Porometry, Porosimetry, and Pycnometry: the 3 P’s You Need for Porous Materials Characterization

For capillary flow porometry, an inert gas is typically utilized. ASTM F316 (“Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test”) only requires a clean gas such as filtered air or nitrogen.

There is an attachment available for our porometer to do his type of work, but the hollow core fiber needs to be sealed. Please see the following applications note:
https://www.anton-paar.com/corp-en/services-support/document-finder/application-reports/measuring-pore-size-in-the-walls-of-hollow-fiber-membranes/

When considering a wetting fluid, there cannot be any chemical or physical interactions with the sample under test. The sample cannot dissolve, swell or reaction with the wetting fluid. Chemically inert solutions such as fluorocarbons (FC) with complete wetting behavior (contact angle of zero with all solids) are commonly utilized. Different FC’s have different vapor pressures. Higher vapor pressure FC’s are useful when running a “wet before dry” porometry analysis because the reduces the change of through-pores being re-wetting by blind pores. However, FC’s with high vapor pressure can prematurely evaporate from still-full pores due to a high gas flow rate. In this case, low vapor pressures FC’s can be used for a “dry before wet” run. Other liquids include mineral oil, isopropanol, ethanol, silicon oil, and water.

The wetting fluid chosen determines the attraction between the fluid and the pore wall and can affect the overall measurement. Please see the answer to Q3 for more information.

Gas porosimetry covers a pore range of 0.35 nm to >100 nm. Mercury porosimetry can capture a larger range of pores, from 3.2 nm to larger than 500 microns.

Krypton gas adsorption at 87K is often utilized for surface area, micropore and mesopore (up to 10 nm) size distribution of thin film materials. Other methodologies include radiation-based techniques such as small angle X-ray scattering (SAXS), grazing-incidence small-angle scattering (GISAXS), ellipsometry, or X-ray/neutron porosimetry. Quantification of porosity in thin films can also be done via elemental analysis techniques such as energy dispersive X-ray spectrometry (EDX) or Rutherford backscattering spectroscopy (RBS). Positron annihilation lifetime spectroscopy (PALS) has also been used to determine pore size distribution and degree of pore filling in closed pore systems.

Yes unless the pinch point is very close to the average pore diameter.

If the sample does not fully seal the chamber, the pressure/flow characteristics are not correct as gas is shunted around the sample. At Covalent, have Universal sample holder for samples with the following diameters: 10 mm, 18 mm, 25 mm, 37 mm, 47 mm, and 50 mm. For some samples, it is possible to seal the area surrounding the sample with a non-porous adhesive. There are also special use sample holders for hollow fibers, tubular samples, and in-plane pores (transverse pores within a material).

If packed granules, gas or mercury porosimetry could be utilized.

Closed pores are isolated from the sample’s external surface and cannot be accessed by external fluids; thus we cannot measure them directly with pycnometry or porosimetry. However, we can estimate the closed porosity (the volume of closed pores within a material) by comparing true density results (from pycnometry) with the expected density of the based material.

It depends on what material property is of interest, as well as the pore sizes in the material. Methods such as SEM, TEM, micro-CT, and confocal laser microscopy can be employed to image pore morphology and structure. Calculations in porometry and porosimetry make some assumptions about linear, cylindrical pores; however, they can still be employed for analysis of other materials. Ultimately it depends on the material: its pore sizes, and what the property of interest is.

A dry curve is generated by plotting the airflow against the dry material vs. applied gas pressure. A second curve is plotted by dividing the dry material’s measured air flow at each pressure by 2; it assumes that airflow is ½ that of the dry curve (“half dry curve”). Next, the material is saturated with wetting fluid and airflow through the material is measured as a function of gas pressure.

The largest pores empty first, defining the “Maximum Pore Size” (Max PS) and bubble point. The “Minimum Pore Size” (Min PS) is defined as the point where the wet curve meets the dry curve. The “Mean Pore Size” or “Mean Flow Pore Size” corresponds to the pore size calculated at the pressure where the wet curve and half-dry curve meet.

Strictly speaking, “pore volume” and “cumulative pore volume” cannot be obtained from a porometry measurement (porosimetry needs to be performed). In a porometry experiment, we measure pressure 𝑃𝑃, from which we can calculate pore size, or radius 𝑟𝑟 (from the Washburn equation). The “cumulative flow curve” is a percentage of the flow that has passed through pores of a certain size or larger. “Differential flow” is an increase in flow rate per unit increase in pore radius (first derivative of cumulative flow). The “mean flow pore (width)” is the pore width at which 50% of the flow is through pores larger than this width, and 50% of the flow is through pores smaller than this width.

Porous materials can adsorb more gas than non-porous/dense materials. The desorption temperature of the gas can also depend on the porosity of a material.