Colloids & Suspensions, Lecture notes of Chemistry

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COLLOIDS,

EMULSIONS AND

DETERGENTS

Types of solution

On the basis of the size of the dispersed particles, solutions may be of three types. (i) True solution (ii) Suspensions (iii) Colloidal solutions. (i) True solution It is a ‘homogeneous’ solution that contains small solute particles (molecules or ions) dispersed through the solvent. The diameter of particles ranges from 1-10Å and it is invisible even under an ultramicroscope. The particles can easily pass through animal or vegetable membrane. For example, solution of sodium chloride in water. (ii) Suspension It is a ‘heterogeneous’ mixture containing suspended insoluble particles. The diameter of these particles is of the order 1000Å or more. The particles in a suspension may be visible to naked eye or may be seen easily under a microscope. They cannot pass through animal or vegetable membrane or through an ordinary filter paper. For example, dirt particles in water.

Comparison

Continued…

Types of Colloids

Depending on the nature of the dispersed phase and that of the dispersion medium

Properties of Colloidal Solutions

The main characteristic properties of colloidal solution are as follows.

1. Heterogeneous nature Colloidal solutions are heterogeneous solutions. They consist of two distinct phases (i) dispersed phase and (ii) dispersion medium. Because of the small size of their particles, they appear homogeneous to the naked eye but their heterogeneity is confirmed by seeing under electron microscope and by experiments like dialysis, ultrafiltration and ultracentrifugation.
2. Filterability As the size of the pores of an ordinary filter paper is bigger than the size of colloidal particles, these particles can easily pass through the ordinary filter paper. However, animal membrane, vegetable membrane and ultrafilters do not permit the colloidal particles to pass through them.

5. Mechanical properties Continued… a) Brownian movement When the colloidal solutions are observed under an ultramicroscope, a zig-zag motion is seen in them. This is termed as the ‘Brownian movement’ after the name of the discoverer. Wiener (1863) stated that the cause of these movements is the constant collisions between the particles of the dispersed phase and the molecules of the dispersion medium. Weiner said that the impact of the dispersion medium particles is unequal. When a molecule of dispersion medium collides with a colloidal particle, it is displaced in one direction until its direction and speed of motion is altered by another collision. This process gives rise to a zig-zag motion. However, as the size of the dispersed particles increases the chances of unequal collision decreases. That is why a suspension does not show any such movement because of their large molecular size.

(b) Diffusion As the size of the colloidal particles is big, they diffuse slowly from higher concentration to low concentration region. (c) Sedimentation Colloidal solutions are quite stable and remain suspended indefinitely. However, colloidal particles of large size settle very slowly under gravity. This phenomenon called sedimentation is used to determine the molecular masses of these particles. Continued…

6. Optical properties: Tyndall effect If a true solution is placed in a dark room and a strong beam of light is passed through it, then the path of light is not visible. However, when a strong beam of light is passed through a sol placed in a dark room and viewed at right angles, the path of light becomes visible. The path of light appears as a hazy beam or a cone. This effect was studied by Tyndall in 1869 and is popularly known after his name as the Tyndall effect. The illuminated path or beam of light is termed as the Tyndall beam or the Tyndall cone.

Continued…

7. Electrical properties : Electrophoresis The colloidal particles have electric charge. They have been classified on the basis of their charge as positive and negative colloids. (i) Electrophoresis (or cataphoresis ) The movement of colloidal particles on the application of an electric field is called electrophoresis or cataphoresis. If the colloidal particles are positively charged, they will be discharged near the negative electrode. On the other hand, if the colloidal particles are negatively charged they will be discharged at the positive electrode.

Emulsion

These are liquid-liquid colloidal systems. In other words, an emulsion may be defined as a dispersion of finely divided liquid droplets in another liquid. Generally one of the two liquids is water and the other, which is immiscible with water, is designated as oil. Either liquid can constitute the dispersed phase. The dispersal of a liquid in the form of an emulsion is called emulsification.

Based On Size Of The Dispersed Particle

Macro emulsions

Size range from 0.2 to 50 micrometers in size and are easily visible under the microscope.

Micro emulsion

Particle Size range from 0.01 to 0.20 micrometers

Based on Nature of Dispersed Phase

These are of two types: (i) oil-in-water type and (ii) water-in-oil type. (i) Oil in water (O/W) type When the dispersed phase (small amount) is oil and the dispersion medium (excess) is water it is termed as oil-in-water type emulsion. These emulsions are also called aqueous emulsions. (ii) Water-in-oil (W/O) type When the dispersed phase is water and the dispersion medium is oil, then the emulsion is termed as water-in-oil type emulsion. These emulsions are also called oily emulsions. It may be noted that the two types of emulsions can be interconverted by simply changing the ratio of the dispersed phase and the dispersion medium.

The emulsions obtained simply by shaking the two liquids are unstable. The droplets of the dispersed

phase coalesce and form a separate layer. To have a stable emulsion, small amount of a third substance

called the Emulsifier or Emulsifying agent is added during the preparation. This is usually a soap,

synthetic detergent, or a hydrophilic colloid.

Emulsifier

Identification of emulsions

The following tests can be used to differentiate the two types of emulsions.

(i) Dilution test Add water to the emulsion. If the emulsion is diluted on addition of water it is an

oil-in-water type emulsion. In case it is not diluted, then it is a water-in-oil type emulsion where oil

is present in excess and is the dispersion medium.

(ii) Dye test Add an oil-soluble dye to the emulsion and shake the mixture. If the color spreads

throughout the whole emulsion then oil is the dispersion medium and if the color of the dye appears

as droplets then the emulsion is oil-in-water type and oil is the dispersed phase.