Research on Electrolyte transport modeling for non-neutral systems in the Monroe Research Group, Department of Engineering Science, University of Oxford

Energy is intrinsic to matter and its contribution must appear in the first law of thermodynamics when considering open systems. The concept of chemical potential is thus used to fulfil this requirement. However, when there are charged species in the system, the energy content of the system will change when subjected to an electric potential field and this demands reconsideration of the terms in the first law. The electrical contribution can be successfully incorporated, as Guggenheim had done [1], by adapting a new thermodynamic quantity called the electrochemical potential, which takes into account both the chemical and electrical contributions of a given species. 

However, the electrochemical potential is not convenient for systems where electroneutrality is violated, such as local system volumes within electrolytic solutions for several reasons. As noted by Guggenheim himself in [1], the chemical and electrical parts are not separately measurable. The splitting of the two parts are in fact arbitrary because there is no absolute reference point for electric potentials. Furthermore, the meaning of electrochemical potential is obscure for charged species such as ions because ions cannot be introduced to the system without adding the counter-charged ions. This presents a fundamental problem in making sense of the definition of the electrochemical potential based on the Gibbs free energy. Also, the fact that electrical information is embedded in the electrochemical potential and the particle number of species complicates the description of non-electroneutral systems. 

To resolve these issues, Prof. Monroe and Dr. Goyal have been developing a new set of potentials more appropriate for multispecies, non-electroneutral systems. We are currently working with these new definitions to study their effects on transport phenomena in non-electroneutral electrolytic systems. 

 

[1] Guggenheim, E. A. (1929). The conceptions of electrical potential difference between two phases and the individual activities of ions. Journal of Physical Chemistry, 33(6), 842–849. https://doi.org/10.1021/j150300a003