Effects of Headgroup Structure on the Incorporation of Anions into Sulfobetaine Micelles. Kinetics and Physical Evidence

Zwitterionic micelles of (tetradecyldialkylammonio) propane-, butane- and pentanesulfonate increase the rate of reaction of Br- with methyl naphthalene-2-sulfonate, MeONs, but this reaction is largely suppressed by addition of NaClO4. The NMR line width of 35Cl in ClO4- increases very sharply on incorporation into micelles of tetradecyldimethylammonio propanesulfonate, SB3-14. The 14N line width of SB3-14 also increases significantly under these conditions, but there is no evidence for strong interactions with 23Na+. Aggregation numbers, Nagg, of SB3-14 increase modestly with increasing [SB3-14], but not with the other surfactants. The increase is greater with added NaClO4, but NaOH, NaCl, and NaNO3 have little effect on Nagg. Interactions of the anions with the micelles are treated with equations of the form of the Langmuir isotherm, and binding parameters estimated kinetically and conductometrically agree. Second-order rate constants of the reaction of Br- with MeONs at micellar surfaces are similar in sulfobetaine and cationic micelles. The reaction of I- with MeONs is also accelerated by micelles of SB3-14.