174 PROTOPLASM 



to them, but they will bear repetition and elucidation. The 

 scientific contributions of Gibbs underlie much of the theories of 

 solution, surface tension, and adsorption. 



Gibbs expressed adsorption reactions mathematically. They 

 may also be expressed as a principle. If we return for a moment 

 to our definition of adsorption as concentration at a surface, we 

 see that if particles in a solution gather at the surface of the 

 solution, this is as much adsorption as when particles are taken 

 out of solution by a porous solid. Gibbs' law may be stated as 

 follows: A substance in solution will become more concentrated 

 at the surface if it lowers the tension there and less concentrated 

 at the surface if it raises the tension. Soaps and fats lower 

 surface tension; they will, therefore, be more concentrated in 

 the surface film than within the solution. We may restate 

 Gibbs' principle thus: A dissolved substance is positively 

 adsorbed if it lowers surface tension and negatively adsorbed 

 if it raises it. This is true because free surface energy tends 

 toward a minimum, and any substance that contributes toward 

 a lowering of the surface energy remains in the surface. If 

 this statement is made general by saying that all energy strives 

 toward a minimum, we have the second law of thermodynamics 

 upon which Gibbs' principle rests. 



Traube's Rule. — Gibbs told us that substances which lower 

 surface tension will be concentrated (adsorbed) at the surface. 

 Traube found that the reduction in surface energy, or the amount 

 of substance adsorbed in the surface film, increases with each 

 higher member of a homologous series of alcohols or fatty acids. 

 Each member of these series differs by one CH2 group from the 

 preceding member, thus: 



Alcohols Fatty Acids 



Methyl, CH3— OH Formic, H— COOH 



Ethyl, C2H5— OH Acetic, CH3— COOH 



Propyl, C3H7— OH Propionic, CH3CH2— COOH 



Butyl, C4H9-OH Butyric, CH3CH2CH2-COOH 



Each successively higher member of a homologous series is 

 more effective in lowering surface tension than the preceding 

 member. It lowers surface tension the same amount when of a 

 concentration but one-third that of the previous member. 



Freundlich's Adsorption Isotherm. — Adsorption constants are 

 mathematical measures of the degree to which a substance is 



