Surface Effects on Cation Transport across Porous Alumina Membranes

Abstract

Transport behavior of monovalent and divalent solutes across mesoporous Anopore γ-alumina membranes was investigated as a function of pore diameter, pH, ionic strength, and nature of the salt or complexing species in solution. Radiotracers 137 Cs, 85 Sr, 22 Na, and 45 Ca were present in the feed solutions at very low concentrations, ranging from 10 -9 to 10 -12 M and total salt concentrations from 0.1 to 10 -4 M. The divalent cations Ca 2+ and Sr 2+ exhibit lower diffusion rates (3-7 times slower) than the monovalent cations Cs + and Na + for membranes with 20 nm diameter pores. Differences between monovalent and divalent cation diffusion rates for the membranes can be explained in terms of a Donnan exclusion effect from the positively charged alumina surface. The rate of Sr 2+ transport across the 20 nm alumina membranes was greatly increased by raising the pH (reducing the membrane surface charge) from 5 to 8 for both the feed and receive sides. Increased ionic strengths and the addition of complexing agents or specific salt solutions also facilitated divalent ion transport. Diffusion coefficients for divalent cations increased 3-fold for the 100 nm pore diameter membranes.

Highly efficient and selective enrichment of phosphopeptides using porous anodic alumina membrane for MALDI-TOF MS analysis

“…However, when we used o-phosphoric acid as the acid dopant in DHB as the matrix, the MS signal of tetraphosphopeptide (m/z 3122) and another phosphopeptide (m/z 2556) were observed°after°the°enrichment°process°with°PAA° (Figure°5).°In°addition°to°the°enhanced°ionization°of°phos-phopeptide, phosphoric acid might decrease the interactions between the PAA membrane and the tetraphosphopeptide. Therefore, the tetraphosphorylated peptide species are efficiently eluted from the PAA membrane and can be satisfactorily detected under°acidic,°neutral,°and°alkaline°conditions°( Alumina is an amphoteric material whose surface electrical°properties°depend°on°solution°pH° [47]°and°the isoelectric point of the amorphous alumina is pH Ӎ 8 [48].°If°the°interactions°between°the°phosphopeptides and the PAA membrane are mainly by electrostatic interactions, the enrichment capacity would be influenced by solution pH. The isoelectric point of the PAA membrane made in phosphate acid is about 5.5 in Tris-HCl°buffer° [42].°Thus,°at°pH°Ͻ°5.5,°the°PAA membrane is protonated and has a positively charged surface.…”

Diffusivities of Tris(2,2′-bipyridyl)ruthenium inside Silica- Nanochannels Modified with Alkylsilanes

“…1, [12][13][14] Martin and co-workers 1, 12 have studied the diffusivity inside nanopores by observing the mass transport across nanopore membranes. In their studies, they estimated the apparent diffusion coefficients of phenol derivatives in columnar alumina pores modified with octadecyltrimethoxysilane; the order of the estimated values was 10 -8 to 10 -6 cm 2 s -1 .…”

“…12 For the alumina membrane system, the diffusion coefficients of metal cations (10 -7 to 10 -6 cm 2 s -1 ) in unmodified alumina pores were reported and the surface effect on the diffusion behavior was discussed. 13 Fu et al 14 applied single-molecule fluorescence correlation spectroscopy to estimate an apparent diffusion coefficient of Nile Red in a surfactant-templated mesoporous silica thin film, and found that the diffusion coefficient was very small (∼10 -10 cm 2 s -1 ). These reported diffusion coefficients in nanopores are smaller than those in the bulk solution phase (∼10 -6 cm 2 s -1 ), and this slow diffusivity has sometimes been ascribed to the adsorption of molecules at the inner wall of a nanopore.…”

“…These reported diffusion coefficients in nanopores are smaller than those in the bulk solution phase (∼10 -6 cm 2 s -1 ), and this slow diffusivity has sometimes been ascribed to the adsorption of molecules at the inner wall of a nanopore. 13,14 In contrast, the purpose of this study is to examine the effects of solvents and the type of alkylsilanes in nanopores on the diffusion process.…”

Nanoporous Alumina Membranes as Diffusion Controlling Systems

“…[7,8] Removing the alumina material by dissolution after filling the pores furnishes metal nanowires or tubes of oxidic materials. [6] Bluhm et al [9,10] have reported on the use of nanoporous alumina membranes for diffusion processes. A wide field of possible applications exist, such as novel drug delivery systems (operational without any mechanical or electrical building blocks).…”