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

## TL;DR

This study compares how NaCl and KCl surfaces react when exposed to an ionic liquid, finding that KCl dissolves significantly while NaCl remains stable.

## Contribution

The paper reveals a strong interaction between BMIM-PF6 and KCl surfaces, leading to dissolution and crystallite formation, which is not observed with NaCl.

## Key findings

- NaCl surfaces show slight erosion and smoothing with increased friction.
- KCl surfaces dissolve rapidly and form crystallites when exposed to BMIM-PF6.
- Molecular dynamics and Raman spectroscopy confirm stronger IL-KCl interactions.

## Abstract

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.