-phase regions are present. The curve for R' = 0.050 corresponds to a molar mixing ratio C12S03Na/NaC1 of 1/10. Static light scattering is a useful probe of nonionic micellar solutions near the miscibility gap.8Jl,30In particular, three features can be identified in such measurements on solutions of C12E6 (Figure 7). (1)Far from the miscibility gap (25 "C) the scattered intensity is independent of the magnitude of q; thus the micelles are small and roughly spherical. (2) Closer to the gap (45 "C), the scattering increases at low q due to the influence of critical (OrnsteinZernike) scattering. (3) Near the gap, the scattered intensity depends on the surfactant concentration, and at 45 "C, there is no longer a steady increase in intensity with concentration. These characteristics are consistent with the nearness of a critical point. Static scattering measured as a function of temperature on C&03Na/C12& mixtures with R = 0.005 exhibits entirely different q dependence at high temperature than that observed for the nonionic surfactant alone (Figure 8, and cf. Figure 4b). In the mixed systems, the samples (30) Wilcoxon, J. P.; Schaefer, D.W.;Kaler, E. W.J. Chem. Phys. 1989,90,1909.
Langmuir, Vol. 7, No. 6, 1991 1101
Phase Behavior of Mixtures of C 1 a 6and C , S O a a 6
dramatically with q, and the intensity increases steadily with concentration.
Discussion
A
0 4 0
A
A
A
A
0
A 0
A 0
A
+
+ + + + + + + + +
0
0
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0
A
0
0
T=25T
0 0 0 0 0 0 0
2
1
0
3
4
A-I )
(
Figure 7. Scattered intensities of aqueous CIZ& solutions at the indicated concentrations. At low temperature, 25 "C, intensity curves are flat as a function of the magnitude of scattering vector, q. At 45 "C, the curves show an increase at low q values and no longer steadily increase with concentration.Both features are characteristic of critical scattering. a) T = 25°C
I
1.5,
+
2.OWlB
A
3.0~1%
I 0
1
2
3
4
( 1 0 - 3 ~ ) b) T=45"C
I
1
- ,
0
1
2
3
4
(1O3k1) Figure 8. Scattered intensities of ClzS03Na/C1& with R = 0.005 as a function of temperature: (a) at 25 O C , curves are flat as a function of q; (b) at 45 O C , a slight increase at high q is becoming apparent; (c) at 65 "C as a phase transition is approached, the dramatic increasein scatteringat high q indicates a buildup of long range order.
examined are approaching two different phase boundaries as temperature increases. Those at low surfactant concentrations will pass into the l& region near 69 "C, while those at higher concentrations will cloud at approximately 69 "C and move into a two-phase region. When the samples are at 25 "C, all the scattering curves are independent of q. Intensities recorded at 45 "C begin to increase slightly at high q. Finally, at 65 "C, the intensity curves rise
The full three-component phase diagram of interest is a Gibbs phase prism with temperature as the ordinate. The present phase diagrams are slices through the phase prism which originate at the water apex and terminate in binary mixtures of nonionic and anionic surfactant. The weight percent of anionic surfactant varies along these slices, but the molar ratio of anionic to nonionic surfactant is constant. In all cases the molar ratio is kept to a small value, and although some nonionic surfactant concentrations reached 75 wt % , the anionic surfactant concentration was less than 0.5 wt 7%. At 25 "C, the aggregation number of C12E6 is on the order of 400,2S1so a molar mixing ratio of 0.005 means that there are at most two anionic molecules per micelle. These few anionic molecules cause the appearance of new phases that replace portions of the miscibility gap present in binary mixtures of water and nonionic surfactant. These new phases are not due to relocation or extension of the liquid crystalline phases present at high surfactant concentrations but rather are extensions of phases within the three-component phase prism. The static birefringence of the higher concentration one-phase region (148) suggests that it is liquid crystalline, and the streaming birefringence of the lower concentration onephase region (14,) is characteristic of an L3 phasee7 Although the two birefringent phases have the characteristics of lamellar and L3 phases, there is no clear evidence of a two-phase region between them. As surfactant concentration increases, there is an abrupt change from a 14, to a 148 region. Near this transition concentration, samples do not cloud, nor do they develop interfaces. However, the neighboring phases are viscous and phase separation may be unobservably slow. Colored samples are not unique to mixed anionic/ nonionic surfactants, as a number of aqueous surfactant solutions are colored over narrow concentration ranges. Low concentration solutions of zwitterionic alkylamine oxides are iridescent, but the phase is not statically birefringent.32 No two-phase region is reported between the nonbirefringent phase and a neighboring birefringent one, and no mention is made of using shear to observe streaming birefringence. In other surfactant systems, highly swollen lamellar phases can be iridescent over a narrow range of omp position.^*^^*^^ In binary C12E~/H20 solutions, the swollen lamellar phase exists in equilibrium with an L3 phase.29 The two-phase region is narrow a t low concentrations and low temperature and broadens at high concentrations and high temperature. In contrast, mixed ~ a narrow surfactant solutions change from 14, to 1 4over concentration range that is independent of temperature (Figures 3 and 4). Static light scattering from micellar phases of C12SO3Na/ClaEs ( R = 0.005) does not suggest the presence of a transition between l& and 14~.Samples that pass into both birefringent phases at high temperature were measured, but all scattering curves show similar q dependence and display a steady increase in intensity as a function of concentration (Figure 8). A t low temperatures, the ~~
~
~
(31) Attwood, D.;Elworthy, P. H.; Kayne, S. B. J. Phys. Chem. 1970, 74. 3529. (32) Imae, T.; Sasaki, M.; Ikeda, S.J. Colloid Interface Sci. 1989,131, 601.
