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Viscoelastic and some colloid chemical properties of partially neutralized alkenylsuccinates in dilute aqueous solutions
Viscoelastic and Some Colloid Chemical Properties of Partially Neutralized Alkenylsuccinates in Dilute Aqueous Solutions KAORU
TSUJII, N A O Y U K I
SAITO, AND T A K A S H I T A K E U C H I
Tochigi Research Laboratories, Kao Corporation, 2606 Akabane, Ichikai-rnachL Haga-gun, Tochigi 321-34 Japan
Received September 6, 1983; accepted November 18, 1983 Viscoelasticityin dilute aqueous solutions of an anionic surfactant, partially neutralized alkenylsuccinate, has been found for the first time. This solution also shows striking spinnability and Weissenberg effect. It is a remarkable feature of this system that these unique properties are not observed at all in the solutions of the fully neutralized agent of the above. The more hydrophobic nature of the surfactant in partially neutralized state than that in fully neutralized one may cause the formation of an elongated rod-like micelle which behaves in the solutions like a high polymer. The frequency dependence of the dynamic rigidity and viscosity suggests that an entanglement coupling or a flexible network structure formation of long cylindrical micelles is the most possible mechanism for viscoelasticity. Some colloid chemical properties of the surfactant have been measured in both viscoelastic and normal (non-elastic) solution regions to obtain the information useful for the practical applications of the agent. All results obtained from the surface tension, solubilization, critical miceflization concentration, and foaming measurements indicate that the viscoelastic solutions of the agent are much more surface active than the normal ones. INTRODUCTION Viscoelasticity in dilute aqueous solutions is one o f the m o s t interesting p h e n o m e n a which are recently f o u n d in the field o f surfactant chemistry. The viscoelastic binary ( 1 3) and single surfactant solutions (4-12) have been f o u n d and reported by several authors. However, only two kinds o f cationic c o m p o u n d , hexadecyltrimethylammon i u m salts (4-9) a n d hexadecylpyridinium salicylate (10-12), are k n o w n to give the viscoelastic solutions in single surfactant systems. We have f o u n d in this work a new viscoelastic solution o f single anionic surfactant, partially neutralized alkenylsuccinates which can be also regarded as the binary mixtures o f alkenylsuccinic acid and its salt, and measured the d y n a m i c rigidity and viscosity o f the solutions. The alkenylsuccinate solutions exhibit
striking viscoelasticity, spinnability, a n d Weissenberg effect in partial degree o f neutralization, but are quite n o r m a l (no elasticity, spinnability, and Weissenberg effect) in fully neutralized state. We are especially interested from the practical point o f view in the difference o f colloid chemical properties between viscoelastic and n o r m a l solutions o f the agent. Then, the surface tension, solubilizafion, electric conductivity, critical micellization concentration, and f o a m - v o l u m e measurements have been m a d e in both viscoelastic and normal solution regions. EXPERIMENTAL M a t e r i a l s . A l k e n y l s u c c i n i c acids were kindly presented by Mr. Y. Ishikawa a n d Dr. N. M o r i y a m a o f W a k a y a m a Research Laboratories, K a o Corporation, and were prepared by the following procedures.
553 0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
554
TSUJII, SAITO, A N D T A K E U C H I
//O CH--C //O R - - C H 2 - - C H = C H 2 + I] / O --~ R - - C H = C H - - C H 2 - - C H - - C ~ CH--C%o [ / O ---* CH2--C%o R - - C H = C H - - C H 2 - - C H --COOH
I
CH2--COOH a-Olefins (1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene) were reacted with maleic anhydride in an autoclave at 220°C for 4 hr. The starting a-olefins were found to be more than 96% pure by gas chromatography. The reaction products were purified by distillation under reduced pressure. The oligomeric olefin-succinic anhydride, main byproduct of the above reaction, was removed together with the other impurities through this procedure. The purified alkenylsuccinic anhydrides were hydrolized with an aqueous NaOH solution. The obtained alkenylsuccinic acids were precipitated at pH 2, and finally purified by repeated recrystallization from acetone. The sample solution was prepared by direct neutralization of alkenylsuccinic acid in a volumetric flask with desired amounts of base solutions. Sodium hydroxide was used as a base unless noted otherwise. The surfactant samples are, hereafter, abbreviated as CnSA, where n is the carbon number of a-olefin. The degree of neutralization is denoted as a, and written in brackets, if necessary, such as CnSA (a = 0.7). Yellow OB (1-o-tolyl-azo-2-naphthylamine) was purchased from Tokyo Kasei Ltd., and purified by recrystallization three times from ethanol. The deionized and distilled water was used to prepare the sample solutions. Methods. Viscoelasticity can be easily detected qualitatively by the simple method of swirling a sample solution and visually observing recoil of small air bubbles entrapped in the sample just after swirling is stopped (3, 5, 8, 9). Quantitative measurements of dynamic rigidities were performed with a ShiJournal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
madzu Model RM-1 rheometer, using a coaxial cylindrical cell. The pH-titrations were carried out with a Horiba Model N-5 pH meter at 39.5 _ 0.2°C. The N2 gas was passed through the cell during the whole titration procedure to avoid the interference of CO2 gas. The temperature was controlled by circulating the thermostated water. Solubilization of water-insoluble dye, Yellow OB, was measured by essentially the same manner as described elsewhere (13). A sealed test tube containing the surfactant solution and the small amounts of solid Yellow OB was shaken at 45°C for 46 hr. The amount of solubilized Yellow OB was determined by the optical density measurement at 447 rim. Qualitative flow birefringence was observed visually using a polariscope at room temperature (9). The sample solutions were rotated at ca. 500 rpm by a magnetic stirrer. The flow birefringence of the CnSA solutions was unfortunately too weak, and we could not take photographs as was done by Gravsholt (9). Surface tension was measured by the drop weight method. Electric conductivity was determined with a Radiometer Model CDM-3 conductivity meter. The temperature was maintained constant within +0.2°C for both measurements. Foaming test was made at room temperature by the same stirring method as used in a previous work (14). RESULTS
Figure 1 shows the pH-titrafion curves of C12SA with 1 N NaOH solution. The apparent
VISCOELASTIC SURFACTANT SOLUTIONS
,2I
50m~,l~
11
'°I
555
.f
5mM~ i
I O
0.5
o6--W--o pH
0,5
oL
1.0
FIG. 1. The pH-titration curves of CtzSA with 1 N NaOH solution at 39.5°C. The concentration of C~2SA are 5 mM (O) and 50 mM (O). degree of neutralization, a, is taken as abscissa. One can see from the figure that the CI2SA shows considerable buffering action at neutral pH in the range of 0.2 < a < 0.8. Roughly speaking, the C12SA solutions are viscoelastic in this range of a. In the degree of neutralization smaller than 0.2, the solubility of CIzSA is not enough and the solutions are turbid, whereas the viscoelasticity is not observed at all in spite of the clear solution formation of the agent when a exceeds 0.8. Similar pH-dependent viscoelastic behaviors can be seen also in the solutions of higher hydrocarbon chain homologs and of CnSA neutralized with other bases than N a O H such as LiOH, KOH, mono-, di-, and triethanolamine. The dynamic rigidities of 2 wt% ( ~ 7 0 raM) CI2SA solutions are plotted against pH in Fig. 2. Elastic solutions appear only in narrow pH range, but in wide range of neutralization, because of the buffering action of C12SA. The viscoelasticity of the solution diminishes with increasing temperature, and finally disappears at ca. 50°C as shown in Fig. 3. Similar tern-
7
FIG. 2. The dynamic rigidity (G') of 2 wt% C~:SAsolutions as a function of pH, measured at 35°C and the frequency of 1.0 Hz.
