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In this article we will discuss about:- 1. Properties of Colloids 2. Separation of Colloids 3. Examples.
Properties of Colloids:
Some of the properties of colloids are as follows:
i. Brownian Movement:
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Due to the impact of the solvent molecules, the colloid particles are continuously moving about.
ii. Faraday-Tyndall Phenomenon (Optical Phenomenon):
When a beam of light is passed through a colloidal solution and observed at right angles, the track of light, which is invisible in a true solution, may be visible here as a white line. This is caused by the dispersal of light rays by the colloid particles. If the particles be sufficiently small, the light, when viewed through a Nicol’s prism, will be found to be polarised at right angles to the beam.
iii. Electrical Phenomenon:
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a. Colloid particles carry electric charge—which may be positive or negative (colloidal ions).
b. Isoelectric pH—By addition of salt, week acid, week alkalies, etc., the hydrogen-ion concentration of the medium can be adjusted to an isoelectric pH where the electric charge of the colloidal ions will be completely neutralised. At this pH the colloid particles become less soluble and may be precipitated. Almost every colloid, particularly proteins, has its characteristic isoelectric pH.
c. Electrophoresis.
iv. Precipitation:
The colloid particles can be precipitated in various ways. Isoelectric precipitation has been mentioned above. In precipitation, there is no intramolecular change and it can be made to go back into solution again.
v. Coagulation:
This also is a property of the colloid. Here, the intramolecular change takes place. The coagulum cannot be redissolved.
vi. Osmotic pressure (Oncotic pressure):
Colloids exert some osmotic pressure. But the particles being large, it is proportionally much smaller. However, it is of great physiological importance.
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vii. Imbibition of Water:
Emulsoid particles can imbibe a good amount of water.
viii. Adsorption:
This also is a colloidal phenomenon.
Separation of Colloids:
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May be done by:
i. Dialysis.
ii. Isoelectric precipitation.
iii. Salting out:
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By adding suitable amounts of various salts. It is a kind of precipitation of a protein from its solution by saturation or partial saturation with such neutral salts as sodium, chloride, magnesium sulphate, or ammonium sulphate.
iv. Electrophoresis:
If an electric current be passed through a colloidal solution, the positively charged colloid ions will accumulate around the negative pole, while the negatively charged particles will accumulate around the positive pole. In this way they can be separated. Even their rate of movement can be measured by noting the concentration of a particular colloid at different points in the electric field.
v. Adsorption:
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By suitable methods, colloids can be isolated by adsorption.
vi. Ultracentrifugalisation:
If centrifugalised at high speed colloid particles collect at the bottom of the centrifuge tubes at different rates, the heavier ones settle out faster.
vii. Ultrafiltration:
By this process the colloidal (larger) and crystalloid (smaller) dissolved molecules can be separated from each other. Colloid of different particle size can be separated from one another by ultrafiltration through membranes of different pore sizes.
Examples of Colloids:
Some examples are as follows:
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i. Cell Protoplasm:
In every cell—exists in a colloidal state—mostly as emulsoid. 90% of organic matter of the body remains as colloid.
ii. Milk, Plasma and Lymph:
They are all emulsoids. Blood is a suspension of red cells in plasma.
iii. Interfacial Reaction:
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Being dispersed in the form of minute particles, colloids afford a very large surface area for various reactions to occur, such as adsorption, surface tension, enzyme action, etc.
For instance:
(a) The emulsification of fats during digestion, provide a large surface of contact for the enzymes to act, and thus quickens the process of digestion,
(b) The total surface area of red cells being very large facilitates rapid exchange of gases, ions, etc., in the blood stream.
iv. Imbibition of Water:
Since emulsoids readily imbibe water, a good deal of water remains stored in the body in this way.
v. Adsorption:
It is of great physiological importance—is a colloidal phenomenon.
vi. Blood Clotting:
Is essentially a colloidal process, in which a sol (plasma) is converted into a gel (Clot).
vii. Colloid Osmotic Pressure:
When human red blood corpuscles are suspended in 0.5% sodium chloride solution (hypotonic), water and sodium chloride pass into the cells due to lower osmotic pressure inside the cells. The red cells are swollen and burst.
If the red cells are suspended in more than 1.0% sodium chloride solution (hypertonic) opposite change occurs, i.e., water and salt pass out of the red cells. The red cells are shrunken and become crenated. If the red cells are suspended in normal saline solution (0.9%) there is practically no change in the cells.
Plasma contains both diffusible and non-diffusible large colloidal particles. The total osmotic pressure of plasma is about 6.5 atmospheres. This is due to crystalloids and colloids. The osmotic pressure of the plasma proteins is 25 mm of Hg. The osmotic pressure of the plasma proteins depends on serum albumin and serum globulin. Serum albumin exerts a greater osmotic pressure than serum globulin.
By virtue of the osmotic pressure of the plasma proteins, the fluid in the blood capillaries is retained and the plasma volume is maintained. The capillary blood pressure is a filtering force tending to drive protein-free fluid into the interstitial spaces, whereas the osmotic pressure of the plasma protein exerts an opposite effect. The osmotic pressure of plasma proteins also plays a great role in glomerular filtration.
