J. Phys. Chem. 1989, 93, 8383-8384
Micelles of Bipolar Surfactants: A Case for the Surfactant-Block Model?
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Sir: In a recent paper, Wong et al. have studied the conformation of the hydrocarbon chain of the bipolar surfactant N,N'-1,20eicosanediylbis(triethy1ammonium bromide) in micellar aggregates.' They found the order parameter along the hydrocarbon to be almost invariant, with the chains being almost extended. The authors mention that it may be difficult to pack rather extended chains into a small spherical micelle of a radius of 13-14
1 4
A.
It is the purpose of the present Comment to call attention to a-seemingly antique-micelle model which is suitable to rationalize the results of Wong et al. quite naturally; the surfactant-block model as proposed in 198 1 .24 It is the basis of the surfactant-block model that some properties of micelles of unipolar surfactants are described surprisingly well by the model of an isometric bilayer of parallel aligned extended molecules, i.e., a fluid variant of McBain's c o n ~ e p t . ~It is the weakness of the bilayer model, as pointed out by Hartley? that it provides no basis for the finite size of micelles. This classical defect is cured in the surfactant-block model by rotation of blocks of parallel aligned molecules by right angles. This rotation, which releases headgroup repulsion, is permitted only if the bilayer is isometric, i.e., if its diameter equals its thickness. This is not the place to discuss the pros and cons of the surfactant-block model with respect to experimental data7o8 and computer simulations9on micelles of usual surfactants. I should like to point out here only how the model may be applied to micelles of bipolar surfactants. I avoid space-filling CPK models or eye-catching computer graphics. To emphasize the architecture of micellar assembly I use, stubbornly, homemade space-filling models of bipolar surfactants (Figure 1). Molecules are represented by rods. Their width as compared to the length is chosen such that their volume matches the volume of a chain in liquid hydrocarbon.I0 A rod does not represent a molecule in all-trans configuration. Even in a compact assembly there is plenty of space for local defects in the chains. An isometric monomolecular layer of eicosanediylbis(triethylammonium) is formed by 19 molecules (Figure 1). Rotation of blocks of parallel correlated molecules by right angles lowers the headgroup repulsion with slight increase of the hydrocarbon/water contact (Figure 1). A rotation of several blocks is allowed only along a single axis at any time. Simultaneous rotation along different axes would lead to an intersection of molecules or to interdigitation of molecules with enormous voids in the hydrophobic core. It may appear that rotation along a single axis limits the growth of an isometric monolayer only with respect to one dimension, such that rodlike micelles may still be formed. However, in an isometric monolayer a rotation of blocks may occur along various directions, in the case of an ideal hexagonal packing along three directions, not simultaneously in a single micelle at a single time, but sequentially in time or in the ensemble of micelles. The thermodynamic stability of the micelle is determined by all configurations attained by the dynamics of packing or by all possible configurations in an ensemble. The "structure" of the micelle is the ( I ) Wong, T. C.; Ikeda. K.; Meguro. K.; Sderman, 0.; Olsson, U.; Lindman, B. J. Phys. Chem. 1989. 93,4862. (2) Fromherz, P. Chem. Phys. .Leu. 1981, 77, 460. (3) Fromherz. P. Ber. Bunsenges. Phys. Chem. 19%1,85, 891. (4) Fromherz. P. I n Surfucrunrs in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1, p 321. (5) McBain, J. W. Colloid Science; Heath: Boston, 1950; p 255. (6) Hartley, G. S. Q. Rev. Chem. Soc. 1948, 2, 152. (7) Walderhaug, H.; S6derman. 0.; Stilbs, P. J. Phys. G e m . 1984, 88, 1655. (8) Cabane, B.; Duplessix, R.;Zemb, T. J. Phys. 1985.46, 2161. (9) Wendoloski, J. J.; Kimatian, S. J.; Schutt, C. E.; Salemme, Science 1989. 243, 636. (10) Tanford, C. J . Phys. Chem. 1972, 76, 3020.
Figure 1. Architecture of the surfactant-block model for a micelle of the bipolar surfactant eicosanediylbis(trimethy1ammonium): isometric monolayer of 19 monomers and one configuration with orthogonal assembly of blocks of parallel correlated molecules. The rods represent molecules in a fairly extended sh:.pe though not all-trans configuration. The black zones indicate the headgroups. The architecture does not symbolize a crystalline solid, but a liquid with parallel correlated molecules!
