Properties of Colloidal Dispersions

Colloidal dispersions show several characteristic properties beyond the general ones noted in Section 11.9.4. Optical property -- the Tyndall effect: when light passes through a true solution, the path of the light cannot be seen, but when light passes through a colloidal dispersion (Fig 11.8, attached to this section), the dispersed particles scatter some of the light in all directions, and the scattered light makes the beam's path visible to an observer standing at right angles to it -- this scattering of light by colloidal particles, making the light's path visible, is the Tyndall effect, and the visible bright cone of scattered light is called the Tyndall cone. It is only observed when (i) the diameter of the dispersed particles is not much smaller than the wavelength of the light used, and (ii) the refractive indices of the dispersed phase and dispersion medium differ significantly; it is used both to estimate the number and size of particles in a colloidal system and to distinguish a colloidal dispersion from a true solution. Colour: the colour a colloidal dispersion shows depends on the wavelength of light scattered by its particles, and also on how the observer views it -- a small amount of milk in a large amount of water looks blue viewed by scattered light but red viewed by transmitted light; colour also depends on particle size, e.g. the finest gold sol appears red, while larger gold particles appear purple. Kinetic property -- Brownian motion: colloidal (microscopic) particles show a ceaseless, random, zig-zag motion in every direction within the fluid (Fig 11.9, attached to this section), first observed by the British botanist Robert Brown in pollen grains under a microscope, and explained theoretically by Albert Einstein in 1905; it is caused by the dispersed particles constantly being struck, from random directions, by the fast-moving molecules of the dispersion medium, which transfers kinetic energy to the dispersed particles and keeps them in constant erratic motion. Electrical properties: colloidal particles in a given sol all carry the same sign of electric charge (either uniformly positive or uniformly negative -- Table 11.6, attached to this section, lists examples of each), caused by preferential adsorption of one type of ion from the medium onto the particle surface. Electrophoresis is the movement of colloidal particles under an applied electric potential (Fig 11.10, attached to this section): positively-charged particles migrate toward the cathode and negatively-charged particles migrate toward the anode, depositing there; this reveals the sign of the particles' charge, is used to measure their migration rate, and can separate a mixture of different colloidal particles that migrate at different rates. If the particles themselves are held in place (e.g. by a membrane) while an electric field is applied, it is instead the dispersion medium that moves -- this reversed effect is c …