DYNAMICS OF POLAR SOLVATION IN LECITHIN WATER/CYCLOHEXANE REVERSE MICELLES/

The solvation dynamics of water in lecithin/cyclohexane reverse micell es have been determined via ultrafast time-resolved fluorescence studi es. At hydration levels w(o) less than or equal to 4.8, the reverse mi cellar samples are nonviscous. Here a single relaxation time is observ ed that is much longer than the response of free or bulk water. In con trast, small additions of water to the samples produces a viscous gel, referred to by others as an organogel or ''living polymer''. At hydra tion levels of w(o) greater than or equal to 5.8, three relaxation tim es are observed with approximate time constants of 0.5, 15, and 200 ps , the shortest of which correlates to free water motion. The dynamics reveal no evidence of micelle crossover or branch points associated wi th gel formation. In comparison to Aerosol OT reverse micelles of simi lar hydration, the water in the lecithin reverse micelles is significa ntly more restricted. It is proposed that lecithin sequesters signific antly more water than previously predicted precluding formation of dis tinct core water pools in the micelles. The results are also compared to models for aqueous structure and dynamics near phospholipid membran es and to bulk water dynamics.