Mixed Micelles with Bolaform Surfactants - ACS Symposium Series

Chapter 19

Mixed Micelles with Bolaform Surfactants R. Zana

Downloaded by STANFORD UNIV GREEN LIBR on August 8, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch019

Institut Charles Sadron (CRM-EAHP), Centre National de la Recherche Scientifique-ULP, 6, rue Boussingault, 67000 Strasbourg, France

This paper reviews studies dealing with the formation of mixed micelles in solutions of three types of mixtures of bolaform surfactants and conventional surfactants : (i) of like electrical charges, (ii) of opposite charges and (iii) where the bolaform surfactant is the counterion of the conventional surfactant. The conditions for mixed micelli zation and the changes of the CMC and micelle aggregation number or molecular weight with the relevant parameters of the mixtures are dis cussed in relation with the results for mixtures of conventional surfac tants. Bolaform surfactants (also named bolaamphiphiles, bolaphiles or α,ω-type surfac tants) refer to surfactants where the alkyl chain (or a more complex hydrophobic moiety) is terminated at both ends by a polar (ionic, zwitterionic or nonionic) group (1). Bolaform surfactants have a number of properties which distinguish diem from conventional surfactants (I). Complex bolaform surfactants are present in the lipids constituting the membrane of archaebacteria (1) . The micellization of bolaform surfactants has been much investigated recently as they represent a new class of surfactants on which can be testai current theories of micellization (2-8). A controversy exists as to whether, in bolaform surfactant micelles, the alkanediyl chains are stretched or folded, the folded (wicket-like) conformation being that adopted at the air-water interface (2,7). Recently the first studies of mixed micelli zation in solutions of mixtures of bolaform and conventional surfactants have been reported. Constraints in the packing of die alkyl chains of the bolaform and conventional surfactants are expected to play an important role in determining whether mixed micelles can form in these mixtures. This paper provides a review of some of the results reported thus far and considers three types of mixtures of bolaform and conventional surfactants (i) having like electrical charges ; (ii) having opposite electrical charges and (iii) where the bolaform surfactant is the counterion of a conventional surfactant. These three types of systems are representative of most of those generated by mixing bolaform and conventional surfactants. Mixed micellar systems involving nonionic or zwitterionic bolaforms have not been described yet.

0097-6156/92/0501-0292$06.00/0 © 1992 American Chemical Society

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

19.

293

Mixed Micelles with Bolaform Surfactants

ZANA

Mixed Micellization in Mixtures of Bolaform and Conventional Surfac tants of Like Electrical Charge The occurrence of mixed micelles in mixtures of alkyltrimethylammonium bro mides, C TAB, and alkanediyl-a,a)-bis(trimethylammonium bromide), C (TAB) , has been systematically investigated as a function of the chain lengths of the conventional and bolaform surfactants, characterized by the carbon numbers m and η of the alkyl and alkanediyl groups, respectively (9). Surfactants with identical head groups were selected in order to eliminate the effect of this parameter on the properties investigated (CMC, micelle aggregation number). Also the micellization of both C TAB and C (TAB) was well characterized (2,10,11). The electrical conductivity method was extensively used. The conductivity κ was measured as a function of the conventional surfactant concentration, C, at constant mole fraction, X, of bolaform surfactant. Comparative measurements where the bolaform was replaced by KBr were also performed to evidence mixed micellization. In mixtures where no mixed micelles formed, the plot of the micelle apparent degree of ioniza tion, 0.5

12

n

C (TAB)

2

C TAB 10

2

No

a

2

2

Yes

C (TAB) 22

2

No

No

Yes at X > 0.25

No No

Yes Yes at X > 0.5

Adapted from ref.9, valid at the CMC of the mixtures

Other conductivity measurements where the bolaform was added to a solution of conventional surfactant showed that mixed micelles formed at higher total sur factant concentration, even when mixed micellization did not occur at the CMC (9). In this respect mixtures of bolaform and conventional surfactants behave simi larly to mixtures of two conventional surfactants.


