Jay Deep Wadhawan

Electron transfer reactions in droplets on a surface is a fascinating platform that can be used for the study of chemistry in confined volumes ranging over 18 orders of magnitude: from yoctolitres to microlitres. However, they also provide a methodology to enable the coupling between biochemical, geochemical and electrochemical systems, which enables opportunities in the redox recovery of strategic and growth minerals, and empowering communities of living organisms in relatively anoxic environments. One of the problems in the study of micro-organisms attached to redox surfaces is the occurrence of non-uniform material flux, which results from the singularity at the basal contact. In this lecture, the chemistry at triple-phase boundaries will first be examined through understanding the role played by the non-uniform electric field that exists. Although this work might appear somewhat specialised and adventurous, it enables the showcasing of the challenges in the continual translation of chemistry in the laboratory to the applied settings of the manufacturing and analytical process industries, where research directions are purposely constrained to drive economic growth. Accordingly, droplet electrochemistry commercialisation journeys and successes will be highlighted through applications in environmentally and socially responsible technologies for (1) new concept pH sensors for oceanology and biomedical point-of-care settings; (2) offshore oil and gas wastewater remediation; (3) green chemistry for jet fuels and mass transportation products; and (4) strategic metal extraction.

Homogeneous redox catalysis within electrochemically supported microdroplets immobilised on an electrode surface and bathed by an immiscible electrolyte solution is characterised using finite difference numerical methods, after conformal transformation of the physical problem. This is shown to be a challenging environment to simulate and model, not least due to the confinement of the heterogeneous electron transfer to the droplet/support/electrolyte boundary, and hence leading to acute convergent/divergent diffusion regimes. Reactivity at the triple phase boundary underpins both the spatial and temporal non-uniformity of the reacting droplet environment. Crucially, through comparison with experimental data reported in the literature, it is demonstrated that there is no droplet-induced acceleration of the redox catalytic reaction. Reasons for this discrepancy with literature are suggested. It is recommended that any inference of reaction rate acceleration through increased rate constants in microdroplets on surfaces be re-examined, lest the multi-dimensional dynamics at the three-phase boundary are unaccounted.

Contact angles of liquid, spherical cap droplets immobilised on an electrode surface and bathed by a fluid are important, quantifiable measures of the liquid/fluid interfacial tension. Optical goniometry, even if computer assisted, suffers when the contact angle is 10° or less. In this work, an alternative method of measurement is considered: electrochemical techniques (voltammetry and chronoamperometry), which rely on the transport of material from within the droplet to the conductive surface. As a result of the reactions that take place at the triple phase boundary, these are demonstrated to provide information on the size and the shape of the droplet, including its contact angle, for the cases when the droplets have a redox analyte and either have a supporting electrolyte, or not. The voltammetric behaviour is seen to change from exhaustive, thin film characteristics, to quasi-steady-state signals as the droplet becomes bigger, or the scan rate becomes larger, or diffusion of the redox material inside the droplet becomes slower. One of the surprising outcomes is that there is a zone of planar diffusion only in the case of the supported droplets, with both the droplet size and its contact angle determining whether this is seen at conventional combinations of scan rates and diffusion coefficients. Experimental data are provided which emphasize key features pertaining to the nature of the redox system and illustrate the facile nature of the contact angle estimation process, albeit to within 10% uncertainty.

This work reports the oxidative breakdown of the LiMn₂O₄structure using alkaline hypochlorite, as a method for recovering both lithium and manganese species.

Active and passive manganese and iron impurities in drinking water-grade lime hydrate are removed through redox treatment.

A surface‐integrated form of the widely used (anharmonic) Lennard‐Jones 12–6 interaction potential, the Lennard‐Jones 9–3 potential, is used to develop a quadratic activation/driving force relationship that gives rise to asymmetric Tafel plots for electron transfer occurring with simultaneous interaction rupture. The Tafel plots are shown to exhibit linearity over a wide potential range, depending on the ratio of the Gibbs interaction well to the solvent reorganisation free energy. The fit of the model to experimental data for a ferrocene‐based self‐assembled monolayer (SAM) bathed by aqueous perchloric acid suggests ion pairing between ferricenium and perchlorate ions. This crude and primitive model readily enables experimentalists to obtain a parametric understanding of the physicochemical dynamics underpinning interaction rupture in concert with electron transfer, which may empower routes to improve the efficiency of a plethora of topical electrochemical technologies.

The ‘ref tracker’ electrode tracks any drift in the reference electrode. The user always knows that measured pH is real as any reference electrode drift is compensated for, in situ, with the ‘ref tracker’.

Ferrocenemethanol is employed as an aqueous, homogeneous redox mediator for hydrogen sulfide. The reaction is seen to follow an EC’ mechanism over the range 6≤pH≤9, with bisulfide reacting four times more rapidly than hydrogen sulfide. In the presence of 10 vol.% hæmolysed blood, greater concentrations of sulfide (as H ₂ S or as HS ⁻ ) are required to achieve the same degree of redox catalysis, compared with the absence of the blood proteins, at both pH 7 and pH 9. It is suggested that this phenomenon derives from a ferrocenemethanol/blood protein interaction, which is first titrated by the sulfide species. This can give rise to a titration‐based electroanalytical assay for sulfide in blood, whilst being important for blood‐based electrochemical bioassays involving hydrophilic ferrocene derivatives.