Physisorption is just a condensation, or more exactly, corresponds to the beginning of the condensation. In usual language, condensation means 3D condensation of liquid molecular or atomic layers on a substrate, whereas physisorption is 2D with a limited amount of condensed layers. But there is a complete continuity between both phenomena. Therefore, physisorption will always occur as soon as the temperature is low enough, otherwise the low interaction between molecules and substrate (Van der Waals type) will be destroyed by thermal agitation. But for measuring physisorption, you also need to have enough adsorbed molecules in dynamic equilibrium with gaseous ones, therefore the relative pressure should not be too low. Each isotherm has a special shape, depending on the nature of the surface, so there is no general answer to your question. All the parameters (temperature, surface coverage and surface nature) are related to each others.

Alain

Physisorption is based on omnipresent intermolecular interaction, irrespective of temperature. The underlying interaction is also described - as Alain correctly wrote - as van der Waals-type interaction. In rough an approximation it obeys a 6-12-distance (R) law: F (strength of interaction) being proportional to (A/R (to the power minus 6) -  B/R (to the power minus 12)) where A and B represent "constants". Any other intermolecular interaction as of the types of dipole-sorption center, quadrupole-sorption center, induced dipole or quadrupole  interaction, etc., being superimposed on van der Waals interaction, additively. These specific interaction forces have specific temperature dependences. -

Yoou may read the textbook on Physicall Adsorption by Young and Crowell published somewhen in the 1960s.

The sticking coefficient will also be coverage dependent. "Sticking" usually includes the stages of "landing", diffusing on the surface and then take off again or stay - maybe still with diffusive motion. An important part in the process is getting rid of the excess kinetic energy. The probability for doing so may be (much) enhanced if collisions with other adsorbates are possible.

 For  physisorption, it is  necessary at low  temperature  and some  cases the  very low vacuum is needed. it depends on  which gas you are using. 

 i think The sticking coefficient is a function of surface temperature, during  the  physisorption the surface coverage up to a monolayers formation.

A simple estimation of the temperature dependence of sticking probability S(T) can be made by means of the adsorption energy Eads: S(T) ~ exp(-Eads/RT). For physisorbed CO2 a value of 20 to 25 kJ/mol is a good first guess. Usually you can expect adsorption of CO2 at temperatures below 80 K if the sample is exposed to a CO2 partial pressure of 1E-8 mbars. 

A word auf caution: The 6-12-distance law, Martin mentions, is correct for the gas phase interaction of molecules. The van-der-Waals interaction with a surface is better represented by a 3-9-distance law. Note that there is a physical rationale only for the 12 and the 9, i.e. interaction of induced point dipoles and of point dipole with semi-infinite polarizable medium, respectively. the 6 and the 3 are just chosen for mathematical convenience to represent the Pauli repulsion, which in fact would exhibit more like an exponential decay with distance.

To E. Bertel:

Thanks! You may be right. There is a number of similar expressions, in literature. Have in mind that I wrote: "...  In rough an approximation ..."..

Thanks a lot for all your answers. I didn't expect so many answers for my first question here.

A few years late for the discussion, but I hope you are still listening.  I have attached 2 papers that describe what I have come to realize is a physisorption process between radon and hydrocarbon molecules.  In heavy oil recovery, this occurs in the vapor cloud that remains after heavy oil is displaced by steam, and manifests itself when a new well is drilled and temporarily chills the annulus around the well.  Gamma ray values in the cooled well can be 200x greater than those observed after the well reaches thermal equilibrium.  The cooled wellbore is about 140 degF and the vapor cloud is 250 degF +. At 140 degF, I think this is an example of physisorption at fairly high temperatures.

As a petrophysicist with 35 years of experience, I consider myself to be a practitioner, rather than a theoretician.  I am very fortunate to have the opportunity to make so many observations of subsurface characteristics, especially in systems that change.

I am very thankful to all of you for the discussion of theory.  I would greatly appreciate your comments on this hypothesis.

Article In situ vapor evaluation of vapor properties using condensed...

Research High Gamma Radiation in Heavy Oil Steam Zones - A Condensati...