Langmuir 1991, 7, 30-35
30
Interactions between Betaine-Type Zwitterionic and Anionic Surfactants in Mixed Micelles Toshihiko Iwasaki, Masataka Ogawa, Kunio Esumi,* and Kenjiro Meguro Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received April 10, 1990. In Final Form: May 31, 1990 The interaction between zwitterionic and anionic mixed surfactants has been investigated by means of surface tension, static and dynamic light-scattering, 1H NMR spectroscopy, rheological measurement, and probing methods. These systems are N-alkyl-N,N-dimethylbetaine(Clz, DNB; (214, TNB) and sodium dodecyl sulfate (SDS) in the presence of 0.01 mol dm-3 NaCl. These systems show a composition dependency in micellar properties. When the molar fraction of DNB and TNB is about 0.6, the cmc values of these systems exhibit minima, whereas the solubilized amount of a water-insoluble dye, viscosity, and NMR line widths show maxima. Furthermore, the size of a micelle is found to increase at the same fraction. The viscosity displays a Newtonian flow at all compositions for the DNB-SDS system, while a strong shear dependence is observed at a certain composition for the TNB-SDS system. In particular, viscoelastic behavior appears at the intermediate composition region for the TNB-SDS system where a sphere-rod transition would occur in micellar shape due to the formation of intramolecular complexes by the electrostatic interaction of oppositely charged head groups between TNB and SDS.
Introduction Zwitterionic surfactants exhibit unique properties in aqueous solutions, such as pH dependence of cmc, foam stability, lowering of Krafft point by addition of salts, and so I n particular, it is interestingSl2 that zwitterionic surfactants have a strong interaction or complex formation with anionic surfactants in aqueous solutions. Tsujii et a1.I3performed measurement of the Krafft points for zwitterionic surfactants of sulfobetaine type-sodium dodecyl sulfate (SDS)mixed systems and found a maximum at a certain composition which can be correlated with formation of a n intramolecular compound in the solid phases. Moreover, it was reportedg that the micellar solution phase of these mixtures of zwitterionic and anionic surfactants displays viscoelastic behavior and extraordinary change in electrical conductivity. Although zwitterionic surfactants have been used as a booster of several anionic surfactants in industrial applications and their mixed properties have been reported,14-21few systematic studies have been carried out up (1) Ikeda, S.;Tsunoda, M.; Maeda, H. J . Colloid Interface Sci. 1978, 67, 336. (2) Okumura, T.; Tajima, K.; Sasaki, T. Bull. Chem. SOC.Jpn. 1974, 47, 1067. (3) Tsujii, K.; Arai, H. J . Am. Oil Chem. SOC.1978, 55, 558. (4) Anderson, D. L. J . Am. Oil Chem. SOC.1957,34, 188. (5) Tsujii, K.; Mino, J. J . Phys. Chem. 1978,82, 1610. (6) Ikeda, S.;Tsunoda, M.; Maeda, H. J . Colloid Interface Sci. 1979, 70, 448. (7) Herrmann, K. W. J . Phys. Chem. 1964,68, 1540. (8) Saul, D.; Tiddy, G. J. T.;Wheeler, B. A.; Wheeler, P. A,; Willis, E. J . Chem.SOC.,Faraday Trans. 1 1974, 70, 163. (9) Aoki, Y.; Gomi, T.;Tokiwa, F. J . Jpn. Oil Chem. SOC.1974,23,737.
(IO) Kolp, D. G.; Laughlin, R. G.; Krause, F. P.; Zimmerer, R. E. J . Phys. Chem. 1963,67,51. (11) Rosen, M. J.; Friedman, D.; Gross, M. J . Phys. Chem. 1964,68,
3219. (12) Tajima, K.; Nakamura, A.; Tsutsui,T. Bull. Chem. SOC. Jpn. 1979, 52, 2060. (13) Tsujii, K.; Okahashi, K.; Takeuchi, T. J . Phys. Chem. 1982,86, 1437. (14) Abe, M.; Kato, K.; Ogino, K. J . Am. Oil Chem. SOC.1988,65,272. (15) Rosen, M. J.; Zhu, B. Y. J . Colloid Interface Sci. 1984,99, 427.
