Probing the early stages of salt nucleation--experimental and theoretical investigations of sodium/potassium thiocyanate cluster anions.

Abstract

Due to the fast solvent evaporation in electrospray ionization (ESI), the concentration of initially dilute electrolyte solutions rapidly increases to afford the formation of supersaturated droplets and generation of various pristine anhydrous salt clusters in the gas phase. The size, composition, and charge distributions of these clusters, in principle, witness the nucleation evolution in solutions. Herein, we report a microscopic study on the initial stage of nucleation and crystallization of sodium/potassium thiocyanate salt solutions simulated in the ESI process. Singly charged Mx(SCN)x+1(-), doubly charged My(SCN)y+2(2-) (M = Na, K), and triply charged Kz(SCN)z+3(3-) anion clusters (x, y, and z stand for the number of alkali atoms in the singly, doubly, and triply charged clusters, respectively) were produced via electrospray of the corresponding salt solutions and were characterized by negative ion photoelectron spectroscopy (NIPES). The vertical detachment energies (VDEs) of these sodium/potassium thiocyanate cluster anions were obtained, and theoretical calculations were carried out for the sodium thiocyanate clusters in assisting spectral identification. The measured VDEs of singly charged anions Mx(SCN)x+1(-) (M = Na and K) demonstrate that they are superhalogen anions. The existence of doubly charged anions My(SCN)y+2(2-) (y = 2x, x ≥ 4 and 3 for M = Na and K, respectively) and triply charged anions Kz(SCN)z+3(3-) (z = 3x, x ≥ 6) was initially discovered from the photoelectron spectra for those singly charged anions of Mx(SCN)x+1(-) with the same mass-to-charge ratio (m/z), and later independently confirmed by the observation of their distinct mass spectral distributions and by taking their NIPE spectra for those pure multiply charged anions with their m/z different from the singly charged species. For large clusters, multiply charged clusters were found to become preferred, but at higher temperatures, those multiply charged clusters were suppressed. The series of anion clusters investigated here range from molecular-like M1(SCN)2(-) to nano-sized K22(SCN)25(3-), providing a vivid molecular-level growth pattern reflecting the initial salt nucleation process.