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Solution properties of mixed surfactant system
Solution Properties of Mixed Surfactant System V. The Effect of Alkyl Groups in Nonionic Surfactant on Surface Tension of Anionic-Nonionic Surfactant Systems MASAHIKO ABE,*' t NOBUYUKI TSUBAKI,* AND KEIZO OGINO*'t *Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba, Japan 278, and tlnstitute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Japan 162 Received March 30, 1984; accepted February 21, 1985 The effect of alkyl groups in a nonionic surfactam on the surface tension of anionic-nonionic surfactant systems is described; these systems are sodium 3,6,9-tfioxaicosanoate (ECL)-alkyl polyoxyethylene ethers (CmPOE, m = 12, 14, 16, and 18). The surface tension of each mixed surfactant solution decreases with increasing mole fraction of anionic surfactant. However, these surface tension vs mole fraction curves are shifted from a monotonous curve to specific curves with increasing alkyl chain length in the nonionic surfactant. Furthermore, in the case of the mixed systems including the nonionic surfactant which has shorter alkyl chain length, the surface tension vs total concentration curve shows two breakpoints in the vicinity of the CMC for ECL alone and for CmPOE alone. In the case of systems including a nonionic surfactant which has longer alkyl chain length, that curve shows only one breakpoint. This may be attributed to the fact that the mixed micelle is formed more easily by a nonionic surfactant including long alkyl chain length than by one having shorter alkyl chain length. Then, in the mixed surfactant systems, there are two kinds of micelles coexisting (one rich in anionic surfactant and the other rich in
nonionic surfactant).
© 1985 Academic Press, Inc.
INTRODUCTION
It has been reported that the effect on surface activity of a mixed surfactant system is superior to that of a single-surfactant system (1-3). A number of solution properties have been measured for various mixed surfactant systems. For example, anionic-anionic (4-7), anionic-nonionic (8-13), anionic-cationic ( 14-15), nonionic-nonionic ( 16, 17), amphoteric-anionic (18, 19), amphoteric-nonionic (20), and amphoteric-amphoteric (21) mixed surfactant systems have been studied. In recent years, the solution properties in fluorocarbon-hydrocarbon mixed surfactant systems have been reported (22-24). It is well known that these mixed surfactant systems show a specific behavior as compared with the hydrocarbon-hydrocarbon mixed surfactant systems and that two kinds of micelles coexist in aqueous solution (25-27).
In the previous paper (28), we have reported the electric properties of the mixed surfactant system: 3,6,9-trioxaicosanoate (ECL, an anionic surfactant)-alkyl polyoxyethylene ethers (CmPOE, a nonionic surfactant; rn = 12, 14, 16, and 18). We found that it could form two kinds of micelles as the alkyl chain length in CmPOE decreased. In the present paper, we report the surface tension of ECL-CmPOE mixed surfactant systems in aqueous solutions. We discuss the differences in the mixed micelle forming due to the different alkyl chain lengths in nonionic surfactants. EXPERIMENTAL
Materials Anionic surfactant. Sodium 3,6,9-trioxaicosanoate [ECL, CI1H2aO(C2HgO)2CH2COONa] was supplied by Nihon Surfactant
503 0021-9797/85 $3.00 Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
Copyright © 1985 by Academic Press, Inc. All fights of reproduction in any form reserved.
504
ABE, TSUBAKI, AND OGINO
Industries Company, Ltd, Tokyo, Japan. The purity was ascertained by thin-layer chromatography (ether carboxylate; >98%), and gasliquid chromatography (methyl ester; >98%). Nonionic surfactant. Alkyl polyoxyethylene ethers [CmPOE, CmHzm+IO(C2H40)aoH; m = 12, 14, 16, and 18] were supplied by Nihon. It has a narrow molecular weight distribution. Water used in this experiment was twice distilled and deionized by using an ion-exchange instrument (NANO pure D-1791 of Barnstead Co., Ltd.); its resistivity was about 18.0 Mohm. cm and its pH was 6.7.
Methods Preparation of mixed surfactant solutions. Into several 100-ml beakers, portions of a given concentration of an ECL solution (5.0 X 10-3 mole/liter) were placed, followed by addition of a given concentration of CmPOE solution (5.0 X 10-3 mole/liter). The mixtures were diluted stepwise with water. These mixtures were stirred for 30 min in a thermostat at 30.0°C in order to establish their equilibria. Determination of the surface tension value. The surface tension of aqueous solutions of single and/or mixed surfactant systems were measured at 30 + 0.1 °C using a Wilhelmytype surface tensiometer (A-3, Kyowa Scientific Co., Ltd, Tokyo, Japan) with a platinum plate.
k
i
i
i
Mixed system : ECL-CI2POE • : ECL-CI4POE I) : ECL-ClfPOE 42
38
°'-.o e.~ ~ 34
Total
concentrot ion 5.0 x 10 -3 mo]/l
26
I
I
0.2 0.14 0,6 018 Mole froction of ECL
1.0
FIG. 1. The surface tension against the mole fraction of ECL in mixed surfactant solutions above CMC.
kyl chain length in nonionic surfactant. In the case of the ECL-C18POE system, the surface tension value decreases with increasing XECL up to 0.3, remains almost constant until XECL = 0.7, then decreases again.
