Distinct Effects of External Electric Field on Interfacial and Bulk-like Water Confined in Reverse Micelles

Nanoscopically confined water under an electric field is widely found in various fields of chemistry and biology. Despite their importance, it remains to be clarified how such water molecules respond to an electric field. In this study, we use reverse micelles as a model system for water confinement and measure the electric field effects on water (D2O) molecules inside reverse micelles with infrared (IR) electroabsorption (EA) spectroscopy. Conventional IR and IR EA spectra of water in anionic and nonionic reverse micelles are interpreted in terms of interfacial and bulk-like water molecules in the reverse micelles. The IR EA signals of these water species that are attributable to field-induced reorientation show opposite signs, indicating that they have distinctly different vibrational properties mainly because of the confined environment. Our results demonstrate that IR EA spectroscopy holds promise for gaining novel insights into the behaviors of confined water such as hydration water and water in porous electrodes.