NEET Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Colloids, Emulsion, Gel and Their Properties With Application

Colloids, Emulsion, Gel and Their Properties With Application

Colloidal state. 

            (1) The foundation of colloidal chemistry was laid down by an English scientist, Thomas Graham, in 1861. The credit for the various advances in this field goes to eminent scientists like Tyndall, Hardy, Zsigmondy, N.R. Dhar, S.S. Bhatnagar and others.

            (2) Thomas Graham classified the soluble substances into two categories depending upon the rate of diffusion through animal and vegetable membranes or parchment paper.

            (i) Crystalloids : They have higher rate of diffusion and diffused from parchment paper.

Examples : All organic acids, bases and salts and organic compounds such as sugar, urea etc.

            (ii) Colloids (Greek word, kolla, meaning glue-like) : They have slower rate of diffusion and can not diffused from parchment paper. Examples : Starch, gelatin, gums, silicic acid and hdemoglobin etc.

            (3) The above classification was discarded i.e., the terms colloid does not apply to a particular class of substances but is a state of matter like solid, liquid and gas. Any substance can be brought into colloidal state.

            (4) The colloidal state depends on the particle size. If is regarded as intermediate state between true solution and suspension.

• True solution : In true solutions the size of the particles of solute is very small and thus, these cannot be

            detected by any optical means and freely diffuse through membranes. It is a homogenous system.

• Suspension : The size of particles is large and, thus it can be seen by naked eye and do not pass through

filter paper. It is a heterogeneous system.

    The size of different solutions are sometimes expressed in other units also as given below: Size (diameter) of particles in particles in different units

True solutions	Colloids	Suspensions	Relation
\[<{{10}^{9}}m\] \[<1nm\] \[<10{\AA}\] \[<1000pm\]	\[{{10}^{9}}mto\text{ }{{10}^{7}}m\] 1 nm - 100 nm \[10\text{ }{\AA}1000\text{ }{\AA}\] \[1000pm{{10}^{5}}pm\]	\[>{{10}^{-7}}\]m > 100 nm \[>1000\text{ }{\AA}\] \[>{{10}^{5}}pm\]	\[1nm={{10}^{9}}m\] \[1\text{ }{\AA}={{10}^{10}}m\] \[1pm={{10}^{12}}m\]

The important distinguishing features of the three types of solutions

Property	Suspension	Colloid solution	True solution
Nature	Heterogeneous	Heterogeneous	Homogeneous
Particle size	> 100 nm	1 nm ? 100 nm	< 1 nm
Separation by (i) Ordinary filtration (ii) Ultra- filtration	Possible Possible	Not possible Possible	Not possible Not possible
Settling of particles	Settle under gravity	Settle only on centrifugation	Do not settle
Appearance	Opaque	Generally transparent	Transparent
Tyndall effect	Shows	Shows	Does not show
Diffusion of particles	Does not diffuse	Diffuses slowly	Diffuses rapidly
Brownian movement	May show	Shows	Negligible

            (5) Roughly speaking the colloidal state is a heterogeneous dispersion of solute particles of size between true solution and suspension.            

Note :q Colloidal particles do not settle down under the force of gravity even an long keeping.

                        q The surface area of colloidal particle is very large in comparison to suspension.

 

Phases of colloids and their classification.    

            (1) Phases of colloids: We know that a colloidal solution is of heterogeneous nature. It consists of two phases which are as follows

            (i) Internal phase or Dispersed phase (Discontinuous phase) : It is the component present in small proportion and is just like a solute in a solution. For example in the colloidal solution of silver in water (silver acts as a dispersed phase)

            (ii) External phase or Dispersion medium (continuous phase) : It is generally component present in excess and is just like a solvent in a solution. For example, in the colloidal solution of silver in water. Water act as a dispersion medium.

           

Note :q When dispersion medium is a gas, the colloidal system is called aerosol. When it is a liquid system is called solution (hydrosol for water, alcosol for alcohol, benzosol for benzene)                      

• Colloidal system is a two phase system. \[Colloidal\,system\,=\,Dispersed\,phase+\,Dispersion\,medium\]

(2) Classification of colloids : The colloids are classified on the basis of the following criteria

(i) Classification based on the physical state of the dispersed phase and dispersion medium  Depending upon the physical state of dispersed phase and dispersion medium whether these are solids, liquids or gases, eight types of colloidal systems are possible.

