Growth of cetyltrimethylammonium chloride and acetate micelles with counterion concentration

Time resolved fluorescence quenching measurements with pyrene as probe are employed to determine aggregation numbers for hexadecyltrimethylammonium ac etate (CTAOAc) and chloride (CTACl) micelles as a function of the concentra tion of detergent and added common-counterion salt. The aggregation numbers , N-A, of CTACl are roughly twice those of CTAOAc micelles at equivalent co ncentrations of detergent and salt, consistent with the known relative coun terion binding affinity (chloride much greater than acetate). For both dete rgents, the increase of N-A with the net concentration of counterions in th e intermicellar aqueous phase ([Y-aq]) follows the relationship found previ ously for anionic micelles: log N-A = log N-o + gamma log[Y-aq], where gamm a and N-o are constants. However, compared to anionic micelles, for which g amma = 0.2-0.25, spherical micelles of both CTAOAc and CTACl exhibit less p ronounced growth, with gamma = 0.1. (C) 1999 Academic Press.