Dissolution of NaCl nanocrystals: an ab initio molecular dynamics study

Abstract

The dissolution of NaCl has been systematically investigated by employing ab initio molecular dynamics (AIMD) on different NaCl nanocrystals as well as on a surface system immersed in water. We discovered a complex dissolution process simultaneously involving multiple ions initiated at the corner sites of the crystal. Our simulations indicated a difference in the dissolution rates of sodium and chlorine. While sodiums readily became partially solvated, chlorines more frequently transitioned into the fully solvated state leading to an overall greater dissolution rate for Cl. We determined that this difference arises due to faster water mediated elongations of individual ionic bonds to Na, but a significantly slower process for the last bond in comparison to Cl. In an attempt to investigate this phenomenon further, we performed metadynamics based free energy simulations on a surface slab presenting corner sites similar to those in cubic crystals, aiming to extract the dissolution free energy profile of corner ions. In qualitative agreement with the nanocrystal simulations, this revealed a shallower first free energy minimum for Na, but no statistically significant difference in the corresponding barriers and inconclusive results for the latter stage. Finally, simulations of smaller NaCl crystals illustrated how dissolution proceeds beyond the point of crystal lattice collapse, highlighting the strength of solvated ion interactions.