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.