Pore Volume Distributions can be determined by either gas adsorption porosimetry (typically N2, Ar or CO2)
or mercury intrusion porosimetry.
Gas porosimetry measures pores from 3.5 Angstroms to about 4000 Angstroms in diameter.
Mercury porosimetry is applicable to pores from 30 Angstroms up to 900 micrometers in diameter.
Gas Adsorption Technique
To measure pore size by gas adsorption, isotherms are recorded from low pressures (approximately 0.00001 torr, minimum) to saturation pressure (approximately 760 torr). The pressure range is determined by the size range of the pores to be measured. Isotherms of microporous materials are measured over a pressure range of approximately 0.00001 torr to 0.1 torr. Isotherms of mesoporous materials are typically measured over a pressure range of 1 torr to approximately 760 torr.
Once details of the isotherm curve are accurately expressed as a series of pressure vs. quantity adsorbed data pairs, a number of different methods (theories or models) can be applied to determine the pore size distribution. Available micropore methods include: Density Functional Theory (DFT), MP-Method, Dubinin Plots (Dubinin-Radushkevich D-R, Dubinin-Astakov D-A), and Horvath-Kawazoe (H-K) calculations. Available Mesopore methods include: Barrett, Joyner and Halenda method (BJH), and Density Functional Theory (DFT). T-Plot analysis is also available for total micropore area as well.
Mercury Intrusion Technique
Mercury intrusion porosimetry involves placing the sample in a special sample cup (penetrometer), then surrounding the sample with mercury. Mercury is a non-wetting liquid to most materials and resists entering voids, doing so only when pressure is applied. The pressure at which mercury enters a pore is inversely proportional to the size of the opening to the void.
As mercury is forced to enter pores within the sample material, it is depleted from a capillary stem reservoir connected to the sample cup. The incremental volume depleted after each pressure change is determined by measuring the change in capacitance of the stem. This intrusion volume is recorded with the corresponding pressure or pore size.
NOTE: The maximum pore size that any mercury porosimeter can characterize depends on a number of factors. The primary limiting factors are 1) the contact angel between mercury and the sample material, and 2) the head pressure gradient associated with a volume of mercury and the volume of sample material subjected to these pressures.