Pore size, shape and distribution

One of the most important adsorbent parameters is the pore size and pore size distribution.

As we discussed in the theory chapter, adsorbent surface area is the factor directly affecting the analyte retention. Pore size defines an ability of the analyte molecules to penetrate inside the particle and interact with its inner surface. This is especially important because the ratio of the outer particle surface to its inner one is about 1:1000. The surface molecular interaction mainly occurs on the inner particle surface.

Pore shape is mainly unknown, but it could be approximated by the model. Three basic pore models exist:
a) cylindrical pores, circular in cross section
b) ink-bottle pores having a narrow neck and wide body
c) slit-shaped pores with parallel plates.

Decision, which model is appropriate for particular adsorbent usually based on the cross-calculations of pore volume and surface area and comparison with the experimentally measured values.

Pore size distribution is the secondary parameter. This could be measured by mercury porosimetry, or low-temperature gas adsorption-desorption (BET method). For HPLC the most important is the absence of micro- and meso-pores. In arbitrary classification micro-pores have diameter less than 10 , meso - is less than 50 . The surface of the micro-pores are usually accounted for in the adsorbent surface area measured by BET method, but most of the analyte molecules could not penetrate in this small pores. Meso-pores are partially accessible for analytes but molecular diffusion into the pore space are restricted by steric hindrance effect, which significantly slows mass transfer and decreases the column efficiency.

Pore volume

Specific pore volume, Vp, is the sum of volumes of all pores in one gram of adsorbent. We have to emphasise that only internal volume inside the adsorbent particles is counted.

Pore volume, Vp, specific surface area , S, and mean pore diameter D are correlated to each other. Specific surface area is said to be inversely proportional to D. But, for the adsorbent with mean pore diameter of 100 and pore volume 0.5 ml/g , the surface area will be ~130 m2/g. For the adsorbent with the same pore diameter but with the pore volume of 1 ml/g the specific surface area will be ~200 m2/g.

There is no exact relationship between these parameters. The correlation strongly depends on the adsorbent pore type and shape.