Name: suspensions, sedimentation, and colloids
Uploaded: 2023-06-23T15:38:43.000Z
Duration: 1 min 17 s
Channel: JoVE

Les colloïdes et les suspensions sont des mélanges hétérogènes de deux ou plusieurs composants physiquement mélangés.
Dans un colloïde, la taille des particules de soluté varie de 1 à 1000 nanomètres, suffisamment petites pour être invisibles individuellement. Cependant, ces particules de soluté peuvent diffuser la lumière, ce qui donne aux colloïdes un aspect translucide ou opaque. Ils restent indéfiniment dans la solution et ne s’installent pas.
Le cytosol, le matériau semi-fluide à l’intérieur des cellules vivantes, est un colloïde en raison des nombreuses protéines et métabolites qui y sont dispersés.
En revanche, les suspensions contiennent de grosses particules, généralement supérieures à 10000 nanomètres, souvent visibles à l’œil nu.
Lorsqu’elles ne sont pas perturbées, ces particules se déposent sous forme de sédiments.
Le sang est un exemple de suspension. Lorsque le sang fraîchement prélevé n’est pas perturbé pendant un certain temps, les globules rouges se déposent au fond du récipient. La partie liquide supérieure, appelée plasma sanguin, est une solution de petits solutés ainsi qu’un colloïde contenant des protéines plasmatiques plus grosses.

les colloïdes et les suspensions

Les enfants en jeu font souvent des suspensions telles que des mélanges de boue et d'eau, de farine et d'eau, ou une suspension de pigments solides dans l'eau connue sous le nom de peinture à la tempera. Ces suspensions sont des mélanges hétérogènes composés de particules relativement grandes visibles à l'œil nu ou observées à la loupe. Ils sont troubles et les particules en suspension se déposent après avoir été mélangées. Les particules en suspension dans une suspension se déposent après un certain temps de mélange. La séparation des particules d'une suspension s'appelle la sédimentation. Un test sanguin pour déterminer la vitesse de sédimentation mesure à quelle vitesse les globules rouges dans un tube à essai se déposent de la partie aqueuse du sang (appelée plasma) sur une durée définie. La sédimentation rapide des globules rouges ne se produit généralement pas dans un corps sain. Cependant, certains aspects de certaines maladies peuvent provoquer l'agglutination des globules rouges, et ces amas lourds de globules rouges se déposent au fond du tube à essai plus rapidement que les globules rouges normaux. Une autre classe de mélanges appelée colloïdes (ou dispersions colloïdales) présente des propriétés intermédiaires entre celles des suspensions et des solutions. Les particules dans un colloïde sont plus grandes que la plupart des molécules simples; cependant, les particules colloïdales sont suffisamment petites pour ne pas se déposer en se tenant debout. Les particules dans un colloïde sont suffisamment grandes pour disperser la lumière; un phénomène appelé effet Tyndall; c'est pourquoi les mélanges colloïdaux semblent troubles ou opaques, comme les faisceaux de projecteurs. Les nuages sont également des mélanges colloïdaux. Ils sont composés de gouttelettes d'eau beaucoup plus grandes que les molécules, mais suffisamment petites pour ne pas se déposer. Certains colloïdes ont la propriété de subir des transformations sol-gel, qui peuvent se transformer de manière réversible d'un état fluide à un état solide (gel). Par exemple, les produits à base de gélatine se solidifient pour former un gel lorsqu'ils sont réfrigérés, puis peuvent être liquéfiés à nouveau lorsqu'ils sont chauffés. La propriété sol-gel du cytosol, le liquide des cellules vivantes, est essentielle pour de nombreuses activités cellulaires, telles que la division cellulaire ou le changement de forme cellulaire. La composante particulaire d'une solution colloïdale, présente en quantité relativement faible, est appelée phase dispersée. La substance ou la solution à travers laquelle les particules sont réparties est appelée le milieu de dispersion. Ce texte est adapté de <a href="https://openstax.org/books/anatomy-and-physiology-2e/pages/2-4-inorganic-compounds-essential-to-human-functioning">Openstax, Anatomy and Physiology 2e, Section 2.4: Inorganic Compounds Essential to Human Functioning</a> and <a href="https://openstax.org/books/chemistry-2e/pages/11-5-colloids">Openstax, Chemistry 2e, Section 11.5: Colloids</a>.

Les colloïdes et les suspensions

Les enfants qui jouent fabriquent souvent des suspensions telles que des mélanges de boue et d’eau, de farine et d’eau, ou une suspension de pigments solides dans l’eau connue sous le nom de tempera...

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Colloids and Suspensions

suspensions, sedimentation, and colloids

Suspensions and colloids are two common kinds of mixtures seen in everyday life. Mud and water, flour and water, and tempera paint are suspensions. These mixtures are heterogeneous, which means their particles are not spread out evenly. The particles are relatively large and can be seen with the naked eye or a magnifying glass.

Suspension particles make mixtures cloudy, and they settle out after mixing. The settling of particles in a suspension is called sedimentation. A blood test can measure the rate of sedimentation by showing how fast red blood cells settle out of plasma, the watery part of blood, in a test tube over time. Rapid sedimentation is not usual in a healthy body, but some diseases can cause blood cells to clump together and sink faster than normal cells.

Colloids, also called colloidal dispersions, sit between suspensions and solutions. Their particles are larger than most simple molecules, but they are small enough that they do not settle out when left standing. Colloidal particles are still large enough to scatter light. This is known as the Tyndall effect, and it makes colloids look cloudy or opaque, like a beam from a searchlight.

Clouds are another example of a colloid. They contain water droplets that are bigger than molecules, but still small enough to stay suspended in the air. Some colloids can also change from a fluid sol to a solid gel and back again. Gelatin is one example. It forms a gel when chilled and can become liquid again when heated. The sol-gel behavior of cytosol, the fluid inside living cells, is important for cell division and for changes in cell shape.

In a colloid, the dispersed phase is the particulate part present in smaller amount. The dispersion medium is the substance that carries the particles throughout the mixture. These terms help describe how colloids are organized and why they behave differently from suspensions and solutions.

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Learn how suspensions settle, how sedimentation works, and why colloids stay cloudy without settling. Explore real examples like blood, clouds, and gelatin.

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Suspensions, Sedimentation, and Colloids

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