Colloids

Crystalloids:

  • Many crystalline substances like common salt, sugar, urea when dissolved in water can pass through parchment membrane are termed as crystalloids and their solution with water is called as a true solution.
  • In true solution particle size of solute is very small. The diameter of particles is about 1 × 10-9 m.
  • They cannot be seen by any instrument.

Colloids:

  • The substances like gum, glue which when dissolved in water do not pass through a parchment membrane.  A solution of colloidal substance with water is known as a colloidal solution.
  • In colloidal solution particle size varies from 5 × 10-9 m to 2 × 10-7 m. in diameter.
  • Colloidal particles can be seen clearly under high-resolution microscope or ultramicroscope.


Types of Solutions on the Basis of Size of Particles:

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True Solution:

  • Crystalline substances like sugar, sodium chloride are called crystalloids and are soluble in water.
  • The aqueous solution of crystalloids is called true solution.

Characteristics of True Solutions:

  • It is a homogeneous solution as the particles are not seen even under ultramicroscope.
  • The solution is clear and transparent.
  • Solution is a single phase, consisting of solute (sugar, sodium chloride) and solvent (water)
  • Size of solute particles is 1 × 10-9 m. Therefore they can pass through both the parchment membrane as well as through filter paper.
  • The particles can be neutral molecules as in the case of sugar or contain cations (Na+) and anions (Cl) as in case of NaCl.
  • The true solution does not exhibit Tyndall effect. When a powerful beam of light is passed through a true solution kept in a dark, the path of the beam of light through the true solution is invisible.
  • A true solution does not exhibit Brownian movement. The zig-zag motion of particles like pollen grains in a random direction in straight lines is called Brownian movement.
  • Particles of solute never settle down under gravity.

Colloidal Solution or Colloidal Dispersion:

  • The word colloids is derived from Greek word Kolla (glue) and oid (like). Thus colloid means glue-like.
  • When a thin paste of amorphous substances like glue, gum or starch etc. is added to boiling water, taken in a beaker, with continuous stirring a solution obtained is called as a colloidal solution or colloidal dispersion.

Characteristics of Colloidal Solutions:

  • It is heterogeneous solution consisting of two immiscible phases. They are visible under ultramicroscope.
  • The solution is turbid.
  • There are two phases present in the solution. One phase is present in the form of small particles, dispersed in the medium is called as the dispersed phase or discontinuous phase. The other phase in which particles are dispersed is called as dispersion medium or continuous phase.
  • The particle size of the dispersed phase (diameter) is in the range of 5 × 10-9 m to 2 × 10-7 m. Hence they can pass through filter paper but not through parchment membrane.
  • All colloidal particles in solution carry the same charge either positive or negative. Due to same nature of charge on all colloidal particles in the solution, the particles repel each other and thus impart stability to the solution.
  • Colloidal solutions exhibit Tyndall effect
  • Colloidal solutions exhibit Brownian movement
  • Particles do not settle under gravity.

Suspension:

When sand is stirred into water, the solution obtained is called a suspension.

Characteristics of Suspension:

  • It is heterogeneous solution consisting of two immiscible phases. They are visible to the naked eye.
  • The solution is turbid.
  • There are two phases present in the solution. One phase is present in the form of small particles. Dispersed in the medium is called as the dispersed phase or discontinuous phase. The other phase in which particles are dispersed is called as dispersion medium or continuous phase.
  • The dispersed particles are aggregates of millions of molecules.  The diameter of particles are aggregates of millions of molecules.  The diameter of particles is greater than 10 -7 m. Hence they can not pass through filter paper and not through parchment membrane.
  • Particles do not carry any charge.
  • Suspensions do not exhibit Tyndall effect
  • Suspensions do not exhibit Brownian movement
  • Particles settle under gravity.


Note:

  • The colloidal state is intermediate between crystalloids and suspensions. The colloidal solution is a solution of colloidal substance.  The colloidal solution is consists of two phases.

Terminology of Colloidal Solutions:

Colloidal State:

  • The colloidal state is a heterogeneous dispersion of two immiscible phases which possess certain distinguishing characteristics.

Colloidal Solution:

  • The colloidal solution is a heterogeneous system consisting of two phases.

Dispersed Phase:

  • The colloidal substance which is dispersed in a solvent is called as dispersed phase or inner phase.
  • e.g. In smoke, carbon particles are dispersed phase.

Dispersion Medium :

  • The medium in which the colloidal substance is dispersed is known as dispersion medium or outer phase.
  • e.g. In starch solution, water is dispersion medium. another e.g. for a copper sol, copper particles constitute dispersed phase and water the dispersion medium.

