Discovery of Oriented Gaseous Water Molecules in the Vapor-Liquid Interface of Water

Abstract

Abstract : We present the first experimental measurements of the orientation and hydrogen bonding character of interfacial water molecules in their gaseous state in coexistence with the bulk liquid water. These interfacial vapor state molecules are distinct from the surface free OH and the interfacial liquid state water molecules and show a preferred orientation with their hydrogen atoms directed towards the liquid surface. The goal to develop a molecular level picture of water orientation and bonding at this vapor-liquid boundary where liquid phase water molecules coexist with vapor phase molecules has fascinated scientists for decades. Whereas most theoretical and experimental studies in recent years are converging on a consistent description of interfacial water on the liquid side of the interface, there is considerable disparity in the theoretical description of water molecules on the vapor side. A primary factor in this lack of consensus is the paucity of available data that unequivocally measures the properties of these interfacial vapor state molecules. As with theoretical efforts in this area, the low density of these vapor state interfacial water molecules is experimentally problematic. In this paper we present the first vibrational spectroscopic measurements of interfacial water vapor species in coexistence with their liquid phase. These detected vapor state molecules have a preferred orientation relative to the surface plane and have minimal hydrogen bonding to adjacent water molecules as manifested in the energy and linewidth of the spectral OH stretching modes examined. These molecules are distinctly different than the water molecules with a dangling bond into the vapor phase, or water in the liquid portion of the interface where hydrogen bonding between water molecules is strong.