COLLOID -> Introduction : A colloid or colloidal dispersion is a type of homogenous mixture. A colloid consists of two separate phases : Dispersed phase & Continuous phase. The Dispersed phase is made of tiny particles or droplets that are distributed evenly throughout the continuous phase. Homogeneous mixtures with a Dispersed phase in the size range 1 nm to 100 nm may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions or hydrosols. If a colloidal has a solid phase dispersed in a liquid, the solid particles will not pass through a membrane, whereas the dissolved ions or molecules of a solution will pass through. Some colloids are translucent due to Tyndall effect (scattering of light by particles in colloid). Some of the examples of colloids include butter, milk, aerosols (fog, smoke), paints, glues etc. This field of study was introduced in 1861 by Scottish scientist “Thomas Graham”.
Colloidal Dispersions >> Solute doesn’t completely dissolve in solvent but not visible to the naked eye. >> Will Scatter light – gives an opaque , turbid appearance >> Exhibits characteristics of a molecular solution (Brownian motion, capacity for diffusion) >> Size 1
nm
- 0.5
nm
Classification of Colloids -> Colloids can be classified on the basis of interaction between Dispersed phase (DP) & Dispersion medium (DM) : a) Lyophillic colloids / Hydrophilic colloids : (i) Strong affinity is present between particles of DP & DM. (ii) It’s reversible because DP & DM can be separated. (iii) It’s stable & don’t get easily precipitated. (iv)When DM is water , it’s called Hydrophilic.
b) Lyophobic colloids / Hydrophobic colloids : (i) No or very low affinity is present between particles of DP & DM. (ii) It’s irreversible because DP & DM can’t be separated.
(iii) It’s very less stable & has tendency to get precipitated out easily. (iv)When DM is water , it’s called Hydrophobic.
-> Colloids can be classified on the basis of size of colloidal particle : Multimolecular colloids - Colloidal particle is aggregate of small atom or molecule in which particles are held by van der Waal’s force.
Macromolecular colloids - Colloidal particles is big molecule or atom like colloidal solution of polymers. This type of colloidal system is very stable. No need of aggregation or subtraction.
Hydrocolloids A Hydrocolloid is defined as a colloidal system wherein the colloid particles are dispersed in water. Hydrocolloids can be either irreversible or reversible. For example, agar, can exist in a gel & sol state , & alternate between states with the addition or elimination of heat. Many Hydrocolloids are derived from natural sources like pectin is extracted from citrus peel & apple pomade. Hydrocolloids are employed in food mainly to influence texture or viscosity (e.g. – sauce). Hydrocolloids are also used in skin-care & wound – dressing.
Dispersed Systems Dispersed systems may have different degrees of dispersion. Suspensions & emulsions are classed as coarsely dispersed systems, as particles of their dispersed phases are comparatively large. Ordinary Solutions are systems having high degrees of dispersion, as the distributed substance is broken down into molecules or
ions. An intermediate position is occupied by dispersed systems, in which size of dispersed particles is larger than in ordinary solutions. Such systems are called colloidal solutions or sols. Suspensions & emulsions contain particles visible under an ordinary microscope. Their size exceeds 100 millimicrons ( 0.1 micron ). The heterogeneity of such a system can be detected by naked eye.
Stabilization of Colloidal Dispersion : Stabilization serves to prevent colloids from aggregating. Steric stabilization & electrostatic stabilization are the two main mechanisms for colloid stabilization. In a stable Colloid, mass of a dispersed phase is so low that its buoyancy or Kinetic energy is too little to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particle can be observed by applying an electric field : all particles migrate towards same electrode possessing same sign charge.
Destabilization of Colloidal Dispersion : Unstable colloidal dispersions form flocs as the particles aggregate due to interparticle attractions. This can be accomplished by different methods : Removal of electrostatic barrier that prevents aggregation of particles.
Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions.
Addition of nonadsorbed polymers called depletants. Physical deformation of the particle (e.g. stretching) may increase van der Waals forces more than stabilizing forces (such as electrostatic), resulting in coagulation of colloids at certain orientations.
Properties of Colloidal Solutions Tyndall Effect
:
Visible scattering of light along the path of a beam of light as it passes through a system containing discontinuities. The Luminous path of beam of light is called Tyndall cone. It shows heterogeneous nature of solution. This effect isn’t shown by true solution due to small particle size. Colloidal solution & true solution are differentiated by this effect.
Brownian Movement
:
Zig zag motion of colloidal particles in solution is called Brownian movement. Colloidal particle is bombarded by particle of dispersion medium & hence rather than following straight line path follows zig zag path. Brownian movement provides stability to colloids as it doesn’t allow the particle to settle down under the influence of gravitational pull.
Colligative property
:
Colloidal particles happen to be aggregate. The number of particles in solution is very small & so all colligative properties are low & under the conditions only osmotic pressure is suitable for measurement of molar mass.
Applications Of Colloids (i) Cleansing action of soaps & detergents
:
A micelle consists of hydrophobic hydrocarbon – like central core. The cleansing action of soap is due to the fact that soap molecules form micelle around the oil droplet in such a way that hydrophobic part of the stearate ions is in the oil droplet & hyrophillic part projects out of the grease droplet. Since the polar groups can interact with water, the oil droplet surrounded by stearate ions is now pulled in water & removed from the dirty surface. Thus soap helps in emulsification & washing away of oils & fats. The negatively charged sheath around the globules prevents them from coming together & forming aggregates.
(ii) Purification of water
:
The water obtained from natural sources often contains suspended impurities. Alum is added to such water to coagulate the suspended impurities & make water fit for dinking purposes.
(iii) Medicines
:
Most of the medicines are colloidal in nature. For example – argyrols is a silver sol used as an eye lotion, colloidal
antimony is used in curing kaalazar etc. Colloidal medicines are more effective because they have large surface area & are therefore easily assimilated.
(iv) Photographic plates & films
:
Photographic plates & films are prepared by coating an emulsion of the light sensitive bromide in gelatin over glass plates or celluloid films.
(v) Tanning
:
Animal hides are colloidal in nature. When a hide, which has positively charged particles, is soaked in tannin, which contains negatively charged colloidal particles , mutual coagulation takes place. This results in hardening of leather. This process in termed as tanning.
(vi) Rubber Industry
:
Latex is colloidal solution of rubber particles which are negatively charged. Rubber is obtained by coagulation of latex.
(vi) Industrial purposes
:
Paints, inks, synthetic plastics, rubber, graphite lubricants, cement etc., are all colloidal solutions.
Bibliography The information of the project “Colloidal Solutions” has been collected from the following sources : www.Wikipedia.org 2.NCERT Chemistry Part I 3. www.encarta.com 1.
Index Introduction 2. Colloidal Dispersions 3. Classification of Colloids 4. Hydrocolloids 5. Dispersed Systems 6. Stabilization of Colloidal Dispersion 7. Destabilization of Colloidal Dispersion 8. Properties of Colloidal Solutions 9. Application of Colloids 10. Bibliography 1.