perature dependence in viscoelasticity has also been observed in hexadecyltrimethylammonium salt solutions (8). Figure 4 shows the dynamic rigidity and viscosity of ClzSA solution at pH = 5.94 as a function of frequency. Opposite dependence of the dynamic rigidity (increasing) and viscosity (decreasing) on frequency increase may suggest that the CnSA solution is a Maxwell fluid. The viscoelastic solutions mentioned above
5.0
2.0
40 3.0 ~1.0
"7
2.0
x
g ~
1.0 10
2'0
T(*C)
6'0
0
FIG. 3. The dynamicrigidityand viscosity(•') of 2 wt% C~2SAsolutions as a function of temperature, measured at pH 5.94 and the frequency of 1.0 Hz. Journal of Colloid and Interface Science,
Vol. 99, No. 2, June 1984
556
TSUJII, SAITO, AND TAKEUCHI
% 2.0
~_~
O/
21
400 ViscoeLasticregTon
~_aoo
/
o/c/ O(=0.7
.LZ 200 J 3.05 o.
I.C
100
q
_o~o~O~°/ 0.2
0.4
0.6 (Hz)
0.8
j / I
I
!
I
20
i
I
I
40 60 C(mM)
./ /
/--~=to
i
,
i
80
100
120
FIG. 6. Solubilization curves of C12SAtoward Yellow OB measured at 45°C, and c~ = 1.0 (e) and 0.7 (O).
1.0
FIG. 4. The dynamic rigidity and viscosity of 2 wt% CI2SAsolutions as a function of frequency, measured at pH 5.94 and 27°C.
exhibit a characteristic nature found also in other solution properties. Flow birefringence, spinnability, and Weissenberg effect are observed in viscoelastic solutions. For example, one can readily spin a liquid thread longer than 30 cm under the appropriate conditions. The non-elastic highly neutralized solutions, however, are quite normal, and do not show any of the remarkable properties above. Surface tension vs concentration curves of C12SA solutions with various degree of neutralization are given in Fig. 5. It is evident from the figure that the C12SA solutions in lower degree of neutralization show higher surface activity (low surface tension and
CMC). The amounts of Yellow OB solubilized into C12SA (a = 0.7 and 1.0) solutions are plotted against the concentration of C12SA in Fig. 6. The solubilizing power of C I2SA (slope of the straight line in Fig. 6) in viscoelastic region is much greater than that in normal solution region. The solubilization curve of C~2SA (a = 0.7) solution breaks at 17.5 m M above which the viscoelasticity of the solution disappears, since the p H value of the solution increases with increasing concentration o f C~2SA (a = 0.7) as expected from Fig. I. It is worth noting that the slope of the curve in C12SA (a = 0.7) solution at the concentration range above 17.5 m M is very similar to that
0
50
\\ •
o.
)
"~ 4C
::,d 3.0
/U °
2.0
08
1.0
20,~
-3,5
-3.0
-2.5
-2.0
-1.5
-1.0
logC
FIG. 5. The surfacetension vs log (concentration)curves of C~2SAmeasured at 45°C, and at the degree of neutralization = 1.0 (©), 0.9 (~), 0.8 (e), and 0.7 (O). Journal of Colloid and Interface Science, VoL 99, No. 2, June 1984
O~
1'0
2'0 30 C(mNI)
40
50
FIG. 7. Specific conductivity vs concentration curves of C~2SA(c~ = 0.7) measured at 50°C (O), 60°C (O), and 7 0 ° C (@).
VISCOELASTIC SURFACTANT SOLUTIONS of non-elastic fully neutralized solution, indicating the existence of similar micelles in both solutions. Electric conductivity data as a function of C l z S A (oL = 0.7) concentration are given in Fig. 7. There appear two breakpoints in the conductivity vs concentration curves. The similar curves having two break points are also obtained at o~ = 0.7 in the solutions of higher hydrocarbon-chain homologs. In the fully neutralized surfactant solutions, however, the n u m b e r of breakpoint depends on the hydrocarbon chain length of the surfactant. The conductivity vs concentration curves o f C12and C I 4 S A (o! = 1 . 0 ) have also two breakpoints, whereas in the C16- and C18SA (oz = 1.0) solutions, the curves have only one breakpoint as shown in Fig. 8. The critical micellization concentration (CMC) can be determined from any methods of the surface tension, solubilization, and electric conductivity techniques. All CMC data are summarized in Table I. The C M C values obtained from surface tension and solubilization are in good agreement with each other. The results from electric conductivity, however, are somewhat complicated. F o a m v o l u m e - p H profile of the 0.5 wt% C~2SA solution containing 1.0 wt% soybean oil is given in Fig. 9. The foaminess o f C~2SA solution is high enough in viscoelastic p H re-
10
~
CMC(mM) Electric conductivity Surface tension
Surfactant
C12SA ( a = 1.0) CI4SA ( a = 1.0) CI6SA ( a = 1.0) CIgSA ( a = 1.0) CI2SA C12SA CI2SA C14SA C]6SA Ct~SA
(a (a (a (a (a (a
= = = = = =
0.9) 0.8) 0.7) 0.7) 0.7) 0.7)
Solubilization
80 ~ 24 a
1st
76 a
24 b 23 c
8.3 c 3.5 c
9.0 c 4.5 d
5& 5.4 ~ 2.5 ~
20 b 19 b 15 b 8.0 b 5.0 c 2.8 c
2.5 ~
5.3 c
2nd
84 b 40 c
75 b 45 b 35 b 32 b 16 ~ 6.4 C
" M e a s u r e d at 4 5 ° C . b At 50°C. c At 60°C. dAt 70°C.
