BSI 21/30372607 DC:2021 Edition

Small-angle X-ray scattering (SAXS) is a well-established method to obtain structural information on inhomogeneities in materials at the nanoscale, typically between 1 nm and 100 nm, and is thus perfectly suited for nanoporous and nanoparticulate systems which include mesoporous and partly macroporous materials. With special instrumentation, and/or by using absolute-scale techniques, the limits can be significantly extended.

The method is not limited to dilute systems, which is the case with particle size determination by SAXS as treated in ISO 17867:2020. The present document specifies the application of SAXS to the measurement of specific surface area. 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 the present 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. In fact, any interfaces between areas with different electron density, not only to air or vacuum, can be probed. Thus, the method can be applied to any nano- or micro-heterogeneous system. In contrast to gas sorption methods described in ISO 9277:2010(E), SAXS also can measure specific surface of inaccessible, closed pores or inclusions. 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 microporous (pore width < 2 nm) can also be analyzed with respect to their specific surface area with SAXS, this document does not cover these materials.