The pore structure of a material can be just as important as its chemistry because pore size, pore volume, and permeability have a major impact on performance in numerous applications from the bioavailability of a pharmaceutical to the adsorption capacity of a filter. These pore measurements can be made using gas adsorption, mercury intrusion, capillary flow, or liquid-liquid displacement techniques.
Gas Adsorption Technique
To measure pore size by gas adsorption, isotherms (typically using N2, Ar, or CO2) 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. Overall, gas adsorption is applicable to pores ranging from 3.5 Angstroms to about 4000 Angstroms in diameter.
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 except when pressure is applied. The pressure at which mercury enters a pore is inversely proportional to the size of the opening to the void. Pressures ranging from 0.2 to 60,000 psi allow for measurement of pores from 30 Angstroms up to 900 micrometers in diameter.
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 angle 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.
Capillary Flow Technique
Capillary flow Porometry (CFP) is used to measure pore sizes of 500 to 0.015 microns in diameter.
With this method, pore properties are calculated by measuring the fluid flow when an inert, pressurized gas is applied to displace an inert and nontoxic wetting fluid impregnated in the porous network of the sample.
Parameters such as first bubble point (corresponding to the largest pores present) can be calculated with accuracy and repeatability according to ASTM F316.
Liquid-Liquid Displacement Technique
Liquid-liquid displacement porometry (LLDP) measures pores 1,000 to 2 nanometers in diameter.
Using this method, we can measure nanopores (1,000 to 2 nm) at low pressures by displacing the wetting liquid with an immiscible liquid at increasing pressure. This eliminates error from collapse or mechanical damage caused by high pressure when measuring materials such as hollow fibers.