Ultrafast Energy Transfer in Water−AOT Reverse Micelles

Abstract

A spectroscopic investigation of the vibrational dynamics of water in a geometrically confined environment is presented. Reverse micelles of the ternary microemulsion H2O/AOT/n-octane (AOT = bis-2-ethylhexyl sulfosuccinate or aerosol-OT) with diameters ranging from 1 to 10 nm are used as a model system for nanoscopic water droplets surrounded by a soft-matter boundary. Femtosecond nonlinear infrared spectroscopy in the OH-stretching region of H2O fully confirms the core/shell model, in which the entrapped water molecules partition onto two molecular subensembles: a bulk-like water core and a hydration layer near the ionic surfactant headgroups. These two distinct water species display different relaxation kinetics, as they do not exchange vibrational energy. The observed spectrotemporal ultrafast response exhibits a local character, indicating that the spatial confinement influences approximately one molecular layer located near the water−amphiphile boundary. The core of the encapsulated water droplet is similar in its spectroscopic properties to the bulk phase of liquid water, i.e., it does not display any true confinement effects such as droplet-size-dependent vibrational lifetimes or rotational correlation times. Unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the hydration shell to the bulk-like core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.

Reduced coupling of water molecules near the surface of reverse micelles

“…This partitioning of water molecules into two sub-ensembles makes the reverse micelle an excellent model for interfacial water (in small, d = 1 nm micelles the fraction of border-effected water is more than 80%) and almost bulk-like water (in large d = 10 nm diameter micelles the fraction of core water approaches 90%). The separation of water into two sub-ensembles is also supported by the results of previous IR nonlinear experiments on the reverse micelles; 38,39,43,44,[52][53][54] however, some deviations from the core-shell model have also been reported. 55,56 Earlier studies of water vibrational dynamics in AOT reverse micelles 38,39 were focused on verifying the core-shell model in the case of H 2 O molecules where OH stretches are strongly coupled.…”

“…This is corroborated by the fact that B90% of water molecules in d = 10 nm reverse micelles find themselves in the ''bulk-like'' environment of the core. 39,44,48 In contrast, absorption of the OH stretching mode for the small reverse micelles, containing B30 waters, 43,51 displays a pronounced blue shift and spectral narrowing. The blue shift of the OH vibrational frequency is usually attributed to weaker (on average) hydrogen bonds between the water molecules.…”

“…45 As the directions of the dipole moments of these transitions are close to orthogonal, their contributions interfere destructively in the anisotropy measurements thereby bringing the anisotropy value to zero. Finally, the long-term ps dynamics become hidden under the in-growing contribution of the essentially isotropic temperature response due to population relaxation of the core water 39 which maximum coincidently has a frequency of B3430 cm À1 . In this respect, the probe wavelength of 3590 cm À1 presents a window of opportunity as it is free of all aforementioned complications allowing the anisotropy dynamics to be followed at a 10 ps time scale.…”

“…the extent of probed distances from the interface) at charged interfaces is still under debate. 37 On the other hand, ultrafast vibrational spectroscopy studies also evidenced a substantial deceleration of population relaxation rates 38 and a decrease in the induced anisotropy decay [39][40][41][42] of water vibrations in the lipid hydration layer. This implies completely different hydrogen-bond dynamics and effect of intermolecular couplings at the water-lipid interface.…”

Ultrafast dynamics of solvation: the story so far

“…Recent MD simulations and ultrafast infrared spectroscopy measurements have indeed shown position-dependent water dynamics as water molecules were distributed in different parts of reverse micelles. [116][117][118] …”