Different types of colloidal systems

Dispersed phase	Dispersion Medium	Colloidal System	Examples
Liquid	Gas	Aerosol of liquids	Fogs, clouds, mists, fine insecticide sprays
Solid	Gas	Aerosol of solids	Smoke, volcanic dust, haze
Gas	Liquid	Foam or froth	Soap lather. Lemonade froth, foam, whipped cream, soda water
Liquid	Liquid	Emulsions	Milk, emulsified oils, medicines
Solid	Liquid	Sols	Most paints, starch in water, proteins, gold sol, arsenic sulphide sol, ink
Gas	Solid	Solid foam	Pumice stone, styrene rubber, foam rubber
Liquid	Solid	Gels	Cheese, butter, boot polish, jelly, curd
Solid	Solid	Solid sols (coloured glass)	Ruby glass, some gem stones and alloys

           

Note :q A colloidal system of gas in gas is not possible as gases are completely miscible and always form homogenous true solution.

            (ii) Classification based on Nature of interaction between dispersed phase and dispersion medium: Depending upon the nature of interactions between dispersed phase and the dispersion medium, the colloidal solutions can be classified into two types as (a) Lyophilic and (b) Lyophobic sols.

            (a) Lyophilic colloids (water loving) : Substances such as proteins, starch and rubber whose molecules are large enough to be close to the lower limit of colloidal range pass readily into colloidal state whenever mixed with suitable solvent. Thus these colloids have strong interaction with the dispersion medium and are called lyophilic colloids (hydrophilic when continuous phase is water)

“or”

            “The colloidal solutions in which the particles of the dispersed phase have a great affinity for the dispersion medium, are called lyophilic collodis.”

            (b) Lyophobic colloids (water hateing) : Substance such as arsenic sulphide, ferric hydroxide, gold and other metals, which are sparingly soluble and whose molecules are much smaller than the lower colloidal limit, change into colloidal state by aggregation of many individual molecules. “These substances; therefore, do not pass into colloidal state readily and are called lyophobic colloids (hydrophobic when continuous phase is water).”

“or”

            “The colloidal solutions in which there is no affinity between particles of the dispersed phase and the dispersion medium are called lyophobic colloids.”

Distinction between Lyophilic and Lyophobic Sols

Property	Lyophilic (suspensoid)	Lyophobic Sols (Emulsoid )
Surface tension	Lower than that of the medium	Same as that of the medium
Viscosity	Much higher than that of the medium	Same as that of the medium
Reversibility	Reversible	Irreversible
Stability	More stable	Less stable
Visibility	Particles can?t be detected even under ultramicroscope	Particles can be detected under ultramicroscope.
Migration	Particles may migrate in either direction or do not migrate in an electric field because do not carry any charge.	Particles migrate either towards cathode or anode in an electric field because they carry charge.
Action of electrolyte	Addition of smaller quantity of electrolyte has little effect	Coagulation takes place
Hydration	Extensive hydration takes place	No hydration
Examples	Gum, gelatin, starch, proteins, rubber etc.	Metals like Ag and Au, hydroxides like \[Al{{(OH)}_{3}}\], \[Fe{{(OH)}_{3}}\]metal sulphides like \[A{{S}_{2}}{{S}_{3}}\] etc.

          

(iii) Classification based on types of particle of dispersed phase: Depending upon the type of the particles of the dispersed phase, the colloids are classified as follows.

            (a) Multimolecular colloids

• When on dissolution, atoms or smaller molecules of substances (having diameter less than 1nm) aggregate together to form particles of colloidal dimensions, the particles thus formed are called multimolecular colloids.
• In these sols the dispersed phase consists of aggregates of atoms or molecules with molecular size less than 1 nm.
• For example, sols of gold atoms and sulphur \[({{S}_{8}})\]molecules. In these colloids, the particles are held together by Vander Waal's forces. They have usually lyophilic character.

