Long distance ion–water interactions in aqueous sulfate nanodrops persist to ambient temperatures in the upper atmosphere

Abstract

The effect of temperature on the patterning of water molecules located remotely from a single SO₄²⁻ ion in aqueous nanodrops was investigated for nanodrops containing between 30 and 55 water molecules using instrument temperatures between 135 and 360 K. Magic number clusters with 24, 36 and 39 water molecules persist at all temperatures. Infrared photodissociation spectroscopy between 3000 and 3800 cm⁻¹ was used to measure the appearance of water molecules that have a free O–H stretch at the nanodroplet surface and to infer information about the hydrogen bonding network of water in the nanodroplet. These data suggest that the hydrogen bonding network of water in nanodrops with 45 water molecules is highly ordered at 135 K and gradually becomes more amorphous with increasing temperature. An SO₄²⁻ dianion clearly affects the hydrogen bonding network of water to at least ∼0.71 nm at 135 K and ∼0.60 nm at 340 K, consistent with an entropic drive for reorientation of water molecules at the surface of warmer nanodrops. These distances represent remote interactions into at least a second solvation shell even with elevated instrumental temperatures. The results herein provide new insight into the extent to which ions can structurally perturb water molecules even at temperatures relevant to Earth's atmosphere, where remote interactions may assist in nucleation and propagation of nascent aerosols.