(16!Takai, M.; Hidaka, H.; Ishikawa, S.;Takada, M.; Morita, M. J . Am. Oil Chem. SOC.1980,57, 382. (17) Ogino, K.; Abe, M.; Kato, K.; Sakama, R. J . Jpn. Oil Chem. SOC. 1987, 36, 129. (18) Jansson, M.; Rymden, R. J . Colloid Interface Sci. 1987,119,185. (19) Jansson, M.; Linse, P.;Rymden, R. J . Phys. Chem. 1988,92,6689.
to date. The elucidation of their mechanisms remains to be found. In this work, mixed solution properties of N-alkyl-NJVdimethylbetaine and sodium dodecyl sulfate were investigated by means of surface tension, NMR, light-scattering, rheological, and probe measurements.
Experimental Section Materials. N-Alkyl-N,N-dimethylbetaines(CnHzn+lN+(CH3)2CH2COO-;n = 12,14) were synthesized according to the method of Beckett and Woodward.22 Here, the betaines having n = 12 and 14 are abbreviated as DNB and TNB, respectively. Sodium dodecyl sulfate (SDS)of high purity was kindly supplied by Nihon Surfactant Industries Co., Ltd., and recrystallized from ethanol, after being washed by refluxing ether in a Soxhlet extractor for 48 h. All surfactants used were confirmed to be highly pure by the absence of a minimum in the surface tension vs the concentration curves. o-Tolueneazo-@-naphthol(OOT) was obtained from Tokyo Kasei Co. Ltd. Pyrene was obtained from Wako Pure Chemical Industries, Ltd., and purified through silica gel in cyclohexane and by evaporation. DzO (99.75%) was obtained from Merck Co. Ltd. The water used in all experiments was purified by passing it through a Milli-Q reagent grade water system until its specific conductivity fell below 0.1 pS cm-l and its pH was 6.5. Measurements. Surface tensions of aqueous solutions of single and mixed surfactant systems in the presence of 0.01 mol dme3NaCl were measured with a Shimadzu ST-1 surface tensiometer by the Wilhelmy vertical plate method. Proton NMR spectra for the line width studies were measured with a Japan Electron Optics JNM-FX100 spectrometer at 100 MHz. D2O was used as an internal reference. Static and dynamic light-scattering measurementswere carried out on an Otsuka Densi DLS-700dynamic light-scattering spectrophotometer. A light of 632.8-nm wavelength from an He-Ne laser was used for static light scattering, and the scattering angle was changed from 30' to 150'. Measurements of specific refractive index increments were performed on an Otauka Densi RM-102 differential refractometer using a light of 632.8-nm wavelength. For dynamic light scattering, a light of 488-nm wavelength from an Ar ion laser was used. Measurements of (20) Hidaka, H.; Toshizawa, S.;Takai, M.;Morita, M. J . Jpn. Oil Chem. SOC.1982. 31. 409. (21) Abe, M.;-Kato, K.; Ogino, K. J . Colloid Interface Sci. 1989, 127, 328. (22) Beckett, A. H.; Woodward, R. J. J . Pharm. Pharmacol. 1963, 7, 422.
