Hydrogen Bonded Structures of Confined Water Molecules and Electric Field Induced Shift of Their Equilibrium Revealed by IR Electroabsorption Spectroscopy

Water confined on a nanometer scale plays an essential role in various chemical and biological processes. Confined water molecules are often exposed to electric fields as manifested by those that occur on protein surfaces or in electrical double layers, but the electric field effects on confined water are not fully understood. We used IR electroabsorption (EA) spectroscopy with unprecedented sensitivity to observe electric-field-induced changes in the OH stretching absorption of water (H2O) molecules dissolved in 1,4-dioxane, which constitute a simple model system for confined water. A multivariate curve resolution analysis of the normal IR spectra (without an electric field) of water in 1,4-dioxane at different concentrations indicates the presence of the monomer and dimer of the confined water molecules and equilibrium between them. We find that the IR EA spectrum that is free from the contribution of field-induced molecular reorientation is mainly attributable to a field-induced shift of the equilibrium toward the dimer. This result demonstrates a possible control of the polarity of confined water by simply applying an external electric field and the ability of our method to elucidate how it is achieved.