            (b) Macromolecular colloids

• These are the substances having big size molecules (called macromolecules) which on dissolution form size in the colloidal range. Such substances are called macromolecular colloids.
• These macromolecules forming the dispersed phase are generally polymers having very high molecular masses.
• Naturally occurring macromolecules are starch, cellulose, proteins, enzymes, gelatin etc.
• Artificial macromolecules are synthetic polymers such as nylon, polythene, plastics, polystyrene etc.
• Their solutions are quite stable and resemble with true solution in many respects.
• They have usually lyophobic character.
• The molecules are flexible and can take any shape.

(c) Associated colloids

• These are the substances which on dissolved in a medium behave as normal electrolytes at low concentration but behave, as colloidal particles at higher concentration due to the formation of aggregated particles. The aggregates particles thus formed are called
• Their molecules contain both lyophilic and lyophobic
• The colloidal behaviour of such substances is due to the formation of aggregates or clusters in solutions. Such aggregated particles are called

Micelles

• Micelles are the cluster or aggregated particles formed by association of colloid in solution.
• The common examples of micelles are soaps and detergents.
• The formation of micelles takes place above a particular temperature called Kraft temperature \[({{T}_{k}})\]and    above a particular concentration called critical micellization concentration (CMC).
• They are capable of forming ions.
• Micelles may contain as many as 100 molecules or more.
• For example sodium stearate \[({{C}_{17}}{{H}_{35}}COONa)\]is a typical example of such type of molecules.
• When sodium stearate is dissolved in water, it gives \[N{{a}^{+}}\]and \[{{C}_{17}}{{H}_{35}}CO{{O}^{-}}\] ions.

                                    \[\underset{\text{Sodium stearate}}{\mathop{{{C}_{17}}{{H}_{35}}COONa}}\,\]\[\underset{\text{Stearate  ion}}{\mathop{{{C}_{17}}{{H}_{35}}CO{{O}^{-}}}}\,+N{{a}^{+}}\]

            The stearate ions associate to form ionic micelles of colloidal size.

• It has long hydrocarbon part of \[{{C}_{17}}{{H}_{35}}\]radical. Which is lyophobic and \[CO{{O}^{-}}\]part which is
• In the figure, the chain corresponds to stearate ion, \[({{C}_{17}}{{H}_{35}}CO{{O}^{-}})\]. When the concentration of the solution is below from its CMC \[({{10}^{-3}}\,mol\,\,\,{{L}^{-1}})\], it behaves as normal electrolyte. But above this concentration it is aggregated to behave as micelles.
• The main function of a soap is to reduce oily and greasy dirt to colloidal particles (an emulsion). Soap therefore, are known as emulsifying agents.

Some other examples of micelles are sodium palmitate \[({{C}_{15}}{{H}_{31}}COONa)\], Sodium lauryl sulphate \[[C{{H}_{3}}{{(C{{H}_{2}})}_{11}}S{{O}_{3}}{{O}^{-}}N{{a}^{+}}]\], Cetyl trimethyl ammonium bromide \[C{{H}_{3}}{{(C{{H}_{2}})}_{15}}{{(C{{H}_{2}})}_{3}}{{N}^{+}}B{{r}^{-}}\] etc.

 

Note :q Polydisperse and Monodisperse colloids : In multimolecular colloids, the colloidal particles consist of aggregates of atoms or small molecules with diameters less than \[{{10}^{-9}}\]m of 1 nm Colloidal solutions in which colloidal particles are of different sizes are called polydisperse colloids. For example, a gold sol may contain particles of various sizes having several atoms of gold. The colloidal solutions in which all the colloidal particles are more or less of identical size are monodisperse colloids.

Emulsion.

            “The colloidal systems in which fine droplets of one liquid are dispersed in another liquid are called emulsions the two liquids otherwise being mutually immiscible.”   or

            “Emulsion are the colloidal solutions in which both the dispersed phase and the dispersion medium are liquids.”

            A good example of an emulsion is milk in which fat globules are dispersed in water. The size of the emulsified globules is generally of the order of \[{{10}^{-6}}\]m. Emulsion resemble lyophobic sols in some properties.

