Mercury intrusion porosimetry (MIP) is a technique for characterizing the porous structure of solid materials. It measures pore size distribution, total pore volume, bulk density, and specific surface area by intruding mercury into pores under controlled pressure. MIP is applicable to a wide range of porous materials including catalysts, ceramics, construction materials, pharmaceuticals, and geological samples.
The technique exploits the non-wetting property of mercury, which does not spontaneously enter pores due to its high surface tension and contact angle. External pressure is required to force mercury into the pores. The Washburn equation relates the applied pressure to the pore diameter. For cylindrical pores, the intrusion pressure is inversely proportional to the pore diameter. Typical instruments cover pore diameters from approximately 3 nm to 400 µm, requiring pressures from vacuum to 60,000 psi or higher.
An MIP experiment proceeds in two stages. Low-pressure analysis fills the sample holder, evacuates the sample, and fills the penetrometer with mercury at near-atmospheric pressure, intruding macropores above approximately 6 µm. High-pressure analysis forces mercury into smaller pores. The intruded volume is recorded as a function of applied pressure, generating the cumulative intrusion curve.
The pore size distribution is derived from the derivative of the cumulative intrusion curve with respect to pressure. Differential and log differential plots reveal the dominant pore sizes and the breadth of the distribution. Total porosity is calculated from the total intruded volume and the bulk volume of the sample. Bulk density is determined from the penetrometer volume and sample mass. Skeletal density is measured by helium pycnometry.
Hysteresis between intrusion and extrusion curves indicates pore network effects. Ink-bottle pores where narrow necks connect wider bodies show mercury entrapment. The extrusion curve provides information about pore throat size distribution. Pore tortuosity and permeability can be estimated from MIP data using empirical models.
Sample preparation requires drying to remove adsorbed water and volatiles without altering the pore structure. Samples are outgassed under vacuum. The penetrometer glassware is cleaned and calibrated for each measurement. Mercury handling requires careful safety procedures including spill containment, vapor monitoring, and proper disposal.
MIP is complementary to gas adsorption (BET/BJH) and microscopy methods. Nitrogen adsorption accurately measures mesopores below 50 nm, while MIP covers both mesopores and macropores. SEM provides direct visualization but limited statistics. MIP offers statistically representative bulk measurements.
