Modeling the pump-down of a reversibly adsorbed phase. I. Monolayer and submonolayer initial coverage

An analysis is presented of the pressure versus time characteristics during the pump-down of a vacuum system with a reversibly adsorbed phase, e.g., water at room temperature, using the conservation of mass equation, where the quantity adsorbed is derived from a suitable adsorption isotherm. This quasistatic approach is suitable for cases of reversible adsorption and takes readsorption into account. Equations for the pressure versus time characteristics are derived for several isotherms, Henry’s law, Langmuir isotherm, Freundlich isotherm, Temkin isotherm, and an extended form of the Temkin isotherm. The p(t) characteristics resulting from these different isotherms are compared and their usefulness considered. For monolayer adsorption the extended Temkin isotherm gives p=kt−γ (where γ is about 1) in good agreement with recent experimental data on the pump-down of water sorbed on stainless steel when reasonable values of the parameters are assumed. For initial coverage less than a monolayer the extended Temkin isotherm shows that γ tends to decrease as the initial coverage is reduced. It is shown that outgassing rates, measured by pumping a system through an orifice of known conductance, may be considerably lower than the true outgassing rate if the sticking probability is not very small, an estimate of this effect is made in terms of the sticking probability.