The Change of Surface Tension with the Total Concentration under a Constant Mole Ratio
The surface tension of ECL-CmPOE mixed surfactant solutions in various mole fractions of ECL are plotted against the total concentration of surfactants in Figs. 2-5: Fig. 2 is the ECL-C12POE system, Fig. 3 the ECL-C14POE system, Fig. 4 the ECL-CI6POE system, and RESULTS Fig. 5 the ECL-C18POE system. The Change of Surface Tension with Mole In the case of the ECL-alone and the Fraction under a Constant Total CmPOE-alone systems, each surface tension Concentration value decreases with increasing the concentraThe surface tensions of ECL-CmPOE mixed tion, but remains constant above the critical surfactant solutions at 5.0 × 10-3 mole/liter micelle concentration (CMC). On the other (above CMC) are plotted against the mole hand, in the case of the ECL-CI/POE system fraction of ECL in Fig. 1. (Fig. 2), the surface tension values of aqueous In the case of the ECL-C~2POE system, the solutions in various mole fractions decrease surface tension value decreases monotonously with increasing total concentration too, but with increasing mole fraction of ECL (XEcL). the line breaks at two points, in the vicinity However, these surface tension vs mole frac- of the CMC for CIzPOE alone and of that for tion curves are shifted from a monotonous ECL alone. The intervals between the two curve to specific curves with increasing the al- breakpoints decrease with increasing alkyl Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
SOLUTION PROPERTY OF MIXED SURFACTANT SYSTEM, V -~O
,
40
~ Mole fractl
-~
o_r_Ec&
The Composition of Mixed Micelle
]
•
o~_
®
®-
o
®
.~
"-.No
The micellar mole fraction of ECL(XmEcL) in a mixed surfactant solution can be expressed by the following equation, suggested by Funasaki and Hada (30): XmECL = ( C t ° XEC L --
@
@
505
CMC"
XbECL)/
( G - CMC).
@-
[11
Here Ct is the total concentration, XECL the overall mole fraction of ECL, and XbECLthe 10-t4 10-3 10-2 Concentration (n~l/l) monomeric mole fraction of ECL. Figure 6 shows the surface tension (open FIG. 2. The surface tension against the total concentracircles) and cmc (solid circles) against XbECL tion in mixedsurfactantsolutionsof ECL-CIzPOEsystem. curve as the calibration curve. We obtained XbECL and XmECL of the ECL-C18POE mixed chain length in nonionic surfactant (Fig. 3- surfactant solution for various XECLby using Fig. 5). Finally, in the case of the ECL-C18POE Eq. [1] and Fig. 6. These values are shown in system (Fig. 5), the surface tension value de- Table I. The mixed micelle whose composition creases with increasing total concentration, is the same as the mixed solution is formed in the ECL-CIsPOE system. and breaks at only one point. 30
•
I
•
I
@-1
Existence of Two Kinds of Micelles
DISCUSSION
Formation of Mixed Micelle As seen in Fig. 1, the surface tension vs mole fraction curves are shifted from a monotonous curve to specific curves shaped like the letter "S," with increasing alkyl chain length in nonionic surfactant. In the previous paper (29) we measured the surface tension in the ECL-hexadecyl polyoxyethylene ether (CI6POE10, a nonionic surfactant in which the number of oxyethylene groups is 10) system. We found that the surface tension vs mole fraction curve has a specific S shape. This is attributed to the fact that the ECL molecules have migrated from the bulk phase to the micellar phase and the CI6POE10 molecules have migrated from the micellar phase to the bulk phase in forming the mixed micelle. Therefore, the mixed micelle is formed more easily with increasing alkyl chain length in a nonionic surfactant. Thus, we have attempted to determine the composition of the mixed micelle which is formed in the ECL-CI8POE system by the surface tension method.
In general, probable surfactant chemical species would be three types in ECL-CmPOE mixed system (namely, anionic ECL, protonated ECL, and nonionic CmPOE). In a previous paper, we found that the degree of ionic dissociation of the mixed micelle is almost 1 for the ECL mixed with CmPOE (hexadecyl polyoxyethylene ether) at a molar ratio of
45
p
O ~ @ \~
t
t
\ ~ \
i
ECL - C ,,POE
'o\ \ \ ~ .
,.
