Polarizable potential window at soft molecular interfaces as a quantitative descriptor for the water content in organic solvents

Abstract

The presence of water in organic solvents is a ubiquitous fact and can affect the reactivity and selectivity of chemical reactions. Traditional physical and chemical methods (IR, NMR, Karl Fischer titration, etc.) for quantitative measurement of water in organic solvents are not very suitable for rapid trace water analysis. Here, we demonstrate that, with hydrated Li⁺ and Cl⁻ as probes to build polarizable potential windows (PPWs) at interfaces between water and more than twenty organic solvents, we can reflect the water content in organic solvents. This method only requires a scan of a cyclic voltammogram for Li⁺ and Cl⁻ transfer (a weak-interaction electrochemical method), at a micro-scale polarized water/oil interface. A hybrid modified Born ionic solvation model was employed by us to compute the theoretical PPWs of LiCl at a series of water/oil interfaces, which match with the experimental results to some extent. Experiments and theories jointly confirm a novel and universal relationship: the PPW width correlates with the water content (in a large range) in organic solvents in a negative natural logarithm way. We postulate that when the organic solvent is different, the water fingers, i.e., ions dragging a string of water molecules, will search for water molecules in the organic phase with different probabilities (or microstate numbers) after crossing the interface. This determines the macroscopic quantities, namely the standard Gibbs free energy of ion transfer and the PPW width. It is envisioned that our work paves the way for a broad spectrum of applications.