Physicochemical Properties of Mixed Surfactant ... - ACS Publications

Chapter 8

Physicochemical Properties of Mixed Surfactant Systems

Downloaded by UNIV MASSACHUSETTS AMHERST on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch008

Keizo Ogino and Masahiko Abe Faculty of Science and Technology, Science University of Tokyo, 2641, Yamazaki, Noda, Chiba, 278, Japan

Within the past several years, a great number of solution properties for mixed surfactant systems have been published. Our research group has also investigated the solution properties of various mixed surfactant systems, based on surface tension and electrical measurements over the past fifteen years. This paper will discuss recent developments of mixed surfactant systems, especially, of anionic-nonionic surfactant systems. Included in the set of examples will be (1) differences in properties of mixed micelles forming with different alkyl chain lengths and / or polyoxyethylene chain lengths in nonionic surfactants, and (2) protonation and fading phenomena of azo oil dyes occurring in aqueous solutions containing mixtures of surfactants.

Recently, it has been reported that the surface activity of mixed surfactant systems is superior to that of single surfactant systems (1-3). In fact, surfactants used in practical applications almost always consist of a mixture of surface-active compounds. Mixed surfactant systems are also of great theoretical interest. It can be assumed that the tendency to form aggregated structures in solutions containing mixtures of surfactants is substantially different from that in solutions involving only the pure surfactants. Therefore, many publications have reported solution properties of mixed surfactant systems, including those cited in references (4-7) and papersfromour laboratory (813). Micelle Formation of Anionic-Nonionic Mixed Surfactant Systems E C L - C P O E systems Surface Tension of Aqueous Solution of Binary Surfactants Systems. We have obtained surface tension data for aqueous mixed surfactant system: sodium 3,6,9-trioxaicosanoate (ECL, Ethercarboxylate, an anionic surfactant which has both nonionic and anionic properties) - alkyl polyoxyethylene ethers (C POEk, a nonionic m

n

m

0097-6156/92/0501-0142$06.75/0 © 1992 American Chemical Society

In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

8. OGINO & ABE


surfactant: m=12, 14, 16 and 18, n=10, 20, 30 and 40) (i2,13). Figure 1 shows the dependence of surface tension on the mole fraction of ECL in mixed surfactant solutions above the CMC. In the case of the E C L - C ^ P O E ^ system, the surface tension decreases monotonically with increasing mole fraction of ECL ( X ) . However, these surface tension vs. mole fraction curves are shiftedfroma monotonic curve to curves with an inflection point with increasing alkyl chain length of the nonionic surfactant. In the case of the ECL-CjgPOE^ system, the surface tension decreases with increasing X up to 0.3, remains almost constant until X =0.7, then decreases again. Figure 2 shows the surface tension of E C L - C ^ P O E ^ mixed surfactant solutions containing various molefractionsof ECL, plotted against the total concentration of surfactant. In the case of the ECL-alone and the C POE2 -alone systems, each surface tension value decreases with increasing concentration, but remains constant above the CMC. On the other hand, in the case of the E C L - C ^ P O E ^ system (Fig. 2), the surface tension values of aqueous solutions at various molefractionsalso decrease with increasing total concentration. However, the line breaks at two points, in the vicinity of the CMC for C ^ P O E ^ alone, and of that for ECL alone. The intervals between the two breakpoints decrease with increasing alkyl chain length in nonionic surfactant. Finally, in the case of the ECL-CjgPOE^ system the surface tension decreases with increasing total concentration, and breaks at only one point, as shown in Fig. 3. Figure 4 shows the relation between surface tension and the concentration in bulk for the ECL-C POE2 y - The concentration in bulk (a) and the surface tension (b) are plotted against the concentration in solution. Here, points A and B represent the CMC values of aqueous solutions for C P O E 2 alone and for ECL alone, respectively. The concentration in bulk phase for ECL and C P O E 2 increases in direct proportion with increasing total concentration up to point A. Then, in the case of C POE2 , the concentration in the bulk phase remains constant at 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 the bulk phase. The additivity of surface tension values occurs because interactions between ECL and C^POE^Q are entirely absent. Thus, a plot of surface tension against total concentration is broken at the two points of CMC for ECL alone and the C ^ P O E ^ alone, as shown by the dashed line in Fig. 4 (b). The solid line in Fig. 4 (b) is the observed value. As can be seenfromFig. 4 (b), there is good agreement between the observed values (solid line) and calculated values (dashed line). This indicates that there are two kinds of micelles are coexisting (one rich-in ECL and the other rich in E C L


E C L

ECL

12

s

12

0

s t e m

0

12

0

12

12

0

0

C POE2 ). 12

0

Results for the ECL-CjgPOE^ system can be treated in a similar way. Figure 5 shows (a) the concentration in bulk phase and (b) the surface tension, plotted against the total concentration in the mixed surfactant solution. As seen in Fig. 5 (b), the observed value (solid line) is different from the calculated value (dashed line). This is attributed to the fact that a mixed micelle is formed in mixtures of ECL and CjgPOE^. Thus, the mixed micelle is formed more easily by a nonionic surfactant including a long alkyl chain than by one having shorter alkyl chains. Similar results were obtained on the effect of oxyethylene chain length in nonionic


143

144

MIXED SURFACTANT SYSTEMS

50

1

1

Mixed system d: E C L - P O E J O ECL-POEJQ €): E C L - P O E J Q O: ECL-POE40

46 9


I •I

42 38 ^ C

34 OÔ-O-O-O-OÔLÎ

30

Total wnrentratton 3

5.0 * 10" mol/l 26

JL 0.2 0.4 0.6 0.8 Mole fraction of E C L

1.0

Figure 1. Relation between the surface tension and the mole fraction of ECL in mixed surfactant solutions above CMC. (Reproduced with permission from ref. 13. Copyright 1985 Academic Press.)