} 40 of E6L 35 I)
~=
®
:0,2 : o,5
@
:0.8 : 1.0
@
~ --®--@--@-
30
@--@--@l
l
l
l
l
10-6
10-5
10-4
10-3
10-2
Concentration (¢ol/1) FIG. 3. The surface tension against the total concentration in m i x e d surfactant solutions of E C L - C , 4 P O E system. Journal of ColloM and Interface Science, Vol. 107, No. 2, October 1985
506
ABE, TSUBAKI, AND OGINO \,\
,
~,
\,
,
\
40 I
,
o u
~
t
'
t
O
ECL- CI6POE
q
--0-- O~--o--O--
H01e fractlon 3s
--~f~
30
• : 0,2 ®:0,5 O:O.8 • : 1,0
~= 35
~k
- ~L®-®--~-~-
•
30 f
I
I
I
I0-5
10-4
10-3
i0-2
I
0
PIG. 4. The surfacetension against the total concentration in mixed surfactant solutions of ECL-CI6POE. CmPOE/ECL = 1/1. Therefore, under this condition, the protonated ECL molecules are absent in the mixed micellar solutions. Therefore, we will describe the case of molar ratio of CmPOE/ECL = 1/1 in the following discussion. As can be seen from Fig. 2, in the case of the ECL-C12POE system, the surface tension vs concentration curve breaks at two points in the vicinity of the CMC for C~2POE alone and that for ECL alone. Now, we shall assume that there is no interaction between ECL and C12POE in mixed surfactant solutions. In this case, the concentration in bulk phase and micellar phase will depend on the mole ratio for mixing. Figure 7a shows the relation between the concentration in bulk phase for ECL and 45
•
I \?\?,
¶
~-c1~PO~
\
0.2
I
I
I
0.4
0.6
0.8
.0
Mole fraction of ECL In bulk phase
Content rotlon (dynlcm)
a, 35
0 -5.0
•--•--•--
.I.
4o
0
"~--~--~
10-6
g
0
\
FIG. 6. Relation between the surface tension and log CMC as a function of the concentrationin bulk phase. C12POE and the total concentration in mixed solution at a given mole ratio CI2POE/ECL = 1/1. Here, points A and B are the CMC of aqueous solution for C~2POE alone and for ECL alone, respectively. The concentration in bulk phase for ECL and C12POE increase in the same proportion with increasing total concentration up to point A. Then, in the case of C12POE, the concentration in bulk phase remained constant above point A by forming the micelle. On the other hand, in the case of ECL, this concentration increases with increasing total concentration up to point B. The surface tension is closely related to the concentration of bulk phase. The additivity of surface tension values will be applied since interaction between ECL and CI2POE is entirely absent. Thus, the surface tension against the total concentration curve will be broken at the two points of CMC for ECL alone and that for C12POE alone, as shown by the dashed line in Fig. 7b. This surface tension value (3'~d) is calculated by using the equation
\o--8~-~@--8~--o--o--o--o--oMole fraction o : o
Xtl)~ID~ID
iD
®~-®~®--®
ffcal = X " ~/ECL -~" ( 1 ®--~-
- X)"
"~CmPOE-
[2]
Here ~/ECLand "~CmPOEare the surface tension values at the given concentration and X is the mole fraction of ECL for mixing (in this case • --•--I-30 we use X= 0.5). I I i ; J I0 -6 10.5 10-4 I0-3 10.2 The solid line in Fig. 7b is the observed Content rot fan (m01/1) value of aqueous solution of ECL mixed with C12POE for mole ratio Cx2POE/ECL = 1/1. PIG. 5. The surfacetension against the total concentration in mixed surfactant solutions of ECL-C,sPOE. As can be seen from Fig. 7b, the good agree-
Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
507
SOLUTION PROPERTY OF MIXED SURFACTANT SYSTEM, V TABLE I The Mole Fraction of ECL in Bulk Phase and Micelle Phase of ECL - C~8POE20 Mixed Surfactant Solutions at 30.0°C Phase
Mole fraction of ECL
Solution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Bulk
0
0.23
0.41
0.50
0.50
0.50
0.51
0.56
0.70
0.84
1.00
Micelle
0
0.10
0.19
0.29
0.39
0.50
0.60
0.71
0.81
0.91
1.00
Note. Total concentration: 5.0 X 10-3 mole/liter.
ment of observed values (solid line) and calculated values (dashed line) indicates that there are two kinds of micelles coexisting (one rich in ECL and the other rich in C12POE). Next, we discuss the ECL-C18POE system in a similar manner to what we did for the ECL-C~2POE system. Figure 8 shows (a) the concentration in bulk phase and (b) the surface tension, against the total concentration in mixed surfactant solution. As seen in Fig. 8b, the observed value (solid line) is different from the calculated value (dashed line). This is at-
I -~
i0-3
tributed to the fact that the mixed micelle is formed by mixing ECL with C~8POE, as mentioned above. Thus, the mixed micelle is formed more easily by a nonionic surfactant including long alkyl chain length than by one having shorter alkyl chain length. Furthermore, in the latter case, there are two kinds ofmicelles which coexist. In the case of hydrocarbon-hydrocarbon mixed surfactant systems, a mixed micelle
I
-. i0-3
I
(0) ~ - -
I
I
v
1o-4
ECL
(a) E C L ~
c
._~
I I S 45
pA
L~B
iI calculated , \i orve !