             

(1) Types of Emulsion : Depending upon the nature of the dispersed phase, the emulsions are classified as;

            (i) Oil-in-water emulsions (O/W) : The emulsion in which oil is present as the dispersed phase and water as the dispersion medium (continuous phase) is called an oil-in-water emulsion. Milk is an example of the oil-in-water type of emulsion. In milk liquid fat globules are dispersed in water. Other examples are, vanishing cream etc.

            (ii) Water-in-oil emulsion (W/O) : The emulsion in which water forms the dispersed phase, and the oil acts as the dispersion medium is called a water-in-oil emulsion. These emulsion are also termed oil emulsions. Butter and cold cream are typical examples of this types of emulsions. Other examples are cod liver oil etc.

            

Note :q The emulsion can be inter converted by simply changing the ratio of the dispersed phase and dispersion medium. For example, an oil-in-water emulsion can be converted to water in oil emulsion by simply adding excess of oil in the first case.

           

(2) Preparation of Emulsions

            (i) Emulsions are generally prepared by vigorously agitating a mixture of the relevant oil and water by using either a high speed mixer or by using ultrasonic vibrators.

            (ii) The emulsions obtained by simple mechanical stirring are unstable. The two components (oil and water) tend to separate out.

            (iii) To obtain a stable emulsion, a suitable stabilizing substance is generally added.

            (iv) The stabilizing substance is called emulsifier of emulsifying agent. The emulsifier is added along with the oil and water in the beginning. For Examples : substances which can act as emulsifiers are soaps, detergents, long chain sulphonic acid, lyophilic colloids like gelatin, albumin, casein etc.

           

(3) Nature of emulsifier : Different emulsifiers may act differently in the case of a particular emulsion.

            For example,

• Sodium oleate is used to prepare oil-in-water (O/W) emulsions.
• Magnesium and calcium oleates are used to prepare water-in-oil (W/O) emulsions. When calcium oleate is added to an emulsion stabilized by sodium oleate, the stability of the system decreases. At a certain ratio of \[N{{a}^{+}}:C{{a}^{2+}}\], the oil-in-water emulsion becomes unstable. If the \[C{{a}^{2+}}\]ions concentration is increased further very quickly, then the reversal of the emulsion type occurs, that is the oil-in-water emulsion gets converted into a water-in-oil type.

 

(4) Identification of emulsions : Several methods are available to find out whether an emulsion is of the oil-in-water type or of the water-in-oil type emulsion. An emulsion can be identified as follows.

            (i) Dilute test : Add water to the emulsion. If the emulsion can be diluted with water this means that water acts as the dispersion medium and it is an example of oil-in-water emulsion. In case it is not diluted, then oil acts as dispersion medium and it is an example of water-in-oil emulsion.

            (ii) Dye test : An oil soluble suitable dye is shaken with the emulsion. If colour is noticed on looking at a drop of the emulsion, it is oil-in-water type emulsion. In case the entire background is coloured, it is an example of water-in-oil type.

            (iii) Conductivity test : Add small amount of an electrolyte (e.g. KCl) to the emulsion. If this makes the emulsion electrically conducting , then water is the dispersion medium. If water is not the dispersed phase.

            (5) Properties of emulsion

            (i) Emulsions show all the characteristic properties of colloidal solution such as Brownian movement, Tyndall effect, electrophoresis etc.

            (ii) These are coagulated by the addition of electrolytes containing polyvalent metal ions indicating the negative charge on the globules.

            (iii) The size of the dispersed particles in emulsions in larger than those in the sols. It ranges from 1000 Å to 10,000 Å. However, the size is smaller than the particles in suspensioins.

            (iv) Emulsions can be converted into two separate liquids by heating, centrifuging, freezing etc. This process is also known as demulsification.

           

(6) Applications of emulsions

            (i) Concentration of ores in metallurgy

            (ii) In medicine (Emulsion water-in-oil type)    

            (iii) Cleansing action of soaps.

            (iv) Milk, which is an important constituent of our diet an emulsion of fat in water.

            (v) Digestion of fats in intestine is through emulsification.

 

Gels.

            (1) “A gel is a colloidal system in which a liquid is dispersed in a solid.”