294

MIXED SURFACTANT SYSTEMS

The micelle aggregation numbers Ν in C T A B - C ( T A B ) mixtures were mea sured by time-resolved fluorescence quenching in a restricted range of bolaform mole fraction and compared to the Ν values measured when KBr substituted the bolaform (9). Figure 2 shows the variations of Ν upon addition of KBr or C ( T A B ) to a 0.1 M C14TAB solution as a function of die concentration C , or mole fraction X of the additive. It is seen that additions of C i ( T A B ) increase Ν just like additions of KBr. This clearly indicates that mixed micelles do not form in C H T A B - C I ( T A B ) mixtures even at concentrations much larger than the CMC of the mixture, at X < 0.4. The same is true for the C i T A B - C ( T A B ) mixtures investigated. However mixed micelles are present in the C i 4 T A B - C ( T A B ) mix tures (9). The Ν values in Figure 2 represent the numbers of C14TAB per micelle. The number of bolaform surfactants per micelle is given by N C B / C . More recendy mixed micellization was investigated in mixtures of alkyltrimethylammonium bromides, C TAB, and chlorhexidine digluconate, CG (12). The latter is a dicationic surfactant with antibacterial properties which forms only small aggregates in water ( N 4) at above an operational CMC of 0.0344x10 M (12). The CMC's of CG mixtures with Ci TAB, C T A B and Ci TAB were obtained from conductivity measurements. The variation of the CMC with the mixture composi tion was used to obtain the mixed micelle composition by the method of Mysels and Otter (13) and by the thermodynamic treatment of Motomura et al., (14). The two methods yielded results in good agreement, as seen in Figure 3. For the CGTAB mixtures, the composition of the micellar and monomelic phases was also determined directiy by gel filtration chromatography (12). The micelle composi tions obtained in this manner were in good agreement with those from the above cited methods at low mole fraction of CG, X G , but not at high X G . The authors reported a lack of stability over the time period required for the measurements 12). This may reflect the fact that the micelles were then of much smaller size (see below) and, thus, had a much shorter lifetime (15). Static light scattering was used to determine the mixed micelle aggregation numbers Ν at the CMC, based on the micellar compositions determined as indicated above. The Ν values at 25 °C are listed in Table II, as a function of X G . The sharp drop of Ν at X G > 0.45 is noteworthy. Thus for mixtures of C TAB with either CG or C (TAB) , two bola form surfactants of much differing chemical structure, the formation of mixed micelles results in a decrease of aggregation number, from that of C TAB micelles to that of the bolaform micelle, as the latter is generally smaller than the former (2,10,11,15). A similar observation has been reported for mixtures of conventional surfactants (16). M

N

2

N

2

B

2

2

2

2

4

16

2


2 2

2

m

2

H

6

C

C

C

C

m

n

2

m

Table Π. Aggregation Numbers of Mixed CG-Cu TAB Micelles XCG

0.00

0.21

0.45

0.71

1.00

Ν

64

51

42

9

4

Mixed Micellization in Mixtures of Conventional and Bolaform Surfactants of Opposite Electrical Charge Mixed micellar properties of the aqueous mixtures of sodium dodecylsulfate (SDS) and dodecanedyl-l,12-bis(triethylammonium bromide), Ci (TEB) , have been investigated by Ishikawa et al (17), at an ionic strength of 0.1 M NaBr, using sur face tension, viscosity , static and dynamic light scattering, Orange OT solubiliza2

2



19. ZANA

295


Figure 1.: Variation of the CMC and 8 (see Figure 5). Finally, the micelle aggregation number is a minimum for n= 8-10 (see Figure 6). These data together with those concerning the molecular area per sur factant at the air-water interface were interpreted on the basis of a change in the location of the alkanediyl chain upon binding to the tetradecanesulfonate micelles. Thus for η < 8 the alkanediyl-a^-bis(pyridinium) dications bind to the micelle surface through electrostatic interactions between pyridinium and sulfonate ions, with the alkanediyl chains remaining essentially at the micelle surface, exposed to water. For η > 8, the alkanediyl chains would fold and penetrate in the micelle core, thus contributing to the free energy change upon micelle formation and resulting in the observed decrease of CMC (20). Note that from the change of CMC with η the authors calculated a free energy of transfer from water to micelles of 1.2 kT per C H group (20). This value is very close to those found for other homologous surfactant series. Regarding the presence of a maximum in the log CMC vs η curve of the above surfactants it must be recalled that a similar observation was made in the study of a peculiar class of bolaform surfactants the alkanediyl-a,a)-bis(dodecyl or hexadecyl dimethylammonium bromide), C -a,c*>-(C N (CH ) Br ) (22). The CMC was also found to be a maximum at around η = 6 and the decrease of CMC at η > 6 was attributed to the folding of the alkanediyl chain and its penetration in the micellar core. +

n

2

4

2

+

n

m

3 2




298

X

X

Figure 4.: Variations of the CMC, mixed micelle molecular weight M , solution viscosity, and solubilization S of Orange OT in solution of SDSCi2(TEB) mixtures with the mole fraction of the bolaform in the mixture, X , and in the micelle (Reproduced with permission from ref.17. Copyright 1991 Academic Press Inc.). 2


ZANA


299


19.

