Properties of Colloids
3) Colloidal solutions are heterogeneous in nature having two distinct phases
5) Non-Setting nature
6) Concentration and density
8) Colligative properties
9) Shape of colloidal particles
10) Optical property
11) Kinetic property
12) Electrical property
13. Coagulation or precipitation
14. Protective action
The colour of a sol is not always the same as the colour of the substance in the bulk. For example bluish tinge is given by diluted milk in reflected light and reddish tinge in transmitted light.
Colour of the sol, generally depends on the following factors.
• Method of preparation
• Wavelength of source of light.
• Size and shape of colloidal particle
• whether the observer views the reflected light or transmitted light.
The size of colloidal particles ranges from 1nm (10-9m) to 1000 nm (10-6m) diameter.
Though experiments like dialysis, ultrafiltration and ultracentrifuging clearly show the heterogeneous nature in the recent times colloidal solution are considered as border line cases.
As the size of pores in ordinary filter paper are large the colloidal particles easily pass through the ordinary filter papers.
Colloidal solutions are quite stable i.e. they are not affected by gravity.
When the colloidal solution is dilute, it is stable. When the volume of medium is decreased coagulation occurs. Generally, density of sol decreases with decrease in the concentration.
Unlike true solution, colloids diffuse less readily through membranes.
The colloidal solutions show colligative properties i.e. elevation of boiling point, depression in freezing point and osmotic pressure. Measurements of osmotic pressure is used to find molecular weight of colloidal particle.
It is very interesting to know the various shapes of colloidal particles. Here are some examples
Colloids have optical property. When a homogeneous solution is seen in the direction of light, it appears clear but it appears dark, in a perpendicular direction.
But when light passes through colloidal solution, it is scattered in all directions. This effect was first observed by Faraday, but investigations are made by Tyndall in detail, hence called as Tyndall effect.
The colloidal particles absorb a portion of light and the remaining portion is scattered from the surface of the colloid. Hence the path of light is made clear.
Robert Brown observed that when the pollen grains suspended in water were viewed through ultra microscope, they showed a random, zigzag ceaseless motion.
This is called Brownian movement of colloidal particles.
This can be explained as follows
The colloidal sol particles are continuously bombarded with the molecules of the dispersion medium and hence they follow a zigzag, random, continuous movement.
Brownian movement enables us,
1. to calculate Avogadro number.
2. to confirm kinetic theory which considers the ceaseless rapid movement of molecules that increases with increase in temperature.
3. to understand the stability of colloids: As the particles in continuous rapid movement they do not come close and
hence not get condensed. That is Brownian movement does not allow the particles to be acted on by force of gravity.
(1) Helmholtz double layer
The surface of colloidal particle adsorbs one type of ion due to preferential adsorption. This layer attracts the oppositely charged ions in the medium and hence at the boundary separating the two electrical double layers are setup. This is called as Helmholtz electrical double layer.
As the particles nearby are having similar charges, they cannot come close and condense.
Hence this helps to explain the stability of a colloid.
When electric potential is applied across two platinum electrodes dipped in a hydrophilic sol, the dispersed particles move toward one or other electrode.
This migration of sol particles under the influence of electric field is called electrophoresis or cataphoresis. If the sol particles migrate to the cathode, then they posses positive (+) charges, and if the sol particles migrate to the anode then they have negative charges(-).
Thus from the direction of migration of sol particles we can determine the charge of the sol particles. Hence electrophoresis is used for detection of presence of charges on the sol particles.
Few examples of charges of sols detected by electrophoresis are given below:
(iii) Electro osmosis
A sol is electrically neutral. Hence the medium carries an equal but opposite charge to that of dispersed particles. When sol particles are prevented from moving, under the influence of electric field the medium moves in a direction opposite to that of the sol particles. This movement of dispersion medium under the influence of electric potential is called electro osmosis.
The flocculation and settling down of the sol particles is called coagulation.
Various method of coagulation are given below:
(i) Addition of electrolytes
(iii) Mixing oppositively charged sols.
(i) Addition of electrolytes
A negative ion causes the precipitation of positively charged sol and vice versa.
When the valency of ion is high, the precipitation power is increased. For example, the precipitation power of some cations and anions varies in the following order
Al3+ >Ba 2+ >Na+ , Similarly [Fe(CN)6 ]3− > SO42− > Cl-
The precipitation power of electrolyte is determined by finding the minimum concentration (millimoles/lit) required to cause precipitation of a sol in 2hours. This value is called flocculation value. The smaller the flocculation value greater will be precipitation.
In the electrophoresis, charged particles migrate to the electrode of opposite sign. It is due to neutralization of the charge of the colloids. The particles are discharged and so they get precipitated.
(iii) By mixing two oppositively charged sols
When colloidal sols with opposite charges are mixed mutual coagulation takes place. It is due to migration of ions from the surface of the particles.
(iv) By boiling
When boiled due to increased collisions, the sol particles combine and settle down.
Generally, lyophobic sols are precipitated readily even with small amount of electrolytes. But they are stabilised by addition of a small amount of lyophillic colloid.
A small amount of gelatine sol is added to gold sol to protect the gold sol.
Zsigmondy introduced the term ‘gold number’ as a measure of protecting power of a colloid. Gold number is defined as the number of milligrams of hydrophilic colloid that will just prevent the precipitation of 10ml of gold sol on the addition of 1ml of 10% NaCl solution. Smaller the gold number greater the protective power.