Département de chimie moléculaire

We report mol. dynamics studies on the solvation of sodium chloride in the 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide ionic liq. ([BMI][Tf2N] IL). We first consider the potential of mean force for dissocg. a single Na+Cl- ion pair, showing that the latter prefers to be undissociated rather than dissocd. (by ca. 9 kcal/mol), with a free energy barrier of ca. 5 kcal/mol (at d ≈ 5.2 {\AA}) for the assocn. process. The preference for Na+Cl- assocn. is also obsd. from a 100 ns mol. dynamics simulation of a concd. soln., where the Na+Cl- ions tend to form oligomers and microcrystals in the IL. Conversely, the simulation of Na13Cl14- and Na14Cl13+ cubic microcrystals (with, resp., Cl- and Na+ at the vertices) does not lead to dissoln. in the IL. Among these, Na14Cl13+ is found to be better solvated than Na13Cl14-, mainly due to the stronger Na+···Tf2N- interactions as compared to the Cl-···BMI+ interactions at the vertices of the cube. We finally consider the solid/liq. interface between the 100 face of NaCl and the IL, revealing that, in spite of its polar nature, the crystal surface is solvated by the less polar IL components (CF3(Tf2N) and butyl(BMI) groups) rather than by the polar ones (O(Tf2N) and imidazolium(BMI) ring). Specific ordering at the interface is described for both Tf2N- anions and BMI+ cations. In the first IL layer, the ions are rather parallel to the surface, whereas in the second "layer" they are more perpendicular. A similar IL structure is found at the surface of the all-neutral Na0Cl0 solid analog, confirming that the solvation of the crystal is rather "apolar", due to the mismatch between the IL and the crystal ions. Several comparisons with water, methanol, or different BMI+-based ILs as solvents are presented, allowing us to better understand the specificity of the ionic liq.-NaCl interactions. [on SciFinder(R)]