Nafion Templated on Ficoll: A Density Graded Ion Exchange Polymer

Krysti Knoche Gupta; Johna Leddy

doi:10.1149/MA2026-01492426mtgabs

Ficoll (R) is a charge neutral, water soluble polysaccharide. Because Ficoll in water generates a density gradient, Ficoll is used to separate biological cells based on cell buoyancy. At 50 % w/v in water, Ficoll relative viscosity is > 500. Viscosity and diffusion coefficient, D, are inversely related; as viscosity increases, D decreases. Ficoll Films: Ficoll in water is cast on a 0.452 cm 2 platinum disk. The Ficoll modified electrode is placed in acetonitrile containing 1.00 mM TMPD (tetramethyl phenylenediamine) and 0.10 M TBABF 4 . The morphology of cyclic voltammograms is sigmoidal at 2 mV s -1 . At 200 mV s -1 , the voltammograms have some small avian peak currents but are predominantly sigmoidal. From confocal microscopy, Ficoll is denser near the surface and density decreases with distance from the surface. The Ficoll layer establishes a density-viscosity gradient. As TMPD diffuses from the solution into the diffuse Ficoll film, the diffusion coefficient is spatially dependent D(x). The gradient in viscosity induces a gradient in the diffusion coefficient. Fick’s second law and the current expression are modified. ∂ c(x,t) / ∂ t = ∂ [ D(x) ∂ c(x,t) / ∂ x ] and i(t) / nFA = D(x) ∂ c(x,t) / ∂ x | x=0 Because D(x) varies spatially and establishes a density gradient, flux of reactant toward the electrode introduces an additional diffusive term into the transport. The additional flux delivers material to the electrode at near steady state so that diffusional tailing in the voltammograms is eliminated. The density gradient introduces a “passive convection” to the transport. Nafion on Ficoll Composites: Ficoll is used as a template to cast density graded Nafion layers. The protocol is similar to the protocol for Ficoll films except Nafion suspension is included in casting the layer on the electrode. Cyclic voltammograms at 2 (solid), 20 (short dash), and 200 (long dash) mV s -1 are shown for a Nafion film (bottom, black) and a Ficoll Nafion composite (top, blue). The avian response at 20 and 200 mV s -1 for Nafion is as expected. For the Nafion on Ficoll composite, the response shifts from avian to sigmoidal as scan rate slows. The maximum currents for the Nafion Ficoll composite are ~10 % of the Nafion film, consistent with lower diffusion coefficients and / or lower concentration of the redox probe. The evolution of voltammetric response for the Nafion on Ficoll composite to the sigmoidal response is driven by the diffusion coefficient gradient induced by the graded viscosity of the Ficoll. The response for the Nafion Ficoll composite is morphologically similar to the response for Ficoll films. Formation of graded density, ion exchange polymers such as Nafion on Ficoll provides means to control flux in ion exchange polymers and perhaps induce passive convection. The steady delivery of ions and molecules through a density graded film may have applications in separations and drug delivery. This work was supported by NSF and ARO. Reference [1] Krysti Knoche, Ph.D., University of Iowa, 2015 Figure 1

Nafion Templated on Ficoll: A Density Graded Ion Exchange Polymer

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