The Roles of Salt Concentration and Cation Charge in Collisions of Ar and DCl with Salty Glycerol Solutions of NaI and CaI₂

Abstract

Gas-liquid scattering experiments are used to investigate the roles of ion concentration and ion charge in reactions of DCl with glycerol containing dissolved NaI and CaI 2 . Previous studies show that DCl molecules follow one of three pathways upon adsorption at the surface of pure glycerol: immediate desorption of DCl back into the gas phase, near-interfacial DCl f HCl exchange, and longtime solvation and dissociation. The electrolytes NaI and CaI 2 enhance immediate DCl desorption and D f H exchange at the expense of bulk solvation. We find that these enhancements rise linearly from 1.2 to 3.9 M NaI (5.5 to 1.4 glycerol molecules per ion), suggesting that the limited solvation of Na + and I -and greater ion-ion association at higher concentrations do not abruptly change or reverse trends in these pathways. DCl desorption and D f H exchange are equally enhanced by 0.7 M CaI 2 and 1.2 M NaI (nearly equal I -concentrations), but 1.3 M CaI 2 is twice as effective as 2.6 M NaI. This departure may be driven by the closer proximity of Ca 2+ to the surface at higher ion concentrations, especially if ion pairing between Ca 2+ and surface-active I -drags the cation toward the interfacial region. Argon atom scattering and surface tension measurements provide independent evidence for the presence of ions at the surfaces of the salt solutions.

Specific Na+ and K+ cation effects on the interfacial water molecules at the air/aqueous salt solution interfaces probed with nonresonant second harmonic generation

“…[3,5,12] In the experimental studies on the reactive and non-reactive molecular beam scattering from the salty liquid solution surfaces, such as from the glycerol salt solution surfaces, various cation and anion effects have been observed. [79,80,81] Since molecular beam scattering would be difficult to be implemented for the more volatile liquids than the glycerol, such as water, it would be worthwhile to investigate the ion solvation and interactions at these non-aqueous liquid surfaces with the nonlinear optical techniques and to compare the results.…”

Atomic and Molecular Collisions at Liquid Surfaces

“…Relative to pure glycerol, solutions containing NaI and CaI 2 slightly increase the peak translational energy of IS, inhibit DCl trapping at high collision energies, and promote trapping-desorption and trapping-exchange-desorption at the expense of trapping-solvation. These observations suggest that NaI, CaI 2 , and NaBr make the glycerol surface mechanically stiffer and smoother, and it is hypothesized that these salts modulate the post-trapping pathways of DCl by decreasing the availability of the glycerol hydroxyl groups that promote DCl dissociation and H + /D + transport (15,105,106). Like NaI, CaI 2 , and NaBr, glycerol solutions of the ionic surfactant tetrahexylammonium bromide (THABr) also promote DCl trapping-desorption and trapping-exchangedesorption at the expense of trapping-solvation.…”

“…High translational energies, specular angular distributions, and reductions in energy transfer with increasing incident angle imply that IS projectiles undergo one or few collisions with the outermost atoms of the interface. For example, hyperthermal Ar scattering experiments on glycerol solutions of 2.6 M NaI, 2.6 M KI, 2.5 M LiI, 2.7 M NaBr, and 1.3 M CaI 2 all show an increase in the flux and average energy of IS Ar leaving the liquid surface relative to pure glycerol (15,105,106). This increased flux is consistent with the formation of a smoother, stiffer interface, likely owing to OH interactions with ions in the interfacial region.…”

Gas–surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers

“…4 Alternatively, projectiles that lose enough energy, through single or multiple collisions, to become trapped at the surface are often assigned to the TD channel. Extensive work by the Nathanson group explored molecular reactions and interactions by scattering a variety of gases from acidic and basic liquids, [11][12][13][14][15][16] salty solutions, [17][18][19] and molten metals. 9,20 Gas-surface collision dynamics studies using SAMs were first performed by Sagiv and co-workers to examine translational energy transfer from atoms and molecules to model organic surfaces.…”