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

Abstract

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K⁺ and Li⁺ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li⁺ adsorbs to the aqueous solution-vapor interface, while K⁺ does not. Thus, we provide experimental validation of the "surfactant-like" behavior of Li⁺ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K⁺ and Li⁺ ions to a difference in the resilience of their hydration shells.

Keywords: Hofmeister series; air−water interface; aqueous ionic solvation; ion adsorption; specific ion effects.

Fig. 1.

Surface (200 eV KE) and bulk (600 eV KE) LJ-XPS spectra for (A and B) 2.0 M KI solutions and (C) LiI solutions. (D) Ratios of normalized cation/O1s signals plotted vs. photoelectron KE (probe depth). The zero of the vertical axis for the surface spectra in A–C has been offset to clearly display the surface and bulk spectra on the same plots. The spectral intensities in A–C have been corrected for the photon flux, and the photoionization cross-section so that direct comparisons of the intensities in these plots are meaningful. Since the I4d and the Li1s peaks occur in the same range of binding energies, the change in the scaling due to the significant difference in cross-sections is shown clearly (e.g., in C) by the discontinuity in the signal-to-noise as one goes from the I4d region to the Li1s region of the spectrum. The error bars shown in D include statistical errors in the determination of peak areas from the fitting routine and the precision of multiple experiments.

Fig. 2.

Ratios of normalized XPS signals (I4d/K2p for KI and I4d/Li1s for LiI) plotted vs. photoelectron KE (probe depth) for 2.0 M KI and 2.0 M LiI solutions. The error bars include contributions from the statistics of the spectral fit to obtain peak areas and the precision of replicated experiments. The error bars do not include any contribution from the unknown uncertainty in the ionization cross-sections.

Fig. 3.

Density profiles of ions and water oxygen atoms from MD simulations of (A) 2 M KI and (B) 2 M LiI solutions. The density profile of each species has been calculated with respect to the instantaneous solution−air interface and divided by the corresponding bulk density, ρ₀. The interface is located at depth = 0.

Fig. 4.

Depth dependence of the electrostatic interaction energies of (A) K⁺ ions in 2 M KI and (B) Li⁺ ions in 2 M LiI. The green curves are contributions from cation−cation interactions, the blue curves are from cation−anion interactions, and the magenta curves are from cation−water interactions. The black curves are the total electrostatic interaction energies. (C) Density profile for K⁺ ions in 2 M KI. (D) Density profile for Li⁺ in 2 M LiI. (E) Number of water molecules in the first solvation shell of K⁺ in 2 M KI. (F) Number of water molecules in the first solvation shell of Li⁺ in 2 M LiI. All quantities in this figure were calculated with respect to the instantaneous solution−air interface. The interface is located at depth = 0.