(85c) Ions, Ion Pairs and Inverse Micelles in Non-Polar Media

This is review of the papers dedicated to the electrochemistry of non-polar liquids during last century. There is a list of dozens liquids that have been studied and variety of electrolytes. There is an overview of 13 different experimental methods, which have been employed for this task. Theoretical part emphasizes work done by Onsager, Debye, Fuoss, Kraus, Bjerrum and others in 1920^th-30^th. They had initiated and justified fundamental ideas that serve as a scientific basis for the modern handbooks on non-aqueous electrochemistry. They pointed out that electrostatic interactions lead to formation of ion-pairs and other structures in non-polar liquids. These notions and related equilibrium constrains allowed then to achieve explanation of many experiments, including conductivity dependence on concentration, dielectric constant of electrolyte solutions, deviation from the Ohm’s law in non-polar liquids. They and their followers were able to extract information on ions dimension for many non-polar electrolyte solutions using conductivity and dielectric permittivity data. It was less than 1 nm in all cases. Many of these papers from 1930^th and later are reviewed here.  

These ideas can be applied for non-polar solutions that contain surfactants. This would logically lead to the formation of inverse micelles ions as a result of ions electrostatic interaction with dipole moments of the surfactant molecule polar part. Overview of the history associated with inverse micelles ions reveals existence of two different models for inverse micelles ions. “Symmetrical model” assumes identical size for cation and anion. “Asymmetrical model” presents cation and anion of very different sizes, one of them possibly bare. There are several arguments offered, theoretical and experimental, for supporting asymmetrical model. This model suggests existence of two different classes of surfactants depending on their role in solvating ions in non-polar liquids. Asymmetrical model explains charging of interfaces by adsorption of the smaller ion, which is supported with experiment as well. 

Independently on model, inverse micelles ions must form ion-pairs. It is imperative to take into account “ions”-”ion-pairs” equilibrium in all electrodynamic effects in  non-polar liquids, as was shown by Onsager. This might yield significant consequences for interpretation transient current experiments.