Anion Exchange at the Liquid/Solid Interface of Ultrathin Ionic Liquid Films on Ag(111)

Thin ionic liquid (IL) films play an important role in many applications. To obtain a better understanding of the ion distribution within IL mixture films, we sequentially deposited ultrathin layers of two ILs with the same cation but different anions onto Ag(111), and monitored their dynamic behaviour by angle-resolved X-ray photoelectron spectroscopy. Upon depositing [C₈ C₁ Im][PF₆ ] on top of a wetting layer of [C₈ C₁ Im][Tf₂ N] at room temperature (RT), we found a pronounced enrichment of the [Tf₂ N]^- anions at the IL/vacuum interface, due to a rapid anion exchange at the IL/solid interface. In contrast, at 90 K, the [Tf₂ N]^- anions remain at the IL/solid interface. Upon heating, we observe a rearrangement of the cations between 140 and 160 K, such that the octyl chains preferentially point towards the vacuum. Above 170 K, the ions start to become mobile, and at 220 K, the anion exchange is completed, with the [Tf₂ N]^- anions enriched at the IL/vacuum interface in the same way as found for deposition at RT. The temperature range for the anion exchange corresponds well to glass transition temperatures reported in literature. We propose two driving forces to be cooperatively responsible for the replacement/exchange of [Tf₂ N]^- at the IL/solid interface and its enrichment at the IL/vacuum interface. First, the adsorption energy of [C₈ C₁ Im][PF₆ ] is significantly larger than that of [C₈ C₁ Im][Tf₂ N], and second, the surface tension of [C₈ C₁ Im][Tf₂ N] is lower than that of [C₈ C₁ Im][PF₆ ].