Abstract: The relationships between the surface pressure(π) and the area(σ) occupied by each adsorbed molecule on the silica gel were calculated by the Gibbs adsorption equation from the adsorption isotherms of vapours of formic, acetic and propionic acid (at 20℃).The resulting π-σ curves were similar to those of the insoluble films on the sarface of solution, there also exist "the liquid-expanded state", "the liquid-condensed state" and "the intermediate state".The surface films are monomolecu-lar layer on water, but become multimolecular layer on the silica gel at a higher vapour pressure.When the monolayer turns to the multilayer, the liquid-expanded state becomes the intermediate.The enthalpy and entropy of adsorption were culculated from the adsorption isotherms at 20℃ and 30℃.The results show that △Hand △Sgo through a minimum at or near monolayer capacity.Near the minimum the state of the film shows a marked change.

Key words: Adsorption, Adsorption of vapours, Surface film, Silica gel