A Fourier transform infrared study of the lamellar liquid crystalline phase of dimethyldodecyl amineoxide-water and dimethyldodecyl amineoxide-gramicidin-D-water systems

Abstract

Abstract The overall conformational order of the alkyl tail of the lyotropic lamellar phase of the dimethyldodecyl amineoxide, (CH3)2ON+(CH2)11CH3 (DDAO)H2O system (75.7 wt.%) has been studied by using Fourier transform infrared spectroscopy. The spectral region 1000–1400 cm−1, covering the CH2 wagging modes and the methyl umbrella modes of DDAO, has been recorded in the temperature interval 3–60°C and at different mole fractions of gramicidin-D with respect to DDAO (Xg = 0.03, 0.05 and 0.1) for both DDAO-H2O and DDAO-D2O systems. It has been shown that the DDAO amphiphile molecules of the lamellar phase reorganise in a phase-like transition near 25–30°C. The DDAO-water system does not show any significant bands corresponding to a double gauche conformation at 1355 cm−1 nor to a gauche-transgauche (kinked) conformation at 1367 cm−1. These bands are probably present but hidden in the broad low-frequency side of the CH3 umbrella band at 1377 cm−1. Upon incorporation of gramicidin into the lamellar phase both head group and acyl chain spectra of the lipid change in such a way as to indicate a decreased “ordering” of the molecules, as judged by comparison with spectra of the same molecule in a micellar environment, and with increased fluidity of the acyl chains.