Abstract
The efficient oxidation of iodide and bromide at the aqueous solution-air interface of the ocean or of sea spray aerosol particles had been suggested to be related to their surface propensity. The ubiquitous presence of organic material at the ocean surface calls for an assessment of the impact of often surface-active organic compounds on the interfacial density of halide ions. We used in situ X-ray photoelectron spectroscopy with a liquid micro-jet to obtain chemical composition information at aqueous solution-vapor interfaces from mixed aqueous solutions containing bromide or iodide and 1-butanol or butyric acid as organic surfactants. Core level spectra of Br 3d, Na 2s, C 1s and O 1s at ca. 160 eV kinetic energy and core level spectra of I 4d and O 1s at ca. 400 eV kinetic energy are compared for solutions with 1-butanol and butyric acid as a function of organic concentration. A simple model was developed to account for the attenuation of photoelectrons by the aliphatic carbon layer of the surfactants and for changing local density of bromide and iodide in response to the presence of the surfactants. We observed that 1-butanol increases the interfacial density of bromide by 25%, while butyric acid reduces it by 40%, both in comparison to the pure aqueous halide solution. Qualitatively similar behavior was observed for the case of iodide. Classical molecular dynamics simulations failed to reproduce the details of the response of the halide ions to the presence of the two organics. This is attributed to the lack of correct monovalent ion parameters at low concentration possibly leading to an overestimation of the halide ion concentration at the interface in absence of organics. The results clearly demonstrate that organic surfactants change the electrostatic interactions near the interface with headgroup specific effects. This has implications for halogen activation processes specifically when oxidants interact with halide ions at the aqueous solution-air interfaces of the ocean surface or sea spray aerosol particles.
Highlights
Introduction a Laboratory of EnvironmentalChemistry, Paul Scherrer Institute, 5232 Villigen, Switzerland
We address the difference between an alcohol and carboxylic acid headgroup of a monofunctional surfactant on the abundance of Na+, BrÀ, and IÀ at the interface within the probe depth using Liquid micro-jet (LJ)-X-ray photoelectron spectroscopy (XPS).[48]
We have attempted to obtain density profiles for bromide and sodium in presence and absence of Bu-ol and Bu-acid by molecular dynamics simulations as described in the Electronic supplementary information (ESI).† The results clearly demonstrate that the photoemission signals estimated from those density profiles are not in agreement with the XPS measurements presented above, especially not the striking difference in the Br/O signal intensity ratios shown in Fig. 3c and d
Summary
We made use of the NAPP endstation with the liquid microjet setup at the SIM beamline of the SLS at PSI.[48]. The even smaller amounts of 3rd order light at 687 eV were used to excite the O 1s orbital for a KE of 148.9 eV for liquid water. This procedure[64] allowed determining photoemission signals for each element within one sweep of the electron analyzer in a relatively narrow kinetic energy range. In mixtures with butyric acid, the pH varied from 3.2 to 2.7 and from 3.0 to 2.6, respectively, for the butyric acid mole fraction range from 0.001 to 0.008, for NaBr and NaI solutions.
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