Ion-Water Interactions of Solvated Multiply Charged Anions and Their Effects on the Extended Hydrogen-Bonding Network of Water Molecules
Ion-water interactions have a large influence on the chemical and physical behavior of water and solutes in a diverse range of environments. These interactions play a role in atmospheric aerosols, desalination methods, enzymatic active sites, and ion stability. For example, multiply charged anions (MCAs), such as sulfate and phosphate, are ubiquitous in the condensed phases, but are intrinsically unstable once desolvated due to the lack of non-covalent interactions. Aqueous ion-containing nanodroplets formed via electrospray ionization can be investigated using high-resolution mass spectrometry. Utilizing the ability to store ion-containing droplets in the ion cell of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, an ion with a discrete number of solvating water molecules can be mass-selected and thereafter probed using blackbody, infrared, and ultraviolet photons, or thermal electrons to measure its chemical and physical properties. Studying the stabilities of hexacyanoferrates as a function of (H2O)n, we find two hydration shells are required to stabilize Fe(CN)64- from spontaneous electron ejection. Although the ability of ions to influence water molecules remotely has been debated in condensed phase experiments, we provide unambiguous evidence for ion-water interactions leading to structural perturbations into a fourth solvation shell. Furthermore, evidence for remote dynamic perturbations in solutions is presented using a novel broadband terahertz spectroscopy technique. Although the nanodroplet experiments are performed in a temperature-regulated ion cell, a key question in the gas phase is to what magnitude does evaporative cooling affect the internal temperature of an ion population? Extensive master equation modeling to fit temperature-dependent blackbody dissociation kinetics reveals that, although the effect is minimal at low temperatures, evaporative cooling can lead to disparities of ΔT > 100 K when storing nanodroplets in an ion cell above room temperature. These findings can be used to more accurately relate experimental nanodroplet studies to the ambient temperatures relevant to atmospheric aerosols and condensed-phase solutions. The research presented here provides unique molecular-level insight into ion-water interactions leading to stability in MCAs as well as remote structural and dynamic perturbations that extend beyond the inner solvation shell.