In order for a filter to be wetted by a liquid, the contact angle must be considered. Water has a high contact angle and will not wet a hydrophobic PTFE filter. On the other hand, the contact angle for acetone is low, and it will readily wet a PTFE filter. Methanol has a high contact angle when compared to acetone. In fact, its contact angle is at the upper limit for a liquid that will still wet PTFE. Methanol readily picks up water. If methanol contains 1% water, it will not completely wet a PTFE membrane. On that basis, if methanol must be used, it should be as dry as possible. Alternatively, the membrane can be pre-wet by a solvent such as acetone or ethanol before wetting with methanol.
Yes. Filters will perform very differently in a gas/air stream than in a liquid stream. These differences can be observed by comparing flow rates and retention ratings between gas /air and liquid for the same medium. In liquid, particles generally follow the flow of the solution as it travels through the filter, and it is primarily particles that are directly intercepted by the filter matrix which will be retained. In gas/air filtration, direct interception also applies, but there are other additional mechanisms that can cause particles to be captured by the filter. These mechanisms include inertial impaction, gravitational settling, electrostatic interaction, and Brownian Motion.