Solvation Structure of I⁻ and Na⁺ on the Surface of NaI Aqueous Solution Studied by Photodetachment Spectroscopy in Combination with Mass Spectrometry

Abstract

We investigated solvation structures of I− and Na+ on an aqueous solution surface by photodetachment spectroscopy and mass spectrometry. An aqueous solution of NaI was introduced into the vacuum as a continuous liquid flow (liquid beam), and the liquid beam was irradiated with a UV laser pulse. The abundance of electrons emitted by the laser excitation was measured as a function of wavelength (photodetachment spectroscopy). For a concentrated aqueous solution of NaI, we observe an absorption peak at longer wavelengths than the charge-transfer-to-solvent band of I− in solution. This feature is assigned to the photoabsorption of I− at the surface. This finding indicates that when the concentration of NaI is high (>1.0 M), I− exists on the solution surface. The identity of the ion clusters ejected from the liquid beam following selective laser excitation of I− on the surface or I− inside the solution was revealed by mass spectrometry. The mass spectra show that Na rich clusters are formed when I− inside the solution is excited, whereas Na rich clusters are hardly formed by the excitation of surface I−. These findings lead us to conclude that Na+ does not exist on the surface of the NaI aqueous solution.

Solvation structure of the halides from x-ray absorption spectroscopy

“…128 Halide polarization is predicted to cause ion migration to, and a strongly preferred residence at, any air-water interface, e.g., that of suspended aerosol droplets. 44,45,[56][57][58][59]120,[122][123][124][125] Greater polarization is predicted to produce greater surface concentrations. Thus, the polarization induced surfaceenhanced solvation asymmetry of halides has been given an important role in the atmospheric chemistry of saline aerosols.…”

“…7,8,38,41,45 Experimental support for this effect has been reported. [56][57][58][59] Minuit X-ray absorption near-edge (MXAN) analysis has emerged as a powerful method of deriving local structure from x-ray absorption spectroscopy (XAS) spectra. [60][61][62][63] MXAN uses extended continuum multiple scattering (ECMS) theory to iteratively fit the entire XAS spectrum, using an input structural model that is relaxed during the course of the fit.…”

Extreme surface propensity of halide ions in water

“…The volume of a single water molecule corresponds to a sphere with a radius of 2.0 Å, while the chloride and iodide anions have radii of 1.81 and 2.2 Å, respectively 34 . Given that no ion pairing occurs for the studied salts 35,36 , that the volume taken by a bare Na þ ion is B6/B11 times smaller than that of a bare Cl À /I À ion (ionic radius of sodium: 1.02 Å), and that the sodium ions are hardly present in the probed surface layer 11,15,16,24 , we neglect the effect of the cation in calculating interfacial concentrations. We derive an effective interfacial concentration of chloride of (2.0/1.81) 3 Our findings are of fundamental importance for processes in atmospheric chemistry.…”

Charge Transfer to Solvent Dynamics at the Ambient Water/Air Interface

“…The pump wavelength was chosen to be on the long-wavelength side of the lowest energy bulk I -CTTS transition since previous SHG 42 and photoelectron spectroscopy 43,44 studies suggested that the transition is red-shifted for I -at the surface compared to the bulk.…”