Figure 2. Sketch of structural modifications of an ideal orthogonal block-assembly of molecules in all-trans configuration: Local bends are introduced in the hydrocarbon, headgroups are bent to reduce their repulsion, and whole blocks are inclined.
"isometric ensemble" of all axial packings as obtained by rotation of blocks of bipolar molecules. It may be pointed out again24 that the "wood-paint-glue" representation of the surfactant-block model emphasizes the basic architecture of packing. The richness of configurations implied in "realityw2+is symbolized in Figure 2: Starting from molecules in all-trans configuration in an orthogonal assembly of blocks we may consider (i) local chain defects, (ii) nicked headgroups, and (iii) oblique block assembly. What are the testable implications of the model? (a) Size: The aggregation number is around 19 for eicosanediylbis(triethy1ammonium). The average radius of the micelle is around 13 A. (b) Structure: The hydrocarbon chains are parallel correlated at least over a considerable part of their central stem. (c) Conformation: The hydrocarbon chains are rather extended. The disorder due to C-C bond rotamerism is dominated by local defects homogeneously distributed along the chain. (d) Kinetics: The molecules may dissociate and associate fast and unhindered from surface sites. (e) Wetting: In a micelle of 19 molecules 12 of them are in contact to the aqueous phase all over their length, 4 are partially exposed, and only 3 of them are buried. What is the crucial experimental evidence? Size: Zana et al. have estimated an aggregation number of 14-20 for docosaneFor m ice1les of eicosa ned i y Id iy 1bis (t r i met h y 1am mon i u m ) . bis(triethy1ammonium) Wong et al. have estimated a radius of 13 A.' Conformation: A high and invariant order parameter of 0.3 all along the hydrocarbon chain is reported by Wong et al.,
F. R. ( 1 I ) Zana, R.;Yiv, S.; Kale, K. M. J. ColloidlnrerfuceSci. 191(0,77,456.
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J . Phys. Chem. 1989,93, 8384-8385
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indicating a fairly extended conformation.' Kinetics: An extremely fast rate constant of dissociation is reported by Zana et al." Zana et al.ll and Wong et a1.I consider two types of packing of bipolar surfactants in spherical micelles: (i) A "radial" packing of V-shaped molecules with a central sharp bend in the hydrocarbon chain. (ii) An entangled packing of extended chains. Zana et al. reject the second model: The fast dissociation constant is inconsistent with a lateral dissociation of entangled chains or a radial dissociation of extended chains with a transfer of headgroups across the hydrophobic core." Wong et al. reject the first model: The high order parameter, constant along the chain, is inconsistent with a sharp bend in the center of the chains.' According to the present state of the literature both models are falsified. Apparently the surfactant-block model is compatible with extended chains as well as with fast dissociation Further suggestive arguments in favor of the surfactant-block model are the following. Block rotation explains the small size of the aggregates.'J' Limitation of size by block rotation explains the limited swelling as indicated by low so1ubilization.l The rugged though compact structure explains the high hydrocarbon/water contact as indicated by the modest change of specific volume in micellization." Data revealing directly the nature of chain packing are not available. Computer simulations, without unsound constraints such as localizing the headgroups on a sphere, may be a valuable tool to clarify this point. Considering the data of Wong et al.' I feel that it is worth digging out the surfactant-block model, at least with respect to micelles of bipolar surfactants. The model is compatible with the data without undue force. It may suggest further experimental and theoretical work. Possibly block assembly is favored by the constraint imposed by two headgroups. As, however, there exists a striking similarity of physicochemical features of micelles of bipolar and of unipolar surfactants,12 it may be useful also to cast again a glance on the arguments in favor of block assembly of unipolar surfactants.24 (12) McKenzie, D. C.; Bunton, C. A,; Nicoli, D. F.;Savelli, G. J . Phys. Chem. 1987, 91, 5709.
