Phase transition behavior of two-component viologen adsorption layers on a HOPG electrode surface

Abstract

Phase transition behavior of two-component viologen adsorption layers at a HOPG electrode was described using the results of voltammetric measurements. In the coexistence of heptyl viologen (HV) and its bis-carboxylated derivative in the solution phase, a well-mixed condensed monolayer of the radical cations was formed at any molar fraction.In sharp contrast, the binary system of HV and butyl viologen (BV) exhibited phase separation in the molar fraction range where BV is saturated in the predominantly formed condensed phase of HV. It was, however, found that this separation, being opposed to the prediction based on the adsorption free energy, occurs only when the time period enough for full condensation of HV is not given. The significant features of phase transition of two-component viologen adsorption layers on a HOPG electrode surface were highlighted in comparison with the formation and reductive desorption of the self-assembled monolayers of alkanethiols.

Formation of viologen radical cation condensed phase through two-dimensional molecular organization process on an HOPG electrode surface in binary viologen solutions

“…CVs for these two combinations is of transient one as in the case of the dA-7 + dA-4 mixture as we reported previously [18]. Potential holding at more negative potential than the potential of more negative couple in the course of the potential scan resulted in the annihilation of the more negative anodic peak and the growth and positive shift of the less-negative anodic peak.…”

“…The start potential of the potential scan in CV measurements was fixed at 0.0 V, where a pre-equilibration time of 5 min was given, unless otherwise stated. Table 1 In order to be consistent with our previous work [18] in regards to experimental conditions, we used commonly 80 mV s -1 as a standard potential scan rate for CV measurements. For the sake of the analysis of CVs, we first list in Table 1 the anodic and cathodic transition peak potentials (E pa and E pc , respectively) of CVs at 80 mV s -1 , together with the midpoint potential, E 1/2,a = (E pa + E pc )/2, and the peak separation, ΔE p = E pa -E pc , for all six viologens used.…”

“…[15,17]. The behavior of the combination of dA-7 + bC-7 has been described in detail in our previous communication [18]: two molecules are well mixed in a condensed phase.…”

“…A binary mixture of viologens also exhibits phase transition on an HOPG electrode surface as demonstrated in our previous paper [18]. In general, in the single component systems, intermolecular homo-interaction is only the factor to be considered in addition to the interaction with the electrode surface and the interplay of solvent.…”

“…The behavior of the mixture of dA-4 + bC-4 is first concerned, and then it is compared with the behavior of the mixture of dA-7 + bC-7, which has already been discussed in our previous paper [18]. values of E pc and E pa than those of the pure phase.…”