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LX. A Kinetic Theory of Adsorption. 

 By D. C. Henry". 



UP to the present time no theory o£ adsorption has been 

 developed which leads to equations valid over the 

 whole range from low to high concentrations. The adsorption, 

 both of gases and of solutes from solution, is well expressed 

 for low concentrations by the empirical " exponential 



formula " 



1 



a = k. c n , 

 where a denotes the quantity adsorbed, c the exterior con- 

 centration, and k and - are constants. As soon as moderately 



high concentrations of adsorbate are reached, this formula 

 gives results greatly in excess of the values observed, which 

 appear to tend to an upper limit. 



In the present paper a theory of adsorption is developed 

 based on the conceptions of surface action introduced by 

 Hardy and Langmuir. An adsorption equilibrium is con- 

 sidered as involving a balance between the rate at which 

 molecules of adsorbate condense on the surface of the 

 adsorbent and the rate at which molecules leave, or evaporate 

 from the same surface. The fundamental assumptions made 

 are two, for both of which Langmuir has produced much 

 evidence. In the first place, it is assumed that the range of 

 action of the forces which bind molecules of adsorbate on to 

 the adsorbing surface is comparable with the diameter of an 

 atom, so that the layer of adsorbate molecules bound by the 

 field of force of the adsorbent will be only one molecule 

 thick. Secondly, it is assumed that the impact of a molecule 

 on a surface is completely inelastic, so that every impinging 

 molecule will condense. 



General Adsorption Equation for n Gaseous components. 



Consider an adsorbing surface, of area w, brought into 

 contact with a homogeneous gaseous phase containing n 

 components S 1? S 2 , ... S„. Whether the surface be crystalline 

 or liquid, it will present a more or less regular arrangement 

 of points of unsaturated field of force, where molecules of 

 adsorbate can condense; if the surface is crystalline, the 

 arrangement will be related to the crystal lattice, if it is 

 liquid, to the packing of the oriented surface molecules. 



* Communicated by Prof. S. Chapman, F.R.S. 

 Phil. Mag. S. 6. Vol. 44. No. 262. Oct.. 1922. 2 Y 