(33) Appell, J.;Bassereau P.; Marignan, J.; Porte, G.Colloid Polym. Sci. 1989, 267, 600.
(34) Satoh, N.;Tsujii, K. J. Phys. Chem. 1987, 91, 6629.
Douglas and Kaler
1102 Langmuir, Vol. 7, No. 6,1991 scattering is q independent as in pure nonionic surfactant solutions, but at temperatures close to the phase transition boundaries, the scattering patterns exhibit a strong increase at high q suggesting an interaction peak a t even higher q values. This dependence of scattering on temperature indicates the micelles do not interact at low temperatures but interact over long distances a t high temperatures. These long-ranged interactions cause micelle order or growth, and a t higher temperatures apparently produce structure that causes birefringence. In the spectra of colored lamellar samples of aqueous C12E6, sharp peaks in intensity are present a t q values between 0.003 and 0.004 A-1.2e The miscibility gap involves large opposing enthalpic and entropic forces whose delicate balance shifts with temperat~re.~6*3~ Introducing anionic headgroups onto the micelle produces intermicellar electrostatic repulsions that can be modeled with a coulombic potential.37 The appearance of the new birefringent phases can be rationalized by comparing the strengths of various potentials. The sum of two Yukawa potentials has been used to model the temperature-dependent balance between attractive and repulsive forces in aqueous solution of the closely ~ . 25 ~ "C,the atrelated nonionic surfactant C I ~ E At tractive potential falls from -0.2kBT at a micelle center separation of 75 A to -(4 x 1 0 - w ~at ~a separation of 1000 A,and the repulsive term falls from 1 . 5 k ~ T to (2 X 10-20)kBTover the same separation interval. In comparison, the Hamaker equation predicts a fall in the attractive (36) Kjellander, R. J. Chem. SOC.,Faraday Tram. 2,1982,78,2025. (36) Claesson, P. M.; Kjellander, R.; Steniue, P.; Christenson, H. K. J. Chem. SOC.,Faraday Tram. 1 1986,82,2736. (37) Manohar, C.; Kelkar, V. K. J. Colloid Interface Sci. 1990, 137, 604. (38)Reatto, L.; Tau, M. Chem. Phys. Letters 1984,108,292.
potential between monodisperse spheres from - 0 . 0 7 k ~ T t o 4 6 X 1e)kBTatthesame separations.39 In comparison, by use of the coulombic potential, a repulsion beginning is obtained for at 0.25k~Tand dropping to (7 X 10-4)k~T a 2 w t % solution of nonionic surfactant containing anionic surfactant at R = 0.005. The repulsive potential begins and falls to (5 X 10-g)kBTforthe same mixture at 0.06k~T with added salt at R' = 0.050. Clearly, the electrostatic repulsions act over much longer distances than do pure nonionic micellar interactions and thus can lead to the buildup of long-range order and birefringent phases seen at higher temperatures.
Conclusions Introducing small amounts of an anionic surfactant to an aqueous nonionic micellar solution substantially alters the balance of forces that controls the miscibility gap. The location of the gap changes and new phases appear. A streaming birefringent one-phase region borders a statically birefringent one-phase region a t low surfactant concentrations and high temperatures. These may be La and lamellar phases, respectively. The new phases occur within the three-component phase prism and are not due to the movement of the liquid crystalline phases present in the binary nonionic/water system. This novel phase behavior indicates that unlike simple salts, anionic surfactants behave as a true third component when mixed with nonionic surfactant even at anionic concentrations below 0.5 wt %. Acknowledgment. This work was supported by the National Science Foundation (PYIA-8351179) and the Shell Development Company. (39) Magid, L. J. In Nonionic Surfactanta;Schick, M. J., Ed.;Dekker: New York, 1987; p 677. Here a value of 5.5 X 10-10 J is aseumed for the Hamaker constant.
