6064
J. Phys. Chem. 1995, 99, 6064-6073
Surface Diffusion in Microstructured, Ion-Exchange Matrices: NafiodNeutron Track-Etched Polycarbonate Membrane Composites Yun Fang? and Johna Leddy*y$ Department of Chemistry and Biochemistry, Queens College and the Graduate Program of the City University of New York, 65-30 Kissena Blvd., Flushing, New York 11367 Received: August 24, 1994; In Final Form: December 9, 1994@
Steady-state rotating disk voltammetry is used to evaluate the flux of redox moieties through composite ionexchange membranes formed by sorbing Nafion into the cylindrical pores of neutron track-etched polycarbonate membranes. As the diameter of the pore decreases, flux through the Nafion portion of the composite increases, as much as 20-fold over flux through simple Nafion films. A model is presented to interpret these results. Nafion concentrates redox material in the pore by partitioning, and facile transport is supported by surface diffusion along the pore wall, at the interface between the Nafion and the neutron track-etched membrane. Parameters characteristic of transport in bulk Nafion and of transport along the pore walls are identified and evaluated. Surface diffusion can be exploited to enhance flux in microstructured composites, and generic flux maximization schemes are outlined. Interfacial gradients in microstructural matrices can be capitalized upon to facilitate transport.
Introduction Systems of controlled chemical architecture are engineered on submicron-to-molecular length scales. The advantages of micro- and nanoscaled systems include shortened response times as well as reduced consumption of reactant materials and generation of waste. In most of these systems, the interfaces of a microstructured, heterogeneous matrix are tailored chemically to orchestrate separations, electron transfers, and molecular recognition. Examples include the clay-based charge separation devices of Mallouk and c o - ~ o r k e r s l -the ~ molecular transistors and electronics of Wrighton et aL5-11and molecular recognition sites incorporated into monolayer a s ~ e m b l i e s ~ *and - ~ polymer ~ layers.20-22Reticulated electron c o n d ~ c t o rand s~~ the~ smallest ~~ microelectrode^^^*^^ may eventually be important components in the design of microsystems. In these systems, the focus has been on building microstructures, shuttling electrons between immobilized molecular species, and designing molecular recognition into a microstructured matrix. If notions of controlled chemical architecture are extrapolated beyond static structures, dynamic devices where molecular moieties move about the microstructure will be designed. The static microstructures designed thus far will be components in microstructured chemical reactors, where the additional complexities of arranging selective and facile transport of reactants and products through a microstructural matrix must be addressed. As outlined below, the microstructure itself can be exploited to orchestrate facile, selective transport of molecular moieties. Microstructured systems differ from bulk systems in several important ways. First, as microstructures are established at interfaces between two phases, microstructured systems are necessarily heterogeneous. Second, different phases have different properties; at the interface between dissimilar materials, gradients will be established to dissipate the energetic differences of the phases. Such gradients are observed, for example, at the interface betwen a polarized electrode and electrolyte t Current address: American Cyanamid, Princeton, NJ.
*
Current address: Department of Chemistry, The University of Iowa, Iowa City, IA 52242. @Abstractpublished in Advance ACS Abstracts, February 15, 1995.
0022-365419512099-6064$09.00/0
solution. Third, as microstructures decrease in size, the ratio of surface area to volume in the system increases, and more interfacial area where gradients can be established is formed. Thus, interfacial properties which are not important in bulk systems may come to play a major role in dictating the behavior of microstructural systems. The interfacial processes most likely to be important in microstructural systems are those associated with steep but short-range gradients at the interface. Examples include electrostatic effects, migration, magnetic gradients, surface tension, and surface diffusion. In designing microstructured transport systems, these interfacial effects can be capitalized upon to build systems of rapid and selective transport. Here, we report on ion-exchange polymer composites formed by sorbing a perfluorinated cation-exchange polymer (Nafion) into the small radii (7.5-300 nm), straight cylindrical pores of neutron track-etched polycarbonate. Steady-state flux of cations and neutrals through the Nafion portion of the composites increases as the pore radii decreases, with the maximum flux exceeding 20 times the flux through a simple Nafion film. A quantitative model for the flux enhancement based on a facile surface diffusion process at the interface between the ionexchange polymer and the surface of the neutron track-etched pores is presented. Flux in the bulk ion-exchange polymer and along the interface, as well as the thickness of the interfacial zone can be determined. The ratio of surface area for diffusion to the volume of ion-exchange polymer for extraction or the inverse pore radius (r0-l) is shown to be a critical parameter in optimizing flux in composites. For extremely high ratios of surface area to volume, packing of the ion exchanger into the microstructured support can limit flux enhancements. Methods are outlined for optimizing flux in composites by capitalizing on the interfacial gradients available in microstructured matrices.