, 0 1991 American Chemical Society 0743-746319112407-0030%02.50/0 I
,
Langmuir, Vol. 7,No. l , 1991 31
Interactions between Zwitterionic Anionic Surfactants dynamic light scatteringwere performed in the homodyne mode, and the normalized correlation function was analyzed by the cumulant method to obtain the average decay rate of the field correlation function. Solvents and surfactant solutions were filtered through a 0.1-pm Corning membrane filter. Viscosity was measured with a Haake Rotovisco RVlOO concentric-cylinder rotational viscometer. The coaxial cylinder sensor system ME-30, being equipped with a temperaturecontrolled outer cylinder, was used. The shear rate was changed from 3 to 300 s-l. Dynamicviscoelasticproperties were measured with a Haake Rotovisco RVICV-20 concentric-cylinder,Couette system viscometer. The instrument was operated in the oscillatory mode having an angular frequency range of 0.3-65.4 s-l. Measurements were performed at an amplitude of 1mrad, which was chosen to ensure operation in the linear viscoelastic region. The complex modulus G*, storage modulus G’, and loss modulus G” were calculated from the stress and strain amplitudes (TOand UO, respectively) and the phase angle shift 6: G* = rO/uO
104
Io=
103
C
/mol dm.’
00
a 02
00
0 0 4 9 06 0 0 8 0 1 0
(1)
G = G* cos 6 (2) G” = G* sin 6 (3) G* = G’ + iG* (4) where i is a constant that is equal to (-l)l/z. Surfactant solutionscontaining an excessamount of OOT were shaken for 2 days in order to reach a solubilization equilibrium. Then, these mixtures were filtered off, following dilution with ethanol. The optical density of the diluted solutionwas measured at 490 nm with a Hitachi 220A double-beam UV spectrophotometer. The amountsof solubilized OOT were determined from the calibration curves. Fluorescence spectra of pyrene were measured with a Hitachi 650-10s fluorescence spectrophotometer. The excitation wavelength was at 356 nm, and the emission intensity was measured from 350 to 600 nm. The concentration of pyrene was kept at 5 X 104 mol dm-3. All measurements were carried out at 25 O C in the presence of 0.01 mol dm-3 NaC1.
50
‘E E \
*
40
30
10’
lo’’
IO’
1[I’
c / moldm”
Figure 1. Plots of surface tension against total surfactant concentration: (a) DNB-SDS; (b) TNB-SDS mixed systems.
0 TNB-SOS
Results and Discussion Parts a and b of Figure 1 show the surface tension as a function of total surfactant concentration for DNBSDS and TNB-SDS mixed systems, respectively. In both systems, the surface tension values for various molar fractions decrease with increasing total surfactant concentration, but these systems show a minimum in the vicinity of the cmc. Such a minimum has also been reported15p21 in studies for zwitterionic-anionic mixed systems. The surface tension at the cmc (Ycmc) can be used as one of the criteria of surface activity;23the lower I the Ycmc, the higher the surface activity of the system. In 0 0.5 1.0 DNB-SDS and TNB-SDS mixed systems, respective -ycmc Xzwitter. values a t 0.6 molar fraction of zwitterionic surfactant are 26.8 and 25.2 mN m-l. Furthermore, the reduction of the Figure 2. Relationship between log cmc and molar fraction of surface tension is larger for the mixed systems than that zwitterionic surfactant for DNS-SDS and TNB-SDS mixed of single surfactant. Plotting the logarithms of cmc values systems. against the molar fraction of zwitterionic surfactant (Figure tween the cationic portion of the betaine and the dodecyl 2), the cmc values of the mixed systems decrease with sulfate ion in the mixed systems. increasing Xzwitkr and then show a minimum a t Xzwitkr = T o elucidate the interaction of mixed micellar solutions, 0.6. A mixture of two surfactants usually forms mixed the micelles in aqueous solution. It has been r e p 0 r t e d ~ J ~ 1 ~ ~ NMR line width a t half height Au1p of N-methyl protons and alkyl methylene protons against Xzwitterwas that the mixed cmc for two oppositely charged surfacmeasured (total concentration, 0.1 mol dm-3). Figure 3 tants becomes notably smaller than that of respective surshows that a dramatic change in the line widths occurs at factants due to the association of the surfactants induced Xzwitbr = 0.6 and that the change is greater at an alkyl by the electrostatic attraction. The present result can methylene group than at a N-methyl group. These results also be explained by assuming that the association of the suggest that T N B interacts with SDS in the hydrophilic surfactants occurs easily by electrostatic attraction bepart stronger than DNB, and the interaction between betaine and SDS in mixed micelles is largest when the mixed (23) Rosen, M. J. J. Am. Oil Chem. SOC.1974,51,461. ratio is about 0.6. Furthermore, a comparison of betaine (24) Hoyer, H. W.; Marmo, A.; Zoellner, M. J. Phys. Chem. 1961,66, and SDS alkyl methylene group line widths t o those of 1804.
