advancing sorption science

Porosity determination by physisorption analysis

The nature of the treatment depends on :

  • Shapes of pores
  • size
  • connectivity of the pores
  • nature of the chemical/physical interactions
    • Both on the surface and inside the pores
    • sorptive - sample interaction
    • sorptive - sorptive interaction

Range of methods, using models ranging from….

Total porosity

The BET method uses a measurement of the physisorption of a gas to derive a value of “surface area” for a sample.

The gas molecules can pass between particles and into all pores, cracks, and surface roughness, so that the measurement probes the full microscopic surface area of the sample.

Most often, the sample in the form of powder or granules, and the result is stated as a Specific Surface Area, in units of area per unit mass. It may also be given as area per unit of volume, or as the absolute area for an object.

The measurement

pore analysis requires measurement of an “Adsorption isotherm”.

A typical measurement involves :

  1. putting a known amount of sample into a sample cell or container.
  2. outgassing or other treatment of the sample, to remove impurities, moisture.
  3. increasing the pressure of the , which involves exposing a clean sample to a number of different pressures of an adsorptive vapour, and measuring the amount adsorbed at each point.

historically, is most widely performed using adsorption of Nitrogen gas at 77 K, the boiling point of liquid nitrogen, but other species and temperatures are used.

  • Argon at 87 K (liquid Argon temperature)
  • Krypton at 77 K
  • Carbon dioxide, CO2 at 0 °C
  • Water at 20 °C using DVS instruments, such as the IGAsorp and the SPS.

, the adsorption isotherm is measured as a graph of “amount adsorbed”, \(n_{adsorbed}\) versus “Relative Pressure”, \(P/P_0\), where \(P\) is the pressure of the adsorbtive, and \(P_0\) is its saturated vapour pressure at the fixed experimental temperature.

Instruments like the BELSORP mini X BET analyser, and the BELSORP MAX II sorption analyser, can easily measure the adsorption isotherm of gases such as Nitrogen, Argon, Krypton, often at cryogenic temperatures.

Adsorption of Water and other volatile substances is often performed by Dynamic Vapour Sorption instruments, such as the IGAsorp from Hiden Isochema, and the multisample SPS from ProUmid.


A graph of the BET isotherm model, plotted for several different values of C, which shows how the knee of the curve moves to lower pressures as the C constant increases. 
The knee corresponds to a monolayer coverage of the surface: higher C implies greater affinity between the molecules and the surface, so that the monolayer forms more easily.
The model: The graph shows …

Mercer Instruments specialises in the supply and support of scientific instruments for measurements of sorption, and characterisation of pore size and surface area.
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for gas and vapour sorption.
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