            (2) The lyophilic sols may be coagulated to give a semisolid jelly like mass, which encloses all the liquid present in the sol. The process of gel formation is called gelation and the colloidal system formed called gel.

            (3) Some gels are known to liquify on shaking and reset on being allowed to stand. This reversible sol-gel transformation is called thixotropy.

            (4) The common examples of gel are gum arabic, gelatin, processed cheese, silicic acid, ferric hydroxide etc.

            (5) Gels may shrink by loosing some liquid help them. This is known as synereises or weeping.

            (6) Gels may be classified into two types

            (i) Elastic gels : These are the gels which possess the property of elasticity. They readily change their shape on applying force and return to original shape when the applied force is removed. Common examples are gelatin, agar-agar, starch etc.

            (ii) Non-elastic gels : These are the gels which are rigid and do not have the property of elasticity. For example, silica gel.

 

Application of colloids.

            (1) Purification of water by alum (coagulation): Alum which yield \[A{{l}^{3+}}\]ions, is added to water to coagulate the negatively charged clay particles.

            (2) In rubber and tanning industry (coagulation and mutual coagulation) : Several industrial processes such as rubber plating, chrome tanning, dyeing, lubrication etc are of colloidal nature

            (i) In rubber platting, the negatively charged particles of rubber (latex) are made to deposit on the wires or handle of various tools by means of electrophoresis. The article on which rubber is to be deposited is made anode.

            (ii) In tanning the positively charged colloidal particles of hides and leather are coagulated by impregnating, them in negatively charged tanning materials (present in the barks of trees). Among the tanning agent chromium salts are most commonly used for the coagulation of the hide material and the process is called chrome tanning.

            (3) Artificial rains : It is possible to cause artificial rain by throwing the electrified sand or silver iodide from an aeroplane and thus coagulating the mist hanging in air.

            (4) Smoke precipitation (Coagulation) : Smoke is a negative sol consisting of carbon particles dispersed in air. Thus, these particles are removed by passing through a chamber provided with highly positively charged metallic knob.

            (5) Formation of deltas (coagulation) : River water consists of negatively charged clay particles of colloidal dimension. When the river falls into the sea, the clay particles are coagulated by the positive \[N{{a}^{+}},\,{{K}^{+}},\,M{{g}^{2+}}\] ions etc. present in sea water and new lands called deltas are formed.

            (6) Blood consists of negatively charged colloidal particles (albuminoid substance). The colloidal nature of blood explains why bleeding stops by applying a ferric chloride solution to the wound. Actually ferric chloride solution causes coagulation of blood to form a clot which stops further bleeding.

            (7) Colloidal medicine : Argyrol and protargyrol are colloidal solution of silver and are used as eye lotions colloidal sulphur is used as disinfectant colloidal gold, calcium and iron are used as tonics.

            (8) Photographic plates : These are thin glass plates coated with gelatin containing a fine suspension of silver bromide. The particles of silver bromide are colloidal in nature. 

           

Note : q            Isoelectric point of the colloid : The hydrogen ion concentration at which the colloidal particles are neither positively charged nor negatively charged (neutral) is known as isoelectric point of the colloids. At this point, the lyophilic colloids are expected to have minimum stability because at this point particles have no charge or equal quantum of positively and negatively charge. For example,

                                                isoelectric point of gelatin is 4.7 (at pH 4.7 gelatin has no electrophoretic motion; at pH<4.7, gelatin moves towards anode)                                    

• Colloidal solution of graphite is called Aqua dug.
• Ultrasonic dispersion : Various substances such as mercury, oils, sulphur, sulphides and oxide of metals can be dispersed into colloidal state very easily with the help of ultrasonic waves.
• Stem technology: Colloidal particles in a sol are very small and most of them are not visible through an ultramicroscope or light microscope. Recently, new techniques have been developed to determine the size and shape of the colloidal particles. These are

(i) Scanning Electron Microscope (SEM), (ii) Transmission Electron Microscope (TEM)

             A modified form of the above methods has also been developed. It is called Scanning Transmission Electron Microscope (STEM). All these techniques are superior to the light microscope because they have greater resolving power.

         Bencroft rule : The phase in which the emulsifier is more soluble becomes outer phase of the emulsion.