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.