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. == 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: d N A d t ≡ φ = α p 2 π m k B T = α p N A 2 π M R T , {\displaystyle {\frac {\mathrm {d} N}{A\mathrm {d} t}}\equiv \varphi ={\frac {\alpha p}{\sqrt {2\pi mk_{\text{B}}T}}}={\frac {\alpha pN_{A}}{\sqrt {2\pi MRT}}},} where: Since the equation result has the units of s−1 per area, it can be assimilated to a rate constant for the adsorption process.