General Properties of Colloids:
- The ultramicroscopic examination indicates that colloidal dispersion is a heterogeneous system consisting of a continuous dispersion medium and discontinuous disperse phase.
- The colloidal particles readily pass through ordinary filter paper. The range of particle size of colloidal substance is in between 5 × 10-9 m to 2 × 10-7 m. The pore size of ordinary filter paper is bigger i.e. of order 10-7 m. So Colloidal particles can pass through it and thus filter paper can be used to separate colloidal particles from coarse suspension.
- Sols and true solutions pass through filter paper. The colloids cannot pass (diffuse) through parchment membrane but crystalloids can pass through parchment membrane.
- The process of separating colloids from other dissolved substance using parchment membrane as the filter is called dialysis. This process is used for purification of colloids.
- When the impure sol is placed in specially created ultrafilter, with small pores, the sol particles being bigger than the pores remain behind while dispersion medium and dissolved electrolyte pass through. This process is known as ultra purification.
- Colloidal particles are of two types i) Multimolecular and ii) Macromolecular
- Multimolecular colloidal particles are aggregates of a number of small molecules or atoms.e.g. Sulphur sol, gold sol.
- Macromolecular particles are very big molecules or polymers. e.g. Starch, proteins.
- As the colloidal particles are aggregates of a number of molecules or a large molecule themselves their molecular mass is very high.
- Osmotic pressure is colligative property which depends upon the number of particles present in the solution and not on the size of the particles.
- Since a colloidal particle is an aggregation of a large number of particles, the number of particles in the sol is less. Therefore, the osmotic pressure of a colloidal solution is low. Polymers have low osmotic pressure.
- Many sols are coloured. Sol particles are able to scatter light rays.
- Colour of the sol depends upon the wavelength of scattered light by the sol particles and which again depends on the size of the sol particles.
- Let us consider silver sol (colloidal solution of the same substance) having different types of particles. It is found that the sols show different colours. Colour of silver sol Diameter of colloidal particles
Violet 15 × 10-8 m
Purple 13 × 10-8 m
Orange-red 9 × 10-8 m
Orange-yellow 6 × 10-8 m
- The blue sky is due to the blue light scattered by small dust particles in the atmosphere. The atmosphere is a colloidal system consisting of dust particles suspended in air.
- The red sky is due to red light scattered by larger dust particles in the atmosphere.
- Depending on the size of the dust and water particles, different colours are seen in the cloud.
- Fine gold sol is red but as particle size increases it becomes blue or purple.
Optical Property of Colloids:
- When an intense beam of light is passed through the colloidal solution (taken in a glass vessel) placed in a dark the path of light through the colloidal solution is clearly visible due to the scattering of light by sol particles. This effect is known as Tyndall effect.
- This fact was first noted in 1857 by Faraday and then studied in details by Tyndall in 1868
- True solutions do not exhibit Tyndall effect.
- The emitted light emerges in the form of a bright cone called Tyndall cone. Through ultramicroscope, each colloidal particle appears a bright point against the dark background, due to the scattering. Thus the colloidal particles become self-luminous. As a result, the path of the beam of light through colloidal solution becomes clearly visible. The nature of scattering depends on the size of the sol particle and the refractive indices of sol particle.
- Colloidal particles are not large enough like suspension particles to reflect the light nor they are small enough, like true solution particles to allow the light to pass through them.
- Due to the intermediate size of colloidal particles, they scatter part of the absorbed light, from their surfaces in all directions. Thus the cause of Tyndall effect is a scattering of light by colloidal particles.
Applications of Tyndall Effect:
- Tyndall effect is useful to distinguish colloidal solution from true solution
- To test the purity of gases in the manufacture of H2SO4 by the contact process.
- Count the number of colloidal particles in colloidal sols using ultra-microscope.
Mechanical Property of Colloids:
Brownian movement :
- The English Botanist Robert Brown, in 1927 observed that colloidal particles exhibit continuous random motion in all directions in a straight line. He found such movement when pollen grains were suspended in water.
- The phenomenon of continuous zig-zag movement of colloidal particles in straight line paths in a random direction is known as a Brownian movement.
