Protonation Behavior and Stability of Micelles of N-Lauroylaminoalkyl-Dimethylamine Oxides-Effects of Added Salt Concentration and Spacer Length

Abstract

The protonation behavior and the stability of micelles of N-lauroylaminopropyl-N',N'-dimethylamine oxide (C12AmCn; n=3) were studied by the hydrogen ion titration and the equilibrium surface tension measurement. The surface potential of C12AmC3 micelles estimated from the results of hydrogen ion titration was lower than that of dodecyldimethylamine oxide micelles at added NaCl concentration, C S , of 0.1M. The critical micelle concentration (cmc) estimated from surface tension measurements revealed that a reversal of the stability of the cationic and the nonionic micelles of C12AmC3 takes place in the range of C S higher than about 0.2M, that is, the cationic micelles become more stable than the nonionic ones in the range C S ≥ 0.2M. In addition, the surface tension measurements were carried out for C12AmCn with different spacer length, n, ranging from 2 to 6 methylene units at C S = 0.1M. The cmc values of nonionic species were kept at almost constant for n ≤ 4, while they decreased with n for n ≥ 4. The cmc values of cationic species also tended to decrease with n for n ≥ 4. These results were compared with those obtained for alkyldimethylamine oxides, and the effect of amide group and spacer groups introduced into amine oxide surfactants was discussed.

Protonation Behavior and Solution Properties of Amine Oxide Surfactants Containing a Pyridyl Group

“…Hydrogen-bonding between protonated and deprotonated species in both the solid state and liquid crystal state has been examined by infrared spectra 27 . Similar protonation behavior is observed for amine oxide surfactants with an amide group 29 . For example, although lauroylaminoalkyldimethylamine oxides C12AmC l ; l hydrocarbon chain length between the amide group and the amine oxide group should be less repulsive than C12DMAO in dipoledipole interactions due to the amino alkyl group introduced, for example, the protonation behavior and cmc values of C12AmC 3 are almost the same as those of C12DMAO 29 .…”

“…Similar protonation behavior is observed for amine oxide surfactants with an amide group 29 . For example, although lauroylaminoalkyldimethylamine oxides C12AmC l ; l hydrocarbon chain length between the amide group and the amine oxide group should be less repulsive than C12DMAO in dipoledipole interactions due to the amino alkyl group introduced, for example, the protonation behavior and cmc values of C12AmC 3 are almost the same as those of C12DMAO 29 . Thus, we think that the hydrogen bonding between protonated and deprotonated species is the most promising attractive interaction in these systems.…”

Solution Behavior of Long–alkyl–chain Amide Amine Oxide Surfactants Having Multiple Hydrogen-bonding Sites

“…Our previous results 13 strongly suggested that the spacer length determined whether the amide group work as the polar head groups or they are incorporated in the hydrophobic part. For N-lauroylaminoalkyl-N¤,N¤-dimethylamine oxides (the spacer length between the amide and amine oxide groups; 26), the dependence of the critical micelle concentration (cmc) values on the spacer length suggested that the amide groups are in the polar head part for n = 3, while they are in the hydrophobic part for n = 6.…”

“…13 Moreover, samples with different spacer lengths between the amide and amine oxide groups were prepared. Whether the amide groups work as the polar head groups or they are incorporated in the hydrophobic part is expected to depend on the spacer length.…”

“…10 In addition to this type of the hydrogen bond, we have introduced amide groups into amine oxide surfactants as new sites for hydrogen bonding. 13 Moreover, samples with different spacer lengths between the amide and amine oxide groups were prepared. Whether the amide groups work as the polar head groups or they are incorporated in the hydrophobic part is expected to depend on the spacer length.…”