66 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



solid and liquid, the surface tension is always lowered ~by dissolving sub- 

 stances in the liquid. Now, at the interfaces between solid particles and 

 liquid there must be a local accumulation of free surface energy, which 

 will be equal to the surface tension multiplied by the surface (inter- 

 face) area. A constant tendency exists for such free energy to be de- 

 creased and, since dissolved substances have this effect, they will become 

 concentrated at the interface. This is known as the principle of Willard 

 Gibbs, and it is of fundamental importance to the biochemist, because 

 on it depends the phenomenon known as adsorption, which in the case 

 of colloidal solutions may therefore be denned as the local concentra- 

 tion or condensation of dissolved substances at the interface between 

 the two phases. The amount of substance concentrated at the interface 

 can be calculated by a formula which takes into account the concentra- 

 tion of the dissolved substance, the temperature, and the surface tension 

 at the interface (the Gibbs formula). After absorption has occurred, vari- 

 ous reactions of a chemical, electrical or purely physical nature (e. g., dif- 

 fusion) may follow at a rate which depends on the amount of the 

 condensation. 



Every-day Reactions Which Depend on Adsorption 



1. Decolorization of liquids by charcoal. That no chemical reaction oc- 

 curs in such a case is readily shown by the ease with which the pigment 

 can be extracted from the charcoal. 



2. Adsorption of gases by such solids as charcoal and spongy platinum. 

 In these cases there must be great condensation, even a liquefaction of the 

 gas, during which heat must be evolved. By adsorbing oxygen and hydro- 

 gen, spongy platinum causes these gases to combine and form water. The 

 hemoglobin of blood may take up oxygen by a similar process. 



3. Formation of solid surface films on solutions of protein, saponin, etc. 

 The condensation may lead to coagulation, which explains why, if the 

 froth produced by beating the white of an egg is allowed to stand, it can 

 not be again beaten into a froth, the albumin having gone out of solution 

 by surface coagulation. 



An interesting phenomenon depending on the surface tension occurs 

 when the protoplasmic contents of a ciliated infusorian is pressed out in 

 water. A new membrane forms on the protoplasm because of surface con- 

 centration of all constituents which lower surface energy. By application 

 of the principle of Willard Gibbs, A. B. Macallum 18 concludes that not only 

 adsorption, as exhibited in a colloidal solution, but also the local accumula- 

 tions of material often seen in cells, are associated with changes in sur- 

 face energy. His conclusions are based largely on microscopic studies 

 of various forms of cell exhibiting different degrees and types of activity, 



