Monday, October 29, 2012

Difference Between Lyophilic and Lyophobic Colloids

S.No.
PROPERTY
LYOPHILIC COLLOIDS
LYOPHOBIC COLLOIDS
1.
Mode of preparation.
Forms easily by mere shaking or warming the dispersed phase with dispersion medium E.g. Gelatine in water.
Forms only by special methods.
Example: AS2S3 in water.
2.
Size of particle
The particles are true molecules and are just bigger in size (Molecular colloids).
The particles are aggregates of thousands of molecules (Associated colloids).
3.
Nature
Reversible and once precipitated can readily pass into colloidal state by direct contact with dispersion medium because particles are solvent loving.
Irreversible and once precipitated can not easily pass into colloidal state because particles are solvent hating.
4.
Visibility
The particles are not easily detected even under ultra microscope.
The particles are easily detected under ultra microscope.
5.
Charge
The charge of particles depends upon PH of medium and it may be positive, negative or neutral.
The particles carry positive or negative charge.
6.
Stability
These are stable and are self – stabilized.
These are unstable and hence require traces of electrolyte for stabilisation.
7.
Action of electrolytes
Co-agulation can be brought about by the addition of large amount of electrolyte.
Co-agulation can be brought about by small amount of electrolyte.
8.
Viscosity
It is much higher than that of dispersion medium.
It is about same as that of dispersion medium.
9.
Surface
It is lower than dispersion medium
It is about same as that of dispersion medium.
10.
Tyndall effect
Less distinct
More distinct.
11.
Electrophoresis
The particles may or may not show electrophoresis.
The particles show electrophoresis.
12.
Hydration
The particles are heavily hydrated due to love for solvent.
The particles are not appreciably hydrated due to hate for solvent.
13.
Conc. Of dispersed phase
Higher concentrations of dispersed phase are possible.
Only low concentrations of dispersed phase are possible.
14.
Colligative property
They have relatively high osmotic pressure, depression in freezing point and high lowering in vapour pressure.
They have high osmotic pressure, less depression in freezing point, less elevation in boiling point and less lowering of vapour pressure.

Classification of Colloids

Since the colloidal solutions consist of dispersed phase and dispersion medium, the components of these phases have certain specific characteristics of their own. In view of these observations, the colloidal solutions can be classified into the various types as under;
(I).       CLASSIFICATION BASED ON PHYSICAL STATE OF DISPERSED PHASE AND DISPERSION MEDIUM
                        Based on the physical state of dispersed phase and dispersion medium, there are eight types of colloidal solutions. Either these phases may be solid, liquid or gas. Since gases gives homogeneous mixture in all proportions, hence colloidal solution can not be obtained in which dispersed phase and dispersion medium are gases.
S.No.
Dispersed Phase
Dispersion Medium
Name of Colloidal Solution
Examples
1.
Gas
Liquid
Foam
Soap lather, whipped cream, soda water, froth on beer.
2.
Gas
Solid
Solid foam
Cake, Bread, Pumice stone lava.
3.
Liquid
Gas
Aerosol
Mist, fog, Clouds.
4.
Liquid
Liquid
Emulsion
Milk, cream, butter, oil in water
5.
Liquid
Solid
Gel.
Curd, Cheese, Jellies, boot polish.
6.
Solid
Gas
Solid Aerosol
Smoke, dust.
7.
Solid
Liquid
Sols
Paints, gold sol, inks.
8.
Solid
Solid
Solid Sols
Alloys, coloured glass, gem stones.

