Quantified Interfacial Affinity of Hydrogen Sulfate and Sulfate Speciation in Model Marine Aerosol Particles

Abstract

Heterogeneous reactions occurring in atmospheric aerosols are crucial components of multiple chemical reaction cycles. Sulfate species exert substantial influence on many heterogeneous reaction mechanisms due to their abundance and speciation. Despite the importance of sulfate species on heterogeneous reactions, quantitative descriptions of interfacial affinity and sulfate species partitioning at interfaces remain incomplete. Here, we quantify the interfacial properties of HSO₄^–, using cetyltrimethylammonium hydrogen sulfate (CTAHS) reverse micelles (RMs), as model marine aerosol particles. Infrared spectra (IR) of CTAHS RMs were collected and analyzed using multivariate curve resolution-alternating least-squares (MCR-ALS). MCR-ALS analysis reveals that four distinct species contribute to the observed IR spectra, which we assign as CTA⁺-HSO₄^– contact pairs (CPs), interfacial HSO₄^–, core HSO₄^–, and core SO₄^2–. Computational studies reveal that the CTA⁺-HSO₄^– CPs exist in two distinct orientations at the interface. Spectral contributions from each chemical species are used to determine the quantitative interfacial affinity (χ_Int.) of HSO₄^–. Our observations indicate that the cationic charge of the interface alters the ionic distribution of sulfate species compared to air–water interfaces, placing H₃O⁺ below HSO₄^–.