Structural Stability of NaCl and KCl Cleavage Surfaces in the BMIM-PF₆ Ionic Liquid

We have investigated the evolution of freshly cleaved NaCl(100) and KCl(100) surfaces exposed to the ionic liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) and repeatedly scraped using atomic force microscopy (AFM). The response of the two surfaces to the IL is completely different. On NaCl, the cleavage step edges are slightly eroded, and the surface is progressively smoothed by the AFM tip. These changes are accompanied by a continuous increase in the friction force. On KCl, a dramatic dissolution of the surface is observed immediately after bringing it into contact with the IL. The surface is then smeared along the fast scan direction in the area scratched by the tip and even beyond. An increase in the friction force is also observed but only in the beginning of the surface modification process. Crystallites (∼100-200 nm in size) are observed all over the unscratched areas of KCl but not of NaCl. This result is supported by molecular dynamics simulations and Raman spectroscopy, which indicate a much stronger interaction of the IL with the KCl surface and, respectively, the formation of a BMIM-PF₆ solid phase on it. The analysis performed on the model systems presented here could be extended to other ionic crystal surfaces in contact with ILs, the possible degradation of which must be evaluated in view of their use in catalysis and energy storage applications.