LONG-RANGE INTERMOLECULAR VIBRATIONAL COUPLING OF WATER-MOLECULES IN CONTACT WITH SURFACTANT VESICLES

Raman spectra of the Parallel component, I parallel-to (upsilon), of the O-H stretching mode region of water (broad maxima at 3250 and 3395 cm-1) are found to be profoundly altered upon the introduction of 80-105 nm diameter surfactant vesicles prepared from 2.0 x 10(-3) mol dm-3 dioctadecyldimethylammonium bromide (DODAB), both at 20 and at 50-degrees-C. The change manifested itself in 21% and 18% increases of the intensity of the low-frequency I parallel-to (upsilon) Raman peak of water relative to the high-frequency I parallel-to (upsilon) Raman peak of water at 20 and 50-degrees-C, as well as in shifts of the high-frequency I parallel-to (upsilon) maximum to lower energy. Incubation in 2.0 X 10(-3) mol dm-3 Na2SO4 resulted in slow fusion to vesicles with mean diameters up to 400 nm, elicited additional increases in the intensity ratio R, and caused further shifts of the high-frequency I parallel-to (upsilon) Raman peak. Raman spectra of the perpendicular component, I perpendicular-to (upsilon), of the O-H stretching mode of water behaved analogously. In contrast, the Raman spectrum of water was found to be identical, at identical temperatures, with that of 2.0 x 10(-3) mol dm-3 aqueous Na2SO4 (at 20 and at 50-degrees-C) or with that of aqueous vesicles prepared from 2.0 x 10(-3) mol dm-3 didodecyl phosphate (DDP) at 50-degrees-C. These results have been rationalized in terms of long-range ordering of water molecules by the DODAB vesicles which provide large surfaces of an appropriate hydrophobic-hydrophilic balance, and by a further increase in the long-range ordering of water molecules upon fusion of the vesicles which increases the hydrophobic to hydrophilic ratio of the DODAB surface.