Influence of Surfactants with Differently Charged Headgroups on the Surface Propensity of Bromide

Abstract

Halide ions in oceans and sea-spray aerosol particles are an important source of reactive halogen species in the atmosphere that impact the ozone budget and radiative balance. The multiphase cycling of halogen species is linked to the abundance of halide ions at the aqueous solution-air interface. Ubiquitously present surface-active organic compounds may affect the interfacial abundance of halide ions. Here, we use liquid jet X-ray photoelectron spectroscopy and molecular dynamics (MD) simulations to assess the impact of surfactants with different headgroups on the abundance of bromide and sodium ions at the interface. Core level spectra of Br 3d, Na 2s, and O 1s are reported for solutions containing tetrabutylammonium, hexylamine (HA), and propyl sulfate. We used a photoelectron attenuation model to retrieve the interfacial concentration of bromide in the presence of these different surfactants. The experimental results confirm the previously reported strong enhancement of bromide in the presence of tetrabutylammonium at the interface. In turn, propyl sulfate had a minor impact on the abundance of bromide but led to a significantly enhanced concentration of sodium cations. The MD simulations performed for bromide solutions containing hexylammonium and propyl sulfate show an enhancement of the interfacial bromide and sodium concentrations, respectively, comparable to the experimental results. The difference between the measured enhancement of bromide for HA and the nearly nonexistent effect of HA on bromide in the MD simulations is ascribed to the small amounts of hexylammonium present in the experimental solution. The present work suggests an important role of electrostatic interactions at the interface, which may guide the assessment of anion and cation abundances in atmospheric particles more generally.

Figure 1

(a) Br 3d and Na 2s and (b) O 1s spectra (not normalized, same y-scale) of mixed 0.1 M NaBr/0.1 M HA/0.55 M NaCl (pH 12), mixed 0.1 M NaBr/0.1 M HA (pH 12), 0.1 M NaBr, mixed 0.1 M NaBr/0.1 M sodium propyl sulfate (PS^–) (neutral pH), and mixed 0.1 M NaBr/0.1 M sodium PS^–/0.55 M NaCl (neutral pH) aqueous solutions at a photoelectron KE of 155 eV.

Figure 2

Signal intensity ratios of Br/O for (a) 0.1 M TBA-Br and mixed 0.1 M TBA-Br/0.55 M NaCl and for (b) mixed 0.1 M NaBr/0.1 M HA, mixed 0.1 M NaBr/0.1 M HA/0.55 M NaCl, mixed 0.1 M NaBr/0.1 M sodium PS^–, and mixed 0.1 M NaBr/0.1 M sodium PS^–/0.55 M NaCl aqueous solutions as a function of electron KE, after normalization to the corresponding intensity ratio for the 0.1 M NaBr solution, photon flux, and cross-section. Dashed lines are fits of the attenuation model to the data. Their error bars reflect the uncertainties related to the parameters characterizing the neat NaBr solution (see the text in the Discussion section).

Figure 3

Snapshots illustrating the composition of mixed sodium bromide solutions under various conditions: (a) with HA, (b) with HAH⁺, (c) pure NaBr, and (d) with PS^–. Na⁺ ions stand in violet, Br^– ions in dark red, and aliphatic carbon atoms in gray. He headgroups of HAH⁺ and HA are in dark blue, while that for PS^– is in yellow, respectively.

Figure 4

Atomic density profiles calculated from MD simulations of (a) mixed 1 M NaBr/0.5 M HA, (b) mixed 0.5 M NaBr/0.5 M hexylammonium (HAH⁺) bromide, (c) 1 M NaBr, and (d) mixed 1 M NaBr/0.5 M sodium propyl sulfate (PS^–) aqueous solutions. Densities of water molecules are shown in red, Na⁺ ions in violet, Br^– ions in dark red, terminal carbon chains in gray, and the headgroups of HAH⁺and HA are in dark blue, and that of PS^– are in yellow, respectively. To seek clarity, water density has been scaled (divided by a factor 30) so that we can more easily identify the bulk and interface regions.