Micelles Made of Rigid Amphiphiles

The above mentioned examples indicate that a flexible hydrocarbon chain is not a prerequisite for micellization. 

The only other well-known micelles possessing a rigid hydrophobic core are those based on bile acids, some of the most important biological detergents. 

Particularly efficient solubilizing agents of the bile acid family are the salts of deoxycholic acid 6 . Their complexes with water-insoluble compounds are called choleic acids . The aggregation number of deoxycholic acid (DCA) in distilled water is approximately 10-12 ( primary micelles ) and rises to about 100 ( secondary micelles ) in more concentrated solutions and/or after addition of electrolytes to primary micelles. The cmc of primary micelles lies in the range of 1-5 x 10 mol/L. Less polar cholic acids are in general much better 

None of the arguments on the micelle structure are conclusive, but all the evidence points to an ultrathin helical cylinder with at least two turns ( = 12) or at most 16 turns. This is also in accordance with the curvature of the NaDCA molecule and related A/B-cw steroids. Contrary to the molecules of classical detergents, bile salts have a lipophilic outer surface which is the convex side of the steroid nucleus and a hydrophilic inner surface, which is the polyhydroxy-lated concave side of the nucleus. It is a structurally satisfactory finding that the concave side of the cholic acid molecule also forms the concave inside of the micelle. 

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