Quest To Demystify Water: Ideal Solution Behaviors Are Obtained by Adhering to the Equilibrium Mass Action Law

Mass action model of solution activity via speciation by solvation and ion pairing equilibria

Solutes and their concentrations influence many natural and anthropogenic solution processes. Electrolyte and solution models are used to quantify and predict such behavior. Here we present a mechanistic solution model based on mass action equilibria. Solvation and ion pairing are used to model speciated solute and solvent concentrations such that they correlate to a solution's vapor pressure (solvent activity) according to Raoult's law from dilute conditions to saturation. This model introduces a hydration equilibrium constant (K_ha) that is used with either an ion dissociation constant (K_id) or a hydration modifier (m) with an experimentally determined ion dissociation constant, as adjustable parameters to fit vapor-liquid equilibrium data. The modeled solvation equilibria are accompanied by molecular dynamics (MD) studies that support a decline in the observed degree of solvation with increased concentration. MD calculations indicate this finding is a combination of a solvent that solvates multiple solutes, and changes in a solute's solvation sphere, with the dominant factor changing with concentration. This speciation-based solution model is lateral to established electrostatics-based electrolyte theories. With its basis in mass action, the model can directly relate experimental data to the modeled solute and solvent speciated concentrations and structures.

IR Spectroscopy of the Cesium Iodide-Water Complex

There has been much interest in I^-(H₂O) as a simple model for a hydrated iodide ion. Here we explore how this fundamental ion-solvent interaction is modified by the presence of a counterion, specifically Cs⁺. This has been achieved by forming the CsI(H₂O) complex in superfluid helium nanodroplets and then probing this system using infrared spectroscopy. The complex retains the ionic hydrogen bond between the I^- and a water OH group seen in I^-(H₂O), but the Cs⁺ ion substantially alters the anion-water interaction through formation of a cyclic Cs⁺-O-H-I^- bonding motif. As with I^-(H₂O), the OH stretching band derived from the hydrogen-bonded OH group shows substructure, splitting into a clear doublet. However, in contrast to I^-(H₂O), where a tunneling splitting arising from hydrogen atom exchange plays a role, the doublet we observe is attributed solely to an anharmonic vibrational coupling effect.

Hydration and ion association of aqueous choline chloride and chlorocholine chloride

Choline hydration occurs predominantly via its hydroxyl group, and weak contact ion pair formation with Cl⁻ is via the onium moiety.