gion, but depresses suddenly at the p H value where the elasticity of the solution disappears. DISCUSSION
Viscoelastic Regions and a Possible Mechanism of Viscoelasticity As mentioned previously, the viscoelasticity of CnSA solutions appears only in the limited
g ~ 100
i
i
p
to
5o
FIG. 8. Specific conductivity vs concentration curves measured at 60°C and a = 1.0 (©), and of C~sSA measured at 70°C and a = 1.0 (O).
o f CI6SA
CMC Values Determined by Three Different Methods
I
30
C(mM)
TABLEI
200
// ~o
557
05
;
-> pH
FIG. 9. Foam volume vs pH curve of 0.5 wt% C12SA solutions containing 1.0 wt% soybean oil. Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
558
TSUJII, SAITO, AND TAKEUCHI
pH regions, which exhibits a striking contrast to the other viscoelastic surfactant systems. The lower pH limit is simply originated from the solubility loss of CnSA into water, but it is indeed interesting to note that the fully neutralized CnSA solutions are quite normal, and show no elasticity at all. This result indicates a drastic structure change of C~SA micelles by pH variations, which is supported by flow birefringence experiments. The appearance of flow birefringenee, of course, results from the existence of the micelles of an asymmetric shape in the solutions. The theories giving the micellar shape of surfactants have been presented by Tanford (15), and Ninham and his co-workers (1618). According to their theory, the packing of hydrocarbon chains in the hydrophobic interior of micelles is the governing factor to determine the spherical or globular or cylindrical or lamellar or vesicle-shaped micelles. Israelachvili et al. present a quantity, V/aol, as the criterion to determine the shape of micelles; where l and v are the length and the volume of the hydrocarbon part of one surfactant molecule, respectively, and a0 the area occupied by one hydrophilic head at the surface of the micelle (16). The usual spherical micelles are formed when V/aol < 1/3. The shape ofmicelle deforms from sphere to globule and toroid with increasing value of V/aol, and finally becomes the infinite cylinder when v~ aol attains 1/2. For a given alkenylsuccinate, v and l remain unchanged and only a0 can be varied with changing pH of the solution. The a0 value of the fully neutralized CnSA must be large enough to form spherical micelles. At lower pH region, however, the alkenylsuccinates are neutralized only in part, and the electric repulsive force between ionic heads of the surfactant molecules is reduced, which should give the smaller a0 values than that of the fully neutralized agents. When ao attains a certain value at definite pH, the long cylindrical micelles may be formed. Remarkable spinnability of CnSA solutions may support the above assumption, since no spinnable solution has Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
been known so far without any long molecules or colloids as solutes. In addition, the presence of long cylindrical micelles in viscoelastic hexadecyltrimethylammonium salt solutions is substantiated by several experimental means (4-9, 19-27). As mentioned previously, the elastic recoil of the CnSA solution can be observed visually, which means that present systems have long relaxation time rheologically, as substantiated by Fig. 4. Similar viscoelastic behaviors with long relaxation time have been known also in some spinnable polymer solutions (28-30), biological fluids (31-33), and colloidal dispersions (34). In such viscoelastic fluids, the entanglement coupling or the flexible network structure of polymer molecules is assumed to account for their long rheological relaxation time (31-34). The network structure was actually substantiated by Danno in the colloidal mercury sulfosalicylate gel (34). Beautiful network structure formed with the elongated rodlike colloids was visualized by means of electron micrograph (34). Interestingly, the frequency dependence of the dynamic rigidity and viscosity of the above gel is quite similar to that of our micellar solutions shown in Fig. 4. It is likely, we suppose, that the long cylindrical micelles may form a temporary network structure or an entanglement coupling, which results in the viscoelasticity of the solutions. Hoffmann and co-workers also interpret the viscoelasticity as due to the overlapping of rod-like micelles for hexadecylpyridinium salicylate solutions (12). Ulmius et al. proposed, recently, a periodic colloid structure model of rod-shaped micelles for the viscoelastic aqueous solutions of hexadecyltrimethylammoniumsalt (8). We believe, however, any periodic colloid structures formed with the long cylindrical miceUes must be a kind of liquid crystalline state, and optically anisotropic. This is not the case in the solutions of hexadecyltrimethylammonium salt. We may safely say anyhow that the polymer solutions (28-30), biological fluids (3133), and colloidal dispersions (34) which show
559
VISCOELASTIC SURFACTANT SOLUTIONS
similar viscoelastic behaviors to the above surfactant solutions cannot form any periodic structures at all, which strongly suggests that the periodic colloid structure is not necessary for the viscoelastic surfactant solutions.
