Solutions of hypobromite (OBr-) have long been used as titrants, and OBr- is a powerful oxidant. Oxidation of bromide (Br-) to OBr- is one of the important reactions to produce HOBr, which initiates the release of bromine out of the aqueous solution of sea water, sea spray or marine aerosols in the form of molecular halogen compounds that later drive O3 depleting chemistry in the troposphere. The multiphase reaction of oxidation of Br- to OBr- involves an intermediate [BrOOO]-, which has recently been shown to occur preferentially at the liquid-vapor interface and in that way provides for the route for efficient oxidation of bromide at the surface. It is crucial to get the surface properties of each species involved. The surface properties of Br- have been investigated since a decade, with some open questions still. In turn, the surface propensities of OBr- have not yet been addressed at all. OBr- is unstable and disproportionates to bromide and bromate, and thus not many studies characterizing its properties in aqueous solution have been done. A common way to prepare OBr- in aqueous solution is by adding bromine to a solution of an alkali metal hydroxide (e.g. adding Br2 into NaOH aqueous solution in an ice-bath). Here, we directly measure the surface propensity of OBr- at the aqueous solution-air interface using the surface sensitive liquid jet X-ray photoelectron spectroscopy (XPS), performed at the Swiss Light Source (SLS), by aiming at depth profile information for OBr- and Br-. We acquired Br 3d core level spectra of 0.08 M aqueous solutions composed of equal amounts of NaOBr and NaBr. The Br 3d region was fit by two spin-orbit split doublets, with the peak assigned to OBr- separated by 2.2 eV from that of Br-. By varying the photoelectron kinetic energy and thus probe depth via variation of the probing photon energy, the Br 3d to O 1s signal intensity ratios increase with photoelectron kinetic energy for both, Br- and OBr-. This indicates that both ions are rather depleted from the interface, in line with surface tension measurements for the case of bromide. This sheds light on mechanistic and structural aspects of the reaction of bromide with ozone. In the future, theoretical calculations as well as the first principle molecular dynamics simulations will be done to further explain the propensity of OBr- at the liquid-vapor surface.

Surface properties of hypobromite at the liquid-vapor interface studied by liquid jet XPS

256th National Meeting (2018)

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