Colloids: Properties, Preparation & Applications | Physical Chemistry PDF Download

Colloids Meaning and Definition

In simple terms, we can define colloids as a mixture where one of the substances is split into very minute particles which are dispersed throughout a second substance. The minute particles are known as colloidal particles.
Alternatively, we can also say that colloids are basically solutions in which solute particle size ranges from 1nm – 1000 nm. Colloids are heterogeneous in nature.

Preparation of Colloid Solutions

Stable colloids are also known as lyophilic sols, in these strong forces of interaction exist between the dispersed phase and the dispersion medium. These are prepared by the following suitable methods.

Condensation Method
In this method, small solute particles are condensed to form a dispersed phase particle.
(i) Chemical methods:

• By oxidation:
  Colloidal sulphur can be obtained by passing oxygen gas through a solution of hydrogen sulphides. In this method any oxidising agent like HNO₃, H₃Br₂ can also be used.
  2H₂S + O₂ → 2H₂O + 2S (Sulphur sol)
• By double decomposition:
  A solution of arsenic sulphide is obtained in this method. In this process hydrogen sulphide is passed through Arsenious oxide cold solution in water.
  AS₂O₃ + 3H₂S → AS₂S₃ + 3H₂O Arsenic sulphide (sol)
• By reduction:
  A number of metals such as gold, silver, and platinum are obtained in a colloidal state by reacting the aqueous solution of these salts with suitable reducing agents such as formaldehyde, phenylhydrazine, hydrogen peroxide, stannous chloride etc.
  2AuCl₃ + 3SnCl₂ → 3SnCl₄ + 2Au (gold sol)
  2AuCl₃ + 3HCHO + 3H₂O → 2Au + 3HCOOH + 6HCl
  The gold sol prepared in the reduction of gold chloride solution has a purple colour and is called purple of Cassius.
• By hydrolysis:
  Many salt solutions are rapidly hydrolysed by boiling a dilute solution of their salts. For example, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chloride.
  FeCl₃ + 3H₂O → Fe(OH)₃ + 3HCl colloidal sol
  Silicic acid sol is obtained by the hydrolysis by sodium silicate.
• By double decomposition:
  A sol of arsenic sulphide is obtained in this method. In this process hydrogen sulphide is passed through Arsenious oxide cold solution in water.
  As₂O₃ + 3H₂S → As₂ S₃ + 3H₂O Arsenic sulphide (sol)
• By excessive cooling:
  A colloidal sol of ice is obtained in this process. Ice is taken in an organic solvent like chloroform ether. Sol of ice is obtained by freezing a solution of water in the solvent. The molecules of water are no longer in the solution separately combine to form particles of colloidal size.
• By exchange of solvent:
  In this process, colloidal sol of certain substances such as sulphur, phosphorus which are soluble in alcohol but insoluble in water can be prepared by pouring their alcoholic solution into water. For enough alcoholic solution of sulphur on pouring into water gives a milky colloidal solution of sulphur.
• By change of physical state:
  Sols of substance like mercury and sulphur are prepared by passing the vapour through cold water containing a suitable stabilizer such as ammonium salt or citrate.

Dispersion Methods
In these methods, large particles of a substance (suspension) are broken into smaller particles. The following methods are employed.

• Mechanical dispersion
  In this method, the substance is first grounded to coarse particles. It is then mixed with dispersion medium to get a suspension. The suspension is then grinded in a colloidal mill.
  It consists of two metallic dyes nearly touching each other and rotating in the opposite direction at a very high speed 7000 revolution per minute. The space between the dyes of the mill is so adjusted that coarse suspension to great shearing force giving rise to particles of colloidal size. Colloidal solution of black ink, paints varnishes dyes are obtained by this method.
• Bredig’s Arc Method or by Electrical Dispersion

  This method is used to prepare sols of platinum, silver copper or gold. The metal whose sol is to be prepared is made as two-electrode which immersed in a dispersion medium such as water etc.
  The dispersion medium is kept cool by ice. An electric arc is placed between the electrodes. The tremendous heat generated by and give colloidal solute. Electrolytes are used for this process for stabilization and cooling.

• Peptization
  The process of converting a freshly prepared precipitate into a colloidal solution is known as peptization. In this method as the electrolyte in smaller amounts is added which is known as peptization agent or peptizing agent. Cause of peptization is the adsorption of the ions of the electrolyte by the particles of the precipitate. Important peptizing agent are sugar gem gelatin and electrolyte.
  Examples
  (i) Freshly prepared ferric hydroxide can be converted into the colloidal state by shaking it with water contains Fe³⁺ or OH^– ions i.e. FeCl₃ or NH₄OH respectively.
  Fe(OH)₃ + FeCl₃ → (Fe(OH)₁ Fe)⁺³ + 3Cl^– precipitate electrolyte
  (ii) A stable sol of stannic oxide is obtained by adding a small amount of dilute HCl to stannic oxide precipitate similarly a colloidal solution of Al(OH)₃ and AgCl are obtained by treating the corresponding freshly prepared with a very dilute solution of HCl and AgNO₃ or KCl respectively.

Purification of Colloids

Colloids contain ionic impurities and other categories of impure substances that decrease the quality of colloids used in various applications. Following are the methods to purify the colloids.

• Dialysis
  Method of separation of ionic substances from the colloidal solution by means of effusion through a suitable membrane is dialysis. The principle is that sol particle cannot pay through parchment paper or semipermeable membrane due to the impurity slowly diffused out of the base leaving pure colloid.
  Precautions:
  The distilled water in the container where a bag is immersed should be changed frequently to prevent the accumulation of crystalloids otherwise there is a possible change of diffusing back of impurities into the bag.
  
