Quantachrome offers instruments for determining pore size by electroacoustics, capillary flow porometry, mercury porosimetry and gas adsorption.
The choice of method depends on the type of pores and the expected pore size, generally with gas sorption being suitable for micro to meso pores, mercury porosimetry for meso to macro pores and flow porometry for most through pores. In practice the structures can contain many different types of pores requiring more than one analytical approach. With the introduction of electroacoustics, mean pore size can be determined very rapidly without mercury, cryogens or vacuum pumps.
Electroacoustics uses an electrokinetic phenomenon called the seismoelectric effect to determine the mean pore size of a material. When high frequency ultrasound is applied to a wetted porous material the resulting electroseismic current is dependent on the capacitance effect of the double layer as well as how they overlap in pores. To make the measurement, the material is fully wetted with a suitable liquid (depending on the type of sample) and can return results in a few minutes.
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Capillary flow porometry, also know as the liquid expulsion technique, uses the simple principle of gas pressure to force a wetting liquid out of through-pores in a sample. Through pores are simply those that connect from one side of the sample to the other. The pressure at which pores empty is inversely proportional to the pore size, larger pores require a lower pressure than do smaller pores. The resulting volumetric flow of gas through emptied pores is also measured. Pore size is calculated using the Washburn equation.
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The operation of all mercury porosimeters is based upon the physical principle that a non-reactive, non-wetting liquid will not penetrate fine pores until sufficient pressure is applied to force its entry. The relationship between the applied pressure and the pore diameter into which mercury will intrude is given by the Washburn equation.
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Pore size determination by gas sorption requires a recognition and understanding of different basic isotherm types. IUPAC Classification recognizes six types of Sorption Isotherms, and the pore size distribution can be calculated from the adsorption or desorption branch of the isotherm. In some cases the desorption curve does not follow the adsorption curve creating the so called hysteresis adding more information about the pore structure.
The interpretation of these isotherms and the use of the appropriate models:
NLDFT, QSDFT, Monte-Carlo, t-plot, alpha-s method, MP method, DR & DA methods, BJH, DH, all included in Autosorb-iQ, Nova and Quadrasorb Series of instruments yield information about the pore structures, pore volume and pore size distribution.
The tendency of all solid surfaces to attract surrounding gas molecules gives rise to a process called gas sorption. Monitoring the gas sorption process provides a wealth of useful information about the characteristics of solids such as surface area and pore size. Surface area is calculated from the monolayer amount, often using the BET method, and pore size is calculated from pore filling pressures.
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