76 DISPERSE SYSTEMS 



Properties of a Colloidal Dispersion. 



The properties of a dispersion depend in general either on the 

 size of the dispersed particles or on their electrical charge, or on both. 



1. PROPERTIES OF COLLOIDS DEPENDING ON 

 SIZE OF PARTICLES 



(i) Optical. 



(rt) Colour. White light is composed of waves of different lengths 

 varying from 760/x/x to 4<50yu,^. When white light is scattered from 

 a surface instead of being reflected as in a mirror, it gives rise to 

 the sensation of white. Ice, in mass, does not appear white 

 because light is not scattered from its surface. If the ice is 

 powdered, light is scattered from the powdered surfaces and the 

 whole appears white. Crystallised copper sulphate appears blue, 

 but the light scattered from the surfaces of the finely powdered 

 crystals is white. The white colour of the lily or of white hair is 

 not due to the presence of a white pigment, but to the scattering 

 of light from the surfaces of innumerable minute air bubbles 

 embedded in the tissue. From this it follows that particles of 

 different sizes will scatter light of different wave-lengths. In 

 short, the colour of the scattered light may serve as an indication 

 of the size of the particle, provided the difference in the indices of 

 refraction of the dispersoid and the dispersant be kept constant. 



The late Lord Rayleigh deduced a formula relating the size of the 

 particle and the wave-length of the scattered light in a quantitative 

 manner. A particle smaller in diameter than half the wave-length 

 of light will scatter light at the blue end of the spectrum about 

 twelve times as copiously as it does the longer red rays. 



He explained the blue colour of the sky by considering that the 

 fine particles of dust, globules of water, etc., suspended in the 

 air, or even the molecules of the various gases of the atmosphere, 

 cause lateral diffusion of light of short wave-length giving a blue 

 colour, while the red rays are transmitted direct, producing the 

 gorgeous sunset colours. In one of Tyndall's experimental verifi- 

 cations of this theory he passed light through a tube containing a 

 mixture of gases (butyl nitrate in air and hydrochloric acid in air), 

 which gradually combined to form a dust-like suspended precipi- 

 tate. At first the particles were exceedingly small and the colour 

 seen from the side of the tube was a delicate tint of blue. As the 

 particles increased in size the blue became more intense, " until at 

 length a whitish tinge mingled with the pure azure, announcing 

 that the particles were now no longer of that infinitesimal size 

 which scatters only the shortest waves." 



