Two-dimensional ionic liquids with an anomalous stepwise melting process and ultrahigh CO2 adsorption capacity

Ultrathin ionic liquid (IL) films have shown wide applications in the chemistry and materials fields. However, the structure feature and quantitative controlling mechanism of thin IL films have rarely been reported to date. Here, computational simulations combined with scanning tunnel microscope experiments are used to quantify the structure and function of the thinnest possible IL films, two-dimensional (2D) ILs, which consist of 2D ordered mono-ionic IL structures. Interestingly, the 2D ILs exhibit anomalous stepwise melting processes, involving localized rotated, out-of-plane flipped, and fully disordered states, which are different from 3D ILs. Meanwhile, a theoretical model and temperature-surface phase diagram are constructed to evaluate the critical stepwise melting behaviors. Furthermore, the 2D ILs also possess ultrahigh CO2 adsorption capabilities and structural robustness during the CO2 adsorption-desorption process. These findings are promising for the rational design of an IL-solid interface for CO2-capture-fixation chemistry and related applications.