Complementing Onsager's Conductivity Theory by Grotthuss Mechanism Mitigation via Ion-Induced Depletion of Hydrogen-Bond-Donating Water.

Modern Onsager-type conductivity models enable low-uncertainty predictions for electrolytes up to a few moles per liter while using a small set of physical parameters. However, their application to acids and bases results in significant deviations from measurement data and has thus remained limited. Here, we present a model that explicitly describes the molar conductivity due to the Grotthuss mechanism and its dependence on the ion concentration, which is not considered in Onsager-type models. In line with the literature, our model considers a single rate-determining step for a successful Grotthuss mechanism that depends on the ability of water molecules at a reaction site to accept or clear hydrogen bonds. Water in the hydration shell of ions loses this ability due to strong ion-dipole forces, leading to a decrease in Grotthuss conductivity as the ion concentration increases. The two introduced model parameters enable excellent agreement between the model and measurement data for strong acids and bases, while their values are physically explainable. The model reproduces the experimentally observed maxima in conductivity data of acids and bases and explains their physical origin as a result of the declining Grotthuss conductivity.