On probing ions at the gas–liquid interface by quantum state-resolved molecular beam scattering: the curious incident of the cation in the night time

Abstract

There has been a long standing controversy over the preferential presence or absence of cations vs. anions at the gas-liquid interface, dating back to early theoretical efforts by Onsager and co-workers [J. Chem. Phys., 1934, 2, 528]. In the present work, we describe our first efforts to selectively probe ions at the interface via a completely novel approach, based on scattering high energy, jet cooled molecular projectiles from the surface of hydrogen bonded liquids with dissolved alkali halide salts as the source of solvated charges. In particular, this work focuses on preliminary results from quantum state- resolved scattering studies as a function of anion (Cl-, I-), and cation identity (Li+, Na+, K+) in alkali halide/glycerol solutions. By way of physical picture, a quadrupolar projectile such as CO2 preferentially aligns parallel (anions) or perpendicular (cations) to the interface, which could therefore make rotational energy transfer sensitive to the sign of the ion charge. Experimentally, we find that the impulsive scattered (IS) CO2 rotational distributions reveal a clear dependence on anion identity (e.g., hotter for [I-] than [Cl-]), but with essentially no corresponding sensitivity to cation species (e.g., [Li+] indistinguishable from [K+]). Detailed trajectory calculations are used to provide additional insight into the anticipated and observed experimental trends.

Stereodynamical Effects by Anisotropic Intermolecular Forces

“…Such orientation effects can influence the dynamic of the charge-exchange process by channeling most of the neutral molecules in narrow angular cones confined around the most attractive configurations of the interacting systems. Similar effects have been also observed at the interface between the gas and liquid phase for solutions containing cations and anions (Gisler and Nesbitt, 2012).…”

Atomic and Molecular Collisions at Liquid Surfaces

“…These studies show that the high-energy component of the scattered CO 2 rotational distribution is enhanced for 2.5 M NaI in glycerol relative to pure glycerol. Furthermore, these high-energy rotational states were observed to be sensitive to anion identity (Cl − versus I − ) but not cation identity (K + versus Na + ) (115). If it is assumed that cations and anions influence the structure of the interface equally and that the observed rotational distributions are equally sensitive to surface-accessible cations and anions, these results suggest that the anions dominate the outermost region of the liquid surface.…”

Inelastic scattering of OH radicals from organic liquids: isolating the thermal desorption channel

“…The majority of inelastic scattering studies have been of non-reactive systems, the most thoroughly investigated of which is CO 2 from a range of low-volatility liquids which include the benchmark hydrocarbon squalane. 7,[18][19][20][21]37,38 For both inelastic and reactive scattering, it is widely accepted that the scattered products may be broadly split, at least empirically, into two categories. 5 Impulsively scattered (IS) products have characteristically high translational energies and strongly directed angular distributions.…”