Hertz–Knudsen equation

In surface chemistry, the Hertz–Knudsen equation, also known as Knudsen-Langmuir equation describes evaporation rates, named after Heinrich Hertz and Martin Knudsen.


Non-dissociative adsorption (Langmuirian adsorption)

The Hertz–Knudsen equation describes the sticking of gas molecules on a surface by expressing the time rate of change of the concentration of molecules on the surface as a function of the pressure of the gas and other parameters:[1][2]


Quantity Description
ASurface area (in m2)
NNumber of gas molecules
φFlux of the gas molecules (in m−2 s−1)
αSticking coefficient of the gas molecules onto the surface, 0 α 1
pThe gas pressure (in Pa)
MMolar mass (in kg mol−1)
mMass of a particle (in kg)
kBBoltzmann constant
TTemperature (in K)
RGas constant (J mol−1 K−1)
NAAvogadro constant (mol−1)

See also


  1. Kolasinski, Kurt W. (2012). Surface Science: Foundations of Catalysis and Nanoscience, Third Edition. p. 203. doi:10.1002/9781119941798.
  2. R. B. Darling, EE-527: Micro Fabrication, Virginia University (retrieved Feb. 9 2015).
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