Emergence of Solvent-Separated Na⁺–Cl^– Ion Pair in Salt Water: Photoelectron Spectroscopy and Theoretical Calculations

Abstract

Solvation of salts in water is a fundamental physical chemical process, but the underlying mechanism remains unclear. We investigated the contact ion pair (CIP) to solvent-separated ion pair (SSIP) transition in NaCl(HO) clusters with anion photoelectron spectroscopy and ab initio calculations. It is found that the SSIP type of structures show up at n = 2 for NaCl(HO) anions. For neutral NaCl(HO), the CIP structures are dominant at n < 9. At n = 9-12, the CIP structures and SSIP structures of NaCl(HO) are nearly degenerate in energy, coincident to the HO:NaCl molar ratio of NaCl saturated solution and implying that the CIP and SSIP structures can coexist in concentrated solutions. These results are useful for understanding the solvation of salts at the molecular level.

Hydration breaking and chemical ordering in a levitated NaCl solution droplet beyond the metastable zone width limit: evidence for the early stage of two-step nucleation

“…It is worth noting that the three interest regions based on Raman spectra show quite similar aspects to the results of ion pairing in the vapor state of salt water; 18 for NaCl(H 2 O) n , CIPs and SSIPs coexist at n = 9–12, and CIPs are dominant at n < 9. Since the average number of water molecules per ion is about 1.95 at supersaturation S = 2.31, this result is relatively comparable to that of the vapor state and thus the change of the hydration structure in this study will be useful to understand micro-hydration in the vapor state.…”

“…, the formation of dense liquid regions at the early stage of nucleation in NaCl solutions at high supersaturation which has been suggested by simulation studies only. 18,19,28 Moreover, the stability limit of the dense liquid regions is governed by the evolution of the hydration structure, and the breakage of the hydration structure can trigger nucleation in the dense liquid regions. These results provide a new perspective for detailed understanding of the early stage of two-step nucleation in aqueous solutions with monatomic ions, and will impact many research fields such as solution chemistry, electrochemistry, mineralogy, geochemistry, and molecular biology.…”

Core and Valence Level Photoelectron Spectroscopy of Nanosolvated KCl

“…The initial cluster size is well above the requirements to favor charge separation with low concentrations. 6 , 59 − 61 In bulk conditions, the first hydration shell of both K + and Cl – ions contains ∼6 water molecules and the ion–water distance is ∼0.3 nm to the first hydration shell and ∼0.5 nm to the second. 62 The cluster size is thus sufficiently large to form multiple hydration layers around the ions, at least when the number of available water molecules is concerned.…”

Molecular dynamics simulation, ab initio calculation, and size-selected anion photoelectron spectroscopy study of initial hydration processes of calcium chloride

“…22 Step by step emergence of SSIP of the NaCl ion pair was observed in salt water clusters, and the energy degeneration of cluster structures in NaCl(H 2 O) n implies the coexistence of CIP and SSIP in saturated NaCl solution. 25 Nevertheless, alkaline earth metal halides are not well-studied, one of the reasons is the difficulties of their accurate force field description.…”