Les colloïdes et les suspensions

Colloidal particles range from 1 to 1000 nanometers in size, making them individually invisible to the naked eye. Despite their small size, these particles scatter light, causing colloids to appear translucent or opaque. This light-scattering property distinguishes colloids from true solutions, where particles are too small to scatter light.

Suspensions contain large particles typically exceeding 10,000 nanometers, often visible to the naked eye or with a magnifying glass. In contrast, colloidal particles range from 1 to 1000 nanometers and are individually invisible. Suspensions appear cloudy and their particles settle as sediment over time, while colloidal particles remain dispersed indefinitely.

The Tyndall effect is the phenomenon where colloidal particles scatter light, making the mixture appear cloudy or opaque, similar to a searchlight beam. This occurs because colloidal particles are large enough to interact with light waves. Clouds exemplify this effect, consisting of water droplets that scatter light while remaining suspended without settling.

When freshly drawn blood sits undisturbed, red blood cells settle at the bottom as sediment in a process called sedimentation. The upper liquid portion, blood plasma, is a solution containing small solutes and a colloid of larger plasma proteins. This settling behavior demonstrates that blood is a suspension, with the rate of sedimentation serving as a diagnostic indicator in blood tests.

A sol-gel transformation is a reversible process where colloidal mixtures shift between fluid and solid states. Gelatin solidifies into a gel when refrigerated and liquefies when heated, exemplifying this property. Cytosol, the colloidal fluid inside living cells, undergoes sol-gel transformations essential for cellular activities such as cell division and changes in cell shape.

The dispersed phase is the particulate component present in relatively minor amounts within a colloid. The dispersion medium is the substance or solution throughout which the particulate is distributed. Together, these components define the structure of colloidal mixtures, with the dispersed phase remaining suspended indefinitely within the dispersion medium.

Cytosol, the semifluid material inside living cells, is classified as a colloid because it contains numerous proteins and metabolites dispersed throughout in the 1 to 1000 nanometer size range. These particles scatter light and remain suspended indefinitely without settling. The colloidal nature of cytosol enables essential cellular functions including sol-gel transformations that support cell division and morphological changes.