Mercury intrusion and nitrogen sorption

The use of complementary techniques is well illustrated by Minihan et al. (Chapter 17) for measurements of pore size and pore size distribution of silica powders. Mercury intrusion and nitrogen sorption techniques are commonly used in the structural characterization of porous solids, often independently, despite the fact that very often the pore size distributions obtained by the two techniques fail to agree. For silicas Minihan et al. were able to demonstrate that mercury intrusion can lead to compression of silica structures and that this compression can account for differences in pore size distribution as measured by nitrogen sorption and may lead to misconceptions regarding the structure of the material under investigation. 

Experimental Procedure. Each sample was first characterized by both mercury intrusion and nitrogen sorption. Mercury intrusion measurements were replicated at least four times, and the solid residues from each analysis were collected and combined after the bulk of the mercury was decanted. These samples were washed free of mercury by using 50% nitric acid (25 mL per 0.5 g of solid) and then washed free of acid by filtering and reslurrying in demineralized water (six times with 50 mL per 0.5 g of solid). The washed samples were then rapidly cooled in liquid nitrogen and freeze-dried (Chemlab SB4). For comparison, samples of material that had not been analyzed with mercury intrusion were washed and dried in a similar manner to test for structural modification caused by the acid-washing technique. 

Kaufmann J., R. Loser and A. Leemann (2009). Analysis of cement-bonded materials by multi-cycle mercury intrusion and nitrogen sorption . Journal of Colloid and Interface Science 336 730-737. 

After being dried, the samples were reexamined by nitrogen sorption and mercury intrusion, and a portion of the material was analyzed to determine the residual mercury levels. This analysis was achieved by acid digestion (10 mL of 50% aqua regia sample sizes were approximately 0.2 g in all cases) in pressure-sealed poly(tetrafluoroethylene) (PTFE) tubes heated to 140 °C for 10 min in a microwave oven (CEM). The solutions were analyzed after suitable dilution in distilled water with a graphite furnace atomic absorption spectrometer (Perkin Elmer 5100-PC). The detection limit for this method is estimated to be 6 ppm of mercury on the dry solid. 

Comparison between Nitrogen Sorption and Mercury Intrusion... 

Fig. 1.18A shows the pore size distribution for nonporous methacrylate based polymer beads with a mean particle size of about 250 pm [100]. The black hne indicates the vast range of mercury intrusion, starting at 40 pm because interparticle spaces are filled, and down to 0.003 pm at highest pressure. Apparent porosity is revealed below a pore size of 0.1 pm, although the dashed hne derived from nitrogen adsorption shows no porosity at aU. The presence or absence of meso- and micropores is definitely being indicated in the nitrogen sorption experiment. 

Table 2 Pore structural parameters of monolithic silicas derived from nitrogen sorption and mercury intrusion experiments.

Gas sorption (nitrogen at 77 K), mercury intrusion (mercury porosimetry) Specific surface area (BET), pore size distribution, average pore diameter, specific pore volume, particle porosity Retention of solutes, mass loadability, column regeneration, column performance, mass loadability, pore and surface accessibility for solutes of given molecular weight, mechanical stability, column pressure drop, pore connectivity... 

The effect of mercury intrusion analysis on structure was examined for a series of silica xerogels with different pore size distributions. This analysis was achieved by applying nitrogen sorption analysis to the silicas both before and after mercury intrusion analysis. The study required the development of a method for the removal of mercury from a sample after the initial intrusion measurement that does not damage the structure. The results show the potential for an elastic deformation of the structure during compression as well as irreversible compression during mercury intrusion. 

As was observed for the C500 silica, reanalysis of the pore structure by nitrogen sorption (Figure 8, curve c) following intrusion and removal of mercury indicates that the porosimetry experiment results in a permanent loss in pore volume and a shift to smaller pore sizes. In C200, however, this loss in pore volume no longer approximates that associated with the broad-diffuse area of the intrusion trace. Instead, the loss represents only... 

When the pore size of silicas is very small (as in C60), the compression of the structure caused by the mercury intrusion decreases the pore size to such an extent that it is outside the range for mercury intrusion analysis. The pore volumes measured represent only the compression region of the curve, and as a consequence the total pore volumes measured are lower than those measured for nitrogen. Nitrogen sorption methods are the only appropriate technique for this type of material. 

---