The interplay of water and the hydrophilic ionic liquid [C4C1Im][BF4] on Pt(111)

• First in situ ARXPS study on ultrathin films of a hydrophilic ionic liquid on clean and water pre-covered Pt(111) • [C 4 C 1 Im][BF 4 ] destabilizes co-adsorbed water on Pt(111), reducing the water desorption temperature by ∼9 K • Comprehensive comparison with similar hydrophobic film systems • Interfacial behavior and thermal evolution – including ionic liquid decomposition of IL in contact with Pt(111) above 185 K – closely resemble those of hydrophobic ILs We investigated the thermal evaluation of ultrathin coadsorbed films of the hydrophilic ionic liquid (IL) [C 4 C 1 Im][BF 4 ] and water (D 2 O) on Pt(111) by temperature-programmed angle-resolved X-ray photoelectron spectroscopy. Ultrathin crystalline water wetting layers were prepared at 140 K, followed by deposition of one or several IL layers at 100 K. When post-deposited at 100 K, the IL primarily adsorbs on top of the water layer; upon heating, first an exchange process occurs at ∼130 K, such that the IL replaces water from the Pt surface into the second layer, as deduced from a lowered water desorption temperature by ∼9 K. At 150 K, water has desorbed, leaving a pure IL layer with a characteristic checkerboard arrangement of alternating anions and flat-lying cations. Upon further heating, this layer behaves identical to an IL layer directly adsorbed on clean Pt(111) at 100 K: Above ∼185 K, the IL decomposes stepwise, with anion-derived fragments fully desorbing until 400 K whereas most cation-derived products remain as carbonaceous residues up to at least 550 K. The observed behavior for [C 4 C 1 Im][BF 4 ] is very similar to that of the previously studied hydrophobic ILs [C 3 CNC 1 Im][Tf 2 N] and [C 1 C 1 Im][Tf 2 N], which demonstrates that IL hydrophilicity does not markedly affect water-IL co-adsorption on Pt(111).