title: Protonation-induced structural change of lyotropic liquid crystals in oley- and alkyldimethylamine oxides/water systems.
abstract: Lyotropic phase behavior of the nonionic and the half-ionized oleyldimethylamine oxide (OlDMAO)/water systems was investigated using polarized light microscopy, small-angle X-ray diffraction, and differential scanning calorimetry. Nonionic OlDMAO formed isotropic micellar solution, nematic, hexagonal, cubic, and lamellar liquid crystalline phases as the surfactant concentration increased. In contrast, half-ionized OlDMAO (i.e., 1:1 mixture of the nonionic and the protonated species) had a greater tendency to form bilayer structures, and the phase diagram became quite similar to those of double-chained ionic surfactants rather than single-chained ones, despite the introduction of positive charges to the nonionic one. The preference of the bilayer structures in the half-ionized OlDMAO was interpreted in terms of the dimers stabilized by the hydrogen bond between the nonionic and protonated species. For alkyldimethylamine oxides with a saturated hydrocarbon chain (CnDMAO, chain length: n = 14, 16, and 18), the phase sequence of lyotropic liquid crystals was hardly affected by the protonation, but an elongation of the cylinders of the hexagonal phase was observed for the half-ionized C14DMAO. Consequently, it can be considered that the dominant bilayer formation of the half-ionized OlDMAO is attributed to the combined effect of the hydrogen-bonded dimer formation and the cis-double-bond configuration of the alkyl chain.
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka 812-8581 Japan.
authors.forename: Hideya
authors.initials: H
authors.lastname: Kawasaki
authors.forename: Akihiko
authors.initials: A
authors.lastname: Sasaki
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Kawashima
authors.forename: Shigeo
authors.initials: S
authors.lastname: Sasaki
authors.forename: Rie
authors.initials: R
authors.lastname: Kakehashi
authors.forename: Itaru
authors.initials: I
authors.lastname: Yamashita
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Fukada
authors.forename: Tadashi
authors.initials: T
authors.lastname: Kato
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Maeda