Colloid Chemical Properties of the Viscoelastic and Normal Solutions of Alkenylsuccinates As understood from Figs. 5, 6, and 9, the surface activity of CnSA in the viscoelastic solutions is much higher than that in the normal ones. Both the surface tension and the CMC of partially neutralized C~2SA solutions are much lower than those of fully neutralized solutions (Fig. 5). It is especially interesting to note that the surface tension of C~2SA (o~ = 0.7 and 0.8) at the concentrations above CMC attains to 25 dyn/cm, which is unexpectedly low value for the aqueous solutions of surfactant with a hydrocarbon chain as a hydrophobic group. The partial degree of neutralization should give the more hydrophobic nature to the alkenylsuccinate molecules and the weaker electrostatic repulsive force between hydrophilic heads of the molecules than the full neutralization of the surfactant. The compact adsorption of C 1 2 8 A molecules at the surface of the solution due to the weak repulsive force between them may result in the low surface tension in partially neutralized state. It has been already known that there exist two breakpoints in the electric conductivity vs concentration curves o f some difunctional surfactants (35-37). In most cases, the first breakpoint appeared at lower concentration coincides with the CMC value determined by other techniques, and the second break is attributed to the change of size or shape of micelles (35-37). In some cases, however, the CMC value determined from surface tension measurements agrees with the second breakpoint in the electric conductivity curve (37). No explanation has been given so far for this latter case, In our C,SA (o~ = 1.0) systems,
there are three types of behaviors, depending upon the hydrocarbon chain length of the surfactant (Table I). In partially neutralized C,SA solutions, neither first or second breakpoints in the electric conductivity curve coincide with the CMC value obtained from surface tension and/or solubilization measurements with the exception of C I 6 S A (o~ = 0.7). Solubilization method may be the most direct way to detect the presence of any kind of aggregate (micelle) of amphiphiles, since the m o n o m e r solutions must not show the solubilization phenomena. Accordingly, we should take the concentration as the CMC at which the solubilization begins, and conclude that the electric conductivity shows abnormal behavior in viscoelastic surfactant solutions. The viscoelastic binary surfactant solutions of sodium dodecylsulfate and N, N- dim ethyl - N- ( 3 - sulfopropyl )alkylamrnonium inner salt (2, 3) also show an abnormal behavior in electric conductivity phenomenon (38). It is reasonable to assume that the electric conductivity is highly affected by the structure o r shape of the micelles present in the solutions, and is not suitable method to determine the CMC of such difunctional and/or viscoelastic surfactant solutions. It is no longer surprising that the sudden depression of solubilizing (Fig. 6) and foaming power (Fig. 9) of C~2SA occurs at the concentration and at the pH value, respectively, above which the viscoelasticity of the solution disappears, since the surface activity of normal solutions is very poor. The surface tension of CI2SA (o~ = 0.9 and 1.0) solutions is lowered to only about 50 dyn/cm even at the concentrations above CMC. The fully neutralized alkenylsuccinates may be too water-soluble to work well as a surfactant. Accordingly, the viscoelastic solutions in partially neutralized state may be recommended for practical applications of this surfactant. ACKNOWLEDGMENTS The authors thank Dr. F. Tokiwa and Dr. M. Saito for their permission to publish this paper.
Journal of Colloid and Interface Science,
Vol, 99, No. 2, June 1984
560
TSUJII, SAITO, AND TAKEUCHI REFERENCES
1. Barker, C. A., Saul, D., Tiddy, G. J. T., Wheeler, B. A., and Willis, E., J. Chem. Soc., Faraday Trans. 1 70, 154 (1974). 2. Saul, D., Tiddy, G. J. T., Wheeler, B. A., Wheeler, P. A., and Willis, E., J. Chem. Soc., Faraday Trans. 1 70, 163 (t974). 3. Tsujii, K., Okahashi, K., and Takeuchi, T., J. Phys. Chem. 86, 1437 (1982). 4. Hyde, A. J., and Johnstone, D. W. M., J. Colloid Interface Sci. 53, 349 (1975). 5. Gravsholt, S., J. Colloid Interface Sci. 57, 575 (1976). 6. Ulmius, J., Lindman, B., Lindblom, G., and Drakenberg, T., J. Colloid Interface Sci. 65, 88 (1978). 7. Johansson, L. B.-A., Lindblom, G., Gravsholt, S., and Nord6n, B., J. Colloid Interface Sci. 69, 358 (1979). 8. Ulmius, J., Wennerstr6m, H., Johansson, L. B.-/~., Lindblom, G., and Gravsholt, S., J. Phys. Chem. 83, 2232 (1979). 9. Gravsholt, S., in "Polymer Colloids" (R. M. Fitch, Ed.), Vol. II, p. 405. Plenum, New York, 1980. 10. Hoffmann, H., Platz, G., Rehage, H., Schorr, W., and Ulbricht, W., Ber. Bunsenges. Phys. Chem. 85, 255 (1981). 11. Hoffmann, H., Platz, G., Rehage, H., and Schorr, W., Ber. Bunsenges. Phys. Chem. 85, 877 (1981). 12. Hoffmann, H., Platz, G., Rehage, H., and Schorr, W., Advan. Colloid Interface Sci. 17, 275 (1982). 13. Tokiwa, F., and Tsujii, K., Bull. Chem. Soc., Jpn. 46, 1338 (1973); 46, 2684 (1973). 14. Tsujii, K., and Arai, H., Yukagaku 22, 211 (1973). 15. Tanford, C., J. Phys. Chem. 76, 3020 (1972). 16. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., J. Chem. Soc., Faraday Trans. 2 72, 1525 (1976). 17. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., Biochim. Biophys. Acta 470, 185 (1977).
Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
18. Mitchell, D. J., and Ninham, B. W., J. Chem. Soc, Faraday Trans. 2 77, 601 (1981). 19. Reiss-Husson, F., and Luzzati, V., J. Phys. Chem. 68, 3504 (1964). 20. Bain, R. M., and Hyde, A. J., Faraday Syrup. No. 5, p. 145 (1971). 2t. Lindblom, G., Lindman, B., and Mandell, L., J. Colloid Interface Sci. 42, 400 (1973). 22. Larsen, J. W., Magid, L. J., and Payton, V., Tetrahedron Lett. 29, 2663 (1973). 23. Johansson, L. B.-/~., Lindblom, G., and Nord6n, B., Chem. Phys. Lett. 39, 128 (1976). 24. Ulmius, J., and WennerstriSm, H., J. Magn. Res. 28, 309 (1977). 25. Henriksson, V., Klason, T., ()dberg, L., and Eriksson, J. C., Chem. Phys. Lett. 52, 554 (1977). 26. Johansson, L. B.-/~., Davidsson, /~., Lindblom, G., and Nord6n, B., J. Phys. Chem. 82, 2604 (1978). 27. Almgren, M., L6froth, J. E., and Rydholm, R., Chem. Phys. Lett. 63, 265 (1979). 28. Nakagawa, T., Bull. Chem. Soc. Jpn. 25, 88 (1952). 29. Nakagawa, T., Bull. Chem. Soc. Jpn. 25, 93 (1952). 30. Nakagawa, T., and Seno, M., Bull. Chem. Soc. Jpn. 29, 471 (1956). 31. Myers, R. R., Negami, S., and White, R. K., Biotheology 3, 197 (1966). 32. Hwang, S. H., Litt, M., and Forsman, W. C., Rheol. Acta 8, 438 (1969). 33. Efiezer, N., Biorheology 11, 61 (1974). 34. Danno, T., Bull. Chem. Soc. Jpn. 32, 1033 (1959). 35. Ueno, M., Hikota, T., Mitama, T., and Meguro, K., J. Amer. Oil Chem. Soc. 49, 250 (1972). 36. Mino, J., Matijevi6, E., and Meites, L., J. Colloid Interface Sci. 60, 148 (1977). 37. Parreira, H. C., Lukenbach, E. R., and Lindemann, M. K. O., J. Amer. OilChem. Soc. 56, 1015 (1979). 38. Aoki, Y., Gomi, T., and Tokiwa, F., Yukagaku 23, 737 (1974).
TSUJII, N A O Y U K I
SAITO, AND T A K A S H I T A K E U C H I
Tochigi Research Laboratories, Kao Corporation, 2606 Akabane, Ichikai-rnachL Haga-gun, Tochigi 321-34 Japan
Received September 6, 1983; accepted November 18, 1983 Viscoelasticityin dilute aqueous solutions of an anionic surfactant, partially neutralized alkenylsuccinate, has been found for the first time. This solution also shows striking spinnability and Weissenberg effect. It is a remarkable feature of this system that these unique properties are not observed at all in the solutions of the fully neutralized agent of the above. The more hydrophobic nature of the surfactant in partially neutralized state than that in fully neutralized one may cause the formation of an elongated rod-like micelle which behaves in the solutions like a high polymer. The frequency dependence of the dynamic rigidity and viscosity suggests that an entanglement coupling or a flexible network structure formation of long cylindrical micelles is the most possible mechanism for viscoelasticity. Some colloid chemical properties of the surfactant have been measured in both viscoelastic and normal (non-elastic) solution regions to obtain the information useful for the practical applications of the agent. All results obtained from the surface tension, solubilization, critical miceflization concentration, and foaming measurements indicate that the viscoelastic solutions of the agent are much more surface active than the normal ones. INTRODUCTION Viscoelasticity in dilute aqueous solutions is one o f the m o s t interesting p h e n o m e n a which are recently f o u n d in the field o f surfactant chemistry. The viscoelastic binary ( 1 3) and single surfactant solutions (4-12) have been f o u n d and reported by several authors. However, only two kinds o f cationic c o m p o u n d , hexadecyltrimethylammon i u m salts (4-9) a n d hexadecylpyridinium salicylate (10-12), are k n o w n to give the viscoelastic solutions in single surfactant systems. We have f o u n d in this work a new viscoelastic solution o f single anionic surfactant, partially neutralized alkenylsuccinates which can be also regarded as the binary mixtures o f alkenylsuccinic acid and its salt, and measured the d y n a m i c rigidity and viscosity o f the solutions. The alkenylsuccinate solutions exhibit
striking viscoelasticity, spinnability, a n d Weissenberg effect in partial degree o f neutralization, but are quite n o r m a l (no elasticity, spinnability, and Weissenberg effect) in fully neutralized state. We are especially interested from the practical point o f view in the difference o f colloid chemical properties between viscoelastic and n o r m a l solutions o f the agent. Then, the surface tension, solubilizafion, electric conductivity, critical micellization concentration, and f o a m - v o l u m e measurements have been m a d e in both viscoelastic and normal solution regions. EXPERIMENTAL M a t e r i a l s . A l k e n y l s u c c i n i c acids were kindly presented by Mr. Y. Ishikawa a n d Dr. N. M o r i y a m a o f W a k a y a m a Research Laboratories, K a o Corporation, and were prepared by the following procedures.
553 0021-9797/84 $3.00 Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
554
TSUJII, SAITO, A N D T A K E U C H I
//O CH--C //O R - - C H 2 - - C H = C H 2 + I] / O --~ R - - C H = C H - - C H 2 - - C H - - C ~ CH--C%o [ / O ---* CH2--C%o R - - C H = C H - - C H 2 - - C H --COOH
I
CH2--COOH a-Olefins (1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene) were reacted with maleic anhydride in an autoclave at 220°C for 4 hr. The starting a-olefins were found to be more than 96% pure by gas chromatography. The reaction products were purified by distillation under reduced pressure. The oligomeric olefin-succinic anhydride, main byproduct of the above reaction, was removed together with the other impurities through this procedure. The purified alkenylsuccinic anhydrides were hydrolized with an aqueous NaOH solution. The obtained alkenylsuccinic acids were precipitated at pH 2, and finally purified by repeated recrystallization from acetone. The sample solution was prepared by direct neutralization of alkenylsuccinic acid in a volumetric flask with desired amounts of base solutions. Sodium hydroxide was used as a base unless noted otherwise. The surfactant samples are, hereafter, abbreviated as CnSA, where n is the carbon number of a-olefin. The degree of neutralization is denoted as a, and written in brackets, if necessary, such as CnSA (a = 0.7). Yellow OB (1-o-tolyl-azo-2-naphthylamine) was purchased from Tokyo Kasei Ltd., and purified by recrystallization three times from ethanol. The deionized and distilled water was used to prepare the sample solutions. Methods. Viscoelasticity can be easily detected qualitatively by the simple method of swirling a sample solution and visually observing recoil of small air bubbles entrapped in the sample just after swirling is stopped (3, 5, 8, 9). Quantitative measurements of dynamic rigidities were performed with a ShiJournal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
