Anion Detection by Electro-Insertion intoN,N,N′, N′-Tetrahexyl-Phenylenediamine (THPD) Microdroplets Studied by Voltammetry, EQCM, and SEM Techniques

Abstract

The electrochemical oxidation and re‐reduction of N′,N′,N,N‐tetrahexylphenylene diamine (THPD) deposited in form of microdroplets on a basal plane pyrolytic graphite or gold electrode is shown to be a chemically reversible process in the presence of aqueous electrolyte media containing NO3−, SCN−, ClO4−, or PF6−. Sharp voltammetric responses with a mid point potential, Emid, characteristic of the type of anion and its concentration are observed. The oxidation product, an ionic liquid, undergoes rapid ion exchange when the anion of the aqueous electrolyte is exchanged with anions of lower Emid replacing anions of higher Emid. Although the effect of the supporting electrolyte cation in the case of the alkali metals K+, Na+, and Li+ is not significant, a considerable change in the voltammetric peak shape and Emid occurs in the presence of protons. This effect, attributed to the protonation of THPD, is also sensitive to the type of anion present with anions of lower Emid causing more facile protonation. After protonation of THPD, oxidation and re‐reduction can be shown to be associated with H+ expulsion and uptake. Deposited onto the rough surface of a gold coated planoconvex quartz crystal oscillator the [THPD+ClO4−]oil deposit can be observed in form of micron‐sized droplets in SEM images. A strong frequency response of the crystal oscillator in an electrochemical quartz crystal microbalance experiment associated with the oxidation and re‐reduction of THPD can be detected but is not related to changes in mass. Rather, this frequency response may be attributed to changes in the viscosity and/or coverage of the oily deposit.

Chromate and Dichromate Electro-Insertion Processes into a N,N,N′,N′-Tetraoctylphenylenediamine Redox Liquid

“…This voltammetric signal remains essentially constant over several potential cycles. The characteristics of the voltammetric signal are very similar to those observed previously for perchlorate and nitrate electro-insertion into THPD [15] and therefore may be interpreted in a similar way (Equation 2).…”

“…It has been demonstrated previously [15] that rapid anion exchange occurs in the case that a redox liquid composed of an organic cation and a hydrophilic anion is immersed into a solution containing a type of anion, which undergoes electro-insertion more readily (less hydrophilic). In fact, the difference in midpoint potentials, E mid (see Table 1), directly reflects the driving force for this ion exchange.…”

Voltammetry of Electroactive Oil Droplets. Part II: Comparison of Experimental and Simulation Data for Coupled Ion and Electron Insertion Processes and Evidence for Microscale Convection

“…Electrochemical studies on droplet deposits on electrode surfaces are of fundamental interest as well as of interest for the characterization of redox active solids [1,2] and liquids [3,4] and for the development of novel analytical procedures [5]. In contrast to the case of monolayers being reduced or oxidized the redox conversion of bulk oils requires uptake or expulsion of ions in order to obey the electroneutrality principle.…”

“…For voltammetric experiments with microdroplets of organic materials deposited on an electrode surface this type of redox process linked to simultaneous ion exchange has been experimentally observed [3,4]. Redox processes may be shifted along the potential axis simply by changing the electrolyte composition [4] and selectivity for certain ions may be designed into the structure of the deposit [5]. There are considerable implications for this type of electrochemically driven ion transfer process at microdroplet deposits in numerous analytical sensing applications.…”

Voltammetry of Electroactive Oil Droplets. Part I: Numerical Modelling for Three Mechanistic Models Using the Dual Reciprocity Finite Element Method

“…The oxidation process observed under these conditions has been shown to correspond to the one electron oxidation of the oil phase. Scheme 1 The uptake of charge compensating counter anions from the surrounding solution phase accompanies the anodic redox process and gives rise to the formation of an ionic liquid [3,4].…”