- Colloidal particles are surrounded by a large number of dispersion medium molecules which constantly bombard the colloidal particles. On unequal bombardment, the colloidal particles get pushed in certain directions.
- Since colloidal particles possess like charge, they repel each other.
Factors Affecting Brownian Movement:
- Brownian movement depends on the viscosity of dispersion medium. Brownian movement is more in less viscous solution.
- Brownian movement depends on the size of the particle. If the particles are of smaller size. The Brownian movement is more rapid.
Applications Brownian Movement:
- Due to the Brownian movement colloidal particles hardly settle down and prevent aggregation of colloidal particles. Thus colloidal solution becomes stable.
- Avogadro’s number can be calculated by Brownian movement.
Electrical Properties of Colloids:
Charge on the colloidal particles:
- The colloidal particles carry an electric charge. The most important property of colloidal solution is that all suspended particles possess either positive or a negative charge. i.e. they carry same nature of the charge.
- The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol.
- The dispersion medium carries opposite charge, hence as a whole, the colloidal solution is electrically neutral.
Origin of the charge:
- A small quantity of electrolyte is always present in the colloidal dispersion. Its presence is necessary for the stability of the sol, as complete removal of the sol causes coagulation of the sol.
- Theory of preferential adsorption suggests that the colloidal particles preferentially adsorb one type of ions from the electrolyte. Usually, ions common with colloidal particles are adsorbed. Due to this all colloidal particles in the colloidal dispersion acquire the same charge.
- If FeCl3 solution is added to the Ferric hydroxide, Fe3+ ions adsorb on Fe(OH)3 molecules, that is why colloidal particles of ferric hydroxide are positively charged.
- When excess dilute KI is added in excess dilute AgNO3, the Ag+ ions are adsorbed on Agl, and [AgI]Ag+ is formed. Thus the colloidal particles have positive charge.
- When excess dilute AgNO3 is added in excess dilute KI, the I– ions are adsorbed on Agl, and [AgI]I– is formed. Thus colloidal particles have negative charge.
- Due to the preferential adsorption of sulphide ions, the As2S3 sol is negatively charged.
Importance of Charge on Colloidal Particles:
- The mutual forces of repulsion between similarly charged colloidal particles prevent them from aggregating and settling under the action of gravity.
- This gives stability to the sol. In case of lyophobic sols charge on colloidal particles is fully responsible for its stability.
- In case of lyophilic sol, the stability is due to the charge on colloidal particles and solvation.
Electrophoresis or Cataphoresis:
- The unidirectional migration of sol particles or dispersed phase particles or colloidal particles towards oppositely charged electrode under the influence of applied electric field is called electrophoresis or cataphoresis.
Cause Of Electrophoresis:
- All sol particle (colloidal particles) carry same electric charge either positive or negative. If an electric potential is applied across two platinum electrodes dipping in a sol, the sol particles move towards oppositely charged electrodes.
- Consider a sol of As2S3 is taken in a ‘U’ shaped glass tube. The sol particles of the sol are negatively charged.
- Now the dispersion medium with little quantity of electrolyte is introduced over the colloidal solution. There should be a sharp boundary between the sol and the dispersion medium.
- Electric potential is applied across the two platinum electrodes dipped in a sol in two limbs, it is observed that the level of sol drops at the negative electrode and rises at positive electrode side.
- This shows that sol particles have migrated to the positive electrode, indicating that the particles are negatively charged.
- If colloidal particles are allowed to reach the electrode, their charges are neutralised and coagulation takes place.
Applications of Electrophoresis:
- Electrophoresis is used to detect the nature of charge on colloidal particles.
- It is used in the removal of carbon particles from chimney gases.
- It is used in electro-deposition of rubber on metal, wood or cloth surfaces from latex.
- It is used to bring about coagulation of sol.
- The migration of dispersion medium of a colloidal solution under the influence of electric field when the movements of colloidal particles are prevented is called as electro-osmosis.
Cause of Electro-osmosis:
- Since the sol as a whole is electrically neutral, dispersion medium has opposite electric charge as compared with that of the sol particles. If the dispersed phase has a positive charge we say that the dispersion medium has a negative charge.
- A sol of As2S3 is filled in a glass tube. The sol particles of the sol are negatively charged. Hence the dispersion medium (water) is positively charged.
