The pores of microporous membrane filters act as small capillaries. When hydrophilic membranes come into contact with water, capillary action associated with surface tension forces causes the water to spontaneously enter and fill the pores. In this manner, the membranes are easily wetted and allow the bulk flow of water through the pores. Once wetted, hydrophilic membranes will not allow the bulk flow of air or other gasses, unless they are applied at pressures greater than the membrane’s bubble point.
Hydrophilic membrane filters are typically used with water and aqueous solutions. They can also be used with compatible non-aqueous fluids. Hydrophilic membrane filters are typically not used for air, gas or vent filtration since the filters would block flow if inadvertently wetted, by condensation for example.
When hydrophobic membranes come into contact with water, surface tension forces act to repel the water from the pores. Water will not enter the pores and the membranes will act as a barrier to water flow, unless the water is applied at pressures greater than the membrane’s water entry pressure. Low surface tension fluids, such as alcohols, can spontaneously enter and fill the pores of hydrophobic membranes. Once all the air in the pores is displaced, there are no longer any surface tension forces and water can easily enter the pores, displace the low surface tension fluid, and pass through the membrane. The membrane will then allow bulk flow of water for as long as the pore remain water filled. If the membrane is allowed to dry (i.e. air enters the pores), then it must be pre-wet with a low surface tension fluid again prior to use with water.
Hydrophobic membrane filters are typically used with compatible non-aqueous fluids. They are also commonly used as air, gas, or vent filters. Hydrophobic membrane filters are sometimes used with water or aqueous solutions; and, in these applications, they must first be prewet with a low surface tension, water miscible fluid prior to use.
Nominal pore size ratings are provided as a general indication of filter retention. It is understood that some quantity of particles greater than, and equal to, the nominal pore size ratings will pass through the filters into the filtrate. Some manufactures may associate nominal pore size ratings with percentage filtration efficiencies. Nominal pore size ratings vary from manufacturer to manufacturer and, consequently, are not necessarily equivalent. Filters from different manufactures with similar nominal pore size ratings may not actually exhibit similar retention characteristics.
Absolute pore size ratings are typically based on retention studies performed using challenge suspensions of standard microorganism cultures or particles of known size. Absolute pore size ratings represent the size of the smallest microorganisms or particles completely retained during these studies. Absolute pore size ratings are almost always correlated to bubble point specifications that are used for quality control during membrane manufacturing. For the most part, absolute pore size ratings, especially those based on microbial retention, are comparable from manufacturer to manufacturer. There is more uncertainty for absolute pore size ratings based on particle retention studies, especially for pore size ratings <0.2µm, since there are no standard methods for these studies.
Regardless of pore size ratings, it is important to understand that application conditions do influence particle retention. Even filters with absolute pore size ratings can be operated in conditions that will allow unexpectedly sized particles to pass.
Cellulose acetate (CA) membrane filters are one of the lowest protein binding filters available. They will generally have greater throughput with proteinaceous solutions when compared to other membrane filters. CA membrane filters are ideal for filtration of protein and enzyme solutions, tissue culture media and serums, biological fluids, and similar applications where maximum recovery of protein is critical.
CA membranes are manufactured with an integral nonwoven polyester support layer resulting in a dimensionally stable strong membrane that is easier to handle and resistant to curling. The filters have superior resistance to tearing and can withstand steam sterilization up to 135°C. They are suitable for use at elevated temperatures.
CA membranes are hydrophilic and readily wet in water and aqueous solutions. They have good chemical resistance and can be used with low molecular weight alcohols.
Q. Will Sterlitech Track Etch filter membranes keep liquid behind the filter and let gases pass through?
A. PVP-Free Polycarbonate membranes have a water contact angle of approximately 90° and will not spontaneously wet out with liquids that have a surface tension equivalent to or greater than water (1 dyne). Due to the low water contact angle, polycarbonate membranes do not make effective vent filters. Low differential pressures will allow liquid water to break through the pores. We recommend membranes with a higher water entry pressure such as Hydrophobic PTFE, Hydrophobic Polyethylene, and Oleophobic Polyester for venting applications. Effective vent filters will allow permeation of gasses, while blocking liquid from entering the pores. Water vapor and other gases will pass through a hydrophobic vent membrane.
The polyethersulfone (PES) membrane filters have asymmetrical pore structure. The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side. When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance. The side with the largest pores will appear more dull or matte than the side with the smallest pores. With a little bit of experience, most users can easily identify the sides. For optimal throughput, the PES membrane filters should be oriented so that side with the largest pores (the duller side) is facing upstream. For applications involving microscopic analyses of captured particles or microbes, the user may choose to orient the filter so that the side with the smallest pores (the shinier side) is facing upstream. This orientation may reduce throughput but it improves the likelihood of capturing particles of interest on the surface of the membrane instead of within the pore structure.
The polyethersulfone (PES) membrane filters have asymmetrical pore structure. The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side. When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance. The side with the largest pores will appear more dull or matte than the side with the smallest pores. With a little bit of experience, most users can easily identify the sides. For optimal throughput, the PES membrane filters should be oriented so that side with the largest pores (the duller side) is facing upstream. For applications involving microscopic analyses of captured particles or microbes, the user may choose to orient the filter so that the side with the smallest pores (the shinier side) is facing upstream. This orientation may reduce throughput but it improves the likelihood of capturing particles of interest on the surface of the membrane instead of within the pore structure.
The polyethersulfone (PES) membranes used in the Sterlitech membrane filters have asymmetric pore structure. The pore structure varies within the thickness of the membrane such that the largest openings occur on one side and the smallest openings occur on the opposite side. When viewing the membrane with reflected light at low incidence angles, each side has a somewhat different visual appearance. The side with the largest pores will appear more dull (or matte) than the side with the smallest pores (which will appear shinier). With a little bit of experience, most users can easily identify the sides. The membranes can be used with either surface oriented upstream without affecting retention. However, orienting the dull side upstream increases total throughputwhile orienting the shiny side upstream allows for better analyses of the retained particles.
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