Structural Analysis of Surface and Interfacial Water by Synchrotron Radiation X-rays

Abstract

This review paper introduces the structure analysis results of water confined in microfluidic devices and ion exchange membranes for fuel batteries. The properties of water confined in these nano-spaces differ from those in bulk water. The differences in physical properties can be directly explained by examining the liquid structure using X-ray diffraction. The use of powerful beams from synchrotron radiation and the development of two-dimensional detectors have made it possible to analyze the structure of water in such special conditions. In the nano-space of a microfluidic channel, water hydrogen bonds develop, strengthening water structure. This cannot be explained by the direct interaction of water molecules with the wall of the channel. It was shown that water molecules distant from the wall are also affected via hydrogen bonds. We have succeeded in observing in-situ the adsorption and desorption processes of water molecules on an anion exchange membrane. It was found that water clusters connected by hydrogen bonds exist within the membrane. A single aqueous droplet ultrasonically levitated in the air was measured by in-situ Raman and synchrotron X-ray scattering methods. The local structure of a supersaturated Mg(NO₃)₂ aqueous droplet was determined. The droplet’s crystallization process was observed, and the crystal phases were characterized using X-ray scattering.