Long-range intermolecular vibrational coupling of water molecules in contact with surfactant vesicles
Abstract
Raman spectra of the parallel component, I∥(ν), 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 × 10–3 mol dm–3 dioctadecyldimethylammonium bromide (DODAB), both at 20 and at 50 °C. The change manifested itself in 21% and 18% increases of the intensity of the low-frequency I∥(ν) Raman peak of water relative to the high-frequency I∥(ν) Raman peak of water at 20 and 50 °C, as well as in shifts of the high-frequency I∥(ν) maximum to lower energy. Incubation in 2.0 × 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∥(ν) Raman peak. Raman spectra of the perpendicular component, I⊥(ν), 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 × 10–3 mol dm–3 aqueous Na2SO4(at 20 and at 50 °C) or with that of aqueous vesicles prepared from 2.0 × 10–3 mol dm–3 didodecyl phosphate (DDP) at 50 °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.