Iwasaki et al.
32 Langmuir, Vol. 7,No. 1, 1991
X zwittcr. 40
b
8
0 DNI-SUI
30
I \
9
20
-a
IO
20
30
C/gI"
10
Figure 4. Plots of light-scattering intensity (Rayleigh ratio for a 90° scattering angle) against total surfactant concentration:(a) DNB-SDS; (b) TNB-SDS mixed systems. 0.5
1.0
Xzwiiter.
Figure 3. NMR line width of (a) N-methyl protons and (b) alkyl methylene protons against molar fraction of zwitterionic surfactant for DNB-SDS and TNB-SDS mixed systems. N-methyl group indicates that the interior of the micelle in the restricted region is more immobile than the head group. Ueno et al.25have explained that a remarkable increment of the line widths is due to the restriction of the molecular motion of surfactant by the micellar formation as well as the increment of nonequivalency of chemical shifts owing to the aggregation in the head group part. Also, lH NMR spectrum of the protons in the alkyl chain of the amphiphile is sensitive to the presence of large aggregates.26 In both DNB-SDS and TNB-SDS mixed systems, broad spectral lines were observed for the samples a t the intermediate composition region. In addition, the lH NMR line widths are well correlated with the diffusion coefficients, and a broadening of alkyl methylene signals is found when the diffusion coefficient is low. Accoringly, the broad spectral lines also indicate a long correlation time, resulting in the presence of large aggregates. Parts a and b of Figure 4 show the concentration dependency of light-scattering intensities a t scattering angle 90° for different mixing ratios in DNB-SDS and TNB-SDS mixed systems. In the DNB-SDS mixed system, R W vs concentration curves have convexity except for XDNB= 0.6, while for XDNB= 0.6 the downward curvature is found in a higher concentration range. These results indicate that the concentration dependencies of the micellar weight are small, and repulsive intermicellar (25) Ueno, M.; Kishimoto, H. Nippon Kagaku Kaishi 1980, 3, 375. (26) Ulmius, J.; Wennerstrom, H.; Johansson, L. B.-A,;Lindblom, G.
J. Phys. Chem. 1979,83, 2232.
interactions are dominant a t least for XDNBC 0.4 and XDNB > 0.8 except a t XDNB= 0.6. On the other hand, in the TNB-SDS mixed system for XTNB= 0.2 and 0.4, a change similar to that of the DNB-SDS mixed system is observed. However, for XTNB = 0.6 and 0.8, the scattering intensity increases with the concentration and then decreases a t a higher concentration. This decrement in the scattering intensity can be correlated with the repulsive intermicellar interaction, which reduces the concentration fluctuations. Since both intermicellar interactions and aggregation number of micelles depend on the concentration, it is difficult to obtain accurate values of the aggregation number. Therefore, we adopt the quantatitive analyses by Kat0 et where an apparent aggregation number (m*) is calculated by the following equations:
m* = M * / M ,
M , = Ml(l - X,)+ M2X2 where M1 and M2 are the molecular weights of SDS and betaine, M Ois the mean molecular weight of the mixture, X2 is the molar fraction of betaine, M* is the apparent micellar weight, ho is the wavelength in a vacuum, N Ais Avogadro's number, n is the refractive index, and Rw and RWOare the reduced scattering intensities for solutions of the surfactant concentration (0.1mol dm-3) and the cmc a t 90°, respectively. Figure 5 demonstrates that in both (27) Kato, T.;Iwai, M.; Seimiya, T. J. Colloid Interface Sci. 1989,130, 439. (28) Rohde, A.; Sackmann, E. J. Colloid Interface Sci. 1979, 70,494.