u
= 3
10-6
"~
50
'x
'-.f
45
! (D 4-,
40
o3
2
C18POE
10-5
IO -4
35
35
(b) I
i0-4
I
i(?-3
Concentrot]on in solution (rml/l) FIG. 7. Relation between the surface tension and the concentration in bulk of ECL-CI2POE system. (a) The concentration in bulk and (b) the surface tension against the concentration in solution.
J B
'\
"%i
i
colculoted
! I
i,
curve x "\'J_' .... (b) I I I 0-6 10-5 10-4 10-3 i0-2 Concentration]n solution(mol/])
FIG. 8. Relation between the surface tension and the concentration in bulk of ECL-C18POE system. (a) The concentration in bulk and (b) the surface tension against the concentration in solution. Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
508
ABE, TSUBAKI, AND OGINO
5. Shinoda, K., J. Phys. Chem. 58, 1136 (1954). 6. Fitch, R. M., and McCarwill, W. T., J. Colloidlnterface Sci. 66, 20 (1978). 7. Van den Bogaert, R., and Joos, P., J. Phys. Chem. E~?~+t~_~T~ ~ mixedmlcelle 84, 190 (1980). , , ._../; \._.../l 8. Nishikido, N., J. Colloidlnterface Sci. 60, 242 (1977). 9. Tokiwa, F., and Tsujii, K., 3. Phys. Chem. 75, 3560 (1971). ',,.,._....,] ",,,......... ."' 10. Meguro, K., Akasu, H., Ueno, M., and Satake, T., Colloid Interface Sci. 2, 421 (1976). ECL CmPOE 11. Schwuger, M. J., and Smolka, H. G., ColloidPolym. m: alkyl chain rich micelle rich micel]e Sci. 255, 589 (1977). FIG. 9. Micellization models for ECL-CmPOE mixed 12. Ogino, K., Tsubaki, N., and Abe, M., J. Colloid Interface Sci. 98, 78 (1984). surfactant system. 13. Abe, M., Tsubaki, N., and Ogino, K., Yukagaku 32, 672 (1983). which has an arbitrary composition is formed 14. Hendfikx, Y., J. Colloid Interface Sci. 69, 493 (1979). (31). O n the other hand, only in the case o f 15. Lucassen-Reynders, E. H., Lucassen, J., and Giles, D., J. Colloid lnterface Sci. 81, 150 (1981). the h y d r o c a r b o n - f l u o r o c a r b o n mixed surfac16. Ribeiro, A. A., and Dennis, E. A., Colloid Interface tant systems, do two kinds o f micelles exist in Sci. 2, 325 (1976). aqueous solutions (32). However, this study 17. Harusawa, F., and Tanaka, M., J. Phys. Chem. 85, suggests that the mixed micelle does not always 882 (1981). 18. Hidaka, H., Yoshizawa, S., Takai, M., and Mofiyama, form in the h y d r o c a r b o n - h y d r o c a r b o n mixed M., Yukagaku 31, 489 (1982). surfactant solutions. This m a y be attributed 19. Zimmerer, R. O., Jr,, and Lindenbaum, S., £ Pharm. to the degree o f interaction between the hySci. 68, 581 (1971). drophobic group in anionic surfactant and that 20. Robson, R. J., and Dennis, E. A., Biochim. Biophys. in nonionic one. The above suggestion is ilActa 573, 489 (1979). lustrated by the micellization models for 21. Yoshida, R., and Shishido, T., Yukagaku 25, 546 (1976). ECL-C,~POE mixed surfactant system shown 22. Ueno, M., Shioya, K., Nakamura, T., and Meguro, in Fig. 9. K., Colloid Interface Sci. 2, 411 (1976). 23. Shinoda, K., and Nomura, T., J. Phys. Chem. 84, 365 ACKNOWLEDGMENT (1980). 24. Yang, A. Y. S., J. Phys. Chem. 80, 1388 (1976). This research was partially supported by a Grant-in-Aid 25. Mysels, K. J., J. ColloidlnterfaceSci. 66, 331 (1978). from Saneyoshi Scholarship Foundation. 26. Funasaki, N., and Hada, S., Chem. Lett. 717 (1979). 27. Lake, M., 3. Colloid Interface Sci. 91, 496 (1983). REFERENCES 28. Abe, M., Tsubaki, N., and Ogino, K., Colloid Polym. 1. Lange, H., and Beck, K. H., Kolloid Z. Z. Polym. Sci. 262, 584 (1984). 251, 424 (1973). 29. Ogino, K., Abe, M., and Tsubaki, N., Yukagaku 31, 2. Moroi, Y., Nishikido, N., and Matsuura, R., J. Colloid 953 (1982). Interface Sci. 50, 344 (1977). 30. Funasaki, N., and Hada, S., .I. Phys. Chem. 83, 2471 3. Moroi, Y., Akisada, H., Sato, M., and Matuura, R., (1979). J. Colloid Interface Sci. 61, 233 (1977). 31. Nakagawa, T., and Inoue, H., Nippon Kagaku Zasshi 4. Mysels, K. J., and Otter, R. J., J. Colloid Interface 78, 104 (1957). Sci. 16, 474 (1961). 32. Tsujii, K., Yukagaku 31, 981 (1982).
Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
nonionic surfactant).
© 1985 Academic Press, Inc.
INTRODUCTION
It has been reported that the effect on surface activity of a mixed surfactant system is superior to that of a single-surfactant system (1-3). A number of solution properties have been measured for various mixed surfactant systems. For example, anionic-anionic (4-7), anionic-nonionic (8-13), anionic-cationic ( 14-15), nonionic-nonionic ( 16, 17), amphoteric-anionic (18, 19), amphoteric-nonionic (20), and amphoteric-amphoteric (21) mixed surfactant systems have been studied. In recent years, the solution properties in fluorocarbon-hydrocarbon mixed surfactant systems have been reported (22-24). It is well known that these mixed surfactant systems show a specific behavior as compared with the hydrocarbon-hydrocarbon mixed surfactant systems and that two kinds of micelles coexist in aqueous solution (25-27).
In the previous paper (28), we have reported the electric properties of the mixed surfactant system: 3,6,9-trioxaicosanoate (ECL, an anionic surfactant)-alkyl polyoxyethylene ethers (CmPOE, a nonionic surfactant; rn = 12, 14, 16, and 18). We found that it could form two kinds of micelles as the alkyl chain length in CmPOE decreased. In the present paper, we report the surface tension of ECL-CmPOE mixed surfactant systems in aqueous solutions. We discuss the differences in the mixed micelle forming due to the different alkyl chain lengths in nonionic surfactants. EXPERIMENTAL
Materials Anionic surfactant. Sodium 3,6,9-trioxaicosanoate [ECL, CI1H2aO(C2HgO)2CH2COONa] was supplied by Nihon Surfactant
503 0021-9797/85 $3.00 Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
Copyright © 1985 by Academic Press, Inc. All fights of reproduction in any form reserved.
504
ABE, TSUBAKI, AND OGINO
Industries Company, Ltd, Tokyo, Japan. The purity was ascertained by thin-layer chromatography (ether carboxylate; >98%), and gasliquid chromatography (methyl ester; >98%). Nonionic surfactant. Alkyl polyoxyethylene ethers [CmPOE, CmHzm+IO(C2H40)aoH; m = 12, 14, 16, and 18] were supplied by Nihon. It has a narrow molecular weight distribution. Water used in this experiment was twice distilled and deionized by using an ion-exchange instrument (NANO pure D-1791 of Barnstead Co., Ltd.); its resistivity was about 18.0 Mohm. cm and its pH was 6.7.
Methods Preparation of mixed surfactant solutions. Into several 100-ml beakers, portions of a given concentration of an ECL solution (5.0 X 10-3 mole/liter) were placed, followed by addition of a given concentration of CmPOE solution (5.0 X 10-3 mole/liter). The mixtures were diluted stepwise with water. These mixtures were stirred for 30 min in a thermostat at 30.0°C in order to establish their equilibria. Determination of the surface tension value. The surface tension of aqueous solutions of single and/or mixed surfactant systems were measured at 30 + 0.1 °C using a Wilhelmytype surface tensiometer (A-3, Kyowa Scientific Co., Ltd, Tokyo, Japan) with a platinum plate.
k
i
i
i
Mixed system : ECL-CI2POE • : ECL-CI4POE I) : ECL-ClfPOE 42
38
°'-.o e.~ ~ 34
Total
concentrot ion 5.0 x 10 -3 mo]/l
26
I
I
0.2 0.14 0,6 018 Mole froction of ECL
1.0
FIG. 1. The surface tension against the mole fraction of ECL in mixed surfactant solutions above CMC.
kyl chain length in nonionic surfactant. In the case of the ECL-C18POE system, the surface tension value decreases with increasing XECL up to 0.3, remains almost constant until XECL = 0.7, then decreases again.