Types of Colloidal Systems:

  • A colloidal system is made up of a dispersed phase and a dispersion medium. Because either the dispersed phase or the dispersion medium can be a gas, liquid or solid. There are eight types of the colloidal system possible,  Since gases are miscible, the gas colloidal system is not possible. Solid
Sr. No. Dispersed Phase Dispersion Medium Name of Solution Examples
1 Solid Gas Solid aerosol Smoke, fumes, dust
2 Solid Liquid Sol Blood, soap solution, gum, water paints, metal sols of Cu, Ag, Au in water,
3 Solid Solid Solid sol Coloured glass, gems, metal alloys, steel, ruby, glass
4 Liquid Gas Liquid aerosols Foog, mist, cloud, rain, insecticide sprays
5 Liquid Liquid Emulsion Milk, cod liver oil, oil paints, creams, buttermilk, medicine, egg albumin, water in oil, oil in water
6. Liquid Solid Gel  Jams, Jellies,  Badami halwa, curd, butter, cheese, boot polish, silica gel, gelatin, agar agar
7. Gas Liquid Foam Whipped cream of milk, soap lather. Foam, fire extinguisher
8 Gas Solid Solid foam Pumice stone, sponge, cake, bread, rubber foam, biscuits, volcanic ash.

Sols and Their Types:

  • A colloidal solution in which dispersed phase is in solid state and dispersion medium is liquid is called as a sol.
  • e.g. Gum solution, starch in water, Au., Ag, etc. in water, blood etc.

Classification on The Basis of  Interaction Between the Dispersed Phase and the Dispersion Medium:

Lyophilic Sols or Reversible Sols:

  • The sols in which there is a strong affinity between the dispersed phase and dispersion medium are called as lyophilic sols.
  • e.g. glue, gelatin, starch, proteins.


Characteristics of Lyophilic Sols:

  • Lyophilic sols are readily formed by mixing together the substance forming disperse phase and solvent forming dispersion medium and heating the mixture if necessary.
  • There is a strong affinity between the dispersed phase and dispersion medium.
  • The colloidal particles forming lyophilic sols are large single molecules or polymers like starch, proteins etc. of high molecular weight.
  • If lyophilic sol is heated or dried we get solid but we get same sol if liquid (solvent or dispersion medium) is added to the solid. Thus lyophilic sols are reversible. After coagulation, they can again be converted into colloidal form.
  • Lyophilic sols have lower surface tension than dispersion medium.
  • Lyophilic sols have a higher viscosity than dispersion medium.
  • Stability of Lyophilic sols is due to solvation or solvation and charge on colloidal particles.
  • Lyophilic sols are stable and require a large quantity of electrolyte for coagulation.
  • The particles cannot be detected easily under ultramicroscope.
  • Lyophilic sols show weak Tyndall effect.

Stability of Lyophilic Sols:

  • In lyophilic sol, a thin film of the dispersion medium is formed around the dispersed phase colloidal particles due to strong affinity between the dispersed phase and dispersion medium. The formation of this film around dispersed phase colloidal particles is called solvation. The stability of lyophilic sol is due to solvation.
  • Similarly, all the particles carry an electrical charge of the same nature, which results in mutual repulsion between the dispersed phase colloidal particles which also adds to the stability of lyophilic sol.
  • Thus the stability of Lyophilic sols is due to solvation or solvation and charge on colloidal particles.

Lyophobic Sols or Irreversible Sols:

  • The sols in which there is no affinity between the dispersed phase and dispersion medium are called as lyophobic sols.
  • e.g. sols of metals like Ag, Au, non-metals like sulphur, hydroxides like Al(OH)3, Fe(OH)3, sulphides like As2S3.

Characteristics of Lyophobic Sols:

  • Lyophobic sols cannot be readily formed by mixing together the substance forming disperse phase and solvent forming dispersion medium.  Special methods like dispersion method or condensation method should be employed for making lyophobic sols.
  • There is no or very little affinity between the dispersed phase and dispersion medium.
  • The colloidal particles forming lyophobic sols are aggregates of a large number of atoms or molecules.
  • If lyophilic sol is evaporated we get solid but we can not get same sol if liquid (solvent or dispersion medium) is added to the solid. Thus lyophobic sols are irreversible. After coagulation, they cannot be converted into colloidal form again.
  • Lyophobic sols have the same surface tension as the dispersion medium.
  • Lyophobic sols have the same viscosity as the dispersion medium.
  • Stability of lyophobic sol is due to charge on colloidal particles.
  • Lyophobic sols are unstable and require a very small quantity of electrolyte for coagulation.
  • The particles can be detected easily under ultramicroscope.
  • Lyophobic sols show strong Tyndall effect.

Stability of Lyophobic Sols:

  • In lyophobic sols, all colloidal particles of the dispersed phase are either positively charged or negatively charged.
  • Colloidal particles remain suspended in dispersion medium, without coagulation due to the repulsion between the particle having same nature of charge,
  • Thus the stability of lyophobic sol is due to charge on colloidal particles.

Classification of Colloidal Solutions on the Basis of the Number of Molecules or Atoms in the Colloidal Particle:

Multimolecular Colloids:

  • Multimolecular colloids are those systems in which the dispersed phase particles are aggregates of many atoms or molecules.
  • e.g. gold sol particles are an aggregation of many gold atoms. other examples are silver sol and sulphur sol.

Macromolecular Colloids:

  • Macromolecular colloids are those systems in which the dispersed phase particles are a single macromolecule. They are lyophilic in character.
  • e.g. sol of starch in water, Aqueous (Water) solution of proteins.

