ISO 20804:2022

Determination of the specific surface area of porous and particulate systems by small-angle X-ray scattering (SAXS)

This document specifies the application of small-angle X-ray scattering (SAXS) for the determination of specific surface area. Both the mass specific surface area in the order of 1 m²g^-1 to 2 000 m²g^-1 and the volume specific surface areas in the range from 0,01 m²cm^-3 to 1 000 m²cm^-3 can be obtained.

The method described is applicable to dilute and concentrated systems.

NOTE:      In ISO 17867:2020, the determination of the particle size by SAXS is limited to dilute systems.

The determination of surfaces with SAXS is straightforward for two-phase systems only. Surface determination in systems with more than two phases is beyond the scope of this document.

The term ‘surface’ refers to any interface between domains of different density (more precisely: electron density) and is not restricted to the external surface of particles. As any interfaces between areas with different electron density, not only to air or vacuum, can be probed, the method can be applied to any heterogeneous system.

SAXS measures not only the specific surface area of open pores but also of inaccessible, closed pores or inclusions.

NOTE:      This is in contrast to gas sorption methods which are described in ISO 9277:2010.

In addition to porous systems, there can be contributions of internal interfaces to the measured specific surface area of any heterogeneous compact solid system, such as between crystalline and amorphous phases, provided there is an electron density contrast. Although materials comprising micropores (pore width < 2 nm) can also be analysed with respect to their specific surface area with SAXS, this document does not cover these materials.

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