Adsorption of Hydrophilic Amine-Based Protic Ionic Liquids on Iron-Based Substrates

We synthesized hydrophilic amine-based protic ionic liquids (PILs) with hydroxy groups in their cations and anions, and characterized their adsorption at a solid (iron-based substrate) / aqueous solution interface. The IL samples employed in this study were triethanolamine lactate, diethanolamine lactate, and monoethanolamine lactate. Quartz crystal microbalance with dissipation monitoring (QCM-D) measurements revealed that the adsorption mass of the hydrophilic PILs was larger than that of the comparative materials, including a non-IL sample (1,2,6-hexanetriol) and an OH-free sample in the cations (triethylamine lactate). Additionally, an increase in the number of hydroxy groups in the cations resulted in an increased adsorption mass. Force curve measurements by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) measurements proved the high adsorption density of the hydrophilic PILs on the iron-based substrate. A decreased kinetic friction coefficient was also observed in the hydrophilic PIL systems. Moreover, hydrophilic PILs are expected to have potential applications as water-soluble lubricants and additives for metal surface treatments.