8/10/2019 Mice Lles

1/1

Information Textbooks Media Resources

JChemEd.chem.wisc.edu Vol. 75 No. 1 January 1998 Journal of Chemical Education 115

An amphiphilic compound contains a polar or ionicgroup (e.g. , a trimethylammonium group) connected to along hydrocarbon tail (e.g. , a hexadecyl chain). When anam phiphile is dissolved in wat er above a certa in concentra -tion, it self-assembles into a n a ggrega te of 50100 moleculescalled a micelle. Micelles ar e mentioned in alm ost a ll or-ganic chemistry and biochemistry texts owing, in part , totheir importa nce in detergency an d a s models of biologicalmembranes.

Un fortun at ely, the m icelle is most commonly portra yedin organic chemistry texts (16) and in biochemistry texts(712) (and, for that mat ter , in this Journa l [13]) as thespokes of a wheel. Figure 1A is typical of wha t t hese draw -ings look like.

Sometimes, but not often, a picture is described as be-ing schematic; but even wit h this cavea t student s receivea totally erroneous concept of reality. A micelle is, in fact, ahighly disorganized s tructure with mult iple bent chains ,cavities, hydrocarbonwa ter conta ct, and d eviations from a nexact spherical sha pe (14). It is thu s far bett er to present amicelle as a brushhea p of molecules (Fig. 1B) tha n a s thespokes of a w heel. Comparison of the tw o models, Figure1A and Figure 1B, reveals clearly one reason why the lat-ter is preferred: one cannot situ at e 50100 termina l methyl

groups at a single point, namely t he center of a sphere. Notetha t even Figure 1B is highly schematic in that i t is a pla-na r representat ion of a three-dimensional object. Moreover,the cha ins are not dra wn w ith their true van der Waa ls radiiowing, in part, to the fact th at micelle structure is unknownin such deta il. Nonetheless, the Figure 1B does more prop-erly depict t he disorder, nonlinearity, and fluctiona l chara c-ter of th e liquid-like micelle int erior.

G. S . Ha rtley, one of the first t o discuss micelle struc-ture, w rote in 1936 the following words (15):

The symmetrical asterisk form has no physical basisand is dra wn for no other reason tha n tha t the human

mind is an organizing instrument a nd finds unorganizedprocesses uncongenia l.

For reasons of artistic convenience, we presume, the so-called Hartley model evolved into the spokes-of-a-wheelstructure that Hart ley discla imed decades ago. Now thatcomputer softwa re a llows Figure 1B to be drawn as rea dilyas Figure 1A, textbooks an d journa ls should use this moreappropriate representation.

Literature Cited

1 . O u el le t t e, R . J . ; R a w n , J . D . Organic Chemist ry;Prentice-Hall :

Upper Saddle River, NJ , 1996;p. 792.

2. Morrison, R. T. ; Boyd, R. N. Organi c Chemist ry, 6th ed . ; P rentice-

Ha ll : Englewood Clif fs , NJ , 1992; p 1125.3 . Wa d e , L . G . Organic Chemist ry, 3rd ed.; P rentice-Ha ll: Englewood

Cliff s, NJ , 1995; p 1216.

4 . Fessend en , R . J . ; Fessend en , J . S . Organi c Chemist ry, 5th ed.;

B rooks/Cole: P acifi c Gr ove, CA, 1994; p 946.

5 . Carey, F. A. Organic Chemist ry, 2nd ed.; McGra w-Hill: New York,

1992; p 771.

6. Volhardt, K. P. C. ; Schore, N. E.Organi c Chemist ry, 2nd ed.; Free-

ma n: New York, 1994; p 780.7 . Mathews , C . K . ; Van Hold e, K . E . Biochemistry, 2nd ed.; Benjam in/

Cumm ings: Menlo P ar k, CA, 1996; p 321.

8. Horton, H. R. ; Moran, L. A. ; Ochs, R. S. ; Rawn, J . D. ; Scr imgeour,

K . G . Pri ncipl es of Bi ochemistr y, 2nd ed.; Prentice-Hall: Upper

Sa ddle River, NJ , 1996; p 35.9. S t r y er, L . Biochemistry, 4th ed.; Freema n: New York, 1995; p 269.

10. Voet , D. ; Voet , J . G . Biochemist ry, 2nd ed. ; Wiley: N ew York, 1995;

p 285.

11 . G a r r e t t , R . H .; G r i s ha m , C . M . Biochemist ry; Saunders College

P ublish ing: F ort Wort h, TX, 1995; p 39.

12. Lehninger , A. L. ; Nelson, D. L. ; C ox, M. M. Pri ncipl es of Bi ochem-

is t ry, 2nd ed .; Worth : New York, 1993; p 87.

13. Goodling, K. ; Johnson, K. ; Lefkowitz , L. ; Will iams, B . W. J. Chem.

Educ.1994,71, A8.

14. Menger , F. M. ; Carnaha n, 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. Langmu i r1993,9, 9; Gr uen, D. W. R. J . Phys.

Chem. 1985,89, 153.

15 . H a r t l ey, G . S . Aqueous Solu t ions of Paraff i n-Chain Sal ts.A Study

of Micell e Form ation; Herma n: P aris, 1936; see especially Fig. 11A

and the a ccompanying discussion.

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

Figure 1. A: the misnamed Hartley micelle as portrayed in most

modern chemistry and biochemistry texts. B: a more realistic sche-

matic attempting to depict the disordered and fluid nature of the

micelle interior.

Editors note: See also the 3-D structure of a micelle depicted

on page 93 of this issue and related discussion on page 94.