05-10-2012, 11:48 AM
COLLOID
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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 phaseis 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, thesolid particles will not pass through a membrane, whereasthe 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”.
Hydrocolloids
A Hydrocolloid is defined as a colloidal system where in 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.
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.
[attachment=35367]
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 phaseis 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, thesolid particles will not pass through a membrane, whereasthe 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”.
Hydrocolloids
A Hydrocolloid is defined as a colloidal system where in 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.
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.