Filter Membrane Pore Size Measurement Methods

The pore size of a filter membrane refers to the diameter of the pores on the membrane surface. During water filtration, water is forced through the pores from the inside of the membrane under pressure, trapping impurities and organic matter within the membrane.

Determining Filter Membrane Pore Size

The pore size and separation efficiency of microporous filter membranes are crucial, making the evaluation of pore size very important. Currently, the following methods are generally used:

Direct Measurement Method

Direct Measurement of Membrane Pore Size

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM):
These methods are used to characterize the pore size, pore size distribution, and morphological structure of the membrane. Sample preparation is critical. Wet membrane samples must be dehydrated, metal-coated, and replicated.

Stepwise Dehydration Method:
The membrane sample is fixed with 5% acetic acid, then dehydrated stepwise in an extractor using CCl4 or ethanol, embedded in epoxy resin, solidified, and finally sectioned into thin slices using an ultramicrotome. This method is suitable for TEM observation.

Cryo-Dehydration Method:
The membrane sample is frozen in liquid nitrogen or another low-temperature medium, rapidly freezing the water in the sample into fine crystals. Under low temperature (at least below -60°C) and low vacuum, the frozen crystals sublimate stepwise. Such prepared membrane samples do not shrink and can be observed under an electron microscope after gold coating or replication.

  • Microfiltration membrane pore sizes range from 0.05 to 10 micrometers; SEM can resolve these.
  • Ultrafiltration membrane pore sizes range from 1 nm to 30 nm; the resolution of SEM is below 5-10 nm, making it difficult to observe the structure of ultrafiltration membranes using SEM.
  • TEM has a much higher resolution than SEM, about 3-4 Å. With correct sample preparation, high-resolution TEM can observe the fine surface structure of ultrafiltration membranes.
  • Environmental Scanning Electron Microscopy (ESEM) overcomes the limitations of conventional SEM, allowing direct observation of wet, oily, dirty, and non-conductive samples without preprocessing.

Indirect Measurement Method

Indirect methods utilize physical phenomena related to pore size. By measuring corresponding physical parameters under the assumption that the pores are uniform straight-through circular holes, the equivalent pore size of the membrane can be calculated. Main methods include the bubble point pressure method, mercury intrusion method, nitrogen adsorption method, liquid-liquid displacement method, gas permeability method, molecular weight cut-off method, and suspension filtration method.

Bubble Point Method:

Principle:
When gas passes through the liquid-filled pores of the membrane, if the gas pressure equals the interfacial tension of the liquid in the pores, the liquid exits the pores. The relationship between bubble point pressure and membrane pore size is given by:

relationship between bubble point pressure and membrane pore size

Procedure:

  • a. Immerse the sample parallel to the liquid surface in distilled water until it is fully wetted.
  • b. Place the wetted filter membrane on the test cell and cover it with a smooth porous plate.
  • c. Add 3-5 mm deep water on the porous plate.
  • d. Gradually increase the gas pressure. When the first continuous bubble appears on the membrane surface, record the gas pressure value. This value can be used in the formula to calculate the maximum pore size of the sample.
  • e. The pressure value at the maximum bubble appearance frequency gives the minimum pore size.
  • f. The average pore size can be calculated from the maximum and minimum pore sizes.

Conclusion:

  1. The electron microscopy method is intuitive but destructive and only provides local information.
  2. The bubble point method (also known as the gas permeability method) is limited to measuring the maximum pore size in the membrane. For ultrafiltration membranes with small pores, the required pressure is much higher than the operational pressure of the membrane, so it is generally considered suitable only for microfiltration membrane measurement.