V. Romero

Changes in Morphology and Ionic Transport Induced by ALD SiO2 Coating of Nanoporous Alumina Membranes

Nanoporous anodic alumina membranes (NPAMs) were produced by the two-step anodization method in sulphuric, oxalic and phosphoric acidic electrolytes displaying a hexagonally ordered spatial arrangement of pores with well controlled nanopore size distribution and low porosity. Some selected NPAMs were further modified by conformal coating their surface and inner pore walls with a thin layer of SiO2 by means of atomic layer deposition (ALD), which reduces both the pore radii and porosity but it also seems to affect to the electric fixed charge on the membranes surface. A comparative study about the influence of silica modification of NPAMs surfaces on the ionic transport through the nanoporous membranes has been performed by measuring membrane potentials and electrochemical impedance spectroscopy with NaCl solutions. According to these results, a direct correlation between the membrane effective fixed charge and the NaCl diffusion coefficient can be established. The coating with a SiO2 thin layer causes a reduction of 75% in the positive effective fixed charge of the NPAMs independently of their pore radii and the increase in counterion transport (cation transport number and diffusion coefficient) even through constrained nanopores, which can be of interest in several applications (microfluidics, drug delivery, nanofilter devices, etc.). Moreover, slight changes in the membrane/solution interface due to the SiO2 cover layer are also indicated.

Ionic transport across tailored nanoporous anodic alumina membranes

Highly ordered Nanoporous Alumina Membranes (NPAMs) with a precise control of the pore size and different porosity but thickness around 63 μm were fabricated by the two-step anodization process using different acidic aqueous solutions, oxalic (Al-Ox), sulfuric (Al-Sf), and phosphoric (Al-Ph) acids, respectively. The pore size was controlled by properly changing the anodization voltage and the electrochemical bath conditions, obtaining the following average pores diameter values, d(p), as determined by scanning electron micrographic analysis: Al-Ox (d(p)=46±2 nm), Al-Sf (d(p)=27±2 nm), and Al-Ph (d(p)=240±20 nm). A pore increasing of around 5% for samples Al-Ox and Al-Sf was obtained after membranes immersion in 5 wt.% phosphoric acid for a certain etching time. Electrochemical characterization of NPAMs was performed with the samples in contact with NaCl solutions at different electrolyte concentrations. Ionic transport numbers and effective membrane fixed charge were determined from membrane potential measurements, which clearly show the significant influence of the pore diameter on ions transport. Moreover, frictional and electrical effects on mass transfer parameters (salt and ions diffusion coefficients) into the pores of alumina membranes were also evaluated from these results.

Effect of Porosity and Concentration Polarization on Electrolyte Diffusive Transport Parameters through Ceramic Membranes with Similar Nanopore Size

Diffusive transport through nanoporous alumina membranes (NPAMs) produced by the two-step anodization method, with similar pore size but different porosity, is studied by analyzing membrane potential measured with NaCl solutions at different concentrations. Donnan exclusion of co-ions at the solution/membrane interface seem to exert a certain control on the diffusive transport of ions through NPAMs with low porosity, which might be reduced by coating the membrane surface with appropriated materials, as it is the case of SiO2. Our results also show the effect of concentration polarization at the membrane surface on ionic transport numbers (or diffusion coefficients) for low-porosity and high electrolyte affinity membranes, which could mask values of those characteristic electrochemical parameters.

Modification of Nafion Membranes by IL-Cation Exchange: Chemical Surface, Electrical and Interfacial Study

Bulk and surface changes in two proton-exchange membranes (Nafion-112 and Nafion-117) as a result of the incorporation of the IL-cation n-dodecyltriethylammonium (or DTA