Specific surface area

Specific surface area (SSA) is a property of solids defined as the total surface area of a material per unit of mass,[1] (with units of m2/kg or m2/g) or solid or bulk volume[2][3] (units of m2/m3 or m−1).

Scratches, represented by triangular-shaped grooves, make the surface area greater.

It is a physical value that can be used to determine the type and properties of a material (e.g. soil or snow). It has a particular importance for adsorption, heterogeneous catalysis, and reactions on surfaces.

Measurement
Ceramic Raschig rings...
...and plastic Białecki rings of increased SSA

Values obtained for specific surface area depend on the method of measurement. In adsorption based methods, the size of the adsorbate molecule (the probe molecule), the exposed crystallographic planes at the surface and measurement temperature all affect the obtained specific surface area.[4] For this reason, in addition to the most commonly used Brunauer-Emmett-Teller (N2-BET) adsorption method, several techniques have been developed to measure the specific surface area of particulate materials at ambient temperatures and at controllable scales, including methylene blue (MB) staining, ethylene glycol monoethyl ether (EGME) adsorption,[5] electrokinetic analysis of complex-ion adsorption[4] and a Protein Retention (PR) method.[6] A number of international standards exist for the measurement of specific surface area, including ISO standard 9277.[7]

Calculation

The SSA can be simply calculated from a particle size distribution, making some assumption about the particle shape. This method, however, fails to account for surface associated with the surface texture of the particles.

Adsorption

The SSA can be measured by adsorption using the BET isotherm. This has the advantage of measuring the surface of fine structures and deep texture on the particles. However, the results can differ markedly depending on the substance adsorbed. The BET has been proven incorrect and yields incorrect answers, However it may yield relative answers if the solid are very similar chemically. It is better to use the Quantum Mechanical model (called chi) and the Thermodynamic Model (called Excess Surface Work) which yield consistent results.[8]

Gas permeability
Main article: Air permeability specific surface

This depends upon a relationship between the specific surface area and the resistance to gas-flow of a porous bed of powder. The method is simple and quick, and yields a result that often correlates well with the chemical reactivity of a powder. However, it fails to measure much of the deep surface texture.

See also

References
  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "specific surface area". doi:10.1351/goldbook.S05806
  2. http://www.owlnet.rice.edu/~ceng402/Hirasaki/CHAP3D.pdf
  3. http://www.rsc.org/suppdata/lc/b8/b812301b/b812301b.pdf
  4. Hanaor, D.A.H.; Ghadiri, M.; Chrzanowski, W.; Gan, Y. (2014). "Scalable Surface Area Characterization by Electrokinetic Analysis of Complex Anion Adsorption" (PDF). Langmuir. 30 (50): 15143–15152. doi:10.1021/la503581e. PMID 25495551.
  5. Cerato, A.; Lutenegger, A. (1 September 2002). "Determination of surface area of fine-grained soils by the ethylene glycol monoethyl ether (EGME) method". Geotechnical Testing Journal. 25 (3): 10035. doi:10.1520/GTJ11087J.
  6. Paykov, O.; Hawley, H. (1 July 2013). "A Protein-Retention Method for Specific Surface Area Determination in Swelling Clays". Geotechnical Testing Journal. ASTM. 36 (4): 20120197. doi:10.1520/GTJ20120197.
  7. "ISO 9277:2010(en) Determination of the specific surface area of solids by gas adsorption — BET method".
  8. Condon, James (2020). Surface Area and Porosity Determinations by Physisorption, 2nd edition. Amsterdam, NL: Elsevier. pp. Chapters 3, 4 and 5. ISBN 978-0-12-818785-2.


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