• Electro Dialysis
  Dialysis is a slow process and takes so much time for the removal of impurities. The process is improvised by an applied electrical force. This is known as electrodialysis. In this method two electrical plates are inserted into the distilled water and are connected to the terminals of source, long moves to the opposite electric plate with greater speed and the sol is purified.
  
• Ultrafiltration
  Normal filter papers cannot be used to filter the impurities of colloid since due to the large size of pores, impurities along with sol particle will be filtered off. The pore size is reduced by impregnating the papers in collodion solution which is 4 – s. Calculate nitrate solution in alcohol – ether mixture and dried with acetaldehyde. This is known as ultrafiltration and such papers are known as ultrafilter papers.

Examples of Colloids

• Blood: A respiration pigment which has albumin protein in water. Pigment part contains albumin that acts as the dispersed phase and the dispersion medium is water. It is a hydrosol.
• Cloud: It contains air which is the dispersion medium and droplets of water as a dispersed phase. These are aerosol.
• Gold sol: It is a metallic sol in which gold particles are dispersed in the water.

Application of Colloids

Colloids are widely useful in industries, medical and domestic applications.
As food items: Syrup, Halwa, Soup belongs to a colloidal type of system.
Medicine: Colloidal silver in the name of Argyrols, it acts as antiseptic for eye infection.

In Purification of air by Cottrell precipitator:
This process involves coagulation of solution particle. Dust or smoke is passed through the inlet of an electrified chamber which has a central electrical plate which is provided with opposite charge of a dent a smoke particle when dust passes the particles are coagulated and pure air is passed through another outlet.

Tanning of leather:
Animal skins are very soft, when these are immersed in the solution of tannin which has the opposite charge of animal skin, particles are coagulated and the skin becomes hard this is known as tanning of leather.

Formation of delta:
It involves coagulation of clay particles of the river with an electrolyte of seawater.

Classification of Colloids

Colloids are classified into many types.
Classification Based on Physical State

• Solid Solution
  In this dispersed phase solid and the dispersion medium. Eg: gemstones.
• Aerosol
  These colloids consist of air as the dispersion medium.
  Example 1: Cloud. This contains air as dispersion medium and water drops as the dispersed phase.
  Example 2: Dust. This contains air is dispersion medium and dent particle as the dispersed phase.
  Example 3: Smoke. This contains carbon particles in the air.
• Gels
  These contain solid dispersion medium and liquid dispersed phase
  Example: Cheese, butter.
• Emulsion
  These are liquid-liquid solutions in which the dispersed phase is liquid and liquid dispersed medium. Emulsion mainly consists of two aspects.

Oil in water type:
Oil is dispersed phase and water is dispersion medium
Example: Milk
Water in oil type:
Water is the dispersed phase and oil/fat is dispersion medium.
Example: Vanishing cream
Sols and gels are reversible and interconvertible. This is known as thixotropy.

Classification Based on Dispersion Medium
On the basis of dispersion medium sol are classified as;

• Hydrosol:
  In these colloids water act as a dispersion medium.
  Example: Starch
• Alcosol:
  In this type, alcohol acts as a dispersion medium.
• Acrosol:
  These contain a dispersed phase particle in the air.
  Example: Smoke

Classification Based on Interaction Forces
On the basis of interaction forces between the dispersion medium and dispersed phase.
Lyophobic Sols: [Emulsoid]

• Surface Tension: It is same as that of the medium.
• Viscosity: It is the same as that of the medium.
• Irreversible: These are irreversible colloids when once dispersion medium is evaporated and again when the solvent is added no new sol is formed.
• Stability: There is weak interaction between the dispersed phase and dispersion medium hence lyophobic sols are unstable.
• Visibility: Particle can be detected using ultramicroscope.
• Migration: Particle migrates either colloid an anode depending upon the charge of sol particle.
• Action of electrolyte: When the electrolyte is added to sol, coagulation takes place.
• Hydration: No hydration of sol takes place.
• Examples: Metallic sols like Ag, gold etc.

Lyophilic Sols
These are stable strong sols. A strong interaction is present between the dispersed phase and the dispersion medium. Following are the characteristics of lyophilic sols.

• Surface tension: Lower than that of the medium.
• Viscosity: Higher than medium.
• Reversibility: These are reversible sols. When dispersion medium is evaporated and the solvent is added again the same type of sol is formed.
• Stability: More stable due to strong interaction between the dispersed phase and the dispersion medium.
• Visibility: The particleS are visible under an electron microscope.
• Addition of electrolyte: Small amount of electrolyte is required for the formation of sol.
• Hydration: Extensive hydration takes place.
• Example: gum, gelatin, starch.

Classification Based on Properties of Sol Particle
Classification of sols on the basis of properties are given as;
Multi Molecular Colloids

• When a dissolution occurs atoms or smaller molecules of substance [having a 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 a molecular size less than 1 atm. For example sols of gold atoms and sulphur (S_f) molecules. In these colloids, the particles are held together by physical forces called Van der Waals forces. Metallic sols are usually multimolecular sols prepared by Bredig’s are melted. These are usually lyophobic unstable, and separation is early possible.

Macromolecular Colloids

• These are substances having big size molecular called macromolecular which on demolition form size in the colloidal ran such substance are called macromolecular colloids. Thus macromolecule forming the dispersed phase are generally polymers having very high molecular masses.
• Naturally occurring macromolecular are starch, cellulose proteins, enzyme gelatin etc. Artificial macromolecular and synthetic polymers such as nylon, polyester, plastics, polishers etc. They have usually lyophobic sols.