Coagulation

Coagulation is the process by which a colloid precipitates out of a solution. The precipitation is brought about by induced aggregation. For example., an iron (III) hydroxide sol can be made to aggregate by addition of an ionic solution. A positively charged particle of iron (III) hydroxide gathers a layer of anions around it. The thickness of this layer is determined by the charge on the anions. The greater the magnitude of the negative charge, the more compact the layer of charge. For example, phosphate ions gather more closely to the positively charge iron (III) particle than do chloride ions.

A. Layers of ions surrounding a charged particle of iron (III) hydroxide. Fe(OH)3 surrounded by Cl- ions

B. Fe(OH)3 surrounded by PO43- ions

If the ion layer is gathered close to the colloidal particles, the overall charge is effectively neutralized and two colloidal particles can approach close enough to aggregate and precipitate out.

The coagulation of colloids by an electrolyte takes place only when the electrolyte has a certain minimum concentration. Different electrolytes have different coagulation values. Smaller the coagulation value of the electrolyte, larger is its coagulating power. According to Hardy and Schulze, coagulation of colloids by electrolytes is governed by two factors, namely :
i) Ions carrying charge opposite to that of the colloidal particles are effective in bringing about coagulation.
ii) Coagulation power of an electrolyte is directly proportional to the valency of its ions.

These two observations made by Hardy and Schulze are called the Hardy Schulze rule. On the basis of this rule, it is possible to predict that for colloidal sols consisting of negatively charged particles such as (AS2S3)Al3+ ions are more effective than Ba2+ or Na+ ions. For positively charged colloidal particles, PO43- is much more effective than Cl- to bring about coagulation

Colloidal sols can be coagulated by mutual precipitation, electrophoresis, repeated dialysis and heating.

Some examples of coagulation which occur in nature are:
1) Curdling of milk

Milk is a colloidal suspension in which the particles are prevented from aggregating because they have electric charges of the same sign. When milk sugar (lactose) ferments, ions responsible for curdling are formed.

2) When river water meets the concentrated ionic solution of an ocean, coagulation of the colloidal suspension of the soil in river water occurs.

Electrophoresis

The colloidal particles are electrically charged and they carry the same type of charge, that is, either the colloids are negatively charged or positively charged. The existence of charge on the colloidal particles is inferred from the observation that the colloidal particles move either towards the cathode or anode when the colloidal sol is placed in an electric field. This phenomenon of charged particles moving towards the oppositely charged electrodes in the presence of an electric field is called electrophoresis.

Electrophoresis is carried out by placing the colloidal solution in a U tube which is fitted with platinum electrodes.

With electrophoresis we can determine the charge of colloid, its can b use to identification DNA of bomber suicides.

Brown Movement

Brownian movement was first discovered by Robert Brown, a botanist, in 1827. He observed that pollen grains in water do not remain at rest but move about continuously and randomly. This random continuous movement (Brownian motion) was observed in colloidal sol when the sol was viewed under a ultra microscope. Brownian motion in colloidal sols arises due to the impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impact of the molecules of dispersion medium on the colloidal particles are unequal. This leads to the zig-zag (random) motion of the colloidal particles.

This random movement decreases as the size of the particles increases because the effect of the impacts average out. When the size of the dispersed particles increases beyond the colloidal range, Brownian motion stops, and no Brownian movement is observed.

Tyndall effect

1. Tyndall effect
a. Definition

British physicists, John Tyndall succeeded in studying colloids and he found a phenomenon called the Tyndall effect. Tyndall effect is the scattering of light by colloidal particles in its path. The Tyndall Effect is caused by reflection of light by very small particles in suspension in a transparent medium.

b. The implementation in daily live
The example in daily live are:
1. The dust in the air when sunlight comes in through a window, or comes down through holes in clouds
2. The light of car in the foggy condition.
The Tyndall effect is different for every light that have different wave length. For example is the yellow light is little scattering, and it’s the reason why the lamp color of car is yellow. This color can penetrate in the foggy condition.

The Comparisons between solution, colloid and suspension.

Solutions	Colloids	Suspensions
Homogeneous	Heterogeneous	heterogeneous
Particle size - small	Particle size - medium	Particle size - large
Do not separate on standing	Do not separate on standing	Separates on standing
Cannot be separated by filtering	Cannot be separated by filtering	Can be separated by filtering
Do not scatter light	Scatters light	May scatter light, but are not transparent