Preparation of Colloidal Solutions (Dispersion methods):

  • In dispersion method particle of larger size are broken down to the colloidal size in the dispersion medium. Starting with the material in massive form, a colloidal solution is prepared by using suitable devices to disintegrate it into particles of colloidal size. Normally this is carried out by physical methods.


Mechanical dispersion method :

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  • The substance which is to be dispersed is finely ground. It is then mixed with the dispersion medium, protective materials or stabiliser is also added when a coarse suspension is obtained.
  • This suspension is then passed through a colloidal mill.  A colloidal mill consists of two heavy metal discs placed one above the other separated by a very small gap from each other.  They are rotated in the opposite directions at a  very high speed of about  7000 r.p.m.
  • The sol results due to the large shearing effect. Protective material used prevents particles from coagulation.
  • Using this method sols of indigo, sulphur, toothpaste, printer ink, paints, ointments etc. are prepared.

Electrical Dispersion or Bredig’s Arc Method:

Colloidal Solution

  • This method used to prepared metal sols like platinum, silver, gold, copper in water.
  • A dispersion medium (conductivity water) and a trace of sodium hydroxide (stabilising agent) isareaken in porcelain or glass (non conducting) vessel. The vessel containing dispersion medium  is surrounded by a freezing mixture.
  • Metal to be dispersed is dipped in the vessel in the form of electrodes. Electrodes are connected to the high voltage source. The ends of electrodes in the dispersion medium are very near to each other.
  • A very high voltage is applied and then an electrical arc is struck between the tips of electrodes. This creates large heat due to which metal rods melt, evaporate and suddenly cooled due to freezing mixture gives rise to the colloidal solution of the metal.
  • Functions of frezing mixtures are
    • Freezing mixture helps in condensation of metal vapours forming colloid
    • It prevents vapourisation of water.
    • It prevents coagulation of colloids, by keeping sol cold.

Peptization or Chemical Dispersion:

  • Redispersion of freshly prepared precipitate into the sol by adding an electrolyte containing common ion is called as peptization. An electrolyte used for peptization is called as peptizing agent. Peptization is reverse process of coagulation.
  • The peptization action is due to the preferential adsorption of one of the ions of the electolyte on the precipitate.
  • Example – 1: Freshly prepared Fe(OH)3 precipitate when treated with dilute solution of FeCl3, reddish brown ferric hydroxide sol is formed (Fe3+ being common ion)

Fe (OH)3 +   FeCl3 →  [Fe(OH)3] Fe3+

  • Example – 2: Silver chloride precipitate when treated with dilute HCl, AgCl sol is formed.

Preparation of Colloidal Solutions (Condensation methods):

  • These methods involve chemical reactions. In these methods factors like temperature, pressure, concentrations.  etc. are properly maintained.  The unwanted ions present in the sol are removed by dialysis , as these ions may eventually coagulate the sol.

Oxidation Method:

Preparation of Colloidal Sulphur:

  • When H 2S in water (aqueous solution) is exposed to air, it slowly gets oxidised to sulphur. The sulphur so formed remains in water in the colloidal state and the solution so formed remains in water in the colloidal state and the solution has a slightly milkish appearance.

H2S   +     O2  →   H2O  +    2S (colloidal)

  • A sol of sulphur can also be prepared when H2S gas is bubbled through an aqueous solution  of SO2.

H2S  +     SO2 →  2 H2O   +  3S (colloidal)



Reduction Method:

Preparation of Gold Sol:

  • A number of metals like silver, gold, platinum, mercury can be obtained in the colloidal state by the reduction of their salt solutions (dilute) using suitable reducing agents like hydrogen sulphide, formaldehyde, stannous chloride, tannic acid etc.
  • Gold sol can be obtained when AuCl3(dil) solution is treated with stannous chloride.

2 AuCl3 +  3 SnCl2 → 3 SnCl4 +  2 Au (colloidal)

  • Similarly, silver, platinum mercury sols are prepared.

AgNO3  +  Tannic acid  →       Ag sol

Applications of Colloids:

Foods:

  • Majority of our food material are colloidal in nature.
  • Emulsion -Milk, Cod Liver oil, Buttermilk
  • Gels – Jelly, Curd, Butter, Cheese,
  • Juicy fruits like mango, apple,
  • Badami halva.
  • Solid foam – Biscuits, Cake, Bread, Dryfruits
  • Foam – Whipped cream

Medicines:

  • A number of medicines are colloidal. Colloidal medicines are easily assimilated by body tissues.
  • Argyrol and protargol are protected colloidal solutions of silver is used against granulation
  • Colloidal gold, calcium injections are used to raise the vitality of the human physiological system against diseases like T.B. and rickets.
  • Colloidal sulphur is used as germs killer in plants
  • Colloidal antimony is used in curing kala-azar
  • Milk of magnesia is used in the treatment of stomach acidity.

Industrial Goods:

  • Many industrial goods in our daily life are colloidal. e.g. soaps, toothpastes, gum , shoe polish, enamels, resins, leather, paints, varnishes, cosmetics etc.

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