Effect of the hydrophilicity of aromatic counterions on the structure of ionic micelles

Small-angle neutron scattering experiments have been carried out on aqueous micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTABr) in the presence of different aromatic salts. The salts with different hydrophilicity of the counterions that have been used are sodium benzoate (NaB), sodium 2-hydroxy benzoate (Na2-HB), sodium 2,6-dihydroxy benzoate (Na2,6-DHB), and sodium 4-hydroxy benzoate (NA4-HB). It is found that cationic CTABr micelles grow strongly with the addition of small amount of NaB, Na2-HB, and Na2,6-DHB salts. The effect of addition of salts on the growth of the micelles is significantly higher for Na2-HB and Na2,6-DHB as compared to that for NaB. On the other hand, the addition of salt Na4-HB is relatively much less effective similar to the effect of inorganic salts (e.g., NaBr) on the growth of the micelles. We explain the propensity of strong growth of the micelles in the presence of NaB, Na2-HB, and Na2,6-DHB because of the high charge neutralization by these salts as the aromatic counterions are adsorbed on the surface of the micelle. In these counterions, the hydrophilicity seems to be important in deciding the location of the charged group of the counterions with respect to the level of the oppositely charged micellar surface. The counterions of Na4-HB are not adsorbed on the micellar surface, and hence, it behaves differently than the above other salts.