Technical Reference
Comprehensive definitions of key terms, abbreviations, and concepts in pore structure characterization. From BET surface area to Washburn equation, find authoritative explanations of porosimetry terminology.
The accumulation of gas molecules on the surface of a solid material. Physical adsorption (physisorption) is governed by weak van der Waals forces and is reversible, forming the basis of gas adsorption porosimetry methods like BET and BJH analysis.
A plot of the amount of gas adsorbed by a material as a function of relative pressure (P/P₀) at constant temperature. The shape of the isotherm provides information about pore structure and is classified according to IUPAC types (I–VI).
The gas that is adsorbed onto the surface of a solid during gas adsorption analysis. Common adsorbates include nitrogen (N₂) at 77 K, argon (Ar) at 87 K, and krypton (Kr) for low surface area materials.
The solid material on whose surface gas molecules are adsorbed during physisorption analysis. The adsorbent's surface area, pore structure, and surface chemistry determine its adsorption characteristics.
The Brunauer-Emmett-Teller theory provides a method for calculating the specific surface area of materials from gas adsorption data. The BET equation assumes multilayer adsorption. The linear range is typically 0.05–0.30 P/P₀ for non-porous and mesoporous solids, and much lower for microporous solids, where it should be chosen with the Rouquerol criteria. Standardized by ISO 9277.
A method for calculating mesopore size distribution from the adsorption or desorption branch of a nitrogen isotherm. BJH assumes cylindrical pore geometry and uses the Kelvin equation to relate pore size to condensation pressure. Applicable to mesopores (2–50 nm); it underestimates the size of narrow mesopores (below about 10 nm) compared with NLDFT.
The minimum pressure required to force gas through the largest pore of a wetted membrane or filter. The bubble point test is the primary method in capillary flow porometry for determining the maximum pore size, as defined in ASTM F316.
The mass of a porous material divided by its total volume, including both solid material and void space (pores). In mercury porosimetry, bulk (envelope) density comes from the volume of mercury the sample displaces at the low filling pressure, before significant intrusion, together with the sample mass.
The phenomenon where vapor condenses to liquid within mesopores at pressures below the bulk saturation pressure. Governed by the Kelvin equation, capillary condensation is the physical basis for BJH mesopore size distribution analysis.
A technique for measuring the through-pore size distribution of membranes and filters based on the bubble point principle. CFP measures only pores that connect both surfaces of the sample and are relevant to fluid transport and filtration performance.
A parameter in the BET equation related to the energy of adsorption. C must be positive for a meaningful fit; a negative C means the pressure range is wrong. Values of roughly 50–200 are common for nitrogen on oxides and non-porous solids. Very low C (below about 20) means weak interaction and an ill-defined monolayer, and very high C usually points to micropore filling.
The angle formed between a liquid and solid surface at the three-phase contact line. In mercury porosimetry, the Hg-solid contact angle (typically 130°) is a critical parameter in the Washburn equation. In CFP, the wetting liquid contact angle determines bubble point pressure.
The process of removing adsorbed moisture, gases, and volatile contaminants from a sample prior to analysis. Proper degassing is critical for accurate surface area and porosity measurements. Typical conditions: 100–300°C under vacuum for 2–24 hours.
The process by which adsorbed gas molecules leave the surface of a solid as pressure is decreased. The desorption branch is often used for BJH analysis, but pore blocking and cavitation (forced loop closure near P/P₀ ≈ 0.42 for N₂ at 77 K) can distort it. For H2- or H5-type loops, the adsorption branch usually gives the more reliable pore-body size distribution.
A modern computational method for analyzing pore size distributions from gas adsorption data. DFT methods provide more accurate results than classical methods (BJH) for micropores and small mesopores by accounting for molecular interactions in confined pore geometries.
The difference between adsorption and desorption isotherms, appearing as a loop when both branches are plotted together. Hysteresis provides information about pore geometry and connectivity. IUPAC classifies hysteresis loops into types H1–H5.
A phenomenon in mercury porosimetry where large pores connected by small pore necks are not detected until pressure is sufficient to penetrate the neck. The volume of the large cavities is assigned to the smaller neck size. This shifts the distribution toward small pores (large pores look too small) and is a basic limitation of intrusion methods.
The forced entry of a non-wetting liquid (typically mercury) into the pores of a material under applied pressure. The cumulative volume of mercury intruded as a function of pressure forms the basis of MIP analysis.
The International Union of Pure and Applied Chemistry system for classifying physisorption isotherms (Types I–VI) and hysteresis loops (H1–H5). This classification scheme, updated in 2015, is the standard framework for interpreting adsorption behavior and pore structure.
