<Abstract>
A research team from Toyohashi University of Technology, the Institute for Molecular Science, and the Institute of Science Tokyo, led by Yu Kinjo and Professor Ryugo Tero of Toyohashi University of Technology, has developed a new method to investigate the state of water near material surfaces using X-rays. By combining X-ray absorption spectroscopy with data analysis, the researchers successfully separated the weak signals of interfacial water from those of the much larger amount of surrounding water. They also revealed differences in the state of interfacial water between inorganic material surfaces and lipid bilayers that mimic biological membranes. This method is expected to contribute to understanding various phenomena occurring at interfaces between materials and water.
The study was published online in Applied Physics Express on October 1, 2026.
https://doi.org/10.35848/1882-0786/aea737
<Main>
Water is essential for our daily lives and biological activities. Near the surfaces of materials in contact with water, water molecules can exhibit structures and properties different from those of ordinary liquid water due to their interactions with the materials. Such water is called "interfacial water" and plays important roles in various phenomena, including molecular adsorption onto materials, chemical reactions, and biological molecular functions. Investigating the state of interfacial water at the molecular level is important for understanding material properties and biological processes.
One method for investigating the state of water molecules is X-ray absorption spectroscopy (XAS). This technique uses X-rays to probe the electronic states of water molecules and their surrounding hydrogen-bonding environments by measuring how much X-ray radiation is absorbed at different energies. However, interfacial water near material surfaces accounts for only a small fraction of the total water. Consequently, in conventional measurements, its weak signal is overwhelmed by that of the much larger amount of ordinary water, known as bulk water, making it difficult to extract information specifically from interfacial water.
To overcome this difficulty, the research team developed a method that combines X-ray absorption measurements at different water-layer thicknesses with numerical data analysis. Using a liquid sample cell, the researchers varied the thickness of the water layer through which the X-rays passed (Fig. 1). As the water layer becomes thicker, the amount of bulk water increases, whereas the amount of interfacial water near the material surface remains nearly constant. By taking advantage of this difference, the researchers measured a series of X-ray absorption spectra at different water-layer thicknesses (Fig. 2(a)). They then applied a data analysis technique called multivariate curve resolution–alternating least squares (MCR-ALS) to separate the spectra associated with bulk water and interfacial water (Fig. 2(b)). The experiments were conducted at the UVSOR-III synchrotron radiation facility of the Institute for Molecular Science.
The researchers applied this method to an oxide layer on the surface of silicon nitride, an inorganic material, and to lipid bilayers used as models of biological membranes. The results showed that interfacial water near the oxide surface exhibited distinct spectral features associated with interactions with the solid surface and hydrogen bonding between water molecules. In contrast, when lipid bilayers were present, the interfacial-water spectrum showed a smoother spectral profile, and the relative contribution of interfacial water increased (Fig. 3). These findings suggest that water molecules near the inorganic surface have relatively constrained structures, whereas water molecules near the soft and fluid lipid bilayer exhibit a wider variety of hydrogen-bonding environments.
This study demonstrates that combining water-layer thickness modulation with data analysis enables the extraction of X-ray absorption spectra of interfacial water, which have been difficult to isolate using conventional measurements.
<Future Directions>
The method developed in this study is expected to be applicable not only to inorganic materials and lipid bilayers but also to interfaces between water and various functional materials, including polymers. In future studies, the research team aims to investigate how interfacial water changes depending on the type and surface chemistry of materials. By combining the present method with molecular simulations and theoretical calculations, the researchers also seek to clarify the relationship between the X-ray absorption spectra and molecular structures of interfacial water. These advances are expected to contribute to the design of materials that function in aqueous environments and to a better understanding of interactions between biological membranes and water.
<Publication>
Y. Kinjo, A. Katase, T. Hayashi, M. Nagasaka and R. Tero (2026). Resolving interfacial water from bulk water by thickness-modulated soft X-ray absorption spectroscopy. Applied Physics Express , 19 (10), 107001. https://doi.org/10.35848/1882-0786/aea737
<Acknowledgement>
This work was supported by JSPS KAKENHI (Grant Nos. JP25K03396, JP26K24036 and JP26K01354) and Nippon Sheet Glass Foundation for Materials Science and Engineering. XAS experiments were conducted at the BL3U of UVSOR Synchrotron Facility, Institute for Molecular Science (IMS program 25IMS6620). The authors acknowledge support from the Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Applied Physics Express
Experimental study
Not applicable
1-Oct-2026