Electric Field Mediated Ion Transport Through Charged Mesoporous Membranes
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Electric Field Mediated Ion Transport Through Charged Mesoporous Membranes Riaan Schmuhl, W.B. Samuel de Lint, Klaas Keizer, Albert van den Berg, Johan E. ten Elshof MESA+ Research Institute, University of Twente P.O. Box 217, 7500 AE Enschede, the Netherlands ABSTRACT The transport of ions from aqueous solutions through a stacked Au/α-Al2O3/γ-Al2O3/Au membrane under the influence of a dc potential difference is reported. The membrane shows high cation permselectivity at ionic strengths of ~10-3 M at pH 4.3-6.5, which is associated with a combination of anion adsorption and double-layer overlap inside the pores of the γ-alumina layer. The cation flux can be controlled by ionic strength, dc potential difference and pH. INTRODUCTION The development of selective barriers that allow active control over the transport of specific molecular or ionic species may lead to new intelligent interconnects between fluid channels in micro-chemical systems (MiCS) and micro-total analysis systems (µTAS).1 The ability to move molecules with high precision, selectivity and temporal control may result in smaller devices, lower power consumption and improved accuracy. Materials with switchable molecular functions can lead to completely new approaches to valves, chemical separation and detection. One of the pioneering works in this field was performed by Martin and coworkers, who showed that the permselectivity of Au-coated track-etched nanotubular membranes can be controlled by manipulation of the membrane electrical potential relative to the potential in the feed.2 The transport of ionic and neutral species through charged γ-alumina membranes mediated by a variable external electric field can be regarded as a model system for some of the abovementioned types of applications.3,4 Due to the small pore sizes of typically ~5 nm in the γalumina layer and the amphoteric surface charge that is present on the internal pore walls, partial or complete overlap of a negatively or positively charged diffuse double-layer may occur inside the pores, depending on the total ionic strength3 and pH.5 This makes γ-alumina a predominantly anion-, cation- or non-selective barrier depending on externally tunable parameters. Further control over the transport of species is obtained by application of an external electric field over a membrane.2,3 In the present study the influence of electrolyte type, ionic strength and dc potential difference on the transport rates of ions and neutral solutes through a stacked Au/α-Al2O3/γAl2O3/Au membrane is investigated. EXPERIMENTAL DETAILS The preparation of γ-alumina coated α-alumina supports has been described elsewhere.6 The support has a thickness of 2 mm, a pore size of 80-100 nm and a porosity of ~30%. The γ-Al2O3 layer is ~1 µm thick and has a porosity of 40-50%. The pore size distribution was measured by permporometry7 and was found to be in the range 4.0-6.5 nm. The same technique was used to check for the absence of defects >10 nm in the γ-Al2O3 layer. Laterally conductive macroporous gold layers were sp
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