Effects of headgroup structure on the incorporation of anions into sulfobetaine micelles. Kinetic 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 Cl-35 in ClO4- increases very sharply on incorporation into micelles of tetradecyldimethylammonio propanesulfonate, SB3-14. The N-14 line width of SB3-14 also increases significantly under these conditions, but there is no evidence for strong interactions with Na-23(+). Aggregation numbers, N-agg, 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 N-agg. 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.