The Change of Surface Tension with the Total Concentration under a Constant Mole Ratio
The surface tension of ECL-CmPOE mixed surfactant solutions in various mole fractions of ECL are plotted against the total concentration of surfactants in Figs. 2-5: Fig. 2 is the ECL-C12POE system, Fig. 3 the ECL-C14POE system, Fig. 4 the ECL-CI6POE system, and RESULTS Fig. 5 the ECL-C18POE system. The Change of Surface Tension with Mole In the case of the ECL-alone and the Fraction under a Constant Total CmPOE-alone systems, each surface tension Concentration value decreases with increasing the concentraThe surface tensions of ECL-CmPOE mixed tion, but remains constant above the critical surfactant solutions at 5.0 × 10-3 mole/liter micelle concentration (CMC). On the other (above CMC) are plotted against the mole hand, in the case of the ECL-CI/POE system fraction of ECL in Fig. 1. (Fig. 2), the surface tension values of aqueous In the case of the ECL-C~2POE system, the solutions in various mole fractions decrease surface tension value decreases monotonously with increasing total concentration too, but with increasing mole fraction of ECL (XEcL). the line breaks at two points, in the vicinity However, these surface tension vs mole frac- of the CMC for CIzPOE alone and of that for tion curves are shifted from a monotonous ECL alone. The intervals between the two curve to specific curves with increasing the al- breakpoints decrease with increasing alkyl Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
SOLUTION PROPERTY OF MIXED SURFACTANT SYSTEM, V -~O
,
40
~ Mole fractl
-~
o_r_Ec&
The Composition of Mixed Micelle
]
•
o~_
®
®-
o
®
.~
"-.No
The micellar mole fraction of ECL(XmEcL) in a mixed surfactant solution can be expressed by the following equation, suggested by Funasaki and Hada (30): XmECL = ( C t ° XEC L --
@
@
505
CMC"
XbECL)/
( G - CMC).
@-
[11
Here Ct is the total concentration, XECL the overall mole fraction of ECL, and XbECLthe 10-t4 10-3 10-2 Concentration (n~l/l) monomeric mole fraction of ECL. Figure 6 shows the surface tension (open FIG. 2. The surface tension against the total concentracircles) and cmc (solid circles) against XbECL tion in mixedsurfactantsolutionsof ECL-CIzPOEsystem. curve as the calibration curve. We obtained XbECL and XmECL of the ECL-C18POE mixed chain length in nonionic surfactant (Fig. 3- surfactant solution for various XECLby using Fig. 5). Finally, in the case of the ECL-C18POE Eq. [1] and Fig. 6. These values are shown in system (Fig. 5), the surface tension value de- Table I. The mixed micelle whose composition creases with increasing total concentration, is the same as the mixed solution is formed in the ECL-CIsPOE system. and breaks at only one point. 30
•
I
•
I
@-1
Existence of Two Kinds of Micelles
DISCUSSION
Formation of Mixed Micelle As seen in Fig. 1, the surface tension vs mole fraction curves are shifted from a monotonous curve to specific curves shaped like the letter "S," with increasing alkyl chain length in nonionic surfactant. In the previous paper (29) we measured the surface tension in the ECL-hexadecyl polyoxyethylene ether (CI6POE10, a nonionic surfactant in which the number of oxyethylene groups is 10) system. We found that the surface tension vs mole fraction curve has a specific S shape. This is attributed to the fact that the ECL molecules have migrated from the bulk phase to the micellar phase and the CI6POE10 molecules have migrated from the micellar phase to the bulk phase in forming the mixed micelle. Therefore, the mixed micelle is formed more easily with increasing alkyl chain length in a nonionic surfactant. Thus, we have attempted to determine the composition of the mixed micelle which is formed in the ECL-CI8POE system by the surface tension method.
In general, probable surfactant chemical species would be three types in ECL-CmPOE mixed system (namely, anionic ECL, protonated ECL, and nonionic CmPOE). In a previous paper, we found that the degree of ionic dissociation of the mixed micelle is almost 1 for the ECL mixed with CmPOE (hexadecyl polyoxyethylene ether) at a molar ratio of
45
p
O ~ @ \~
t
t
\ ~ \
i
ECL - C ,,POE
'o\ \ \ ~ .
,.
} 40 of E6L 35 I)
~=
®
:0,2 : o,5
@
:0.8 : 1.0
@
~ --®--@--@-
30
@--@--@l
l
l
l
l
10-6
10-5
10-4
10-3
10-2
Concentration (¢ol/1) FIG. 3. The surface tension against the total concentration in m i x e d surfactant solutions of E C L - C , 4 P O E system. Journal of ColloM and Interface Science, Vol. 107, No. 2, October 1985
506
ABE, TSUBAKI, AND OGINO \,\
,
~,
\,
,
\
40 I
,
o u
~
t
'
t
O
ECL- CI6POE
q
--0-- O~--o--O--
H01e fractlon 3s
--~f~
30
• : 0,2 ®:0,5 O:O.8 • : 1,0
~= 35
~k
- ~L®-®--~-~-
•
30 f
I
I
I
I0-5
10-4
10-3
i0-2
I
0
PIG. 4. The surfacetension against the total concentration in mixed surfactant solutions of ECL-CI6POE. CmPOE/ECL = 1/1. Therefore, under this condition, the protonated ECL molecules are absent in the mixed micellar solutions. Therefore, we will describe the case of molar ratio of CmPOE/ECL = 1/1 in the following discussion. As can be seen from Fig. 2, in the case of the ECL-C12POE system, the surface tension vs concentration curve breaks at two points in the vicinity of the CMC for C~2POE alone and that for ECL alone. Now, we shall assume that there is no interaction between ECL and C12POE in mixed surfactant solutions. In this case, the concentration in bulk phase and micellar phase will depend on the mole ratio for mixing. Figure 7a shows the relation between the concentration in bulk phase for ECL and 45
•
I \?\?,
¶
~-c1~PO~
\
0.2
I
I
I
0.4
0.6
0.8
.0
Mole fraction of ECL In bulk phase
Content rotlon (dynlcm)
a, 35
0 -5.0
•--•--•--
.I.
