28 INTERFACIAL PHENOMENA 



The difference in concentration between the surface layer and interior 

 of a liquid are in general much less in negative adsorption than in positive 

 adsorption. 



Adsorbed molecules are not static, but exhibit a continuous although re- 

 duced kinetic activity. Molecules adsorbed at surfaces or interfaces are in 

 dynamic equilibrium with the molecules of the same species in the body of 

 the liquid. An adsorption equilibrium is attained when the number of ad- 

 sorbed molecules passing out of the interface in a unit of time is equal to 

 the number passing into the interface. 



When adsorption occurs from a liquid containing many solutes, as is 

 true of most biological liquids, all solutes are adsorbed to a greater or lesser 

 degree, depending upon their specific properties in relation to the adsorbing 

 surface. In general, however, under such conditions no one solute will be 

 adsorbed as completely as if it alone were present. If a substance is intro- 

 duced into an aqueous solution which will lower the surface tension of water 

 more than another substance, already adsorbed, the first compound will largely 

 displace the second compound in the interfaces of that solution. 



Interfacial Adsorption. — Adsorption occurs at all types of interfaces. 

 Many solids adsorb gas molecules very powerfully at solid-air interfaces. 

 Solutes may be adsorbed at interfaces between immiscible liquids, and at 

 interfaces between liquids and solids, as well as at liquid-gas interfaces ("sur- 

 faces"). Water molecules are strongly adsorbed at many solid-liquid inter- 

 faces. Adsorption is, therefore, a phenomenon of a very general occurrence. 



Adsorption at liquid-gas interfaces involves only the forces of attraction 

 between the solvent and solute molecules, since the molecules of a gas are 

 too far apart to have any appreciable attraction for the solute molecules. 

 Similarly adsorption at solid-gas interfaces involves only the forces of attrac- 

 tion between the molecules of the solid and those of the adsorbed gas. At 

 liquid-liquid, or solid-liquid interfaces, however, adsorbed molecules are sub- 

 ject to the attraction of the molecules of the two abutting substances at the 

 interface. At an interface between carbon particles and water, molecules are 

 attracted by both the carbon and the water. If the carbon exerts a greater 

 attraction for the solute than the water — as is usually the case — a greater 

 accumulation of solute molecules will occur in the interface, than if the oppo- 

 site is true. The concentration of adsorbed molecules which will be attained 

 at any solid-liquid or liquid-liquid interface is therefore controlled by the 

 relative magnitude of the attractive forces of the two phases for the molecules 

 of the adsorbate (substance adsorbed). Inorganic salts which are negatively 

 adsorbed at surfaces (water-air interfaces) are usually positively adsorbed 

 at water-liquid interfaces, and almost always so at water-solid interfaces. 