20

10t

0

2

H

6

8

10

12

11

CARBON NUMBER OF ALKANEDIYL GROUP

Figure 6.: Variation of the aggregation number of C (Pyr) , 2C14SO3 micelles at the CMC with the alkanediyl carbon number η at 35 °C (Reproduced from ref. 21. Copyright 1990 American Chemical Society.) v2+

n

2


300



Conclusions This paper reviewed the most significance studies of mixed micellization between various conventional and bolaform surfactants. As pointed out the reported results are in many respects similar to those found for mixtures of two conventional surfactants and the thermodynamic treatments used for the latter also apply to bolaform-conventional surfactant mixtures. This similarity is also apparent in a recent study (23) of the interaction between bolaform surfactants and fairly hydrophobic polyelectrolytes (poly-L-lysine and poly-L-ornithine hydrobromides) which can be considered as an extreme case of mixed micellization. The bolaform surfactant binding induced conformational changes similar to those associated to the binding of conventional surfactants. Literature Cited 1. Fuhrhop,J.-H.; Fritsch D. Acc. Chem. Res. 1986, 19, 130 and references therein. 2. Zana, R.; Yiv, S.; Kale, K . M . J. Colloid Interface Sci. 1980, 77, 456 and references therein. 3. Ikeda, K.; Yasuda, M . ; Ishikawa, M . ; Esumi, K.; Meguro, K.; Binana-Limbelé, W.; Zana, R. Colloid Polym. Sci. 1989, 267, 825. 4. Ikeda, K.; Ishikawa, M . ; Yasuda, M . ; Esumi, K.; Meguro, K.; Binana-Limbelé, W.; Zana, R. Bull. Chem. Soc. Jap. 1989, 62, 1032. 5. Abid, S.K.; Hamid, S.M.; Sherrington, D.C. J. Colloid Interface Sci. 1987, 120, 245. 6. Nagarajan, R. Chem. Eng. Comm. 1987, 55, 251. 7. Wong, T.C.; Ikeda, K.; Meguro, K.; Söderman, O.; Olsson, U.; Lindman, B. J. Phys. Chem. 1989, 93, 4861. 8. Jayasuriya, N.; Bosak, S.; Regen, S.L. J. Am. Chem. Soc. 1990, 112, 5844. 9. Zana, R.; Muto, Y.; Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1988, 123, 502. 10. Zana, R. J. Colloid Interface Sci. 1980, 78, 330. 11. Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100. 12. Attwood, D.; Patel, H.K. J. Colloid Interface Sci. 1989, 129, 222. 13. Mysels, K.; Otter, R. J. Colloid Interface Sci. 1961, 16, 462. 14. Motomura, K.; Yamanaka, M . ; Aratono, M . Colloid Polym. Sci. 1984, 262, 948. 15. Lang, J.; Zana, R. in Surfactant Solutions : New Methods of Investigations, R. Zana, Ed.; M . Dekker, New-York, N.Y. 1987, pp 405-452. 16. Malliaris, Α.; Binana-Limbelé, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. 17. Ishikawa, M . ; Matsumura, K.; Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1991, 141, 10. 18. Bury, R.; Souhalia, E.; Treiner, C. J. Phys. Chem. 1991, 95, 3824. 19. Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098 and references therein. 20. Moroi, Y.; Matuura, R.; Kuwamura, T.; Inokuma, S. J. Colloid Interface Sci. 1986, 113, 225. 21. Moroi, Y.; Matuura, R.; Tanaka, M . ; Murata, Y.; Aikawa, Y.; Furutani, E.; Kuwamura, T.; Takahashi, H . ; Inokuma, S. J. Phys. Chem. 1990, 94, 842. 22. Zana, R.; Benrraou, M . ; Rueff, R. Langmuir 1991, 7, in press. 23. Hayakawa, K.; Fujita; M . ; Yokoi, S.-L; Satake, I. J. Bioactive Compatible Polym. 1991, 6, 36. RECEIVED January 6, 1992


Mixed Micelles with Bolaform Surfactants - ACS Symposium Series

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