A General Classification of Electrolyte Solutions Based on Their Charge and Mass Transport Properties

TL;DR

This paper introduces a comprehensive classification of electrolyte solutions based on both charge and mass transport properties using Onsager formalism, revealing new categories especially for highly concentrated solutions.

Contribution

It develops a new classification scheme incorporating mass transport, extending beyond traditional charge-based categories, applicable to highly concentrated electrolytes.

Findings

Identifies three classes of electrolytes based on charge and mass transport coefficients.

Reveals many highly concentrated electrolytes fall into a distinct class with weak mass transport.

Provides a framework for better understanding electrolyte behavior beyond classical models.

Abstract

The conventional classification of electrolyte solutions as 'strong' or 'weak' accounts for their charge transport properties, but neglects their mass transport properties, and is not readily applicable to highly concentrated solutions. Here, we use the Onsager transport formalism in combination with linear response theory to attain a more general classification taking into account both charge and mass transport properties. To this end, we define a molar mass transport coefficient {\Lambda}_mass, which is related to equilibrium center-of-mass fluctuations of the mobile ions and which is the mass-transport analogue of the molar ionic conductivity {\Lambda}_charge. Three classes of electrolyte solution are then distinguished: (i) 'Strong' electrolytes with 4{\Lambda}_mass \approx {\Lambda}_charge; (ii) 'weak charge transport electrolytes' with {\Lambda}_charge << 4{\Lambda}_mass; and (iii) 'weak mass transport electrolytes' with 4{\Lambda}_mass << {\Lambda}_charge. While classes (i) and (ii) encompass the classical 'strong' and 'weak' electrolytes, respectively, many highly concentrated electrolytes fall into class (iii) and thus exhibit transport properties clearly distinct from classical 'strong' and 'weak' electrolytes.