Ionic Partition Diagrams:  A Potential−pH Representation

Abstract

A general method to predict and interpret the transfer mechanisms of ionizable compounds at the interface between two immiscible electrolyte solutions (ITIES) is presented. The approach is based on the construction of the ionic partition diagram of the solute. It consists in defining equiconcentration boundaries as a function of the Galvani potential difference and aqueous pH by taking into account the thermodynamic equilibria governing the distribution of the various acid/base forms of the molecules involved. The method defines the domains of predominance of each species either in the aqueous or in the organic phase. The application of ionic partition diagrams to quinidine offers a global and direct visualization of all the species and demonstrates the validity and efficiency of the method in helping to understand the transfer and partition mechanisms of ionizable drugs.

Electrochemistry at the interface between two immiscible electrolyte solutions (IUPAC Technical Report)

“…From the electrochemical point of view, this approach is unsatisfactory, because the transfer of even highly lipophilic ions across the liquid and bilayer lipid membranes is possible. A more convenient approach that considers the partition of the neutral and the ionic drug forms, as well as the acidbase equilibria in both phases of the partition system has been proposed [140]. The approach is based on the construction of an ion partition diagram of the solute, which is analogous to Pourbaix's pH-potential diagrams widely used in the electrochemistry of corrosion.…”

Electrochemical Study of Ion Transfer of Acetylcholine Across the Interface of Water and a Lipid-Modified 1,2-Dichloroethane

“…3,[5][6][7][8][9] Besides, a great number of electrochemical studies at ITIES have been devoted to the determination of the kinetics of ion transfer, [10][11][12][13][14][15][16] as well as to the elucidation of various electrode mechanisms involving the ion and/or electron transfers. [17][18][19][20][21][22] The standard Gibbs energy of ion transfer is an important physical parameter which can be deduced from the electrochemical experiments at ITIES, closely related to one of the most important characteristic of the compounds, that is, their lipophilicity. [1][2][3][4] Lipophilicity is a specific property of the compounds which decides to a great extent their distribution, transport, metabolism, toxicity, and thus their biological activity.…”

Ionic partition diagram of tetraphenylporphyrin at the water|1,2-dichloroethane interface

“…Ionic partition diagrams have proved to be a rather useful representation of thermodynamic equilibria involving ionisable species in biphasic liquid systems [21][22][23][24]. The method consists in representing the domains of predominance of the various species as a function of applied potential and aqueous pH.…”