What is Bubble Point Pressure?

Bubble point pressure is the minimum differential pressure required for a liquid to penetrate and flow through a dry membrane. This pressure is influenced by two primary factors:

• The surface tension of the liquid
• The pore size of the membrane

The required pressure may be generated by positive pressure above the membrane, vacuum below the membrane, or a combination of both.

Why Hydrophobic Membranes Can Be Difficult to Wet

Hydrophobic membranes are designed to repel water and other highly polar liquids. As a result, aqueous solutions may not readily pass through a dry hydrophobic membrane because the available pressure or vacuum is insufficient to overcome the membrane's bubble point pressure.

When this occurs, filtration can often be initiated by first wetting the membrane with a solvent that has a lower surface tension, such as methanol or another compatible organic solvent.

Using a Wetting Solvent to Initiate Filtration

A common approach for filtering aqueous solutions through hydrophobic membranes is:

1. Wet the membrane with a low-surface-tension solvent.
2. Apply vacuum or pressure to initiate flow through the membrane.
3. Maintain membrane wetting at all times.
4. Replace the wetting solvent with water or the aqueous sample to be filtered.

Once flow has been established and the membrane remains wetted, filtration of higher-surface-tension liquids can often continue without difficulty.

Important Considerations

When switching from the wetting solvent to an aqueous sample, verify that the solvent and sample are compatible.

Caution: Some samples may form precipitates when mixed with residual wetting solvent. When necessary, flush the membrane with purified water after wetting and before filtering the final aqueous solution. This helps minimize the risk of precipitation while maintaining membrane wetting.

Practical Applications

Understanding bubble point pressure is important when working with:

• Hydrophobic PTFE membranes
• Mobile phase preparation
• Solvent filtration
• HPLC sample preparation
• Laboratory vacuum filtration systems
• Buffer and reagent preparation

Knowledge of bubble point behavior can help laboratories select the proper membrane and wetting procedure for successful filtration of both aqueous and organic solutions.