High Perfluorooctanoic Acid Adsorption Performance of Sub-monolayer Fluoroalkylsilane on Silicon Wafers

In recent years, per/polyfluoroalkyl substances (PFAS) have gained widespread attention owing to their potential hazards to human health and the environment. However, their chemical characteristics render detection and adsorption challenging. In this study, we controlled the density of polyfluoroalkylsilane (PFS) on silicon wafers from sub-monolayers to monolayers. Two PFSs, namely, (CF₃(CF₂)₅(CH₂)₂SiCl₃ and CF₃(CF₂)₉(CH₂)₂SiCl₃), were used to investigate the effect of the -CF₂- length on the adsorption characteristics. For each surface, we measured the change in surface energy due to the adsorption of perfluorooctanoic acid (PFOA) in an aqueous solution via contact angle measurements. Then, the maximum surface excess (Γ_max), Langmuir coefficient (K_L), and initial surface energy (σ₀) were determined based on a Szyszkowski-Langmuir model. We found that Γ_max and K_L have maxima in the sub-monolayer region and were smaller in monolayers or over monolayer regions. Interchain interactions in polyfluoroalkylsilane monolayers and their interaction mechanisms with PFOA were logically deduced. These findings strongly suggest that the side of the PFS chain rather than the top is a strong adsorption site for PFAS. Recent studies have revealed that the stratified dipole array (SDA) model can explain intermolecular PFAS interactions, and the results of this study are consistent with these findings. This study provides a new direction for the development of functional surfaces for the efficient detection and adsorption of PFAS.