madzu Model RM-1 rheometer, using a coaxial cylindrical cell. The pH-titrations were carried out with a Horiba Model N-5 pH meter at 39.5 _ 0.2°C. The N2 gas was passed through the cell during the whole titration procedure to avoid the interference of CO2 gas. The temperature was controlled by circulating the thermostated water. Solubilization of water-insoluble dye, Yellow OB, was measured by essentially the same manner as described elsewhere (13). A sealed test tube containing the surfactant solution and the small amounts of solid Yellow OB was shaken at 45°C for 46 hr. The amount of solubilized Yellow OB was determined by the optical density measurement at 447 rim. Qualitative flow birefringence was observed visually using a polariscope at room temperature (9). The sample solutions were rotated at ca. 500 rpm by a magnetic stirrer. The flow birefringence of the CnSA solutions was unfortunately too weak, and we could not take photographs as was done by Gravsholt (9). Surface tension was measured by the drop weight method. Electric conductivity was determined with a Radiometer Model CDM-3 conductivity meter. The temperature was maintained constant within +0.2°C for both measurements. Foaming test was made at room temperature by the same stirring method as used in a previous work (14). RESULTS
Figure 1 shows the pH-titrafion curves of C12SA with 1 N NaOH solution. The apparent
VISCOELASTIC SURFACTANT SOLUTIONS
,2I
50m~,l~
11
'°I
555
.f
5mM~ i
I O
0.5
o6--W--o pH
0,5
oL
1.0
FIG. 1. The pH-titration curves of CtzSA with 1 N NaOH solution at 39.5°C. The concentration of C~2SA are 5 mM (O) and 50 mM (O). degree of neutralization, a, is taken as abscissa. One can see from the figure that the CI2SA shows considerable buffering action at neutral pH in the range of 0.2 < a < 0.8. Roughly speaking, the C12SA solutions are viscoelastic in this range of a. In the degree of neutralization smaller than 0.2, the solubility of CIzSA is not enough and the solutions are turbid, whereas the viscoelasticity is not observed at all in spite of the clear solution formation of the agent when a exceeds 0.8. Similar pH-dependent viscoelastic behaviors can be seen also in the solutions of higher hydrocarbon chain homologs and of CnSA neutralized with other bases than N a O H such as LiOH, KOH, mono-, di-, and triethanolamine. The dynamic rigidities of 2 wt% ( ~ 7 0 raM) CI2SA solutions are plotted against pH in Fig. 2. Elastic solutions appear only in narrow pH range, but in wide range of neutralization, because of the buffering action of C12SA. The viscoelasticity of the solution diminishes with increasing temperature, and finally disappears at ca. 50°C as shown in Fig. 3. Similar tern-
7
FIG. 2. The dynamic rigidity (G') of 2 wt% C~:SAsolutions as a function of pH, measured at 35°C and the frequency of 1.0 Hz.
perature dependence in viscoelasticity has also been observed in hexadecyltrimethylammonium salt solutions (8). Figure 4 shows the dynamic rigidity and viscosity of ClzSA solution at pH = 5.94 as a function of frequency. Opposite dependence of the dynamic rigidity (increasing) and viscosity (decreasing) on frequency increase may suggest that the CnSA solution is a Maxwell fluid. The viscoelastic solutions mentioned above
5.0
2.0
40 3.0 ~1.0
"7
2.0
x
g ~
1.0 10
2'0
T(*C)
6'0
0
FIG. 3. The dynamicrigidityand viscosity(•') of 2 wt% C~2SAsolutions as a function of temperature, measured at pH 5.94 and the frequency of 1.0 Hz. Journal of Colloid and Interface Science,
Vol. 99, No. 2, June 1984
556
TSUJII, SAITO, AND TAKEUCHI
% 2.0
~_~
O/
21
400 ViscoeLasticregTon
~_aoo
/
o/c/ O(=0.7
.LZ 200 J 3.05 o.
I.C
100
q
_o~o~O~°/ 0.2
0.4
0.6 (Hz)
0.8
j / I
I
!
I
20
i
I
I
40 60 C(mM)
./ /
/--~=to
i
,
i
80
100
120
FIG. 6. Solubilization curves of C12SAtoward Yellow OB measured at 45°C, and c~ = 1.0 (e) and 0.7 (O).
1.0
FIG. 4. The dynamic rigidity and viscosity of 2 wt% CI2SAsolutions as a function of frequency, measured at pH 5.94 and 27°C.
exhibit a characteristic nature found also in other solution properties. Flow birefringence, spinnability, and Weissenberg effect are observed in viscoelastic solutions. For example, one can readily spin a liquid thread longer than 30 cm under the appropriate conditions. The non-elastic highly neutralized solutions, however, are quite normal, and do not show any of the remarkable properties above. Surface tension vs concentration curves of C12SA solutions with various degree of neutralization are given in Fig. 5. It is evident from the figure that the C12SA solutions in lower degree of neutralization show higher surface activity (low surface tension and
CMC). The amounts of Yellow OB solubilized into C12SA (a = 0.7 and 1.0) solutions are plotted against the concentration of C12SA in Fig. 6. The solubilizing power of C I2SA (slope of the straight line in Fig. 6) in viscoelastic region is much greater than that in normal solution region. The solubilization curve of C~2SA (a = 0.7) solution breaks at 17.5 m M above which the viscoelasticity of the solution disappears, since the p H value of the solution increases with increasing concentration o f C~2SA (a = 0.7) as expected from Fig. I. It is worth noting that the slope of the curve in C12SA (a = 0.7) solution at the concentration range above 17.5 m M is very similar to that
0
50
\\ •
o.
)
"~ 4C
::,d 3.0
/U °
2.0
08
1.0
20,~
-3,5
-3.0
-2.5
-2.0
-1.5
-1.0
logC
FIG. 5. The surfacetension vs log (concentration)curves of C~2SAmeasured at 45°C, and at the degree of neutralization = 1.0 (©), 0.9 (~), 0.8 (e), and 0.7 (O). Journal of Colloid and Interface Science, VoL 99, No. 2, June 1984
O~
1'0
2'0 30 C(mNI)
40
50
FIG. 7. Specific conductivity vs concentration curves of C~2SA(c~ = 0.7) measured at 50°C (O), 60°C (O), and 7 0 ° C (@).