4o
0
"~--~--~
10-6
g
0
\
FIG. 6. Relation between the surface tension and log CMC as a function of the concentrationin bulk phase. C12POE and the total concentration in mixed solution at a given mole ratio CI2POE/ECL = 1/1. Here, points A and B are the CMC of aqueous solution for C~2POE alone and for ECL alone, respectively. The concentration in bulk phase for ECL and C12POE increase in the same proportion with increasing total concentration up to point A. Then, in the case of C12POE, the concentration in bulk phase remained constant above point A by forming the micelle. On the other hand, in the case of ECL, this concentration increases with increasing total concentration up to point B. The surface tension is closely related to the concentration of bulk phase. The additivity of surface tension values will be applied since interaction between ECL and CI2POE is entirely absent. Thus, the surface tension against the total concentration curve will be broken at the two points of CMC for ECL alone and that for C12POE alone, as shown by the dashed line in Fig. 7b. This surface tension value (3'~d) is calculated by using the equation
\o--8~-~@--8~--o--o--o--o--oMole fraction o : o
Xtl)~ID~ID
iD
®~-®~®--®
ffcal = X " ~/ECL -~" ( 1 ®--~-
- X)"
"~CmPOE-
[2]
Here ~/ECLand "~CmPOEare the surface tension values at the given concentration and X is the mole fraction of ECL for mixing (in this case • --•--I-30 we use X= 0.5). I I i ; J I0 -6 10.5 10-4 I0-3 10.2 The solid line in Fig. 7b is the observed Content rot fan (m01/1) value of aqueous solution of ECL mixed with C12POE for mole ratio Cx2POE/ECL = 1/1. PIG. 5. The surfacetension against the total concentration in mixed surfactant solutions of ECL-C,sPOE. As can be seen from Fig. 7b, the good agree-
Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
507
SOLUTION PROPERTY OF MIXED SURFACTANT SYSTEM, V TABLE I The Mole Fraction of ECL in Bulk Phase and Micelle Phase of ECL - C~8POE20 Mixed Surfactant Solutions at 30.0°C Phase
Mole fraction of ECL
Solution
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Bulk
0
0.23
0.41
0.50
0.50
0.50
0.51
0.56
0.70
0.84
1.00
Micelle
0
0.10
0.19
0.29
0.39
0.50
0.60
0.71
0.81
0.91
1.00
Note. Total concentration: 5.0 X 10-3 mole/liter.
ment of observed values (solid line) and calculated values (dashed line) indicates that there are two kinds of micelles coexisting (one rich in ECL and the other rich in C12POE). Next, we discuss the ECL-C18POE system in a similar manner to what we did for the ECL-C~2POE system. Figure 8 shows (a) the concentration in bulk phase and (b) the surface tension, against the total concentration in mixed surfactant solution. As seen in Fig. 8b, the observed value (solid line) is different from the calculated value (dashed line). This is at-
I -~
i0-3
tributed to the fact that the mixed micelle is formed by mixing ECL with C~8POE, as mentioned above. Thus, the mixed micelle is formed more easily by a nonionic surfactant including long alkyl chain length than by one having shorter alkyl chain length. Furthermore, in the latter case, there are two kinds ofmicelles which coexist. In the case of hydrocarbon-hydrocarbon mixed surfactant systems, a mixed micelle
I
-. i0-3
I
(0) ~ - -
I
I
v
1o-4
ECL
(a) E C L ~
c
._~
I I S 45
pA
L~B
iI calculated , \i orve !
u
= 3
10-6
"~
50
'x
'-.f
45
! (D 4-,
40
o3
2
C18POE
10-5
IO -4
35
35
(b) I
i0-4
I
i(?-3
Concentrot]on in solution (rml/l) FIG. 7. Relation between the surface tension and the concentration in bulk of ECL-CI2POE system. (a) The concentration in bulk and (b) the surface tension against the concentration in solution.
