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Portraying the Structure of Micelles F. M. Menger Department of Chemistry, Emory University, Atlanta, GA 30322 R. Zana Institut Charles Sadron, 6, rue Boussingault, F67083 Strasbourg Cedex, France B. Lindman Chemical Center, University of Lund, Physical Chemistry 1, Lund, Sweden An amphiphilic compound contains a polar or ionic group (e.g., a trimethylammonium group) connected to a long hydrocarbon tail (e.g., a hexadecyl chain). When an amphiphile is dissolved in water above a certain concentration, it self-assembles into an aggregate of 50–100 molecules called a “micelle”. Micelles are mentioned in almost all organic chemistry and biochemistry texts owing, in part, to their importance in detergency and as models of biological membranes. Unfortunately, the micelle is most commonly portrayed in organic chemistry texts (1–6) and in biochemistry texts (7–12) (and, for that matter, in this Journal [13]) as the “spokes of a wheel”. Figure 1A is typical of what these drawings look like. Sometimes, but not often, a picture is described as being “schematic”; but even with this caveat students receive a totally erroneous concept of reality. A micelle is, in fact, a highly disorganized structure with multiple bent chains, cavities, hydrocarbon–water contact, and deviations from an exact spherical shape (14). It is thus far better to present a micelle as a “brushheap” of molecules (Fig. 1B) than as the “spokes of a wheel”. Comparison of the two models, Figure 1A and Figure 1B, reveals clearly one reason why the latter is preferred: one cannot situate 50–100 terminal methyl groups at a single point, namely the center of a sphere. Note that even Figure 1B is highly schematic in that it is a planar representation of a three-dimensional object. Moreover, the chains are not drawn with their true van der Waals radii owing, in part, to the fact that micelle structure is unknown in such detail. Nonetheless, the Figure 1B does more properly depict the disorder, nonlinearity, and fluctional character of the liquid-like micelle interior. G. S. Hartley, one of the first to discuss micelle structure, wrote in 1936 the following words (15): The symmetrical asterisk form…has no physical basis and is drawn for no other reason than that the human mind is an organizing instrument and finds unorganized processes uncongenial.
For reasons of artistic convenience, we presume, the socalled Hartley model evolved into the spokes-of-a-wheel structure that Hartley disclaimed decades ago. Now that computer software allows Figure 1B to be drawn as readily as Figure 1A, textbooks and journals should use this more appropriate representation. Editor’s note: See also the 3-D structure of a micelle depicted on page 93 of this issue and related discussion on page 94.
Figure 1. A: the misnamed “Hartley” micelle as portrayed in most modern chemistry and biochemistry texts. B: a more realistic schematic attempting to depict the disordered and fluid nature of the micelle interior.
Literature Cited 1. Ouellette, R. J.; Rawn, J. D. Organic Chemistry; Prentice-Hall: Upper Saddle River, NJ, 1996; p. 792. 2. Morrison, R. T.; Boyd, R. N. Organic Chemistry, 6th ed.; PrenticeHall: Englewood Cliffs, NJ, 1992; p 1125. 3. Wade, L. G. Organic Chemistry, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1995; p 1216. 4. Fessenden, R. J.; Fessenden, J. S. Organic Chemistry, 5th ed.; Brooks/Cole: Pacific Grove, CA, 1994; p 946. 5. Carey, F. A. Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1992; p 771. 6. Volhardt, K. P. C.; Schore, N. E. Organic Chemistry, 2nd ed.; Freeman: New York, 1994; p 780. 7. Mathews, C. K.; Van Holde, K. E. Biochemistry, 2nd ed.; Benjamin/ Cummings: Menlo Park, CA, 1996; p 321. 8. Horton, H. R.; Moran, L. A.; Ochs, R. S.; Rawn, J. D.; Scrimgeour, K. G. Principles of Biochemistry, 2nd ed.; Prentice-Hall: Upper Saddle River, NJ, 1996; p 35. 9. Stryer, L. Biochemistry, 4th ed.; Freeman: New York, 1995; p 269. 10. Voet, D.; Voet, J. G. Biochemistry, 2nd ed.; Wiley: New York, 1995; p 285. 11. Garrett, R. H.; Grisham, C. M. Biochemistry; Saunders College Publishing: Fort Worth, TX, 1995; p 39. 12. Lehninger, A. L.; Nelson, D. L.; Cox, M. M. Principles of Biochemistry, 2nd ed.; Worth: New York, 1993; p 87. 13. Goodling, K.; Johnson, K.; Lefkowitz, L.; Williams, B. W. J. Chem. Educ. 1994, 71, A8. 14. Menger, F. M.; Carnahan, D. W. J. Am. Chem. Soc. 1986, 108, 1297; Smit, B.; Esselink, K.; Hilbers, P. A. J.; van Os, N. M.; Rupert, L. A. M.; Szleifer, I. Langmuir 1993, 9, 9; Gruen, D. W. R. J. Phys. Chem. 1985, 89, 153. 15. Hartley, G. S. Aqueous Solutions of Paraffin-Chain Salts. A Study of Micelle Formation; Herman: Paris, 1936; see especially Fig. 11A and the accompanying discussion.
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
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