Abteilung Biophysik der Universitat Ulm 0-7900 Ulm- Eselsberg, West Germany
Peter Fromherz
Received: July 14. 1989
Mlcelle Structure for Blpolar Surfactants. Reply to Comment by Fromherr Sir: One important motivation behind studies of novel surfactants is to help in answering fundamental questions of surfactant self-assembly. Thus a change in the chemical structure of a surfactant from a model should give predictable changes in phase diagram, micelle structure, etc. Therefore, studies of doublechained and branched-chain surfactants, studies of surfactants with different head-group sizes or charges or counterion valences or solvation become important and have been helpful in discussions of surfactant aggregate structure. Surfactant molecules are amphiphilic and built up of one long nonpolar chain and one (small or large) polar head. A rather drastic change in surfactant structure is to introduce polar groups at both ends. This can be expected to strongly change the conditions of self-assembly. The class of bipolar a,w-surfactants has been little studied, however. In a recent paper' we examined the conformational and molecular dynamic aspects of micellar aggregates of eicosane- 1,20-bis(triethylammonium bromide) and Fromherz in his comment2 correctly notes that such data offer a unique possibility of returning to (1) Wong, T. C.; Ikeda, K.;Meguro, K.;SWerman, 0.; Olsson, U.; Lindman, B. J . Phys. Chem. 1989. 93, 4861. (2) Fromherz, P. J . Phys. Chem., preceding paper in this issue.
0022-3654/89/2093-8384$01 S O / O
controversial points of micelle structure and a discussion of different models of micelle structure. One significant observation in ref 1 was that the order parameter profile is quite different for bipolar and unipolar ~urfactants.~This shows that the packing of the alkyl chains is quite different for micelles of the two types of surfactants. According to Fromherz, the order parameter profile we observe is as predicted for his surfactant-block (SB) model, which was originally deduced for unipolar surfactants. We agree. with Fromherz that the SB model is consistent with these findings and a possible candidate for the description of bipolar surfactant micelles. However, it remains to show what conventional micelle models, introducing the constraint of having both ends of the surfactant at the micelle surface, would predict for this case. It is our contention that they would predict these observations but this is a task of future research. Additionally, it follows from the major difference in packing for unipolar and bipolar surfactant micelles that the SB model would not well describe monopolar surfactant micelles. For these, the standard picture of ionic micelles, as analyzed in detail by Gruen," appears to offer an accurate description of experimental observations. With regard to the present discussion it should be mentioned that it accurately predicts correct order parameter profiles for unipolar surfactants. We have recently studied also other aspects of the self-assembly of N,N'- 1,20-eicosanediylbis(triethylammoniumbromide).5 In mixtures with water the micellar phase extends up to very high surfactant concentrations while a hexagonal phase forms between 58 and 83 wt %. The micelles appear to be small and closely spherical over a wide range of concentrations and somewhat more hydrated than unipolar surfactant micelles; however, the hydration of micelles of triethylammonium surfactants is not available for comparison. Notable is the low solubilization capacity of the bipolar surfactant micelles. It is unclear to us if the SB model predicts these observations but it seems not obvious to us that it would give the low solubilization. (We should remember that, for unipolar surfactants, lamellar phases with high amounts of solubilization appear to be a common observation.) In summary, we feel that the SB model may explain certain features of the micellization of bipolar surfactants but this is probably also the case for some alternative micelle models. However, a detailed discussion of this matter seems a little premature as to date firstly very few systems have been studied and secondly the number of experimental approaches is very limited. (3) See, e&: SMerman, 0.; Carlstrom, G.; Olsson, U.;Wong, T. J. Chem. SOC., Faraday Trans.1 1988,84,4475. (4) Gruen, D. Prog. Colloid Polym. Sci. 1985, 70, 6. (5) Ikeda, K.; Khan, A.; Meguro, K.; Lindman, B. J . Colloid InterfaceSci.,
in press.
Department of Chemistry University of Missouri Columbia. Missouri 6521 I
T. C. Wong
Department of Applied Chemistry Institute of Colloid and Interface Science Science University of Tokyo Tokyo 162, Japan
K. Ikeda K. Merguro
Physical Chemistry I Chemical Center University of Lund, P 0 B 124 S- 22007 Lund, Sweden
0. SGderman U. Olsson B. Lindman*
Received: August 30, 1989
Lack of Molecular Hydrogen as a Product of the Elementary Reactlon HO, HO,
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Sir: Production of H2 as a channel in the gas-phase disproportionation of H 0 2 has been a source of controversy in recent 0 1989 American Chemical Society
