Nucleation in alkali metal chloride solution observed at the cluster level

Abstract

Nucleation processes of alkali metal chlorides (MCl) in methanol were studied by specially designed electrospray ionization mass spectrometry at the cluster level. From solutions of LiCl, NaCl, KCl and RbCl, the M⁺(MCl)_n clusters with n = 4, 13 and 22 were observed prominently as magic number clusters. These clusters correspond to a square-planar 3 × 3 × 1, cubic 3 × 3 × 3, and cuboid 3 × 3 × 5 structure, respectively. On the other hand, from the solution of CsCl, the Cs⁺(CsCl)_n clusters were almost monotonically decreased with an increase in the cluster size n, without showing any magic number species. This has a good correlation with the difference in the crystalline structures. When nucleation occurs in the condensed phase, the difference in the crystalline structure will be realised at the cluster level. Moreover, it was demonstrated that the mass distributions of K⁺(KCl)_n and Cl⁻(KCl)_n′ clusters look very similar. When these inter-cluster interactions take place efficiently, the crystal growth will be accelerated rapidly.