- The colloidal solution and pure dispersion medium in a glass tube are separated by a semipermeable membrane.
- When an electric potential is applied across the platinum electrodes dipping in each arm, sol particles cannot pass through the semipermeable membrane but dispersion medium (water) move to the negative electrode through the semipermeable membrane. The level of sol drops at the +ve electrode and rises at -ve electrode.
- This movement of dispersion medium towards -ve electrode shows that the charge on the dispersion medium is positive.
Applications of Electro-osmosis:
- Electro-osmosis is used in dewatering of moist clay
- It is used in drying of dye-pastes
- It is used in the removal of water from peat.
- The process of precipitation of colloidal particles due to aggregation the particles is called as coagulation or flocculation.
- The presence of the same type of electric charge on colloidal particles causes repulsion and keep them in a suspended state.
- If by some means, the charge on the colloidal particles is removed, these particles come together, aggregated, become large enough to settle down in the form of a precipitate. Thus the charge on the colloidal particles is neutralised.
- In this process, the dispersion medium and dispersed phase are separated from each other.
- If the precipitate formed instead of settling down, floats on the surface of the dispersion medium then the phenomenon is called as flocculation.
Different Methods of Coagulation:
By Mutual Coagulation:
- If two sols of opposite charge are mixed together, their charges are mutually neutralised and both sols get coagulated.
Fe(OH)3 (+ve sol) + As2S3 (- ve sol) → Precipitate
- On the heating thermal energy of colloidal particles increases so much that it overcomes the repulsive forces between them, and particles unite to form larger particles.
- The charge on colloidal particles is due to preferential adsorption. The phenomenon of adsorption is inversely proportional to temperature. Due to heating desorption of adsorbed ions from the surface of colloidal particles takes place.
- Thus due to above two reasons, the neutral sol particles aggregate to form a precipitate.
- Example :
- If an egg is boiled egg albumin gets coagulated.
Egg albumin → Precipitate.
- Under the influence of electric field, sol particles move to the electrode where they get discharged as a neutral particle. Neutral sol particles aggregate to form a precipitate.
- Example: Rubber from latex is coagulated by electrophoresis on cloth or metal.
By Addition of Electrolyte:
- When an excess of an electrolyte is added to a sol, oppositely charged ions of electrolyte removes the charged on sol particles. Neutral sol particles aggregate to form a precipitate.
- Any negatively charged sol is coagulated by the cations of electrolyte and vice-versa.
- Example: If BaCl2 is added to the As2S3 sol which is negative sol, Ba2+ ions causes coagulation of sol.
Hardy- Schulze Rule:
Statement: (Part – I)
- The ions of electrolyte having opposite charge with respect to as that of the sol particles are responsible for the coagulation.
- For positively charged sol like Fe(OH)3, the anions cause coagulation. Similarly, for negatively chargedsol like As2S3 the cations cause coagulation.
Statement: (Part – II)
- Coagulating power of ions of electrolyte which cause coagulation increases with the valency (charge) of ions. More the valency of the effective ions the greater is its precipitating power.
- Example :
- Coagulating power of cations, coagulating any negative sol is in the order, Al3+ > Ba2+ > Na+
- Coagulating power of anions, coagulating any +ve sol is in the order, PO43- > SO42- > Cl–
Applications of Coagulation:
- Dirty and muddy water from gutters and drainages is called sewage is in colloidal form (colloidal solution).
- Sewage water containing colloidal particles of mud, rubbish etc. is collected in a tank fitted with electrodes.
- On applying an electric field, colloidal particles are attracted towards oppositely charged electrodes. As their charge gets neutralised, they settle as a precipitate. The precipitated or coagulated matter called sludge is used as manure while clear water is used for irrigation.
- Smoke is a colloidal solution of negatively charged carbon particles in air (aerosol)
- These carbon particles may condense water vapour on them and thus cities may have a thick cover of smog (smoke + fog). This smog causes air pollution.
- Smoke is passed between metal electrodes at high voltage (about 50.000 V.) The charged particles are neutralised at the oppositely charged electrode and get deposited there. This process was developed by Cottrell.
- The gases free from carbon particles are passed to a chimney or for further purification.