Speaker
Description
Aqueous solution-vapor interfaces play a major role in atmospheric processes, for example in the interaction of the oceans or of aqueous aerosols with trace gases [1]. The largest contiguous aqueous-vapor interface is that of the oceans with air, covering more than 70% of the Earth’s surface [2]. Studies have shown that the ocean-air interface is covered by a thin film of amphiphilic compounds, including surfactants [3]. This so-called sea surface microlayer significantly influences many processes with importance to the global ecosystem, such as the exchange of trace gases (e.g., CO2) and heat transfer. Additionally, it contributes to the production of sea spray aerosols (SSAs), which are among the largest natural sources of aerosols globally [4,5].
Our previous studies on the interaction of ions and surfactants at liquid-vapor interfaces [6] demonstrated that surfactants can influence the behavior of dissolved ions like Mg²⁺and SO₄². These effects arise mainly from electrostatic interactions between ions and the charged functional groups of surfactants (e.g., -CNH₃⁺, -COO⁻). The goal of our current research is to achieve a molecular-scale, quantitative understanding of how organic surfactants interact with inorganic ions in ocean water. By investigating the directional photoemission propensities, termed photoelectron angular distributions (PADs), of these interfacial components using liquid-jet X-ray photoelectron spectroscopy (XPS), we aim to gain deeper insights into their depth distribution and infer structure properties. Specifically, measuring PADs provides valuable information with Ångstrom resolution on the distance between the charged functional groups of surfactants and the dissolved ions [7].
In this study, we examine the relative distance of divalent (SO₄²⁻) ions to the interface in the presence of surfactants. As prototypes for the latter, we use sub monolayer coverages of octyl ammonium (-CNH₃⁺, positively charged) and octanoate (-COO⁻, negatively charged). Our findings indicate that the presence of positively and negatively charged surfactants can modify the relative depth of species at the interface, which is crucial for understanding molecular-level heterogeneous chemical reactions in the atmosphere.
