Structure of Self-Assembled Monolayer Interfaces during Vapor Adsorption

Abstract The modification of materials with self-assembled monolayers (SAMs) enables control of interfacial interactions, making SAMs relevant in catalytic, electronic, and biological systems. For control of the interfacial properties in these applications, it is important to understand the interactions between the SAM ligand tail and its chemical environment. In this work, we use in situ X-ray reflectivity to probe the structure of octadecyl phosphonic acid (ODPA) on sapphire in gaseous 1-hexene, n-hexane, and water environments. We control the relative pressure of each vapor to probe a range of chemical potentials, varying from vacuum to near-saturation pressure. Moreover, we measure the evolving SAM structure during increasing and decreasing vapor pressure to understand adsorption and desorption, respectively. n-Hexane and 1-hexene exhibit disordered adsorption on ODPA, while there is comparatively minimal interaction with water. Examining the correlation between changes in layer thickness (d) and interface roughness (σ) as a function of chemical potential provides insight into the affinity of hydrocarbon adsorption at the ODPA interface. For ODPA, when all vapor is removed from the SAM environment, the SAM is restored to its initial state, consistent with nearly complete vapor desorption. In comparison, PEGylated phosphonic acid (PEG–PA) indicates extensive water adsorption near saturation and irreversible structural changes on the 10 min time scale. Cumulatively, this work elucidates the nature of vapor adsorption and its role in diffusion within and across SAM layers.