TYNDALL PHENOMENON 41 



Special filters have been devised, however, with pores of such a small 

 diameter that the disperse phase can be separated from the dispersion medium 

 by filtering a sol through them. Such filters are known as ultrafilters. The 

 process of filtering through such a filter is known as ultrafiltration. The most 

 commonly used types of ultrafilters are those made of collodion or gelatin. 

 The size of the pores in such filters can be controlled by the length of time 

 allowed for drying and in other ways. It is possible to prepare ultrafilters 

 with pores of such dimensions that solutes can pass through them, but col- 

 loidal micelles cannot. 



Tyndall Phenomenon. — Suppose that a clean glass vessel, preferably one 

 with flat, parallel sides, be filled with pure water and held so that a strong 

 pencil of light passes laterally through the vessel. If an observation be made 

 laterally and at right angles to the path of the beam of light, no distinctive 

 trace of its path through the water can be detected. Such a liquid is said to 

 be optically empty. The same will be true if the water in the vessel be re- 

 placed by a sugar or salt solution, or in fact, by any true solution.^ Suppose, 

 however, that the vessel be filled with a hydrophobic sol and observed in the 

 manner just described. The results are strikingly different. The path of the 

 light will be clearly delineated as a cloudy, often opalescent track through the 

 sol. Even a colloidal system which seems perfectly transparent to the unaided 

 eye will usually show some turbidity when submitted to this test. The intensity 

 of this effect varies greatly with the specific colloidal system, and with the 

 concentration of the disperse phase, but it is universally shown by hydrophobic 

 sols. 



The phenomenon just described is called the Tyndall phenomenon, and is 

 due to the scattering or diffraction of light. The difference in the index 

 of refraction between the two phases of the colloidal system is also a factor 

 determining the intensity of the Tyndall effect. The greater this difference, 

 the stronger the effect. Since in the diffraction of light, the short wave lengths 

 (blue end of the spectrum) are bent more than the longer wave lengths, a 

 partial separation of the spectrum results. For this reason a sol with a color- 

 less disperse phase often appears to be pale blue when viewed in the path of a 

 strong beam of light. 



Similar Tyndall phenomena are exhibited by hydrophilic sols, but usually 

 the effect is less striking when sols of this type are viewed in the path of a 

 beam of light than the effect observed when hydrophobic sols are employed. 



^ Actually a trace of the light track will usually be perceived even in water or 

 true solutions, due to the presence of contaminating dust particles. In order to 

 prepare a truly optically empty liquid, provision must be made for the removal of 

 such dust particles. 



