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1.3.2.3. Properties of colloidal dispersions

Properties of colloidal dispersions

Most of the liquids of living beings are colloidal dispersions, hence it is important to see some of their properties:

Tyndall effect

The colloidal dispersions are transparent, but when illuminated laterally on a dark background turbidity is observed as colloidal particles, due to their size, scatter light. On the contrary, in true solutions and gases without suspended particles they are transparent, since they practically do not scatter light.

Brownian movement

The Brownian motion is the random movement observed in some microscopic particles are in a fluid medium. This arbitrary and disorderly movement is characteristic of colloidal particles, when they are displaced by the molecules of the dispersing phase. The Brownian motion helps the colloidal particles stay suspended in the medium without settling. Thanks to the Tyndall effect , this movement is observable under the microscope.

Sedimentation

Colloidal particles remain in suspension, but when subjected to a strong gravitational field, sedimentation (flocculation) is possible.

The ultracentrifugation of a colloidal dispersion allows the sedimentation of the particles, whose molecular mass can be determined by calculating the time it takes to settle and the number of revolutions reached by the ultracentrifuge.

High viscosity

The viscosity is the resistance of a fluid to the motion of the molecules that comprise it . Colloidal dispersions are very viscous, as they contain large molecules, and their viscosity increases as the molecular mass or the number of colloidal particles increases. Colloids in the gel state are more viscous than those in the sun state.

High adsorption

The adsorption is the ability attraction of the surface of a solid on the molecules of a liquid or a gas. Colloidal particles have a great adsorbing power on other molecules present in dispersions, which facilitates the verification of chemical reactions.

Diferencia entre absorción y adsorción

Daniele Pugliesi [CC BY-SA 3.0], via Wikimedia Commons

Electrophoresis

The electrophoresis is the transport of the colloidal particles through a gel by the action of an electric field. It is generally used to separate the different proteins that are extracted together in a tissue. The speed is greater the higher its overall electrical charge and the smaller its size (molecular weight). Starch or polyacrylamide gels are often used.

In summary, colloidal dispersions differ from true solutions in that the particles of the solutions do not form gels, their viscosity is generally low, they are not adsorbent, they are optically empty, they do not settle by ultracentrifugation and the solutes cannot be separated by electrophoresis .