VISCOELASTIC SURFACTANT SOLUTIONS of non-elastic fully neutralized solution, indicating the existence of similar micelles in both solutions. Electric conductivity data as a function of C l z S A (oL = 0.7) concentration are given in Fig. 7. There appear two breakpoints in the conductivity vs concentration curves. The similar curves having two break points are also obtained at o~ = 0.7 in the solutions of higher hydrocarbon-chain homologs. In the fully neutralized surfactant solutions, however, the n u m b e r of breakpoint depends on the hydrocarbon chain length of the surfactant. The conductivity vs concentration curves o f C12and C I 4 S A (o! = 1 . 0 ) have also two breakpoints, whereas in the C16- and C18SA (oz = 1.0) solutions, the curves have only one breakpoint as shown in Fig. 8. The critical micellization concentration (CMC) can be determined from any methods of the surface tension, solubilization, and electric conductivity techniques. All CMC data are summarized in Table I. The C M C values obtained from surface tension and solubilization are in good agreement with each other. The results from electric conductivity, however, are somewhat complicated. F o a m v o l u m e - p H profile of the 0.5 wt% C~2SA solution containing 1.0 wt% soybean oil is given in Fig. 9. The foaminess o f C~2SA solution is high enough in viscoelastic p H re-
10
~
CMC(mM) Electric conductivity Surface tension
Surfactant
C12SA ( a = 1.0) CI4SA ( a = 1.0) CI6SA ( a = 1.0) CIgSA ( a = 1.0) CI2SA C12SA CI2SA C14SA C]6SA Ct~SA
(a (a (a (a (a (a
= = = = = =
0.9) 0.8) 0.7) 0.7) 0.7) 0.7)
Solubilization
80 ~ 24 a
1st
76 a
24 b 23 c
8.3 c 3.5 c
9.0 c 4.5 d
5& 5.4 ~ 2.5 ~
20 b 19 b 15 b 8.0 b 5.0 c 2.8 c
2.5 ~
5.3 c
2nd
84 b 40 c
75 b 45 b 35 b 32 b 16 ~ 6.4 C
" M e a s u r e d at 4 5 ° C . b At 50°C. c At 60°C. dAt 70°C.
gion, but depresses suddenly at the p H value where the elasticity of the solution disappears. DISCUSSION
Viscoelastic Regions and a Possible Mechanism of Viscoelasticity As mentioned previously, the viscoelasticity of CnSA solutions appears only in the limited
g ~ 100
i
i
p
to
5o
FIG. 8. Specific conductivity vs concentration curves measured at 60°C and a = 1.0 (©), and of C~sSA measured at 70°C and a = 1.0 (O).
o f CI6SA
CMC Values Determined by Three Different Methods
I
30
C(mM)
TABLEI
200
// ~o
557
05
;
-> pH
FIG. 9. Foam volume vs pH curve of 0.5 wt% C12SA solutions containing 1.0 wt% soybean oil. Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
558
TSUJII, SAITO, AND TAKEUCHI
pH regions, which exhibits a striking contrast to the other viscoelastic surfactant systems. The lower pH limit is simply originated from the solubility loss of CnSA into water, but it is indeed interesting to note that the fully neutralized CnSA solutions are quite normal, and show no elasticity at all. This result indicates a drastic structure change of C~SA micelles by pH variations, which is supported by flow birefringence experiments. The appearance of flow birefringenee, of course, results from the existence of the micelles of an asymmetric shape in the solutions. The theories giving the micellar shape of surfactants have been presented by Tanford (15), and Ninham and his co-workers (1618). According to their theory, the packing of hydrocarbon chains in the hydrophobic interior of micelles is the governing factor to determine the spherical or globular or cylindrical or lamellar or vesicle-shaped micelles. Israelachvili et al. present a quantity, V/aol, as the criterion to determine the shape of micelles; where l and v are the length and the volume of the hydrocarbon part of one surfactant molecule, respectively, and a0 the area occupied by one hydrophilic head at the surface of the micelle (16). The usual spherical micelles are formed when V/aol < 1/3. The shape ofmicelle deforms from sphere to globule and toroid with increasing value of V/aol, and finally becomes the infinite cylinder when v~ aol attains 1/2. For a given alkenylsuccinate, v and l remain unchanged and only a0 can be varied with changing pH of the solution. The a0 value of the fully neutralized CnSA must be large enough to form spherical micelles. At lower pH region, however, the alkenylsuccinates are neutralized only in part, and the electric repulsive force between ionic heads of the surfactant molecules is reduced, which should give the smaller a0 values than that of the fully neutralized agents. When ao attains a certain value at definite pH, the long cylindrical micelles may be formed. Remarkable spinnability of CnSA solutions may support the above assumption, since no spinnable solution has Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
been known so far without any long molecules or colloids as solutes. In addition, the presence of long cylindrical micelles in viscoelastic hexadecyltrimethylammonium salt solutions is substantiated by several experimental means (4-9, 19-27). As mentioned previously, the elastic recoil of the CnSA solution can be observed visually, which means that present systems have long relaxation time rheologically, as substantiated by Fig. 4. Similar viscoelastic behaviors with long relaxation time have been known also in some spinnable polymer solutions (28-30), biological fluids (31-33), and colloidal dispersions (34). In such viscoelastic fluids, the entanglement coupling or the flexible network structure of polymer molecules is assumed to account for their long rheological relaxation time (31-34). The network structure was actually substantiated by Danno in the colloidal mercury sulfosalicylate gel (34). Beautiful network structure formed with the elongated rodlike colloids was visualized by means of electron micrograph (34). Interestingly, the frequency dependence of the dynamic rigidity and viscosity of the above gel is quite similar to that of our micellar solutions shown in Fig. 4. It is likely, we suppose, that the long cylindrical micelles may form a temporary network structure or an entanglement coupling, which results in the viscoelasticity of the solutions. Hoffmann and co-workers also interpret the viscoelasticity as due to the overlapping of rod-like micelles for hexadecylpyridinium salicylate solutions (12). Ulmius et al. proposed, recently, a periodic colloid structure model of rod-shaped micelles for the viscoelastic aqueous solutions of hexadecyltrimethylammoniumsalt (8). We believe, however, any periodic colloid structures formed with the long cylindrical miceUes must be a kind of liquid crystalline state, and optically anisotropic. This is not the case in the solutions of hexadecyltrimethylammonium salt. We may safely say anyhow that the polymer solutions (28-30), biological fluids (3133), and colloidal dispersions (34) which show
559
VISCOELASTIC SURFACTANT SOLUTIONS
similar viscoelastic behaviors to the above surfactant solutions cannot form any periodic structures at all, which strongly suggests that the periodic colloid structure is not necessary for the viscoelastic surfactant solutions.