(II).     CLASSIFICATION BASED ON ELECTRICAL CHARGE ON DISPERSION PHASE
                        Based on the nature of charge on the dispersed phase, there are two types of colloidal solution;
            (a).       Positive Colloids
                                     When dispersed phase in a colloidal solution carries a positive charge, it is known as Positive Colloid.
                        Examples :- Metal hydroxides like Fe(OH)3, Al(OH)3, TiO2, Methylene blue sols etc.
            (b).       Negative colloids
                                     When dispersed phase in a colloidal solution carries a negative charge, it is known as Negative Colloid.
                        Examples: AS2S3 Sol, Cu or Ag Sol.
(III). CLASSIFICATION BASED ON APPEARANCE OF THE COLLOIDS
                        On this basis, the colloidal solutions can be divided into two types;
            (a).       Sols
                                    When a colloidal solution appears as fluid it is referred to as Sol. The sols are generally named after dispersion medium. For Instance, when the dispersion medium is water, the sol is known as Hydrosol or Aquosol. When the dispersion medium is alcohol and benzene, the sols are called Alcosol and Benzosol respectively.
            (b).       Gels
                                    When a colloidal has solid like appearance it is called Gel. The rigidity of gels varies from substance to substance. Some examples of gels are: Jelly, butter, cheese, curd.
(IV).    CLASSIFICATION BASED ON MOLECULAR SIZE OF THE DISPERSED PHASE
                        On the basis of molecular size the colloids can be divided into three types:
            (a).       Multimolecular Colloids       
                                    In these colloids, the Individual particles of the dispersed phase consists of aggregates of atoms or small molecules having diameter less than 10–7cm. For instance a gold sol consists of particles of various size containing several atoms. Likewise, a sulphur sol contains particles each having eight sulphur atoms (S8). In these colloids, the particles are held by weak vanderwaal’s forces.
            (b).       Macro Molecular Colloids:
                                    In these colloids, the particles of dispersed phase are sufficiently large in size enough to be of colloidal dimensions. These are called Natural Polymers. Some typical examples are starch, cellulose and proteins. Some artificial or synthetic polymers are polythene, polystyrene, nylon and plastics.
            (c).       Associated Colloids
                                    These colloids behave as normal electrolytes at low concentrations but behave as colloids at higher concentration. These associated colloids are also referred as Micelles. Sodium Stearate behaves as electrolyte in dilute solution but as colloid at higher concentration. Some other examples of associated colloids are; Soaps, higher alkyl sulphonates, polythene oxide etc.
(V)      CLASSIFICATION BASED ON THE INTERACTION OR AFFINITY OF TWO PHASES
                        The colloidal solutions can also be classified in terms of the attraction between the dispersed phase and dispersion medium as:
            (a).       Lyophilic Colloids
                                    Colloidal solutions in which the dispersed phase has a great affinity or love for the dispersion medium are termed as Lyophilic Colloids. Such substances have a tendency to pass directly into colloidal solution when brought in contact with dispersion medium. The lyophilic colloids are also called Intrinsic Colloids.
                                    If the dispersion medium is water, they are called Hydrophilic or Emulsoids. The lyophilic colloids are generally self-stablised, reversible in nature and are heavily hydrated. The typical examples of lyophilic colloids are starch, geletin, rubber, protein etc.
            (b).       Lyophobic Colloids
                                    Colloidal solutions in which the dispersed phase has no affinity or hates the dispersion medium are called Lyophobic Colloids. These are also referred as Extrinsic Colloids. Such substances have no tendency to pass into colloidal solution when brought in contact with dispersion medium. The lyophobic colloids are relatively unstable and readily tend to change into a suspension. They are irreversible by nature and are stablised by adding small amount of electrolyte. They are poorly hydrated.
                                    If the dispersion medium is water, the lyophobic colloids are called Hydrophobic or Suspensoids. The important typical instances of lyophobic colloids are sols of metals like Au, Ag, sols of metal hydroxides like Al(OH)3, Fe(OH)3 etc and sols of metal sulphides like AS2S3.

