colloids_prop

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Colloids Properties

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The properties of Colloidal Solutions are as follows: (1) Physical properties Heterogeneous nature : Colloidal sols are heterogeneousin nature. They consists of two phases; the dispersed phase and the dispersion medium. Stable nature : The colloidal solutions are quite stable. Their particles are in a state of motion and do not settle down at the bottom of the container. Filterability : Colloidal particles are readily passed through the ordinary filter papers. However they can be retained by special filters known as ultrafilters (parchment paper). Colloids - Properties

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The main properties of Colloidal Solutions are as follows: (2) Colligative properties Due to formation of associated molecules, observed values of colligative properties like relative decrease in vapour pressure, elevation in boiling point, depression in freezing point, osmotic pressure are smaller than expected. (ii) For a given colloidal sol the number of particles will be very small as compared to the true solution . Colloids - Properties

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The main properties of Colloidal Solutions are as follows: (3) Kinetic properties • Brownian motion. • Diffusion. • Sedimentation. • Viscosity. Colloids - Properties

1) Brownian Motion::

1) Brownian Motion: Definition : Colloidal particles are subjected to random collision with molecules of the dispersion medium (solvent) so each particle move in irregular and complicated zigzag pathway. First observed by Robert Brown (1827) with pollen grains suspended in water. The velocity of particles increases with decreasing particle size and viscosity.

2) Diffusion::

2) Diffusion: Definition: As a result of Brownian motion particles pass (diffuse) from a region of higher concentration to one with lower conc. Rate of diffusion is expressed by; Fick’s first law: dm/dt = -DA dc/dx Where dm is the mass of substance diffusing in time dt across an area A is area under the influence of a concentration gradient dC/dx. The minus sign denotes that diffusion takes place in the direction of decreasing concentration. D is the diffusion coefficient.

3) Sedimentation::

3) Sedimentation: Stoke’s law states that V = 2r 2 ( p-po) g / 9 η v: velocity of sedimentation of spherical particles. p: density of the spherical particles. po : density of the medium. η: viscosity of the medium. g: acceleration due to gravity. At small particle size (less than 0.5 um) Brownian motion is significant & tend to prevent sedimentation due to gravity & promote mixing in stead. So, we use an ultracentrifuge which provide stronger force so promote sedimentation in a measurable manner.

4) Viscosity:

4) Viscosity Definition : The resistance to flow of a system under an applied pressure. Viscosity of colloid allows 1- calculation of the molecular weight. 2- Provide useful information about the shape of the colloidal particles. Spherocolloidal dispersions are of relatively low viscosity. On the other hand Linear colloidal dispersions are of high viscosity.

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(4) Optical properties : Tyndall effect (i) When light passes through a sol, its path becomes visible because of scattering of light by particles. It is called Tyndall effect. This phenomenon was studied for the first time by Tyndall. The illuminated path of the beam is called Tyndall cone. (ii) The intensity of the scattered light depends on the difference between the refractive indices of the dispersed phase and the dispersion medium. Colloids - Properties

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(4) Optical properties : Tyndall effect (iii) In lyophobic colloids, the difference is appreciable and, therefore, the Tyndall effect is well - defined. But in lyophilic sols, the difference is very small and the Tyndall effect is very weak. (iv) The Tyndall effect confirms the heterogeneous nature of the colloidal solution. (v) The Tyndall effect has also been observed by an instrument called ultra – microscope. Colloids - Properties

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Some example of Tyndall effect are as follows (a) Tail of comets is seen as a Tyndall cone due to the scattering of light by the tiny solid particles left by the comet in its path. (b) Due to scattering the sky looks blue. (c) The blue colour of water in the sea is due to scattering of blue light by water molecules. (d) Visibility of projector path and circus light. (e) Visibility of sharp ray of sunlight passing through a slit in dark room. Colloids - Properties

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Colloids - Properties Tyndall Effect

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Colloids - Properties Tyndall Effect

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Colloids - Properties Tyndall Effect

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