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The Properties of Colloidal Systems. — II. On Adsorption as 



Preliminary to Chemical Reaction. 

 By W. M. Bayliss, F.E.S., Institute of Physiology, University College, 

 London. 



(Received April 7 ,— Eead May 18, 1911.) 



When a reaction takes place in a heterogeneous system certain preliminary 

 processes occur, so that the velocity of the reaction as measured is naturally 

 that of the slowest of all the stages, including the chemical reaction proper. 



In the first place, since the reacting bodies are not uniformly distributed, 

 one of them is compelled to travel a certain distance in order that contact 

 with the other one may take place, e.g. in the case of a sheet of zinc 

 immersed in dilute hydrochloric acid, it is necessary that the ions taking- 

 part in the reaction diffuse from the distant parts of the liquid phase in 

 order to reach the surface of the solid phase. Hence, diffusion is the first 

 stage of the reaction as a whole. 



In the second place, at the interface, where the separate phases are in 

 contact, there exists a local accumulation of energy, surface energy, as it is 

 called. Now it has been shown by Willard G-ibbs* that if a substance in 

 solution in either phase by concentration at the surface of contact will 

 reduce the surface energy there, such a process will of necessity take place, 

 if it is possible. This theorem is a case of the general result of Gibbs, 

 expressible in the following way : Increase of concentration at a surface will 

 always occur when the potential of any form of energy at this surface can be 

 diminished by the process. Electrical, thermal, and chemical changes are 

 included in this statement, and not only mechanical changes such as those 

 of surface tension. The name " adsorption " has been given to this form of 

 condensation of bodies on the surfaces of contact between the phases of 

 heterogeneous systems, and the name should be confined to this use. It has 

 unfortunately been applied by some writers to any " loose " combination of 

 an ill-defined chemical nature; thereby unnecessary confusion has been 

 caused. There is, no doubt, evidence that the chemical configuration of the 

 surface itself plays a part in the phenomenon.f but it is none the less 

 essentially due to action at the surface. 



* 'Connecticut Acad. Proc.,' 1876; 'The Scientific Papers of Willard Gibbs,' 1906, 

 vol. 1, p. 219. 



t See Starling, 'Recent Advances in the Physiology of Digestion/ London, 1906, 

 p. 40. 



VOL. LXXXIV. — B. G 