Colloid Chemical Properties of the Viscoelastic and Normal Solutions of Alkenylsuccinates As understood from Figs. 5, 6, and 9, the surface activity of CnSA in the viscoelastic solutions is much higher than that in the normal ones. Both the surface tension and the CMC of partially neutralized C~2SA solutions are much lower than those of fully neutralized solutions (Fig. 5). It is especially interesting to note that the surface tension of C~2SA (o~ = 0.7 and 0.8) at the concentrations above CMC attains to 25 dyn/cm, which is unexpectedly low value for the aqueous solutions of surfactant with a hydrocarbon chain as a hydrophobic group. The partial degree of neutralization should give the more hydrophobic nature to the alkenylsuccinate molecules and the weaker electrostatic repulsive force between hydrophilic heads of the molecules than the full neutralization of the surfactant. The compact adsorption of C 1 2 8 A molecules at the surface of the solution due to the weak repulsive force between them may result in the low surface tension in partially neutralized state. It has been already known that there exist two breakpoints in the electric conductivity vs concentration curves o f some difunctional surfactants (35-37). In most cases, the first breakpoint appeared at lower concentration coincides with the CMC value determined by other techniques, and the second break is attributed to the change of size or shape of micelles (35-37). In some cases, however, the CMC value determined from surface tension measurements agrees with the second breakpoint in the electric conductivity curve (37). No explanation has been given so far for this latter case, In our C,SA (o~ = 1.0) systems,
there are three types of behaviors, depending upon the hydrocarbon chain length of the surfactant (Table I). In partially neutralized C,SA solutions, neither first or second breakpoints in the electric conductivity curve coincide with the CMC value obtained from surface tension and/or solubilization measurements with the exception of C I 6 S A (o~ = 0.7). Solubilization method may be the most direct way to detect the presence of any kind of aggregate (micelle) of amphiphiles, since the m o n o m e r solutions must not show the solubilization phenomena. Accordingly, we should take the concentration as the CMC at which the solubilization begins, and conclude that the electric conductivity shows abnormal behavior in viscoelastic surfactant solutions. The viscoelastic binary surfactant solutions of sodium dodecylsulfate and N, N- dim ethyl - N- ( 3 - sulfopropyl )alkylamrnonium inner salt (2, 3) also show an abnormal behavior in electric conductivity phenomenon (38). It is reasonable to assume that the electric conductivity is highly affected by the structure o r shape of the micelles present in the solutions, and is not suitable method to determine the CMC of such difunctional and/or viscoelastic surfactant solutions. It is no longer surprising that the sudden depression of solubilizing (Fig. 6) and foaming power (Fig. 9) of C~2SA occurs at the concentration and at the pH value, respectively, above which the viscoelasticity of the solution disappears, since the surface activity of normal solutions is very poor. The surface tension of CI2SA (o~ = 0.9 and 1.0) solutions is lowered to only about 50 dyn/cm even at the concentrations above CMC. The fully neutralized alkenylsuccinates may be too water-soluble to work well as a surfactant. Accordingly, the viscoelastic solutions in partially neutralized state may be recommended for practical applications of this surfactant. ACKNOWLEDGMENTS The authors thank Dr. F. Tokiwa and Dr. M. Saito for their permission to publish this paper.
Journal of Colloid and Interface Science,
Vol, 99, No. 2, June 1984
560
TSUJII, SAITO, AND TAKEUCHI REFERENCES
1. Barker, C. A., Saul, D., Tiddy, G. J. T., Wheeler, B. A., and Willis, E., J. Chem. Soc., Faraday Trans. 1 70, 154 (1974). 2. Saul, D., Tiddy, G. J. T., Wheeler, B. A., Wheeler, P. A., and Willis, E., J. Chem. Soc., Faraday Trans. 1 70, 163 (t974). 3. Tsujii, K., Okahashi, K., and Takeuchi, T., J. Phys. Chem. 86, 1437 (1982). 4. Hyde, A. J., and Johnstone, D. W. M., J. Colloid Interface Sci. 53, 349 (1975). 5. Gravsholt, S., J. Colloid Interface Sci. 57, 575 (1976). 6. Ulmius, J., Lindman, B., Lindblom, G., and Drakenberg, T., J. Colloid Interface Sci. 65, 88 (1978). 7. Johansson, L. B.-A., Lindblom, G., Gravsholt, S., and Nord6n, B., J. Colloid Interface Sci. 69, 358 (1979). 8. Ulmius, J., Wennerstr6m, H., Johansson, L. B.-/~., Lindblom, G., and Gravsholt, S., J. Phys. Chem. 83, 2232 (1979). 9. Gravsholt, S., in "Polymer Colloids" (R. M. Fitch, Ed.), Vol. II, p. 405. Plenum, New York, 1980. 10. Hoffmann, H., Platz, G., Rehage, H., Schorr, W., and Ulbricht, W., Ber. Bunsenges. Phys. Chem. 85, 255 (1981). 11. Hoffmann, H., Platz, G., Rehage, H., and Schorr, W., Ber. Bunsenges. Phys. Chem. 85, 877 (1981). 12. Hoffmann, H., Platz, G., Rehage, H., and Schorr, W., Advan. Colloid Interface Sci. 17, 275 (1982). 13. Tokiwa, F., and Tsujii, K., Bull. Chem. Soc., Jpn. 46, 1338 (1973); 46, 2684 (1973). 14. Tsujii, K., and Arai, H., Yukagaku 22, 211 (1973). 15. Tanford, C., J. Phys. Chem. 76, 3020 (1972). 16. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., J. Chem. Soc., Faraday Trans. 2 72, 1525 (1976). 17. Israelachvili, J. N., Mitchell, D. J., and Ninham, B. W., Biochim. Biophys. Acta 470, 185 (1977).
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