[Porosity] Adsorption Hysteresis

It is widely accepted that there is a correlation between the shape of the hysteresis loop and the texture (e.g., pore size distribution, pore geometry, connectivity) of a mesoporous adsorbent. An empirical classification of hysteresis loops was given by the IUPAC, which is based on an earlier classification by de Boer.

[Porosity] Classification of Adsorption Isotherms

Based upon an extensive literature survey, performed by Brunauer, Demming, Demming and Teller (BDDT), the IUPAC published in 1985 a classification of six sorption isotherms.

[Porosity] Pore Size and Adsorption Potential

The shape of sorption isotherms of pure fluids on planar surfaces and porous materials depends on the interplay between the strength of fluid-wall and fluid-fluid interactions as well as the effects of confined pore space on the state and thermodynamic stability of fluids confined to narrow pores.

[Porosity] Physical and Chemical Adsorption

Depending upon the strength of the interaction, all adsorption processes can be divided into the two categories of chemical and physical adsorption.

[Porosity] Gas Adsorption Introduction

Gas adsorption is one of many experimental methods available for the surface and pore size characterization of porous materials.

[Porosity] Adsorption in Microporous Materials

According to IUPAC (International Union of Pure and Applied Chemistry), pores are classified as macropores for pore widths greater than 500 A, mesopores for the pore range 20 to 500 A and micropores for pore widths less than 20 A.

[Porosity] Adsorption in Mesopores

The sorption behavior in mesopores (2- 50 run) depends not only on the fluid-wall attraction, but also on the attractive interactions between fluid molecules. This leads to the occurrence of multilayer adsorption and capillary (pore) condensation. Pore condensation is the phenomenon whereby a gas condenses to a liquid-like phase in a pore at a pressure Pless than the saturation pressure Po of the bulk liquid.

[Porosity] Comparison of Porosimetry and Gas Sorption

The useful range of the Kelvin equation (and its derivative methods e.g. BJH, HK) is limited at the narrow pore end by the question of its applicability and at the wide pore end measurements are limited by the rapid (logarithmic) change of the core radius with relative pressure. Modem methods of calculation from gas sorption data (e.g. OFT) confidently extend accurate pore size determination well into the micropore region. Pore diameters in excess of 500 nm are rarely reported from gas sorption data.

[Porosity] Outgassing of the Adsorbent Samples

In order to obtain correct data it is required to remove all physically adsorbed material from the adsorbent surface to ensure a reproducible initial state of the adsorbent surface, especially one in which pores are obstructed by foreign species. This can be accomplished by vacuum pumping or purging with an inert gas at elevated temperatures.

[Porosity] Coolant Level and Temperature Control

In an open dewar the cryogenic coolant such as liquid nitrogen and/or argon will evaporate, and will therefore change the level of cryogen around the sample cell stem and consequently the cold zone and warm zone volumes. Therefore, it is crucial that the specific position of the cryogen level on the sample cell stem is kept constant during the measurement. It should be maintained - unless otherwise compensated - at least 20 mm above the sample and constant to within at least 1-2 mm.

[Porosity] Adsorptives Other Than Nitrogen for Super-and Ultramicroporosimetry

Micropore analysis of microporous materials (e.g., activated carbon and zeolites) has mainly been performed by nitrogen adsorption at 77 K, but this is not satisfactory with regard to a quantitative assessment of the microporosity, especially in the range of ultra micro pores (pore widths < 0.7 nm). A pore width of 0.7 nm corresponds to the bilayer thickness of the Nz molecule.

[Porosity] Micropore Analysis and Void Volume Determination

At the low pressures, where micropore analysis is performed, the void volume correction is relatively small, but because adsorption isotherms are usually performed over the complete relative pressure range up to 1, the void volume needs to be determined carefully.

[Porosity] Mesopore and Micropore Analysis Requirements

Physical adsorption in micropores occurs at relative pressures substantially lower (very often down to a relative pressures of 10-7) than in case of sorption phenomena in mesopores. Physical adsorption in microporous adsorbents can span a broad spectrum of pressures (up to seven orders of magnitude) hence special care is necessary for the pressure measurements. Consequently, more than one pressure transducer is necessary to measure all equilibrium pressures with sufficient accuracy.