In surface chemistry, the Hertz–Knudsen equation, also known as Knudsen–Langmuir equation describes evaporation rates, named after Heinrich Hertz and Martin Knudsen.
Definition
Non-dissociative adsorption (Langmuirian adsorption) The Hertz–Knudsen equation describes the non-dissociative adsorption of a gas molecule on a surface by expressing the variation of the number of molecules impacting on the surfaces per unit of time as a function of the pressure of the gas and other parameters which characterise both the gas phase molecule and the surface:R. B. Darling, EE-527: Micro Fabrication, Virginia University (retrieved Feb. 9 2015).
where: Quantity DescriptionA Surface area (in m2)N Number of gas moleculestTime (in s) Flux of the gas molecules (in m−2 s−1)α Anomalous evaporation coefficient, 0 ≤ α ≤ 1, to match experimental results to theoretical predictions (Knudsen noted that experimental fluxes are lower than theoretical fluxes)p The gas pressure (in Pa)M Molar mass (in kg mol−1)m Mass of a particle (in kg)kB Boltzmann constantT Temperature (in K)R Gas constant (J mol−1 K−1)NA Avogadro constant (mol−1) Since the equation result has the units of s−1 per area, it can be assimilated to a rate constant for the adsorption process.
See also Langmuir (unit)
References