Monomer micelle exchange rate, surfactants

The main technique used to look at exchange processes in equilibrium systems employs labeled surfactants, particularly with ESR spectroscopy. Fox s ESR study [92] of a paramagnetic surfactant in micellar solution was the first of its kind, and yielded a solution-micelle monomer exchange rate of 10 s" at room tanperature for 2,2,6,6-tetramethylpiperidine-oxidedodecyldimethylammonium bromide. These techniques, along with time-resolved luminescence quenching, have shown that the entry of surfactant molecules into micelles is near-diffusion controlled, whereas loss from micelles is rate limiting, and hence kinetically controlled [93]. A decade later (1981), Bolt and Turro [94] were able to find the separate exit and reentry rate constants for 10-(4-bromo-l-naphthoyl)decyltrimethylammonium bromide as 3.2 x 10 s and 5.7 x 10 mok s", respectively. 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

However micelles are very mobile structures. Monomeric surfactant, counterions and solutes exchange rapidly between micelles and solvent [27-29], and it appears that monomers and incorporated solutes enter the micelle at a diffusion-controlled rate. 

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