ISO 20804:2022

INTERNATIONAL ISO
STANDARD 20804
First edition
2022-05
Determination of the specific surface
area of porous and particulate systems
by small-angle X-ray scattering (SAXS)
Détermination de la surface spécifique pour des systèmes poreux et
particulaires par diffusion des rayons X aux petits angles (SAXS)
Reference number
© ISO 2022
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 3
5 Principle of the method . 4
5.1 General . 4
5.2 Ideal two-phase model . 5
5.3 Porod law - Specific surface area . 7
6 Apparatus . 8
6.1 Optics - Focusing - Collimation – Resolution . 8
6.2 Additional requirements for the absolute-scale method . 10
7 Preliminary procedures and instrument set-up .10
8 Sample preparation .11
8.1 General . 11
8.2 Degassing . 11
9 Determination of the specific surface area .11
9.1 K/Q (‘Invariant’) method . 11
9.2 Absolute-scale method . 13
10 Documentation and test report .15
10.1 Test report . 15
10.2 Technical records . 16
Annex A (informative) Example of a typical experimental protocol.17
Bibliography .22
iii
Foreword
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iv
Introduction
Small-angle X-ray scattering (SAXS) can be used to determine the specific surface area of nanoporous
(presence of nanopores) and nanoparticulate systems which include mesoporous and partly
macroporous materials. 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, i.e. materials comprising nanopores and nanoparticulate systems
which include mesoporous (presence of mesopores) and partly macroporous (presence of macropores)
materials. With special instrumentation, and/or by using absolute-scale techniques, the limits can be
significantly extended. User-friendly commercial instruments are available worldwide from a number
of manufacturers for both routine and more sophisticated analyses, and state-of-the-art research
instruments are available at synchrotron radiation facilities.
As in all measurement techniques for surface area, care is required in all aspects of the use of
the instrument, collection of data, and further interpretation. Therefore, there is a need for an
International Standard that allows users to obtain good inter-laboratory agreement on the accuracy
and reproducibility of the technique.
SAXS can be applied to any hetero-phase system, in which the two or more phases have a different
electron density. A ‘phase’ is in this context understood as a homogeneous electron density domain in
the typical size range for SAXS between about 1 nm and 100 nm. State-of-the-art SAXS instruments
and synchrotron SAXS beamlines allow significantly extending the limit of 100 nm to several hundred
nanometres. Special instrumentation for ultra-small angle X-ray scattering (USAXS) pushes the upper
size limit even up to the µm range. This document describes two different evaluation approaches for
determining the specific surface area: The Invariant (K/Q) method has an upper size limit for the
structure of up to several hundred nanometres, whereas for the absolute-scale method the size of the
structure can even be in the µm range.
Because SAXS is sensitive to the squared electron density difference, it does not matter whether the
scattering system is composed of pores or particles within a matrix, respectively.
Small-angle neutron scattering is not described in this document but can be used without restriction
because the theory and application are similar.
v
INTERNATIONAL STANDARD ISO 20804:2022(E)
Determination of the specific surface area of porous and
particulate systems by small-angle X-ray scattering (SAXS)
1 Scope
This document specifies the application of small-angle X-ray scattering (SAXS) for the determination of
2 -1 2 -1
specific surface area. Both the mass specific surface area in the order of 1 m g to 2 000 m g and the
2 -3 2 -3
volume specific surface areas in the range from 0,01 m cm to 1 000 m cm 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.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
nanopore
pore with width of 100 nm or less
[SOURCE: ISO 15901-2:2021, 3.10]
3.2
macropore
pore with width greater than 50 nm
[SOURCE: ISO 15901-1:2016, 3.7]
3.3
mesopore
pore of internal width between 2 nm and 50 nm
[SOURCE: ISO 15901-1:2016, 3.8]
3.4
micropore
pore of internal width less than 2 nm
[SOURCE: ISO 15901-1:2016, 3.9]
3.5
surface area
extent of accessible surface area as determined by a given method under stated conditions
[SOURCE: ISO 15901-1:2016, 3.30]
3.6
mass specific surface area
surface area of the sample divided by sample mass
3.7
volume specific surface area
surface area of the sample divided by sample volume
3.8
external (outer) surface
envelope surface of particles in the micrometre and sub-micrometre range
3.9
internal (inner) surface
surface of pores, cavities, or any other heterogeneity within particles or bulk materials
3.10
closed pore
pore totally enclosed by its walls and hence not interconnecting with other pores and not accessible to
fluids
[SOURCE: ISO 15901-1:2016, 3.10]
3.11
open pore
pore not totally enclosed by its walls and open to the surface either directly or by interconnecting with
other pores and therefore accessible to fluid
[SOURCE: ISO 15901-1:2016, 3.11]
3.12
powder
porous or nonporous solid composed of discrete particles with maximum dimension less than about
1 mm, powders with a particle size below about 1 µm are often referred to as fine powders
[SO
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