Experimental Section Formation of Composites. Composites were formed by sorbing Nafion into neutron track-etched polycarbonate membranes (Costar and Nuclepore). These membranes are transversed by numerous (- lo8pores/cm*),parallel, small-diameter cylindrical pores, with nominal pore radii of 7.5, 15, 25, 50, and 300 nm. The membranes were soaked for 30 min in a 0 1995 American Chemical Society
Surface Diffusion in Ion-Exchange Matrices
J. Phys. Chem., Vol. 99, No. 16, 1995 6065
commercially available suspension of DuPont Nafion (5 g/100 TABLE 1: Solution Parameters mL) in a mixture of water and alcohols (Solution Technologies). DOP' HzQ HzQ FcN(CH3)3+ R u ( N H ~ ) ~ ~ + The composites were blotted briefly upon removal from the parameter HNo3 HN03 HzS04 H2S04 HzS04 soaking suspension to remove excess suspension from the outer c* @moVcm3) 2.0 4.0 4.0 2.0 2.0 surfaces of the membrane and then dried overnight at room n 2 2 2 1 1 temperature in a vacuum desiccator. D,(10-6cm2/s) 6.5 9.4 9.3 6.7 7.3 molecvol(nm3) 0.151 0.113 0.113 0.230 0.045 To perform electrochemical measurements, a composite membrane was placed flush against a glassy carbon disk (NH3)63+ was approximated from the radii of C O ( N H ~ ) and ~~+ electrode inlaid in Teflon. A plastic cap with a hole in its center Cr(NH3)63+,35allowing a 10% larger radius for the ruthenium larger than the disk was then snapped over the membrane onto ion. the Teflon shaft. This held the composite tightly against the electrode surface. Membranes containing no Nafion were Results similarly mounted. .Interpretationof the Rotating-Disk Voltammetric Data. Formation of Nafion Films. Nafion films were formed by Numerous studies of transport through thin films and polymer pipeting 10 p L of Nafion suspension onto a glassy carbon layers on electrodes have been performed by steady-state electrode and allowing the solvents to evaporate. Electrodes rotating-disk ~ o l t a m m e t r y . ~If~ -a~redox ~ species is moving were dried overnight in a vacuum desiccator. Film thicknesses through a film at steady state and if it undergoes neither chemical were calculated from the wetted density (1.58 g/cm3) of Nafion reactions nor physical interactions which cause its concentration 1 The film thickness was 6.9 pm, a value similar to the gradient normal to the surface of the electrode to deviate from roughly 6 pm thick neutron track-etched membranes. a constant value, then the Koutecky-Levich plot, i h - l versus Electrochemical Measurements. All measurements were o-ll2, will be linear. is, is the mass-transport limited steadymade with a Pine ASR Rotator, PAR 273 Potentiostat, and state current measured at the electrode surface at a given rotation Norland IQ 400 Digital Processing Oscilloscope. Composites, rate, w (rad/s): unmodified membranes, and films were examined in an aqueous solution of 2.0 mM 3-hydroxytyramine (dopamine) hydrochlo1 1 - . 1 ride (Aldrich Chemicals) and 0.1 M nitric acid at a 0.458 cm2 llim 'memb 'so111 glassy carbon electrode. At this acid concentration, dopamine28 is a protonated cation, DOP+; pK1 = 8.87. Composites and where imemb is rotation rate independent and characteristic of films were allowed to equilibrate overnight in a redox solution. transport through the modifying layer. isoln is characteristic of Cyclic voltammetric sweeps were performed to verify the quality transport in the solution and is the steady-state current measured of the composites and films. Mass-transport rates through the at the same electrode with no modifying layer: composites were determined by the steady-state, mass-transportlimited oxidation of dopamine at a rotating disk electrode isoln= 0.62OnFAD,2/3~-'/~c*w''~ (2) without resistive compensation. Rotation speeds between 200 and 1500 rpm were employed. DOP+ was electrolyzed through A plot of versus o - l I 2 for a modified electrode will have the film for at least 30 min before the steady-state values were the same slope as that for the unmodified electrode. The slope recorded. is described by c*, the bulk concentration (moles/cm3); Y,the Previously, flux measurements for similar composites were kinematic viscosity (cm2/s);D, (cm2/s);n;F, Faraday's constant; reported,29but no explanation of the observed flux enhancements and A, the electrode area (cm2). For unmodified electrodes, was provided. These composites were formed similarly to the the intercepts of the Koutecky-Levich plots were zero, and the composites described here, except the excess Nafion suspension slopes were used to determine D, for each redox species, as was not blotted from the surface of the membranes following cited in Table 1. the soaking step, and the composites were dried in air and not For modified electrodes, unless the transport processes in the in a vacuum. In the electrochemistry measurements, resistive modifying layer are decoupled from the transport processes in compensation was employed. (The layer adsorbed on the outer solution, the Koutecky-Levich plots cannot be linear. This surface of these membranes is less than 10% of the total conditions is satisfied when the structural features of the membrane thickness and will not significantly limit transport modifying layer in the plane parallel to the electrode surface at steady state because the porosity of the neutron track-etched (and normal to the direction of transport) are small compared materials are sufficiently low ( e10%) and their t h i c k n e ~ s e s ~ ~ to the thickness of the hydrodynamic boundary layer in solution, sufficiently high that the flux to the total surface area of the do, where 60 = 1.61D,1/30-1/2~1/6. In water, where IJ 0.01 electrode is limited by transport through the pores as opposed cm2/s and D, cm2/s, 0.005 5 60 5 0.014 cm for 1500 to through the thin outer layer.) Four redox systems were 2 o 2 200 rpm. For Nafion films, the only structural features studied: 4 mM hydroquinone (H2Q) in 0.1 M HN03, 4 mM are the inverted micelles formed when the hydrophobic fluoH2Q in 0.1 M HzSO4, 2 mM (trimethy1amino)methylferrocene rocarbon backbone segregates from the hydrophilic sulfonic acid iodide (FcN(CH&+) in 0.1 M H2S04, and hexaammineruthesites,51 but these structures are much smaller (55 nm) than nium(II1) chloride (Ru(NH&j3+)in 0.1 M H2S04. All redox 60. For neutron track-etched membranes and their composites, processes were oxidations, with the exception of R u ( N H ~ ) ~ ~ + o was chosen such that ro and N-'I2
