Effects of Protonation on Tetradecyldimethylamine Oxide Micelles

Solution and surface properties of tetradecyldimethylamine oxide (C14DAO) aqueous solutions were examined and compared with those of dodecyldimethylamine oxide (C12DAO). The following two important behaviors of C12DAO were both observed with C14DAO in the present study. (1) The cmc of the cationic species, cmc+, was lower than that of the nonionic species cmc0 in the range of NaCl concentration C s higher than about 0.2 M. (2) The micelle size was the largest at about 1:1 composition. On the other hand, the following three important aspects different from C12DAO were found for C14DAO. (1) Values of pK M were greater by 0.4−0.5 than those of C12DAO at three NaCl concentrations C s examined. This finding is quantitatively explained in terms of different shapes of the two nonionic micelles. (2) The monomer concentration evaluated from the hydrogen ion titration with the Gibbs−Duhem relation differed from the cmc. This difference gave the concentration dependence of the regular solution theory parameter β. (3) At the composition of 1:1 (αM = 0.5), the cooperative diffusion mode was observed in the dynamic light scattering indicative of entanglement of long micelles. The mesh size was about 10 nm. The following quantitative differences were also observed. (1) The salting-out constant, (−d log cmc0/dC s), was 0.40 M-1 (0.32 M-1 for C12DAO). (2) The slope d log cmc+/d log(counterion concentration) in the range C s < 1 M was 0.71 (0.64 for C12DAO). (3) The dependence of the cmc on pH at 0.1 M NaCl was approximately described in terms of the regular solution theory with the parameter β = −1.4 ± 0.2, which is less negative than that of C12DAO (β = −2.12).