Lateral diffusion in organized bilayer assemblies of electroactive amphiphiles: Influence of the oxidation state of the amphiphile investigated by steady-state methods involving an interdigitated micro-electrode array device

Abstract

Interdigitated micro-electrode array (IMA) devices were used to investigate lateral diffusion processes in amphiphilic bilayer assemblies. Photolithographically fabricated arrays consisted of 50 pairs of 800 μm long, 4 μm wide electrodes spaced by 4 μm. The bilayer formation involved two self-assembly steps in which a monomolecular layer of octadecyltrichlorosilane (OTS) is first covalently anchored to the IMA surface, and then a monolayer of octadecylviologen (C 18MV 2+) is formed. The typical coverage of the viologen amphiphile was 2–3 × 10 −10 mol/cm 2 in 0.5 M KCl electrolyte, and was demonstrated to be uniform across the glass inter-electrode gap and Au micromelectrode surfaces of IMA devices. Lateral charge transport dynamics were investigated by generator/collector voltammetry. Computer simulations showed how the shape of the generator/collector current depends on the ratio of the microelectrode and the gap width ( w/d), the ratio of the diffusion coefficients D o/ D r, and the scan rate. Fast scan cyclic voltammetry was used to resolve spatially the coverage of C 18MV 2+ and C 18MV .+ at the electrodes and in the inter-electrode gaps. This technique was used to interpret the steady-state generator/collector current in terms of the individual fluxes of the oxidized and reduced octadecylviologen. The dissimilarity of the diffusion coefficients ( D o > D r) leads, at steady-state, to a net migration of octadecylviologen from the anode to the cathode so that a larger gradient of C 18MV .+ compensates for the lower value of D r. The diffusion coefficients D o and D r were determined as a function of the mole fraction of the reduced octadecylviologen ( x r ) in the bilayer. The magnitude of D o decreases linearly from 7.1 × 10 −8 cm 2/s to 1.2 × 10 −8 cm 2/s as x r increases from 0 to 1. The value of D r = 1.2 × 10 −8 cm 2/s is constant over the entire range of x r. These data reveal the dependence of D o on the micro-fluidity of the octadecylviologen assembly which decreases linearly with the increasing concentration of C 18MV +. The independence of D r on the redox composition of the assembly suggests that low effective charge in the head group region of the reduced species is responsible for their low mobility which is, apparently, limited by drag interactions with the underlying OTS layer. Computer simulations of chronocoulometric experiments were also used to probe the effect of changing diffusion coefficient with the redox composition in the diffusion layer throughout the progress of an experiment. The simulation results demonstrated that it is difficult to differentiate between the negative curvature in Anson plots caused by the decreasing diffusion coefficient, and a similar curvature expected due to the progressive break-down of the semi-infinite character of diffusion in an electrode film of finite thickness. This highlights the utility of the steady-state approach developed in this work in obtaining a more detailed understanding of the molecular dynamics involved in lateral charge transport in bilayer assemblies.