COLLOIDS AND COLLOIDAL SOLUTIONS 383 



Properties of Emulsions. 



(a) The properties of the first kind of emulsions in which a small 

 quantity of liquid is present in another liquid I part in 10,000 are 

 almost the same as those of suspensoids. The globules show Brownian 

 movement, they are precipitated or coagulated by electrolytes and can 

 be retained by ultra-filters. The particles are comparatively rigid and 

 are separated from one another by thick films or layers of the continu- 

 ous phase. 



(&) The properties of the second kind of emulsions in which the 

 quantity of disperse phase is large are very different. 



(1) Viscosity. They are very viscous, an extreme case being the 

 soap and oil emulsion mentioned above, which is almost a solid. 



(2) Closeness together of the Particles. If particles of a solid or 

 rigid sphere be put together so that they touch, they will occupy 74 per 

 cent, of the volume. Such a condition gives a thick paste, which is a 

 solid. If particles of a liquid or a deformable sphere be put together 

 so that they touch, the particles will become flattened and their face 

 will have the shape of a dodecahedron. The whole system remains a 

 viscous liquid. There is no limit to the ratio of the disperse phase to 

 the total volume. 



(3) Surface Tension of the Continuous Phase. In these emulsions 

 the continuous phase must be a solution of an emulsoid colloid. Such 

 solutions froth when they are shaken. Frothing is due to a lowering 

 of the surface tension of the solvent by the substance in solution. This 

 lowering of the surface tension takes place at the points of contact 

 between the two phases, i.e. the interfacial tension is lowered, which pre- 

 vents tearing of the films of continuous phase between the particles. 



(4) Structure of an Emulsion. The globules are flattened and form 

 polyhedra, and they are separated by thin films of continuous phase. 

 The whole system will be represented by a honeycomb structure filled 

 with globules. On shearing, the whole surface of an emulsion becomes 

 enlarged, the polyhedra moving over one another. Surface energy in 

 spite of the lowered surface tension will be developed and it appears 

 as viscosity. 