Colloids

Selmi (1843) was first to study colloids systematically. He prepared colloidal solutions of sulphur, prussian blue, casein etc. According to him, these colloids were not true solutions but suspensions of extremely small particles in water. Later on Thomas Graham in 1861 studied the process of diffusion of dissolved substances through a Parchment membrane. He observed that some substances like glue, geletin starch etc in solution form diffused very slowly and rates of diffusion were found to be very small in comparison to those other substances like sugar, salt and urea in water. 
In view of his observation, Thomas Graham divided substances into two distinct classes as under:-
            (i).        Crystalloids   
            (ii).       Colloids
(I)        Crystalloids:- Substances which diffuse readily in solution and can easily pass through a parchment membrane or animal membrane are known as Crystalloids. For instance, inorganic acids, bases, salts and organic compounds like sugar, urea etc.
(II)       Colloids:- Substances which diffuse very slowly in solution and cannot pass through parchment paper or animal membrane are called Colloids. For instance, starch, glue, geletin, albumin etc.
In recent years, Graham’s classification of substances into Crystalloids and Colloids has undergone major change because it has been observed that every substance irrespective of its nature can be a colloidal under suitable conditions. For instance, NaCl a typical crystalloid in water behaves like a colloid in benzene. Soap, a typical colloid in water, behaves crystalloid in alcohol. Similarly insoluble metal like copper, silver and gold can be converted into colloidal form by suitable methods.
COLLOIDAL STATE OF MATTER
                        A colloidal is not a substance but it is a particular state of substance which depends on the particle size. Any substance can be brought into colloidal state by suitable methods. There are three distinct types of solutions of substances based on the size of particle. These are:–
1.         True Solution :-
                        It is a homogeneous solution which contains small solute particles like molecules or ions dispersed throughout the solvent. The solute particle in a true solution are invisible even under microscope and these particles pass through a filter paper or an animal membrane. The particle size is 0.1 nm – 1 nm.
2.         Suspension :-
                        It is a heterogeneous mixture which contains large insoluble particles. The particles of a suspension can be seen by naked eye and these particles can neither be passed through filter paper nor through animal membrane. The size of particles is greater than 100 nm.
3.         Colloidal solution :-
                        It is a heterogeneous solution which contains solute particles of intermediate size. The particles of colloidal solution can pass through ordinary filter paper but not through animal membrane. These particles cannot be seen with a naked eye but can be seen under ultramicroscope. The particle size is 1nm – 100 nm.
DISTINGUISHING CHARACTERISTICS
OF
TRUE SOLUTION, COLLOID AND SUSPENSION

S.No.
PROPERTY
TRUE SOLUTION
COLLOIDAL SOLUTION
SUSPENSION
1.
Nature
Homogenous
Heterogeneous
Heterogeneous
2.
Size of Particle
0.1 nm – 1nm.
1nm – 100nm.
> 100 nm
3.
Filterability
Passes through ordinary filter paper as well as animal membrane.
Passes through filter paper but not through animal membrane.
Does not passes through filter paper or animal membrane.
4.
Separation
It can not be separated by ordinary filter paper nor by ultra filtration.
It can not be separated by ordinary filter paper but can be separated by ultra filtration.
It can be separated by both.
5.
Appearance
Transparent
Generally transparent
Opaque.
6.
Setting
Does not settle.
Settles in centrifuge.
Settles under the action of gravity.
7.
Ionisation
May be ionised.
Not ionised.
Not ionised.
8.
Diffusion
Diffuses readily
Diffuses slowly
Does not diffuse.
9.
Tyndal Effect
Does not show Tyndal effect.
Shows Tyndal effect.
Does not show Tyndal effect.
10.
Brownian Movement
Does not show Brownian movement.
Shows Brownian movement.
May show.
11.
Electrical Charge
Uncharged
Charged either positively or negatively
Uncharged.

PHASES OF COLLOIDAL SOLUTION
                        A colloidal solution is heterogeneous in nature. It consists of two phases, dispersed phase and dispersion medium.
1.         DISPERSED PHASE
                          In a colloidal solution, the component present in smaller proportion (solute) is known as Dispersed Phase. Example: In colloidal solution of silver in water, the component silver acts as dispersed phase.
2.         DISPERSION MEDIUM
                        In a colloidal solution, the component present in larger proportion (solvent) is called Dispersion Medium. Example: In colloidal solution gold in water, the component water acts as a dispersion medium.