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XLIII. An Experimental Investigation of Gibbs's Theory of 

 Surface-Concentration, regarded as the basis of Adsorption. 

 (Second Paper.) By Wm. C. McC. Lewis, M.A* 



[From the Muspratt Laboratory of Physical and Electro- 

 chemistry, University of Liverpool.] 



IN a previous paper f upon this subject a number of 

 . determinations were recorded, which had been carried 

 out with a view to investigate experimentally the expression 

 deduced by Gibbs for the surface condensation of one of the 

 components of a two-phase system at the boundary separating 

 the phases. Two immiscible liquids — namely a hydrocarbon 

 oil and distilled water — were brought into contact, there 

 being a definite interfacial tension existing at the boundary 

 of separation. To the water there were added various sub- 

 stances soluble in water, but insoluble in oil, viz. sodium 

 glycocholate, methyl-orange, and Congo-red. These sub- 

 stances all produced a lowering effect on the interfacial 

 tension — such lowering effect increasing with corresponding 

 increase in concentration of the solute. 



Gibbs's expression connecting the surface-concentrating 

 of the solute with the lowering effect on the tension and the 

 concentration of the solute in the bulk of the (aqueous) 

 phase is 



r _ c da 



KT dc 



where T = the mass (in grams) of the solute per square centi- 

 metre of the dividing surface in excess of what 

 would be there supposing the concentration to 

 remain the same as in the bulk of the solution ; 



c =the bulk concentration of the solute in the aqueous 

 phase ; 



H = the gas constant; 



T = the absolute temperature ; 



cr = the interfacial tension ; and 



— ~ =the rate of change of the tension with the concen- 

 tration of the solute. 



The negative sign on the right-hand side indicates that 



-j- must be negative in order that Y may be positive, i. e. 

 tic 



those substances which lower the interfacial tension will 



increase in concentration at the interface. 



* Communicated by the Physical Society : read November 27, 1908. 

 1 Lewis, Phil. Mag. April 1908. 



