COLLOIDS AND COLLOIDAL SOLUTIONS 



The particles may possibly be molecules with a high molecular 

 Weight, e.g. albumin, complex dye-stuffs. In the case of metallic and 

 other inorganic sols the particles probably consist of aggregates of 

 molecules. 



The particles have been shown to behave like gases, filling the space 

 in which they are contained and obeying definite laws. 



Visibility of the Particles. The Ultramicroscope. 



The particles in a colloidal solution are too small to be seen 

 with an ordinary microscope, but in most cases the particles can be 

 seen with the so-called ultramicroscope. With this instrument a strong 

 beam of light is sent horizontally through the solution, which is viewed 

 with a microscope. The particles reflect the light into the microscope 

 and appear as bright specks. Instead of the ultramicroscope arrange- 

 ment, many colloidal solutions will show particles by reflected light if a 

 cardioid condenser be used with an ordinary microscope. 



Brownian Movement of Particles. 



The small particles visible in the ultramicroscope, like many larger 

 particles under a microscope, show Brownian movement. 



Non-Settling of Particles due to Electric Charge. 



The mere smallness of the particles is not sufficient to account for 

 the long time taken for a suspensoid to settle, nor is the fact that the 

 particles are in Brownian movement. 



The non-settling of the particles is due mainly to the fact that they 

 are electrically charged and are thus repelled from one another, 

 preventing coalescence to form larger particles or aggregates. 



Almost any substance in contact with water assumes an electric 

 charge; most substances become negatively charged. The charge 

 can be reduced to zero or even reversed in direction by the addition of 

 a suitable electrolyte. The particles in a coarse suspension are also 

 electrically charged. ^ 



Determination of the Electric Charge of the Particles. 



The electric charge on the suspensoid particles may be determined 

 by placing the sol in a U tube ; above the sol on each side is put a 

 layer of distilled water. An electric current is passed through the 

 contents of the U tube, the poles being in the water. The particles 

 will travel to the positive or negative pole. 



This may also be. done on a microscope slide furnished at each 

 side with a platinum wire connected with an electric current. A drop of 

 the sol is put on the slide and the particles, when the current is passed, 

 will travel to one side or the other. 



