Membrane pore densities

An alumina matrix may be prepared with high pore density (more than 60 %) and pore diameters ranging from 5 to 250 nm. Ruiz-Hitzky et al. [214] immobilized GOD in nanoporous alumina membranes with regular hexagonal arrays of highly ordered cylindrical pores aligned perpendicularly to the membrane surface. GOD was anchored in the membrane by the highly hydrophilic chitosan biopolymer. Full activity was maintained for at least 50 hours. 

Nanoelectrode ensembles were prepared by electroless deposition of Au within the pores of polycarbonate membrane filters (Poretics). Filters with pore diameters of 10 and 30 nm were used [25]. The pore densities and average center-to-center distances between pores for these membranes are shown in Table 1. Multiplying the pore density (pores cm ) by the cross-... 

The pores in a commercially available polycarbonate filtration membrane (Poretics) were used as templates to form the nanotubules (pore diameter = 50 nm pore density = 6 X 10 pores cm thickness = 6 pm). As before, the electrolessly plated Au deposits both on the pore walls and the membrane faces [71]. The gold surface layers on the membrane faces allow us to make electrical contact to the Au nanotubules within the pores. The thickness of the gold layers deposited on the pore walls can be controlled... 

The Nuclepore membranes used for this work were standard polyester membranes 25 mm in diameter. The pore number, N, is calculated from the pore density and the effective flowing area of... 

Membrane pore size/density, generally 1.0-8.0 pm, with smaller pore size filters having greater pore density ... 

Membrane filter (Nuclepore Corp.) pore diameter 5-10 pm, pore density 4 x 105 pores/cm2. 

Water Flux The permeability of a UF membrane is determined by pore size, pore density, and the thickness of the membrane active layer. Water flux is measured in the absence of solute, generally on a newly made or freshly cleaned sample. The test is simple, and involves passing water through the membrane generally in dead-end flow under carefully controlled conditions. In a water flux test, the membrane behaves as a porous medium with the flow described by Darcy s law. Adjustments for viscosity and pressure are made to correct tne results to standard conditions, typically the viscosity of water at 25°C and the pressure to 50 psi (343 kPa). The water flux will be many multiples ofthe process flux when the membrane is being used for a separation. Virgin membrane has a standard water flux of over 1 mm/sec. By the time the membrane is incorporated into a device and used in an application, that flux drops to perhaps 100 pm/s. Process fluxes are much lower. 

Mesoporous alumina membranes ( anodic aluminium oxide , or AAO) are prepared by anodic oxidation of aluminium metal [1,2]. The cylindrical pores, perpendicular to the membrane surface, form hexagonal arrays of straight non-intersecting channels with pore densities up to lO Vcm. Their diameters are controllable within the range 5 - 100 nm as a linear function of anodisation voltage. These membranes are used as molecular sieves, and have also found application as templates for metallic nanowires [3,4,5,6], metal elusters and colloids [7,8], and carbon nanotubes [9,10]. 

Other methods have also been adopted to produce inorganic membranes with essentially straight pores. Witte [1988] subjected a metal foil such as nickel to a two-step photolithographic procedure. The pore density of the final membrane exceeds 150,(X)0/cm and the pore sizes form fall under the microfiltration and ultrafiltration ranges. Although both flat and cylindrical shapes can be handled, the flat shape is preferred. 

D-amino acid oxidase apoenzyme (apo-D-AAO)-loaded membrane. The membrane had pores of 400 nm diameter and (B) as (A) but using a membrane with pores of 30 nm diameter (pore density 6 x 10 pores cm ). (From Lakshmi, B.B. and Martin, C.R., Nature, 388, 758, 1997. With permission.)... 

In these membranes, pore size, shape, and density can be varied in a controlled way and, as a consequence, a membrane with the required transport and retention characteristics can be produced. 

Finally, track-etched MF membranes are made from polymers, such as polycarbonate and polyester, wherein electrons are bombarded onto the polymeric surface. This bombardment results in sensitized tracks, where chemical bonds in the polymeric backbone are broken. Subsequently, the irradiated film is placed in an etching bath (such as a basic solution), in which the damaged polymer in the tracks is preferentially etched from the film, thereby forming cylindrical pores. The residence time in the irradiator determines pore density, and residence time in the etching bath determines pore size. Membranes made by this process generally have cylindrical pores with very narrow pore-size distribution, albeit with low overall porosity. Furthermore, there always is the risk of a double hit, i.e., the etched pore becomes wider and could result in particulate penetration. Such filter membranes are often used in the electronic industry to filter high-purity water. 

The difference in the pore volumes measured for the two membranes, 0.17 mlg (A20) and 0.39 mlg (A200), may be ascribed to the different anodic oxidation conditions employed in the preparations. This can influence the pore density in the membranes as earlier discussed [2-4]. 

Pore size and density are also important in selecting a membrane. Some membranes such as polycarbonate have very high pore densities for maximum fluid transport, but are fragile and rather opaque. The pore size is also critical, depending on the experiment. For example, a 0.45- xm pore size will prevent cells from migrating through the membrane, while 3 jxm can be used for transmembrane... 

Type of Membrane Pore Diameter (nm) Ai/erage Density (m... 

In order to facilitate the coating of a homogeneous thin layer on the support, the pore size must be adapted to the grain size of the layer which is to be deposited. The presence of large pores on the internal surface of channels could lead to penetration of the grains into the support and thus to defects in the membrane. The density of the support must be sufficient to ensure an excellent mechanical resistance. However, a low density shows a resistance to the flux through the support, so a compromise must be found for the choice of grain size. 

As in the case of all membrane separation processes, the choice of an appropriate module type is based on feed type (viscosity, suspended solids content, and particle size), required membrane packing density (based on flux, total throughput, and available floor space), good flow hydrodynamics (for minimization of concentration polarization, effective cleaning and sanitation), and module cost. The various types of membrane modules and their fabrication have been reviewed by Strathmann.f Hollow fiber modules, which have the highest membrane packing density of all module types, are the most suitable for use in OD because of the inherently low flux of this process. However, the membranes that have provided the best fluxes and volatiles retention because of their relatively large pore diameters and porosities, that is those fabricated from PTFE, have not yet become available in hollow fibers with an acceptably low wall thickness. 

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