Surface Chemistry

Colloids

Colloids, in chemistry, are a mixture of two substances, in which one substance is divided into minute particles aka colloidal particles (ranging from 1 to 1000 nm in diameter) and dispersed or suspended over another substance. These insoluble particles are inseparable either by filtering or centrifuging. Types of Colloids are Sol, Emulsion, Foam, and Aerosol. Let us learn what exactly colloids, there applications, and method of preparation.

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Colloids

A colloid is a mixture in which one substance of microscopically dispersed insoluble particles are suspended throughout another substance. Sometimes the dispersed substance alone is called the colloid. The colloid consists of a dispersed phase and a continuous phase.

Unlike solutions, colloids do not constitute a solute dissolved in the solvent phase. Rather, the solute phase is dispersed in the solvent phase. The dispersed-phase particles have a diameter between approximately 1 and 1000 nanometers. Such particles are normally easily visible in an optical microscope, although at the smaller size range (r < 250 nm), an ultramicroscope or an electron microscope may be required.

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

Learn more about  Classifications of Colloids here.

Types of Colloids

  • Sol – It is a suspension of minute solid particles in a liquid.
  • Emulsion – It is a colloid between two or more liquid with one consisting a dispersion of another liquid.
  • Foam – It consists of gas dispersed in solid or liquid.
  • Aerosol – It consists of a minute liquid or solid particles in a gas.

Methods of Preparation

There are two principal ways of preparation of colloids:

  • Dispersion of large particles or droplets to the colloidal dimensions by milling, spraying, or application of shear (e.g., shaking, mixing, or high shear mixing).
  • Condensation of small dissolved molecules into larger colloidal particles by precipitation, condensation, or redox reactions. Such processes are used in the preparation of colloidal silica or gold.

Stabilization of Colloids

The colloid is said to be stable when particles remain suspended in the solution without settling down i.e. the dispersed phase. Stability is hindered by aggregation and sedimentation phenomena, which are driven by the colloid’s tendency to reduce surface energy. In order to stabilize the colloidal system, we need to reduce the interfacial tension between the colloidal particles.

colloidsSource: Wikipedia 

Aggregation is due to the sum of the interaction forces between particles. If attractive forces (such as van der Waals forces) prevail over the repulsive ones (such as the electrostatic ones) particles aggregate in clusters. Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.

  • Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that an electrical double layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, the mass of a dispersed phase is so low that its buoyancy or kinetic energy is too weak to overcome the electrostatic repulsion between charged layers of the dispersing phase.
  • Steric stabilization consists in covering the particles in polymers which prevents the particle to get close in the range of attractive forces.

Applications of Colloids

Colloids have varied applications. Some of them include:

  • Medicines: Medicines in colloidal form are easily absorbed by the body tissues and hence are more effective.
  • Cleansing action of soap: Soap solution is colloidal in nature. It removes the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth
  • Purification of water: The precipitation of colloidal impurities present in water can be done by adding certain electrolytes like alum etc. The negatively charged colloidal particles of impurities get neutralized by the Al3+ ions and settle down and pure water can be decanted off.
  • Rubber industry: Latex is a colloidal solution of negatively charged rubber particles. From latex, rubber can be obtained by coagulation. Rubber plated articles are prepared by depositing negatively charged rubber particles over the article to be rubber plated by making that article an anode in a rubber plating bath

Solved Questions For You

Que: Which of the following is not a use of colloids?

  1. Purification of water
  2. Removing the dirt particles either by adsorption or by emulsifying the greasy matter sticking to the cloth
  3. As Medicines
  4. In Textile Industry

Ans: The correct answer is option “D”. .All the other three are known applications of colloids. But it has no widespread application in the textile industry.

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