Photochemistry in Micelles

A micelle is an assembly of amphiphilic molecules dispersed in water. Such molecules are made of two parts, a polar head group and a non-polar tail . The polar head is for example a carboxylic acid which can dissociate into ions (—COO- and H+) the non-polar tail is a saturated hydrocarbon chain. Since the non-polar parts are insoluble in a polar solvent, these molecules aggregate in water to form micelles which are microscopic droplets with a non-polar interior and polar groups at the water interface. This picture of micelles is probably an oversimplification, because water penetrates to some extent between the molecules it is however sufficient for an understanding of the special properties of micellar suspensions in photochemistry. 

1 Micellar Catalysis. Non-polar molecules such as aromatic hydrocarbons are practically insoluble in water. In a micellar suspension they concentrate in the non-polar interior of the micelles where they can reach relatively high concentrations, even though their overall concentration may be very low. The quantum yields of bimolecular reactions like photoadditions are therefore greatly increased in micellar suspensions. 

2 Orientational Effects. Solute molecules which have distinct polar and non-polar parts take up specific orientations in micelles, such that their non-polar end stays in the non-polar interior of the micelle, the polar group residing at the water interface. Photocycloaddition of these molecules will therefore lead preferentially to the head-to-head dimer, even if the head-to- 

Example of head-to-tail and head-to-head photodimerization of an enone in solution and at the interface of a micelle 

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Turro N.J., Cox G.S., Paczkowski., Photochemistry in micelles, Current Chemistry — Photochemistry and organic synthesis . 129, 57-97. (1985)... 

Another problem is measurement in turbid solutions. This problem arises in biological systems, e.g. in lipid membranes or in the examination of photo cross-linking of proteins. The photochemistry in micelles is also frequently examined. In all these cases the chromophore which is supposed to show the photoreaction can interact with the lipids. This interaction can either cause... 

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