Reorientation of the Cation Radical of Heptyl Viologen on Mercury in Water/DMSO Mixed Media

Abstract

In aqueous mixed media containing DMSO in fractions by volume above 0.50 the cation radical of heptyl viologen (HV•+) can be absorbed in two different orientations on a mercury electrode. Thus, the cation radical can take a planar conformation relative to the surface; however, depending on the applied potential, it can reorient itself to adopt a different conformation where the bipyridine group lies normal to or at an angle with the electrode. In this work, the reorientation was found to take place via a two-dimensional phase transition mechanism. The experimental conditions under which the phenomenon occurs are analyzed.

Double potential step chronoreflectometry approach to bi-stable potential region of the faradaic phase transition of heptyl viologen at a HOPG electrode

“…Molecular level clarification of the condensed phase structure is the key for the modeling. The 2D condensed layer structure of HV Åþ was previously proposed by Mill an et al [19,20] at a Hg electrode in water + DMSO using the results of voltammetric measurements and by Arihara et al [22,23] at HOPG and Hg electrodes in water using IR reflection measurements. We may point out that MonteCarlo (MC) simulations [32,47,52,53] are necessary to include real molecular alignment with an anisotropic lattice structure and both short and long-range interactions.…”

“…However, it is known that the use of the Frumkin isotherm is, in many cases, not adequate to represent the phase transitions [10,12,32,[42][43][44][45][46][47]. The Frumkin isotherm is equivalent to the mean field approximation based on the assumption that the adsorbed molecules in the condensed phase are randomly distributed, although the well-ordered alignment of HV Åþ is more likely on the electrode surface [19,20,22]. The pair interaction at short range (i.e., the nearest neighbor interaction), which governs the transition, is not explicitly considered at all in the Frumkin isotherm.…”

“…Faradaic phase transitions are triggered by interconversion of oxidation states. A typical faradaic transition of organic molecules is of viologens observed at Hg, polycrystalline Ag and Pt, single crystal Au, and basal plane HOPG electrodes [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Because viologens have found widespread applications in many fields as electron transfer mediators or chromophores [29], a quantitative description of their interfacial phase transitions from both thermodynamic and kinetic viewpoints is an important issue.…”

Two-Dimensional Condensation and Reorientation of the Bromide Salt of the Heptyl Viologen Cation Radical at the Hg/DMSO Interface

“…In previous work, the cation radical of heptyl viologen (HV •+ ) was found to form two different types of 2D phase on Hg in water/DMSO mixed solvents with a volume fraction X DMSO > 0.55 in the presence of Br - ions. At potentials between −300 and −350 mV, a phase is formed such that the surface concentration of HV •+ , Γ β , is about 1.04 × 10 -10 mol/cm 2 (phase β in Scheme ).…”

Molecular Structure Dependence of the Phase Transition Spike Response of Viologens at an HOPG Electrode Using Bisviologen and Carboxylated Viologen

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] This phenomenon has been understood as being a two-dimensional (2D) faradaic phase transition. One-electron reduction of viologen to mono-radical mono-cation is accompanied by the formation of the condensed phase.…”