Investigating Water Behavior on Repellent Self-Assembled Materials via Soft X-Ray Emission Spectroscopy

Self-assembled monolayers (SAMs) are nanostructured materials that can easily impart various functions by bonding to a substrate^[1]. In particular, SAMs containing fluorine are known to be able to impart unique functions to substrates, such as water repellency and slipperiness, due to their low surface tension, which are not found in other elements^[2]. Understanding the repellent mechanism of materials at the molecular level is useful for the development of functional materials, but little is known about the influence of the chemical structure of SAMs on water adsorption behavior.
In this study, we employed soft X-ray emission spectroscopy (XES), a powerful tool for examining the occupied electronic structure of light elements including water, to acquire a deeper understanding of water adsorption behavior on SAMs with different water repellency and chain structures. We conducted XES measurements on hydrocarbon and fluorocarbon chain structures of SAMs with different water repellency by using a setup developed by our research group that enables XES measurements under precise humidity control. The results demonstrated that under medium humidity conditions, water molecules were progressively trapped on the surface and within the hydrocarbon chains as the supply of water molecules increased. However, such behavior was not observed in the fluorocarbon chains. In contrast, under low humidity conditions, there was no significant change in the initial adsorption phase of water for both hydrocarbon and fluorocarbon chains.
Based on the results of XES and molecular dynamics simulations, we discuss in detail the behavior of water adsorption on molecular chains with different water repellency.

References:
[1] Love J. C. et al., Chem. Rev., 2005, 105, 1103–1170.
[2] Sidharam P. P. et al., Langmuir, 2012, 28, 17690–17700.
[3] Yamazoe, K. et al., Langmuir, 2017, 33, 3954–3959.