COLLOIDS AND COLLOIDAL SOLUTIONS 389 



(1) The Large Boundary Surface. 



The surface of a liquid against its vapour or another liquid is in 

 tension, known as surface tension or interfacial tension. Such a tension 

 also exists between a gas and a solid, and a liquid and a solid. 



Work is required to produce or to enlarge a surface. A surface is 

 a seat of energy and on this account a surface tends to become a 

 minimum. The surface energy >is measured by the product of the 

 surface and surface tension per unit length. 



The surface tension tends to reduce the surface and establish 

 equilibrium with other forces acting in the body of a liquid. 



Gases in contact with a surface produce a lowering of the surface 

 tension, the amount of lowering being characteristic for each gas. With 

 rising gas pressure or concentration there is a lowered surface tension. 

 It is accompanied by condensation of the gas on the surface. The 

 same occurs at the boundary of a solid and a gas. 



In a froth which has a large surface there is a higher concentration 

 than in the liquid, and with froth formation there is lowered surface 

 tension. There is thus an increase in concentration in the surface or 

 adsorption with a lowered surface tension. If a dissolved substance in 

 increasing concentration increases surface tension, it is less concentrated 

 in the surface than in the liquid. If a dissolved substance in increas- 

 ing concentration lowers surface tension, it accumulates on the surface. 

 A small amount of a substance in solution can increase the surface 

 tension only slightly, but a small amount can lower the surface tension 

 greatly. 



The amount of adsorption is proportional to the active surface. It 

 proceeds to a definite end point or equilibrium. 



This is expressed mathematically by 



y - 



= ac* 

 m 



where m = amount of adsorbent, y = quantity adsorbed and c is the 

 end or equilibrium concentration in the liquid after adsorption, a and 

 n are constants depending on the solution and adsorbent. This equa- 

 tion shows the peculiarities of adsorption. 



(2) The Electric Charge. 



In precipitating colloids by electrolytes the charge of opposite sign 

 is the effective ion : equi-valent amounts of the ions produce the same 

 effect. In the mutual precipitation of colloids the precipitation occurs 

 with colloids of opposite charge. In the first case the ion is carried 

 down with the colloid ; in the second case both colloids are pre- 

 cipitated. The large surface of the colloid further influences the electric 

 charge and in the case of emulsoids the viscosity also. 



