72 PRINCIPLES OF GENERAL PHYSIOLOGY 



SUMMARY 



The surface of contact between a liquid and another phase solid, immiscible 

 liquid, or gas has properties differing from those of the main body of either phase. 



In the first place, the surface film behaves as if stretched, so that it is the seat 

 of a special kind of energy. 



This surface tension has its origin in the forces of attraction between the 

 molecules of the liquid, the forces which give rise to the internal pressure of 

 Laplace. 



The amount of this surface energy varies with the chemical nature of the 

 liquid. 



All solutes, with the exception of certain inorganic salts, lower the surface 

 tension at the interface between liquid and air; these particular salts do so at 

 the interface between liquids. 



The interface between phases is also nearly always the seat of electrical forces, 

 the origin of which is usually from electrolytic dissociation in one or other of the 

 phases. But the possibility of phenomena akin to those of frictional electricity 

 cannot as yet be definitely excluded. 



Solubility is also changed in the surface film. 



Since any process that diminishes free energy tends to occur, a solute will bo 

 found in higher concentration in the surface film than in the body of the liquid 

 if it has the power of reducing surface energy. By this means, a greater fall in 

 surface energy is ensured. (Principle of Willard Gibbs.) 



This surface condensation is known as " adsorption " and plays an important 

 part in physiological phenomena. The surface energy spoken of in the previous 

 statement of the Gibbs principle may be of many kinds, mechanical, electrical, 

 chemical, etc. 



In certain cases, surface concentration leads to the formation of a more or 

 less rigid film, as, for example, with saponin or proteins (Ramsden). 



When a solute has an electrical charge, either as an ion or as a colloidal 

 particle, and the surface in contact with the solution has also a charge, the 

 degree of adsorption depends on the relative sign of the two charges ; no decrease 

 of free energy would be produced by adsorption of a negatively charged substance 

 on a similarly charged surface, but the reverse. On the other hand, adsorption 

 of an oppositely charged substance leads, by neutralisation of the charge, to 

 decrease of free energy. 



If the surface has no charge, while the adsorption of an electrically charged 

 ion would lead to diminution of mechanical surface energy, such adsorption will 

 take place and cause the appearance of an electrical charge on the surface. 



Adsorption of a similarly charged ion or colloid can be increased by reversing 

 the sign of the charge on the surface by allowing it previously to adsorb ions 

 of sign opposite to itself. If the surface and the solute have already opposite 

 signs, it is clear that previous adsorption by the surface of an opposite charge 

 will decrease subsequent adsorption of the particular solute. 



These phenomena of electrical adsorption play a considerable part in the 

 processes of dyeing and of histological staining. 



Chemical reactions which lower chemical potential are also favoured at 

 a surface. The rate of such reactions is not controlled by the total mass of the 

 reagents, as in true solution, but by the extent of active surface. The law of 

 mass action, in its simple form, does not apply quantitatively, since the surface 

 of one or both of the reagents has to be taken into account. 



There is some evidence that the chemical configuration of the surface may 

 play a part in adsorption and lead to the appearance of "specific" action. But 

 the question needs further investigation. 



