The Influence of Mercury Contact Angle, Surface Tension, and Retraction Mechanism on the Interpretation of Mercury Porosimetry Data

Abstract

The use of a semi-empirical alternative to the standard Washburn equation for the interpretation of raw mercury porosimetry data has been advocated. The alternative expression takes account of variations in both mercury contact angle and surface tension with pore size, for both advancing and retreating mercury meniscii. The semi-empirical equation presented was ultimately derived from electron microscopy data, obtained for controlled pore glasses by previous workers. It has been found that this equation is also suitable for the interpretation of raw data for sol–gel silica spheres. Interpretation of mercury porosimetry data using the alternative to the standard Washburn equation was found to give rise to pore sizes similar to those obtained from corresponding SAXS data. The interpretation of porosimetry data, for both whole and finely powdered silica spheres, using the alternative expression has demonstrated that the hysteresis and mercury entrapment observed for whole samples does not occur for fragmented samples. Therefore, for these materials, the structural hysteresis and overall level of mercury entrapment is caused by the macroscopic (>∼30 μm), and not the microscopic (<∼30 μm), properties of the porous medium. This finding suggested that mercury porosimetry may be used to obtain a statistical characterization of sample macroscopic structure similar to that obtained using MRI. In addition, from a comparison of the pore size distribution from porosimetry with that obtained using complementary nitrogen sorption data, it was found that, even in the absence of hysteresis and mercury entrapment, pore shielding effects were still present. This observation suggested that the mercury extrusion process does not occur by a piston-type retraction mechanism and, therefore, the usual method for the application of percolation concepts to mercury retraction is flawed.