Langmuir, Vol. 7, No. 1, 1991 33
Interactions between Zwitterionic Anionic Surfactants
I onnI-:Ds
I
t 0.5
0
e
1.0 1
0
-
0.5 7
e
o
l
1.0 I
Xzwitter.
Figure 5. Relationship between apparent aggregation number and molar fraction of zwitterionic surfactant for DNB-SDS and TNB-SDS mixed systems. 40 I
of zwitterionic surfactant for DNB-SDS and TNB-SDS mixed systems. Total concentration is 0.1 mol dm-3.
1 XTNB 00
0 0.2
a 0.4 e 0.5
A
0
Xzwitter
Figure 7. Plots of diffusion coefficient against molar fraction
10
n
90
30
c / gem-3 Figure 6. Plots of diffusion coefficient against concentration for TNB-SDS system. Dashed line is quoted from data of Rohde and Sackmann.ls
systems m* has a maximum a t Xzwitter= 0.6, and these values are 72 and 203 for DNB-SDS and TNB-SDS mixed systems, respectively. Needless to say, these values are underestimated because of the above procedure. It is likely that the change in the size of micelle with the mixing ratio is associated with the neutralization of the micelle charge by the attractive interaction in hydrophilic head groups between betaine and SDS. The difference in the aggregation number between TNB-SDS and DNB-SDS is also affected by the chain length of the betaines. Figure 6 shows the concentration dependence of the diffusion coefficient (D)obtained by dynamic lightscattering measurement for various mixing ratios in the TNB-SDS mixed system. The diffusion coefficient increases linearly with surfactant concentration for XTNB= 0 and 0.2, whereas D-concentration curves for XTNB= 0.4 and 0.5 have convexity. For XTNB,the downward curve is obtained. These changes of D with concentration for mixing ratios may be caused by differences in the intermicellar interaction. In other words, the slope of D vs concentration changes from positive to negative due to a change in the net intermicellar interaction from repulsive to attractive. In particular, for XTNB= 0.6, it is suggested that the rod micelles are randomly oriented, and rotation of the rods results in a considerable mutual interference,
so that the diffusion coefficient decreases. These results are consistent with those of NMR. Further, Figure 7 shows that the dependence of the diffusion coefficient D on the mixing ratio is almost the same for the two systems, and there is a minimum a t Xzwitter= 0.6 (total concentration 0.1 mol dm-3). However, the diffusion coefficient is significantly lower for the TNB-SDS mixed system than for the DNB-SDS mixed system over the whole composition range. This result reflects a difference in the size and shape of micelles. If the Stokes-Einstein relation is applied, the apparent hydrodynamic radii a t Xzwitter = 0.6 are obtained as 35 and 78 A for DNB-SDS and TNB-SDS mixed systems, respectively. These static and dynamic light-scattering data provide some idea that in the DNBSDS mixed system the micelles are spherical a t all compositions, while in the TNB-SDS mixed system the sphere-rod transition in micellar shape occurs a t the intermediate composition region (XTNB = 0.5 -0.8). The rodlike micelles are probably not so large. The difference in micellar shape between TNB-SDS and DNB-SDS mixed systems may be caused by the packing parameters of the surfactant molecule in the micellar assembly. The transition from sphere to rod in the micellar shape would affect a rheological behavior of the mixed system. Parts a and b of Figure 8 show the relative viscosity vs Xzwitter in DNB-SDS and TNB-SDS mixed systems. In the DNB-SDS mixed system, the viscosities display a Newtonian flow at all the composition ranges studied but increase sharply a t XDNB= 0.6. On the other hand, the viscosity for the TNB-SDS mixed system displays a strong shear dependence a t a certain composition. The plots of shear stress against shear rate and of shear stress against time a t XTNB= 0.6 in the TNB-SDS mixed system are shown in Figure 9. The stronger downward curvature is found which is characteristic of a non-Newtonian flow of dilatant. This curve exhibits a strong shear-thickening effect with increasing shear rate range from 10 to 30 s-l. From the shear stress-time plot a t a constant shear rate (30 s-l), a steady shear stress value is reached a t 100 s, suggesting that these effects require times of the order of 100 s to build up and relax. Thus, the existence of shearthickening effects implies that there is a considerable interference between motion of rodlike micelles. The TNB-SDS mixed system may show three steps of rheological behavior depending on the shear rate: for very small shear rates, the solutions behave like Newtonian liquid with relatively low viscosity; with increasing shear rates,
Iwasaki et al.
