Mercury Intrusion Porosimetry: Principles, Data Interpretation, and Applications
What Is Mercury Intrusion Porosimetry?
Mercury Intrusion Porosimetry (MIP) is a characterization technique that determines the pore size distribution, total pore volume, and pore connectivity of porous materials by incrementally forcing mercury under pressure into the pore network of a dried sample. Because mercury is a non-wetting liquid with respect to most materials, it does not enter pore openings spontaneously—external pressure must be applied, and the pressure required is inversely proportional to the pore diameter by the Washburn equation.
MIP is widely used for characterizing catalysts, ceramics, concrete and building materials, coal and rock formations, pharmaceuticals, food products, and porous polymers across the materials science, oil and gas, construction, and pharmaceutical industries.
The Washburn Equation
The relationship between the applied pressure (P) and the accessible pore diameter (D) is given by the Washburn equation:
D = −4γ cosθ / P
Where:
- γ = surface tension of mercury (480 mN/m at 25°C)
- θ = contact angle of mercury with the pore wall (typically 130–140° for most materials)
- P = applied pressure (Pa)
At low pressures (0.1–10 kPa), large pores and macropores (>10 µm diameter) are filled. As pressure increases to 200 MPa, pores down to approximately 3–6 nm are accessible. This makes MIP a versatile technique covering the mesopore to macropore range.
MIP Measurement Procedure
- Sample preparation: The sample is dried (typically 60–110°C) to remove moisture from pores, then evacuated under high vacuum to remove gas
- Low-pressure filling: Mercury fills the penetrometer (sample cell) and fills large surface pores and inter-particle voids under low pressure
- High-pressure intrusion: Pressure is progressively increased from <1 psia to >30,000 psia (200 MPa), and the volume of mercury intruded at each pressure step is recorded
- Extrusion: Pressure is released and the mercury volume extruded is measured—hysteresis between intrusion and extrusion curves reveals pore connectivity and ink-bottle pore geometry
Key Parameters Derived from MIP
|
Parameter |
Definition |
|
Total intrusion volume (mL/g) |
Total pore volume accessible to mercury |
|
Median pore diameter |
Pore diameter at 50% of total intrusion volume |
|
Modal pore diameter |
Most frequently occurring pore diameter (peak of dV/d(logD) curve) |
|
Threshold pore diameter |
Pore diameter at onset of intrusion—represents pore network connectivity |
|
Porosity (%) |
Total pore volume / bulk volume × 100 |
|
Hysteresis |
Difference between intrusion and extrusion curves—indicates pore connectivity |
Limitations of MIP
- Ink-bottle pores: Pores with narrow necks and larger inner cavities are filled at the pressure corresponding to the neck diameter, not the body diameter—MIP underestimates the true body pore size
- Pore shape assumption: The Washburn equation assumes cylindrical pores; real pores are rarely cylindrical
- Mercury toxicity: Mercury requires careful handling, containment, and disposal per environmental regulations
- Minimum pore size: Standard MIP instruments are limited to pores ≥3–6 nm at maximum pressure; smaller mesopores and micropores require nitrogen adsorption (BET) methods
Applications of MIP
- Catalyst characterization: Pore size distribution governs reactant diffusion and active site accessibility
- Concrete and cement: Total porosity and pore size distribution predict durability and permeability
- Pharmaceutical tablets: Porosity affects tablet disintegration and drug dissolution rate
- Reservoir rock: Pore throat size distribution controls oil and gas permeability in petroleum formations
Why Choose Infinita Lab for Mercury Intrusion Porosimetry?
Infinita Lab offers comprehensive pore characterization services including MIP, nitrogen BET surface area and pore size distribution, and gas pycnometry through its nationwide accredited analytical laboratory network. Our materials characterization specialists provide expert interpretation of pore structure data for R&D, quality control, and regulatory submission programs.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you. Request a Quote
Frequently Asked Questions (FAQs)
What pore size range does MIP cover, and how does it compare to BET nitrogen adsorption? MIP covers macropores and large mesopores: approximately 3 nm to 500 µm diameter, depending on the instrument's pressure range. BET nitrogen adsorption covers micropores and mesopores: approximately 0.4 nm to 100 nm. The two techniques are complementary—MIP provides the large-pore end of the distribution; nitrogen adsorption provides the small-pore end. Together they characterize the complete pore network.
What contact angle value should be used in the Washburn equation for MIP? The standard contact angle for mercury on most inorganic materials and polymers is 130°. However, the actual contact angle depends on the surface chemistry—hydrophilic materials (silica gel, zeolites) may have contact angles of 135–140°; hydrophobic materials (carbon, PTFE) may have angles up to 150°. Using an incorrect contact angle shifts all pore size values proportionally. For comparative studies within the same material type, the standard value is usually acceptable.
What is the significance of the intrusion-extrusion hysteresis in MIP? Hysteresis between the intrusion and extrusion curves reflects pore geometry and connectivity. Large hysteresis indicates: ink-bottle pores (narrow neck trapping mercury in the large body during extrusion), or poor pore connectivity preventing mercury drainage. Mercury permanently trapped after extrusion (non-extruded volume) represents the pore volume occupied by isolated or ink-bottle pores.
How must samples be prepared before MIP analysis? Samples must be thoroughly dried to remove all moisture from pores (oven drying at 60–110°C or freeze drying for moisture-sensitive materials) and then degassed under high vacuum (< 50 µmHg) before mercury filling. Any residual moisture or gas in the pores will compress during pressurization and interfere with mercury intrusion volume measurements.
Is MIP suitable for soft, compressible materials such as polymer foams? MIP is less reliable for compressible materials (soft polymer foams, hydrogels, aerogels) because the applied mercury pressure may compress or crush the sample structure, altering the pore network being measured. For such materials, low-pressure gas adsorption, X-ray computed tomography (CT), or optical/SEM image analysis are better alternatives.
