Dynamics in a Water Interfacial Boundary Layer Investigated with IR Polarization-Selective Pump-Probe Experiments

The dynamics of water molecules near the surfactant interface in large Aerosol-OT reverse micelles (RMs) (w₀ = 16-25) was investigated with IR polarization-selective pump-probe experiments using the SeCN^- anion as a vibrational probe. Linear absorption spectra of RMs (w₀ = 25-2) can be decomposed into the weighted sum of the SeCN^- spectra in bulk water and the spectrum of the SeCN^- anion interacting with the interfacial sulfonate head groups (w₀ = 1). The spectra of the large RMs, w₀ ≥ 16, are overwhelmingly dominated by the bulk water component. Anisotropy decays (orientational relaxation) of the anion for w₀ ≥ 16 displayed bulk water relaxation (1.4 and 4.5 ps) plus an additional slow decay with a time constant of ∼13 ps. The amplitude of the slow decay was too large to be associated with SeCN^- in contact with the interface on the basis of the linear spectrum decomposition. The results indicate that the observed slow components arise from SeCN^- in a water boundary layer, in which water molecules are perturbed by the interface but are not directly associated with it. This layer is the transition between water in direct contact with the interface and bulk water in the large RM cores. In the boundary layer, the water dynamics is slow compared to that in bulk water.