Interfacial processes and mechanisms

The articles in this special issue jointly celebrate recent advances in interfacial science and the 75th birthday of Prof. John Albery, who has contributed greatly to this field of research.

The non-specialists amongst the wide readership of PCCP might reasonably ask why an issue should be devoted to this topic and in particular why electrochemistry should feature so strongly. A frivolous response—to be justified in more serious terms—is the often used statement amongst electrochemists that “there are only three types of reaction, namely electron transfers, proton transfers and the rest—and electrochemistry encompasses all of the first two and some of the third”. While this might seem a little provocative at first reading, the reality is that chemical reactions involve the making and breaking of chemical bonds, which may be considered as the addition, removal or transfer of electrons, that is to say reductions and oxidations—in short, electrochemistry. For organic reactants, the response to changing the charge on a molecule by an electron transfer is commonly restoration of electroneutrality by a proton transfer. This provides a bridge between the two motivating factors for this issue.

If one looks back about 30 years, electrochemical research was focused on measurements of current and voltage, either in terms of their direct relationship to each other or as a transient response of one to a perturbation of the other. This was the era of so-called i–V–t techniques. Electrochemical research then had two characteristics. First, there was no opportunity for structural interpretation, since there was no direct evidence to permit identification of intermediates or products. Second, it was the exclusive province of electrochemists. Neither of these is now the case.

During the 1980's advances in instrumentation for a number of structural probes, predominantly spectroscopic, gave them capabilities of value to electrochemists. In some cases these advances related to enhancements in sensitivity, but in others it was selectivity (for surface-immobilised species) or adequate temporal resolution to permit exploration of surface dynamics. A good snapshot of this transformation may be seen in a special electrochemistry issue of the forerunner of this journal, Faraday Transactions (incidentally, carrying the name of a great experimental electrochemist), from the 1990's. In that issue (J. Chem. Soc., Faraday Trans., 1996, 92(20)), there are examples of infra-red and Raman spectroscopies and second harmonic generation, as well as a range of (then) emerging physical techniques based on surface stress and acoustic waves, and imaging techniques such as AFM. The present issue shows how these and other techniques have been applied and exploited in unexpectedly diverse fields to spectacular effect and subtlety.

As a reader, it is interesting to look down a journal contents page and observe how authors attempt to encapsulate their accomplishment. The point at issue is that the single image of a graphical abstract commonly represents aspiration or speculation, rather than realization per se. The introduction of spectroscopic, imaging and other structural probes into interfacial science has removed this ambiguity. One can now represent the population, identity, organization and orientation of molecules at interfaces with confidence and, in the case of electrochemically controlled interfaces, reveal their responses to applied potential. This link between microscopic—dare one say nanoscopic?—structural aspects and the macroscopic performance of a device, as parameterised through electrochemical response, allows rational design and fabrication of materials, interfaces and devices in a manner one could not have imagined a few decades ago.

Turning to the second aspect of this special issue, the commemoration of John Albery's 75th birthday, one may ask how his career and scientific activities have contributed to this transformation. From a historical perspective, it is interesting to note that his scientific research began in the 1960's, under the guidance of R. P. Bell, with the study of proton transfer dynamics. There then occurred one of those fortuitous moments, a meeting with S. Bruckenstein who was stopping briefly in Oxford en route from the USA to a sabbatical period in Moscow with A. M. Frumkin. From their discussions it became clear that electrochemistry and the controlled mass transport systems used by electrochemists to study electron transfer rates could be applied to the study of proton transfers—a critical link between the electron and proton transfer facets of electrochemistry. The outcome of this meeting was a series of seminal papers, mostly published in Faraday Transactions, on the rotating disc and rotating ring disc electrodes.

As the reader looks through the papers in this special issue, largely contributed by former students, colleagues and collaborators of John Albery, three things will emerge. First, there is a drive for a balance between mathematical rigour and physical insights leading to tractable and useable solutions. John's colleagues will recall the concept of case diagrams (see Fig. 1), which manifest his ability to divide parameter space into characteristically distinct regions, within each of which a linear approximation allowed a first-order solution that might be applied to experimental data.

Fig. 1 Case diagram summarizing mechanistic possibilities for mediated charge transfer at a modified electrode in terms of transport and kinetic parameters. The concept is described in W. J. Albery and A. R. Hillman, J. Electroanal. Chem., 1984, 170, 27–49.

This leads to the second feature, an enthusiasm for the integration of theory and experiment. The case diagrams or the approximations on which they were based frequently provided key inputs into experimental design, highlighting regions of potential, concentration, pH, light intensity or mass transport—according to the system at hand—where interesting, unusual or practically useful behaviour might be seen. While the skill of deriving analytical solutions to complex problems has to some extent been superseded by the use of powerful numerical methods, the latter do not universally provide the same physical insights or provocation to explore selected aspects.

The third feature of John Albery's career is its diversity; today, we might label it multi-disciplinarity. In the same laboratory there might be projects on fundamental electrochemistry and methodological development running alongside others on kinetic isotope effects in proton transfer reactions, enzymology, (bio)medical electrochemical sensors and liquid/liquid interfacial transfers. The informal interchanges between these researchers enhanced their understanding of biology, electronics and engineering skills as well as of (electro)chemistry, giving them unique portfolios of ideas and skills with which to launch independent research careers. The “academic genealogists” will no doubt enjoy unravelling this.

The additional, more personal, aspect is the sheer drive and energy John put into his science and, as he continues to do in more social aspects of his life, the immense enjoyment he took from the outcomes of its success. In this last regard, his co-workers will recognize his generosity and the equal satisfaction he took from his own achievements and those of his students and co-workers.

I would like to end by unifying the two strands explored above. Electrochemistry plays a central role not merely in chemistry, but in the whole of science, spanning the natural environment, through the biology of all who occupy it, to a range of processes and devices developed by mankind. Within the broad definition of electrochemistry asserted above, interfaces—by their very name—define our interaction with our surroundings, transduction in (bio)chemical sensors, how energy is stored and converted in numerous devices and how information is moved between data domains; every reader will have utilised all these in perusing this editorial! What, then, is the take-home message? As chemistry continues to resolve reactions in ever finer detail, the fundamental and universal role of electron transfers will become ever more prominent. The challenge for interfacial scientists and electrochemists is to avoid this all-pervasiveness becoming synonymous with convective dilution of the subject. Rather, science—and those who prosecute, promote and fund it—must retain the interconnectivity of interfacial electrochemistry's many and diverse strands.

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