J B
'\
"%i
i
colculoted
! I
i,
curve x "\'J_' .... (b) I I I 0-6 10-5 10-4 10-3 i0-2 Concentration]n solution(mol/])
FIG. 8. Relation between the surface tension and the concentration in bulk of ECL-C18POE system. (a) The concentration in bulk and (b) the surface tension against the concentration in solution. Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985
508
ABE, TSUBAKI, AND OGINO
5. Shinoda, K., J. Phys. Chem. 58, 1136 (1954). 6. Fitch, R. M., and McCarwill, W. T., J. Colloidlnterface Sci. 66, 20 (1978). 7. Van den Bogaert, R., and Joos, P., J. Phys. Chem. E~?~+t~_~T~ ~ mixedmlcelle 84, 190 (1980). , , ._../; \._.../l 8. Nishikido, N., J. Colloidlnterface Sci. 60, 242 (1977). 9. Tokiwa, F., and Tsujii, K., 3. Phys. Chem. 75, 3560 (1971). ',,.,._....,] ",,,......... ."' 10. Meguro, K., Akasu, H., Ueno, M., and Satake, T., Colloid Interface Sci. 2, 421 (1976). ECL CmPOE 11. Schwuger, M. J., and Smolka, H. G., ColloidPolym. m: alkyl chain rich micelle rich micel]e Sci. 255, 589 (1977). FIG. 9. Micellization models for ECL-CmPOE mixed 12. Ogino, K., Tsubaki, N., and Abe, M., J. Colloid Interface Sci. 98, 78 (1984). surfactant system. 13. Abe, M., Tsubaki, N., and Ogino, K., Yukagaku 32, 672 (1983). which has an arbitrary composition is formed 14. Hendfikx, Y., J. Colloid Interface Sci. 69, 493 (1979). (31). O n the other hand, only in the case o f 15. Lucassen-Reynders, E. H., Lucassen, J., and Giles, D., J. Colloid lnterface Sci. 81, 150 (1981). the h y d r o c a r b o n - f l u o r o c a r b o n mixed surfac16. Ribeiro, A. A., and Dennis, E. A., Colloid Interface tant systems, do two kinds o f micelles exist in Sci. 2, 325 (1976). aqueous solutions (32). However, this study 17. Harusawa, F., and Tanaka, M., J. Phys. Chem. 85, suggests that the mixed micelle does not always 882 (1981). 18. Hidaka, H., Yoshizawa, S., Takai, M., and Mofiyama, form in the h y d r o c a r b o n - h y d r o c a r b o n mixed M., Yukagaku 31, 489 (1982). surfactant solutions. This m a y be attributed 19. Zimmerer, R. O., Jr,, and Lindenbaum, S., £ Pharm. to the degree o f interaction between the hySci. 68, 581 (1971). drophobic group in anionic surfactant and that 20. Robson, R. J., and Dennis, E. A., Biochim. Biophys. in nonionic one. The above suggestion is ilActa 573, 489 (1979). lustrated by the micellization models for 21. Yoshida, R., and Shishido, T., Yukagaku 25, 546 (1976). ECL-C,~POE mixed surfactant system shown 22. Ueno, M., Shioya, K., Nakamura, T., and Meguro, in Fig. 9. K., Colloid Interface Sci. 2, 411 (1976). 23. Shinoda, K., and Nomura, T., J. Phys. Chem. 84, 365 ACKNOWLEDGMENT (1980). 24. Yang, A. Y. S., J. Phys. Chem. 80, 1388 (1976). This research was partially supported by a Grant-in-Aid 25. Mysels, K. J., J. ColloidlnterfaceSci. 66, 331 (1978). from Saneyoshi Scholarship Foundation. 26. Funasaki, N., and Hada, S., Chem. Lett. 717 (1979). 27. Lake, M., 3. Colloid Interface Sci. 91, 496 (1983). REFERENCES 28. Abe, M., Tsubaki, N., and Ogino, K., Colloid Polym. 1. Lange, H., and Beck, K. H., Kolloid Z. Z. Polym. Sci. 262, 584 (1984). 251, 424 (1973). 29. Ogino, K., Abe, M., and Tsubaki, N., Yukagaku 31, 2. Moroi, Y., Nishikido, N., and Matsuura, R., J. Colloid 953 (1982). Interface Sci. 50, 344 (1977). 30. Funasaki, N., and Hada, S., .I. Phys. Chem. 83, 2471 3. Moroi, Y., Akisada, H., Sato, M., and Matuura, R., (1979). J. Colloid Interface Sci. 61, 233 (1977). 31. Nakagawa, T., and Inoue, H., Nippon Kagaku Zasshi 4. Mysels, K. J., and Otter, R. J., J. Colloid Interface 78, 104 (1957). Sci. 16, 474 (1961). 32. Tsujii, K., Yukagaku 31, 981 (1982).
Journal of Colloid and Interface Science, Vol. 107, No. 2, October 1985