Lois Anne Zook

Density and solubility of nafion: recast, annealed, and commercial films.

The density of Nafion membranes wetted in water and acetonitrile and the solubility of Nafion in a 50:50 ethanol/water mixture are reported. Commercially available membranes, solution recast membranes, and recast membranes thermally processed for varying lengths of time were examined. Unheated recast membranes were found to be ∼20% less dense in water than both commercial membranes and recast membranes heated at 140 °C for 40, 50, and 60 min. For heating times of ≥40 min, the density of recast membranes in water was invariant and equal to the density of commercial films. Similarly, in acetonitrile, commercial membranes and films heated for 1 h had the same density; unheated recast films were ∼25% less dense. Nafion density in acetonitrile was ∼15% less than that in water. Acid pretreatment reduced Nafion film density by ∼15%. Unheated recast films were 22-100% soluble in a sonicated ethanol/water mixture. Commercial membranes and recast films heated for as little as 10 min were insoluble under the same conditions.

Models for mediated reactions at film modified electrodes: controlled electrode potential

Abstract Models have been developed previously for redox reactions mediated in films on electrode surfaces. As developed by Saveant and coworkers for steady state rotating disk voltammetry, a redox species, P, confined in the film is electrolyzed at the mass transport limited rate to form Q. Q is able to undergo reaction with a species, A, present in solution and able to permeate the film, to regenerate P and form product B; A + Q →k1 B + P. The reaction may occur either in the bulk of the film or at the film-solution interface. Here, the models are modified to include control of the electrode potential such that the concentration of P and Q at the electrode surface are parameterized by the Nernst equation. As long as no P and Q are lost from the film, the Nernstian condition will govern most electrode systems at steady state. Control of the electrode potential allows (1) simplified determination of the kinetic characteristics for some reaction schemes, and (2) evaluations where a second electrolysis occurs at a potential sufficiently close to the redox potential of P/Q that there are advantages in not applying potential sufficient to electrolyze P at the mass transport limited rate. This includes reactions where the formal potentials for P/Q and A/B are within 200 mV of each other as well as cases where a larger applied potential exceeds the solvent limit or leads to film decomposition. Equations are also provided for the electrolysis of A to B at the electrode surface as governed by the Nernst equation. Methods for evaluating experimental data are outlined.

Nano- and Micro-Structured Materials:The Role of the Interfaces in Tailoring Transport

Scientists have long studied the properties of bulk materials. Recently, nanoand micro-structured systems and matrices have received significant attention. Small distances between components in nanomaterials offer rapid response times, while the small volumes of nanosystems have advantages of generating small quantities of waste. It is improbable that the behavior, properties, and transport characteristics of heterogeneous microand nano-structured materials will be fully described by models appropriate to homogeneous bulk systems. Here, several ideas about what dominates the transport characteristics of heterogeneous microstructures are outlined. Essentially, the interfacial forces and gradients established along the high internal surface area of nanoand micro-structured materials strongly influence the flux of ions and molecules through heterogeneous structures. Several systems are described which illustrate these notions. The systems used to illustrated these ideas are predominately microstructured ion exchange polymer composites, however, the ideas presented are likely to extrapolate to a wide variety of nanostructured environments. Design paradigms for tailoring transport in nanoand micro-structured environments are outlined.