Gas sorption porosimetry

Gas sorption analysis probes the interaction between a gas or vapor (adsorptive) and the adsorbent this interaction can occur upon adsorption in micropores, mono- and multilayer adsorption at the inner surface of the specimen or via liquid/solid interaction when the adsorptive is condensing in the pores of the adsorbent. The probe most commonly used to characterize aerogels is N2 sorption at 77.3 K. When varying the gas pressure from vacuum to 0.1 MPa (1 bar) rel. gas pressures from almost 0 to 1 can be covered hereby typically information on specific surface areas down to 0.01 m /g and pore widths in the range from 0.3 to 100 nm can be derived. 

Most of the basic information on the sample under investigation can already be deduced qualitatively from the isotherm itself  

A steep increase at low relative pressures indicates the presence of micropores while large slopes in the intermediate range (before a hysteresis takes over) signal high 

Common evaluation tools that provide quantitative information are the Density Functional Theory (DFT) for micropores and mesopores, Hie Dubinin-Radushkevich equation for the extraction of characteristic parameters on micropores, the t-plot and the oLg-plot for the separation of surface area located in micro- and nonmicropores, the method to calculate the so-called BET surface area and the BJH relationship that provides access to the mesopore size distribution. 

Total pore volume. The total accessible pore volume in micro- and mesopores can be derived by using the Gurvich rule. The assumption hereby is that the maximum specific amount of gas adsorbed, defined by the value of the plateau in the hysteresis at a rel. pressure of about 0.99, can be converted into a specific pore volume by assuming the adsorbate to be present in a liquid-like state in all pores. As a consequence the specific pore volume Vpore,, is derived from the amount of gas adsorbed Fads given in cm (STP)/g by 

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Gas sorption porosimetry is a standard method for the characterization of the pore size distribution (PSD) of porous solids. To interpret the experimental isotherm and obtain the adsorbent PSD, one must adopt a model for the pore structure, and a theory that estimates the adsorption that will occur in pores of a particular size. If the porous solid is represented as an array of independent, noninterconnected pores of uniform geometry and identical surface chemistry, then the excess adsorption, /JP), at bulk gas pressure P is given by the adsorption integral equation... 

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