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 cetyltrimet hylammonium chloride (C(16)CA), cetyl alcohol (C16OH), and water was elucid ated on the basis of electron spin resonance (ESR) spin probe method. ESR s pectra for the ternary assembly incorporated with 5- and 16-doxylstearic ac ids (5-NS, 16-NS) showed definite anisotropic patterns, indicating restrict ed molecular motions in the bilayers. The order parameter, S-33, and the hy perfine splitting constant ANI were higher in the 5-NS system than in the 1 6-NS. These findings suggest more marked restriction of molecular motions n ear the hydrophilic group in the bilayers and relatively free molecular mot ions toward the alkyl terminal. S-33 and A(N)' for the 16-NS system were co nstant on aging, indicating invariance of the molecular motion and the degr ee of polarity near the terminal of alkyl group. A slight change of S-33 fo r the 5-NS system was observed after aging for a few days. An empirical P p arameter, a ratio of a fluid-like bilayer component to a gel-like one, howe ver, decreased after 3 days. Furthermore, AN' for the 5-NS system started t o increase after 3 days. These suggest the phase transition of the assembly is associated with the change in the association state near the hydrophili c group in bilayers. When the penetrated water increases beyond a critical level, vesicles as closed bilayers no longer change into open lamellae.