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.