34 Langmuir, Vol. 7, No. 1, 1991 Tima
100
50
t
/s 0
150
i
IO
20
30 Shear rate/
40
50
s-1
Figure 9. Plots of shear stress against shear rate and of shear stress against time (shear rate 30 8-l) at XTNB= 0.6 for TNBSDS mixed system.
0
50 C-Ccmc/mol
100
dnr3
Figure 8. Plots of relative viscosity against micellar concentration: (a) DNB-SDS; (b) TNB-SDS mixed systems. the rods begin to overlap and entangle together; and for larger shear rates, the entanglement effect becomes weak and the rods are aligned. Furthermore, oscillatory flow measurements were conducted to detect the prescence of elastic effects in fluids. Figure 10 shows the viscoelastic data, storage modulus G’, loss modulus G”, complex modulus G*, and 6 against the angular frequency a t XTNB= 0.6 in the TNB-SDS mixed system (total concentration 0.1 mol dm-3). From w a’ these data, it is obvious that for low angular frequencies Figure 10. Variations of storage modulus G’, loss modulus G”, a viscous property is predominant, while for high angular complex modulus C*, and 6 against angular frequency at X T ~ frequencies the system has mainly an elastic property. = 0.6 for TNB-SDS mixed system. Similarly, for low angular frequencies the G” is larger than the G’, both values being relatively low. In other words, ticity can be observed in a TNB-rich and SDS-rich region. the mixed surfactant solutions behave as a viscous fluid. However, since rodlike micelles are not so large, entanWith increasing angular frequencies, the G’ and the G” glements would be formed by shearing forces. Also, there come into a closer proximity with each other, and then at is a possibility that rodlike micelles would grow by shearing about 50 s-l the G’ is identical with G”. These curves forces. To understand these phenomena, dynamic meashow the transition from a viscous to an elastic liquid as surements under shear are needed. Recently, Jindal et the angular frequency increases. It has been reported that al.31 have performed small-angle neutron scattering of viscoelastic behavior is caused by the presence of superrodlike micelles in a surfactant solution under shear and molecular network structures, Le., a three-dimensional reported that type I micelles existing in the absence of the network which can be formed from rodlike mi~elles.~6~~9~30 shear are partly converted into type I1 micelles which are These rheological data suggest that T N B and SDS interact ordered like a liquid crystal. strongly due to the electrostatic attractive force, so that The fluorescence spectrum of micelle-bound pyrene is rodlike micelles might be formed at the intermediate sensitive to the polarity of the microenvironment at the composition region. Further, non-Newtonian flow and site of solubilization of f l u ~ r o p h o r . In ~ ~this work, the viscoelastic behavior are caused by the formation of an entanglement of rodlike micelles because no viscoelas(29) Hoffmann, H.; Ebert, G. Angew. Chem. 1988,27, 902. (30) Strivens, T. A. Colloid Polym. Sci. 1989, 267, 269.
