Conformational behavior of nonionic surfactants in the organized phases studied by vibrational spectroscopy

Abstract

Conformational behavior of the alkyl chain in α-dodecyl-ω-hydroxytris(oxyethylene) (C₁₂E₃) in the aqueous organized phases has been studied by a newly developed technique of C-D stretching vibrational spectroscopy. This method is based on the fact that the wavenumbers of the isolated C-D stretching vibrations are sensitive to the conformation in the vicinity of the C-D bond. The C-D stretching infrared spectra of five selectively monodeuterated species of C₁₂E₃ in the lamellar (L_α) and isotropic solution (L₂) phases were analyzed and the fractions of the trans conformation around the dodecyl C-C bonds and the oxyethylene-adjoining O-C bond and the fractions of the consecutive trans conformations around the two adjoining bonds were evaluated. The conformational change at the phase transition from L₂ to L_α is not significant and only a small increase in the trans fraction is observed for the C-C bonds close to the alkyl/oxyethylene interface, implying that the conformational states of the alkyl chain in the L₂ and L_α phases in the vicinity of their boundary are substantially not different. In the L_α phase, when the composition or the temperature approaches the region of the phase separation or transition, the trans fractions for the C-C bonds closer to the alkyl/oxyethylene interface and those closer to the chain terminal decrease. These observations indicate that the conformational transformation from trans to gauche at these chain positions makes the lamellar structure less stable and leads eventually to the structural destruction. The conformational order as evaluated from the fractions of the consecutive trans conformation is the highest in the middle of the chain in the L_α and L₂ phases. This vibrational spectroscopic observation, together with the previous NMR observations, indicates that the alkyl/oxyethylene interface is flexible with respect to the conformation and the orientation of the chain.