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For liquid filtration, a membrane must be wettable with the fluid being filtered. The wettability of a membrane is tied to the chemical properties of the membrane surface. Most polymers used to manufacture microporous membranes are naturally hydrophobic, meaning they will not wet out with water.
Some exceptions are nylon and cellulose which are naturally hydrophilic and will wet out with water. The distinction between hydrophobic and hydrophilic relates to the surface energy of the polymer. If the surface energy is >70 dynes/cm, the polymer is hydrophilic. Below 70 dynes/cm, the polymer is hydrophobic.
Hydrophobic membranes are wettable with alcohols. If the polymer is compatible with alcohols, it can be wet first with the alcohol and then equilibrated in water prior to filtering the aqueous fluid. For many applications, this is impractical; the membrane must be directly wettable with the aqueous fluid.
To overcome the hydrophobicity of the polymer, the membrane can be treated with a secondary chemistry that coats the base polymer. The secondary chemistry becomes primary in determining wettability. It is important to recognize that the base polymer remains hydrophobic unless the secondary chemistry is a covalent modification of the polymer.
Membrane Coatings to Change Wettability
Hydrophilic membranes wet spontaneously with pure water (at a surface tension approximately 72 dynes/cm2 at ambient conditions) and require some elevated pressure to allow intrusion of water into the pores of the structure. Reduction of surface tension of the wetting fluid either through addition of solutes, such as surfactants or low surface tension miscible solvents such as alcohol to water, will have an impact on how quickly dry membranes will wet.
In addition, solvents or solvent mixtures which possess a relatively low surface tension will wet hydrophobic membranes spontaneously.
For instance, the 0.2 µm PTFE Fluoropore® membrane requires an intrusion pressure of approximately 50 psig to affect wetting with pure water. However, organic solvents with a surface tension of less than approximately 32 dynes/cm2 will spontaneously wet Fluoropore membranes at 0 psig pressure. There are situations when chemical compatibility requires that a hydrophobic filter be used for a solution that will not wet the filter. In such instances, the filter needs to be pre-wet with an alcohol (i.e. ethanol, methanol). Then the filter should be rinsed, if necessary.
In venting applications, the filter is used as a porous barrier that allows escape of gas bubbles from a liquid stream or exchange of gases between a liquid stream and the external atmosphere. To ensure that the filter does not wet out under any circumstances, it can be treated with a secondary chemistry that renders it superhydrophobic or oleophobic. This reduces the surface energy to <20 dynes/cm. The membrane cannot be wet with water or alcohols.