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 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:


 * $$\frac{\mathrm{d}N}{A \mathrm{d}t} \equiv \varphi = \frac{\alpha p}{\sqrt{2\pi m k_\text{B} T}} = \frac{\alpha p  N_A}{\sqrt{2\pi M RT}},$$

where: Since the equation result has the units of s-1 it can be assimilated to a rate constant for the adsorption process.