Operando Observation of Liquid and Liquid-Liquid Interface by Soft X-ray Absorption Spectroscopy

Soft X-ray absorption spectroscopy (XAS) is an element specific method to reveal local structures of liquid samples since soft X-ray below 1 keV has chemically important absorption edges such as C, N, and O K-edges, and L-edges of transition metals. In aqueous solutions, interactions of organic molecules are measured in C and N K-edges and solvent water is separately observed in O K-edge. However, it is difficult to measure XAS of liquid in transmission mode since the thickness of a liquid layer should be less than 1 µm due to large soft X-ray absorption coefficient of liquid. We have successfully developed a liquid flow cell for XAS in transmission mode, where the absorbance of liquid samples can be easily optimized by controlling the liquid thickness. In this review, we report on the temperature dependent change of hydrogen bond in liquid water by O K-edge XAS and intermolecular interactions of pyridine with water in aqueous pyridine solutions at different molar fractions by using XAS in C, N, and O K-edges. For the operando observation of electrochemical reaction, we have developed an electrochemical cell with built-in electrodes and measured Fe L-edge XAS of aqueous iron sulfate solutions during electrochemical reaction under the same scan rate as in cyclic voltammetry. For understanding the phase transition mechanism, not only the mixed phase but also liquid-liquid interfaces after phase transition should be investigated by spatially resolved XAS. We have developed a liquid flow cell that is set in a scanning transmission X-ray microscope. The mechanism of a lower critical solution temperature in aqueous triethylamine solution is studied by spatially resolved XAS of the liquid-liquid interfaces between triethylamine and water phases with the spatial resolution of 140 nm.