Hydrodynamic voltammetry at the liquid–liquid interface: Application to the transfer of ionised drug molecules

Liquid/liquid electrochemistry has been used to study the transfer of weakly ionised species across the interface between water and 1,2-dichloroethane. It is shown that while transfer of a fully ionised species can be readily used for determination of its diffusion coefficient, transfer of a partially ionised species, such as many common pharmaceutical agents, involves complex ionisation/distribution behaviour, which invalidates the conventional analysis. As a result, the aqueous diffusion coefficient of the transferred species is underestimated by at least one order of magnitude. An alternative method to study the transfer of partially ionised drug molecules employing a rotating liquid/liquid interface is proposed and reported. The alternative approach, which is based on a previously reported rotating diffusion cell approach, employs a lipophilic membrane that stabilises the liquid/liquid interface and allows stirring. This hydrodynamically controlled configuration was successfully applied to transfer of partially ionised drug species, and expected values of the aqueous diffusion coefficient were obtained.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Electrochemical study of transfer of an ionised drug across liquid/liquid interface. ► Ionisation/distribution behaviour at the interface observed and studied using UV–vis. ► Alternative method of using membrane-immobilised rotating interface presented. ► The method is applied to determine aqueous diffusion coefficients of studied drugs.