An electron spin resonance study on the phase transition of the molecular assembly comprising cetyltrimethylammonium chloride/cetyl alcohol/water

The phase transition process of a molecular assembly comprising cetyltrimethylammonium chloride (C(16)CA), cetyl alcohol (C16OH), and water was elucidated on the basis of electron spin resonance (ESR) spin probe method. ESR spectra for the ternary assembly incorporated with 5- and 16-doxylstearic acids (5-NS, 16-NS) showed definite anisotropic patterns, indicating restricted molecular motions in the bilayers. The order parameter, S-33, and the hyperfine splitting constant ANI were higher in the 5-NS system than in the 16-NS. These findings suggest more marked restriction of molecular motions near the hydrophilic group in the bilayers and relatively free molecular motions toward the alkyl terminal. S-33 and A(N)' for the 16-NS system were constant on aging, indicating invariance of the molecular motion and the degree of polarity near the terminal of alkyl group. A slight change of S-33 for the 5-NS system was observed after aging for a few days. An empirical P parameter, a ratio of a fluid-like bilayer component to a gel-like one, however, decreased after 3 days. Furthermore, AN' for the 5-NS system started to increase after 3 days. These suggest the phase transition of the assembly is associated with the change in the association state near the hydrophilic group in bilayers. When the penetrated water increases beyond a critical level, vesicles as closed bilayers no longer change into open lamellae.