Surface Propensities of Atmospherically Relevant Ions in Salt Solutions Revealed by Phase-Sensitive Sum Frequency Vibrational Spectroscopy

Abstract

Supporting Information: Experimental Details and Spectral Analysis NaCl (99.999% trace metals basis), KCl (99.999% trace metals basis), NaNO 3 (99.995% trace metals basis), NH 4 Cl (99.998% trace metals basis), Na 2 SO 4 (99.99% trace metals basis), and (NH 4 ) 2 SO 4(99.999% trace metals basis) were purchased from Sigma-Aldrich. NaCl, KCl and Na 2 SO 4 were baked at 500 o C before use. For the aqueous solutions, proper amount of salts were dissolved in ultra-pure distilled water (with resistivity of 18.3 MΩ/cm, Thermo Fisher-EASYpure). Hydrophobic PTFE syringe filter (VWR) was then used to filter out residue particles from solutions. All glassware was soaked in concentrated H 2 SO 4 mixed with NoCromix, and then rinsed thoroughly with ultra-pure water.The details of our PS-SFVS setup have been described elsewhere. 1 We used collinear geometry with an incident angle of 45 o for the green (532nm, 20ps) and IR (3000-3800 cm -1 , 20ps) beams with input energies of ~300 µJ/pulse and ~120 µJ/pulse, respectively. The two beams overlapped in time and space at the sample surface, one focused to an area of 0.2 mm 2 and the other to 0.1 mm 2 . The SF output was collected after proper filtering by a gated detection system. The signal is proportional to χ eff

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

“…In line with the results of spectroscopic experiments performed over much longer time scales, 36 we find that anions are effectively segregated as if those having larger propensities for the air-liquid interface are enriched in the thinner sectors of the stretched bags. In other words, ion gradients over the span of stretched bags approach those expected from equilibrium distribution considerations.…”

Hofmeister effects in micromolar electrolyte solutions

“…This is borne out by the negative surface potential of most electrolyte solutions, 50, 51 by surface-specific spectroscopic studies, 34,[52][53][54] and by model calculations.…”

Specific cation effects at aqueous solution−vapor interfaces: Surfactant-like behavior of Li + revealed by experiments and simulations

“…For example, surface tension increments (STIs; differences between the surface tension of a salt solution and that of neat water) of sodium salts at the same concentration decrease in the order: SO 4 2− > Cl − > Br − > NO 3 − > I − (15, 16). Molecular dynamics (MD) simulations have predicted that the propensity of anions to adsorb to the solution-vapor interface follows the Hofmeister series in reverse (7,14,17), and this prediction has largely been confirmed experimentally (14,(18)(19)(20)(21)(22). Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2 SO 4 (23)], anions adsorb more strongly to the solution−air interface than their counter cations, and, consequently, electrical double layers are formed near the interface, with the anions residing in or near the topmost layer of the solution, and the cations residing below the anions (14,24,25).…”

“…Moreover, MD simulations have shown that, with few exceptions [e.g., SO 4 2− in (NH 4 ) 2 SO 4 (23)], anions adsorb more strongly to the solution−air interface than their counter cations, and, consequently, electrical double layers are formed near the interface, with the anions residing in or near the topmost layer of the solution, and the cations residing below the anions (14,24,25). Surface potentials (26), phase-sensitive vibrational sum frequency generation (PS-VSFG) spectra (22,27,28), and X-ray photoelectron spectroscopic (XPS) data (19,(29)(30)(31)(32) are consistent with the double layer picture.Compared with anion-specific effects, cation-specific effects at the solution−air interface are generally observed to be relatively weak. For example, the concentration dependence of the STIs of LiCl, NaCl, and KCl are very similar (33).…”

“…In one of the few studies that directly determined cation-specific effects on ion distributions in the interfacial region, XPS spectra and MD simulations revealed that Na + approaches the solution−air interface more closely than Rb + , and that the interfacial population of Cl − is greater in NaCl vs. RbCl solutions (31). PS-VSFG measurements, which provide indirect information on interfacial ion distributions via surface electric fields inferred from the imaginary part of the nonlinear susceptibility, have provided evidence of cation-specific effects on the strength of the electric double layer at the solution−air interfaces of nitrate, sulfate, and halide salt solutions (22,27,28).In almost all aqueous salt solutions of salts containing alkali metal cations, the cations are excluded from the topmost layer of the solution (14). It has been suggested, based on MD simulations (34)(35)(36), that Li + may be an exception.…”