Historical Perspective
Coupling between electrokinetics and electrode kinetics by bipolar faradaic depolarisation processes in microfluidic channels

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Highlights

  • Bipolar faradaic process leads to nonlinear electrokinetics of flat metallic surfaces.

  • Bipolar electronic conduction suppresses partly or entirely streaming potentials.

  • Bipolar faradaic reactions reversibility is not uniform along metallic surfaces in lateral flow.

  • A method is given for ζ-potentials evaluation with account of bipolar electronic conduction.

  • Theories are available for (ir)reversible and quasi-reversible bipolar electrodics.

Abstract

This article is concerned with the nature and impact of bipolar faradaic electron transfer processes in the context of measuring electrokinetic parameters at the interface between an electronically conductive substrate such as a solid metal layer, and a liquid medium. More specifically, it analyses the steady state electric current through the electrodic substrate layer in terms of its short-circuiting effect on the system's electrokinetic quantities, such as the streaming potential. Ample attention is paid to the electrodic behaviour of the chosen metal and its electron transfer characteristics with respect to redox functions in the medium. The electrochemical reversibility of redox couple species is expressed in terms of their oxidation and reduction rate constants as compared to their diffusive transport rates under lateral flow conditions. High values for rate constants lead to high reversibilities and large bipolar leaking currents through the metal substrate. In turn, high electron transfer rate constants generate large reductions in measured values for electrokinetic quantities such as streaming potentials that further become a non-linear function of the pressure gradient applied through the fluidic chamber. The present article presents an overview of theoretical and experimental approaches of this intricate coupling between bipolar electrode kinetics and electrokinetics and the impact from Hans Lyklema's contributions. It highlights not only the implications of bipolar faradaic depolarisation processes in electrokinetics but also the importance of bipolar electrochemistry principles in various electroanalytical applications reported for e.g. the control of microfluidic flows, for surfaces functionalisation, particles manipulation or for the wireless detection of electroactive analytes.

Introduction

Electrokinetic phenomena such as the generation of streaming potentials and the electrophoresis of dispersed particles have played crucial roles in the development of the physical chemistry of colloids [1,2]. Those who have known Prof. Hans Lyklema (1930–2017) and his team at Wageningen University (The Netherlands) will not be surprised that Hans has often found himself immersed in fundamental electrokinetic research issues, from the very beginning [3] till the end [4] of his career. The few electrodic electrochemists in the group generally had a hard time following the electrokinetic line of thinking since the transversal orientation of electrodic electron transfer processes at the substrate surface does not easily match with the tangential orientation of the external electric field as applied in electrokinetic measurements. Yet, there are conditions under which electrokinetic phenomena, as a part of colloid chemistry, are intimately coupled with the electrodic processes occurring in electrochemistry [5].

The measured electrokinetic quantities reflect the double layer properties of the surface/solution interface, in particular the zeta (ζ)-potentials for hard impermeable surfaces [[1], [2], [3], [4], [5], [6]], and the space charge density and hydrodynamic softness for soft surfaces [[7], [8], [9], [10], [11]]. Streaming potential is by far the most widely used experimental quantity to probe double layer properties of flat surfaces [1]. It is the steady state potential difference generated by the flow of liquid along a charged surface in a capillary or a thin-layer cell. The feasibility of the technique for electron-conducting surfaces like metals has been questioned because the strong electronic conduction in the bulk substrate is often believed to annihilate the generation of any measurable streaming potential even though ζ-potential values are reported in literature for metallic surfaces [5,[12], [13], [14], [15], [16], [17]]. Already in the 1930s, the famous Kruyt and Overbeek school of physics and chemistry of colloids at Utrecht University (The Netherlands) recognized the option of electron exchange between an electronically conducting substrate and an electrolytic aqueous medium under lateral flow conditions, as well as the pertaining consequences for the traditional methodologies of measuring electrokinetic quantities [[18], [19], [20], [21]]. Still, in those early days the fundamental aspects of the corresponding conduction via metallic substrates in terms of their impact on the apparent magnitudes of the electrokinetic parameters were not available. With the current insight into surface conduction effects on electrokinetic parameters [[22], [23], [24], [25], [26], [27], [28]] and a well-developed state of electrodic chemistry of bipolar faradaic reactions [[29], [30], [31]], it seems timely to get back to the subject and aim for a description of the basic processes in situations where electrode kinetics interfere with the generation of streaming potential by electron-conducting substrates. This article aims to outline these basic concepts as well as some practical utilities that emerged.

Section snippets

Electrokinetics of metallic surfaces in inert electrolytes

When referring to electrokinetics of (polarisable) metallic surfaces, one thinks about a number of non-linear electrokinetic phenomena that are now extensively detailed in literature, e.g. the superfast electrophoresis of metallic particles [32], the self-assembly of colloids at electrode surfaces [33], the AC electroosmosis at electrode arrays [34], or induced charge electroosmotic processes and their applications in microfluidics [35]. This article focuses on the situation where deviations

Bipolar electrochemistry beyond the context of electrokinetics

The fundamental processes governing the non-linear coupling between bipolar electrodic kinetics and electrokinetics are reviewed in this study within the context of streaming potential generation at metallic surfaces. The possibility to control electroosmotic flow pattern in metallic chambers via exploitation of the intricate connection between bipolar faradaic current features (modulated by playing with the nature of the redox species in solution) and lateral electric field distribution has

Conclusions

Hans Lyklema has insisted a lot on the importance of surface conduction processes for a proper interpretation of electrophoresis data, analysis of the low frequency domain in dielectric spectra of colloids, and also for solving what he called the ‘electrokinetic incongruence’, i.e. without account of surface conduction term, electrokinetic measurements performed on a given material type with different equipments may provide different values with some of them being physically unrealistic or

Declaration of Competing Interest

None.

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