J

L 4

1©-

3

10-

2

10-

Concentration (moffl) Figure 2. Relation between the surface tension and the total concentration in mixed surfactant solutions of ECL/C12POE system. (Reproduced with permission from ref. 12. Copyright 1985 Academic Press.) In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


8. OGINO & ABE


I

Li

io

6

I

15 O

E

o 4 Mixed molar ratio; SDS/C, POE o=l/l 6

_j i i 1111 10-3

•ml

]

I I I III

150

2

I

|

L_

10-2 100

Concentration of surfactants (mol/l)

Figure 17. Dissolved oxygen (DO) value and fluorescence lifetime of pyrene monomer against total concentration of surfactants in SDS/CjgPOEn mixed systems at 30 °C.

2

pyrene fluorescence lifetime is increased at surfactant concentrations above 1.0 * 10' mol/L. The fact that the amount of oxygen in the mixed micelle is decreased by increasing the total concentration of surfactants above 1.0 * 10" mol/L shows that the fading phenomenon of 4-OH is not related to the amount of oxygen in the mixed micelle. The dissolved oxygen concentration in the mixed surfactant solution and/or in the mixed micelle is almost constant and independent of the alkyl chain lengths and poly(oxyethylene) chain length. Thus, although the fading rate is dependent on the alkyl and poly(oxyethylene) chain length, the concentration of dissolved oxygen in the micelle is independent of either chain length in the nonionic surfactant. We have studied the possible effects of formation of each active oxygen species on the fading phenomena in the mixed micelle. First of all, the possibilities of hydrogen peroxide and hydroxyl radical forming in the mixed micelle were studied. The hydroxyl radical is the most reactive of the active oxygen species. In general, irradiation with light is required to form the hydroxyl radicalfromhydrogen peroxide or water molecules. Figure 18 shows that the fading rate of 4-OH in the dark is as fast as that under the natural light. The fading phenomenon of 4-OH is therefore independent of the formation of hydroxyl radicals in the mixed surfactant solution. Secondly, the possibility of superoxide formation in the mixed surfactant solution was investigated. Superoxide dismutase (SOD) is a typical quencher for superoxides. Figure 19 shows that the fading behavior of 4-OH in SDS-C POE mixed surfactant systems is the same as that without SOD and it is independent of SOD concentration. It can be concluded that the superoxide does not form in the mixed micelles of SDS/C^POPv 2

16

10


159


160


10

20 Time (h)

Figure 18. Time dependence of absorbance at 480 nm in SDS/CigPOEjo mixed surfactant solutions at 30 °C.

Superoxide Disimitase

0« 0

—. 10 20 Time (h)

30

Figure 19. Effect of Superoxide Dismutase (SOD) on the fading rate of 4phenylazo-1 -naphthol in SDS/C15POE10 mixed surfactant solutions at 30 °C.


8. OGINO & ABE



T

0 I 0

r

i

,

i

10

20

30

I

T i m e (Ii)

Figure 20. Effect of l,4-diazobicyclo[2,2,2]octane ( D A B C O ) on the fading rate of 4-phenylazo-1 -naphthol in S D S / C ^ P O E J Q mixed surfactant solutions at 30 ° C .

Next, the possibility of a fading reaction attributable to singlet oxygen was investigated. DABCO is a well-known quencher for singlet oxygen. Figure 20 depicts the time dependence of absorbance at 480 nm in SDS/C^POE^ containing DABCO. The fading rate of 4-OH is decreased with increasing die concentration of DABCO solubilized in the mixed surfactant solution. Therefore, the fading phenomenon of 4OH may be caused by the singlet oxygen. The following experiment was performed in order to confirm these results. The lifetime of the singlet oxygen in H 0 is approximately 2-4.2 ps (23). However, in D 0 (24), the lifetime of the singlet oxygen is about 55-68 ps, 10 times longer than in H 0 . The effect of replacement of H 0 by deuterium oxide on the fading rate of 4OH was studied. As can be seenfromthe Fig. 21, the decrease in the optical densities for the solutions containing D 0 is faster than that in solutions without D 0 . The fading rate of 4-OH is increased as the mixed molar ratio of D 0 increased. This may reflect the fact that the lifetime of the singlet oxygen is lengthened in the D 0 surfactant solution. Additionally, the effect of the singlet oxygen sensitizer on the fading behavior of 4OH has been investigated in the mixed surfactant solution. The singlet oxygen sensitizers used in this study were TPP and TPPS, which are water insoluble and soluble, respectively. As is shown in both Fig. 22 and 23, the fading rate of 4-OH is accelerated on addition of the singlet oxygen sensitizer and increases with an increase in the concentration of sensitizer. Consequently, even though the amount of dissolved oxygen in the micelle is almost the same in each surfactant solution, the activated oxygen species, singlet oxygen, forms in the mixed micelle, causing the fading phenomenon of 4-OH. 2

2

2

2

2

2

2

2


161

162


•

;H o 2

O ; H20/D20=5/5


O ; H20/D20=l/9

10

20 Time (h)

30

40

Figure 21. Effect of Deuteriumoxide (D2O) on the fading rate of 4-phenylazo-lnaphthol in SDS/CigPOEio mixed surfactant solutions at 30 °C.


8. OGINO & ABE

Physicochemical Properties of Mixed Surfactant Systems 163

TPPS (mol/l) o;0

Physicochemical Properties of Mixed Surfactant ... - ACS Publications

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