A thermodynamic equation relating the vapor pressure of a curved liquid interface to pore radius. The Kelvin equation is the theoretical foundation for BJH analysis, correlating the capillary condensation pressure to mesopore diameter.
Pores with diameters greater than 50 nm according to IUPAC classification. Macropores are most effectively characterized by mercury intrusion porosimetry, which can measure pores up to 1100 μm.
Pores with diameters between 2 and 50 nm according to IUPAC classification. Mesopores exhibit capillary condensation and can be characterized by both gas adsorption (BET/BJH) and mercury intrusion methods.
Pores with diameters less than 2 nm according to IUPAC classification. Micropore analysis requires gas adsorption methods using nitrogen, argon, or carbon dioxide, often analyzed via t-plot, α-s plot, or DFT methods.
A technique for characterizing pore structure by forcing mercury into a porous material under controlled pressure. MIP provides pore size distribution (3 nm–1100 μm), total pore volume, bulk and skeletal density, and tortuosity data according to ISO 15901-1 and ASTM D4284.
The amount of adsorbate required to form a complete monolayer (single molecular layer) on the surface of a material. The monolayer capacity, determined from BET analysis, is used to calculate specific surface area.
The most common gas adsorption technique, using nitrogen at liquid nitrogen temperature (77 K) as the adsorbate. N₂ adsorption is the standard method for BET surface area (ISO 9277) and mesopore analysis (ISO 15901-2).
An advanced computational method for pore size analysis that accounts for the non-uniform density distribution of adsorbate molecules in pores. NLDFT provides more accurate pore size distributions than classical methods, especially for micropores and narrow mesopores.
A measure of the ability of a porous material to transmit fluids, typically expressed in units of darcys or m². Permeability depends on pore size, porosity, pore connectivity, and tortuosity. Often estimated from MIP data using the Katz-Thompson equation.
Adsorption governed by weak van der Waals interactions between adsorbate and adsorbent. Physisorption is reversible, non-specific, and occurs in multilayers. This is the type of adsorption used in BET and BJH analysis.
A plot showing the volume or area of pores as a function of pore diameter. The PSD is the primary output of porosimetry measurements and provides information about the range, frequency, and distribution of pore sizes within a material.
The measurement of through-pores—pores that connect both surfaces of a membrane or filter and permit fluid flow. Porometry techniques (like CFP) are distinct from porosimetry, which measures all open pores regardless of connectivity.
The analytical measurement of pore characteristics in materials, including pore size distribution, pore volume, surface area, and porosity. The main porosimetry methods are mercury intrusion (MIP) and gas adsorption (BET/BJH). Capillary flow porometry (CFP) is the related technique for through-pores.
The fraction of a material's total volume occupied by void space (pores), typically expressed as a percentage. Porosity is calculated as: ε = (1 - ρbulk/ρskeletal) × 100%, where ρbulk is bulk density and ρskeletal is skeletal density.
The total volume of void space within a porous material, typically expressed in cm³/g or mL/g. Cumulative pore volume is measured directly by MIP or calculated from gas adsorption isotherms.
The density of the solid material excluding all void space (pores). Skeletal density is usually measured by helium pycnometry. MIP gives an apparent skeletal density from the sample volume at maximum pressure, which still includes pores smaller than about 3 nm and closed pores. Used with bulk density to calculate porosity.
The total surface area of a material per unit mass, typically expressed in m²/g. Specific surface area is most accurately determined by the BET method from gas adsorption isotherms according to ISO 9277.
The cohesive force at the surface of a liquid, typically expressed in mN/m or dyn/cm. Surface tension is a critical parameter in both the Washburn equation (mercury: 485 mN/m) and the bubble point equation in CFP.
A pore that connects both surfaces of a material, permitting fluid flow from one side to the other. Through-pores are the only pores measured by capillary flow porometry and are the dominant factor in filtration performance and membrane permeability.
A measure of the complexity of the pore network pathway, defined as the ratio of the actual flow path length through a porous medium to the straight-line distance. Tortuosity values range from 1 (straight pores) to greater than 10 for highly tortuous networks. Can be estimated from MIP data.
An analysis method for determining micropore volume and external surface area from gas adsorption data by plotting adsorbed volume versus statistical thickness of the adsorbed layer. The t-plot method distinguishes between micropore filling and multilayer adsorption on external surfaces.
Weak intermolecular forces arising from induced dipole interactions. Van der Waals forces are responsible for physisorption in gas adsorption analysis and are the basis for BET and BJH methods.
The fundamental equation governing mercury intrusion porosimetry: d = −4γcosθ / P, where d is pore diameter, γ is mercury surface tension (485 mN/m), θ is contact angle (typically 130°), and P is applied pressure. This equation relates applied pressure to the diameter of pores being intruded.