PNNL

Abstract

Due to fast solvent evaporation in electrospray ionization (ESI), the concentration of initially dilute electrolyte solutions rapidly increases to afford formation of supersaturated droplets and generating 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 "M" _"x" ?"(SCN)" ?_"x+1" ^"–" , doubly charged "M" _"y" ?"(SCN)" ?_"y+2" ^"2–" (M = Na, K), and triply charged "K" _"z" ?"(SCN)" ?_"z+3" ^"3–" anion clusters 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 sodium thiocyanate clusters in assisting spectral identification. The measured VDEs of singly charged anions "M" _"x" ?"(SCN)" ?_"x+1" ^"–" (M = Na and K) demonstrate they are superhalogen anions. The existence of doubly charged anions "M" _"2x" ?"(SCN)" ?_"2x+2" ^"2–" (x = 4 and 3 for M = Na and K, respectively) and triply charged anions "K" _"3x" ?"(SCN)" ?_"3x+3" ^"3–" (x = 6) were initially discovered from the photoelectron spectra for those singly charged anions of "M" _"x" ?"(SCN)" ?_"x+1" ^"–" with the same mass-to-charge ratio (m/z), and later independently confirmed by 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 are found to become preferred, but at higher temperatures those multiply charged clusters are suppressed. The series of anion clusters investigated here range from molecular-like "M" _"1" ?"(SCN)" ?_"2" ^"–" to nano-sized "K" _"22" ?"(SCN)" ?_"25" ^"3–" , providing a vivid molecular-level growth pattern reflecting the initial salt nucleation process.