(31) Jindal, V. K.; Kalus, J.; Pilsl, H.; Hoffmann, H.; Lindner, P. J. Phys. Chem. 1990, 94,3129. (32) Kalyanasundaram,K.; Thomas, J. K. J. Am. Chem. SOC.1977,99, 2039.
Langmuir, Vol. 7, No. 1, 1991 35
Interactions between Zwitterionic Anionic Surfactants
1 I
0
0.5
Y
I
1.0
X zwittcr.
Figure 12. Solubilized amount of OOT against molar fraction of zwitterionic surfactant for DNB-SDS and TNB-SDS mixed systems.
L 1 1 . 1 1
106
I
I
I
I
I
1
1
1
1
1
1
1
1
1
IO4
1
1
1
1
103
c /mol
1
1
1
1
1
1
1
~
10-2
6
>lI
dm'
Figure 11. Variations of 11/13 ratio of pyrene against total surfactant concentration for (a)DNB-SDS and (b)TNB-SDS mixed systems.
polarity of micelle interior was evaluated by the intensity ratio 11/13 of the first and third vibronic bands of monomeric pyrene. Parts a and b of Figure 11 show the variations of 11/13 as a function of total surfactant concentration in DNB-SDS and TNB-SDS mixed systems. As the total surfactant concentration increases above the cmc, pyrene is solubilized in the hydrophobic interior as illustrated by the decreased 1 1 / 1 3 ratio. The 11/13 ratio of betaine is higher than that of SDS. Furthermore, the steady values of 11/13 increase monotonously with increasing Xzwitkr in both systems. Thus, the hydrophobicity of the micelles decreases with increasing mixing ratio. The observed 11/13ratios can also be affected by a magnitude of the compactness of the head group structure.32 In other words, the 11/13 ratios indicate the degree of water penetration into micellar systems. In a micelle with a compact head group such as SDS, the 11/13 ratio is lower, indicating a sn-aller water penetration in the micelle compared with micelles with larger head group such as DNB and TNB. The solubilization of OOT into the mixed surfactant systems was studied in order to evaluate the mixing effect.
Figure 12 shows the solubilized amounts of OOTvs Xktbr in DNB-SDS and TNB-SDS mixed systems (total concentration 0.05 mol dm-3). If these surfactants are mixed, the solubilized amounts become greater than that of respective surfactants and show a maxima at X&tbr = 0.6. These behaviors have been reported for some different zwitterionic-anionic mixed s y s t e m ~ . ~ 6 1Furthermore, ~~-~ the strong dependencs of ,A, of OOT on the solvent polarity is used to estiriLatethe polarity in the micelle-dye interaction for these systems; i.e., the dielectric constant is obtained for the environment of OOT from the corresponding Am=. The dielectric constant values for SDS, XDNB= 0.5, and DNB are 58, 33, and 31, respectively. Although this result is inconsistent with that of fluorescence probing, it is reasonable to'understand, taking into consideration the different sites of solubilization of OOT and pyrene: OOT is solubilized a t the micellar surface since OOT interacts with the hydrophilic region of surfactant, while pyrene penetrates fairly into the hydrocarbon region.
Summary The interaction between DNB, TNB, and SDS in the mixed micelles changed remarkably when the molar fraction of betaine was about 0.6 since intramolecular complexes are formed in the mixed micelles by the electrostatic interaction of oppositely charged head groups of betaine and sodium dodecyl sulfate. In particular, for the TNB-SDS mixed system a significant micellar growth and sphere-rod transition occurred, resulting in the appearance of viscoelastic behavior. (33) Tokiwa, F.; Ohki, K. J. Jpn. Oil Chem. SOC.1970,19,901. (34) Abe, M.; Kubota, T.;Uchiyama, H.; Ogino, K. Colloid Polym. Sci. 1989,267, 365.
