Classification of Colloids, and a Comparative Account of their General Properties.

CLASSIFICATION OF COLLOIDS

Colloids are classified on the basis of the physical states of the dispersed phase and the continuous phase or on the basis of charge on the dispersed phase (dispersed particles). The following are the types of colloidal dispersion.

1. Classification of Colloids on the Basis of the Dispersed and Dispersion Medium (Continuous Phase)

• · Sol (Solid in Liquid)

  • Dispersed phase: Small solid particles or large molecules,

  • Continuous phase: Liquid. Examples: Paints, ink, blood.

• · Gel (Liquid in Liquid)

  • Dispersed phase: liquid droplets,

  • Continuous phase: Liquid. Examples: Gelatin, butter, Ghee.

• · Emulsion (Liquid in Liquid)

  • Dispersed phase: Liquid Droplets,

  • Continuous phase: Liquid. Examples: Milk, Mayonnaise, Vinaigrette.

• · Foam: (Gas in Liquid)

  • Dispersed phase: Gas bubbles,

  • Continuous phase: Liquid. Examples: Whipped cream, shaving cream, Meringue.

• · Solid Foam: (Gas in Solid)

  • Dispersed phase: Gas bubbles,

  • Continuous phase: Solid. Examples: Sponge, Pumice Stone

• · Aerosol (Solid or Liquid in Gas)

  • Dispersed phase: Solid or Liquid Particles,

  • Continuous phase: Gas. Examples: Fog, Smoke, Air Pollution

2. Classification of Colloids on the Basis of the Interaction between Dispersed Phase and Dispersion Medium

• · Lyophilic Colloids (Solvent Loving): Dispersed Phase: Affinity with the dispersion Medium. Examples: Starch in water, Protein in water.

• · Lyophobic Colloids: Dispersed Phase: Little or no affinity with the dispersion medium. Examples: Metals in water and hydrophobic Sols.

• Association Colloids: Dispersed particles in micellar form.

3. Classification of Colloids on the Basis of the Charge on Dispersed Phase.

• · Positive Colloids (Cationic Colloids): Dispersed Phase: P0sitively charged dispersed particles. Examples: Ferric Hydroxide Sol.

• · Negative colloids (Anionic Colloids): Dispersed Phase: Negatively charged dispersed particles. Examples: Silica sol, gum Arabic sol.

• · Nonionic Colloids: Dispersed Phase: Electrically neutral particles. Examples: Starch paste, Polythene sols.

Lyophilic Colloids

The term lyophilic comes from the Greek words “Lyo” meaning “to dissolve” and “Philos” meaning “loving” or “affinity”. Lyophilic means solvent-loving. Colloidal particles (Dispersed phase) have a strong affinity with the dispersion medium in the lyophilic colloids. The dispersion medium forms a continuous layer around the colloidal particles. Dispersion of colloidal particles in lyophilic colloids is reversible. Colloidal particles can be easily reconstituted.

Colloidal particles in lyophilic colloids may be hydrophilic or lipophilic. Dispersion medium is selected on the basis of dispersed particles nature. Lyophilic colloids containing water as a dispersion medium and hydrophilic colloidal particles as a dispersed phase are also called hydrophilic colloids. Examples of hydrophilic colloids are acacia, gelatine, starch, and protein dispersion in water. Lyophilic colloids containing organic solvent as dispersion medium and lipophilic colloidal particles as dispersed phase are also called lyophilic colloids. Examples of lyophilic colloids are rubber and polystyrene dispersion in organic solvent.

Lyophilic colloids may be with or without charge. Acacia colloidal particles dispersion in water possesses a negative charge. Gelatin colloidal particles in water possess a positive charge at a pH below 4.5. Gelatin colloidal particles in water are neutral at the isoelectric point.

The following are characteristic features of lyophilic colloids

• 1. Stability: Lyophilic colloids are thermodynamically stable and exhibit a high degree of stability. They do not readily coagulate or settle out of the colloid. This stability is due to strong attractive forces between the dispersed phase (colloidal particles) and the surrounding dispersion medium.

• 2. Affinity: The stability of lyophilic colloids depends on the affinity of the dispersed phase with the dispersion medium in the colloidal system. The stronger the affinity more the stability.

• 3. Particle Size: Colloidal particle size ranging from 1 to 100 nanometers. These colloidal particles are not affected by the Gravitational force.

• 4. Preparation: It is easy to prepare lyophilic colloids. It does not require additional stabilizing agents. Colloidal particles may be polar or charged which promotes interaction with dispersion medium molecules.

Types of lyophilc colloids

• · Protein colloids: Proteins like gelatine and albumin form protein colloids. These proteins are hydrophilic and interact favorably with water molecules.

• · Polymer colloids: Hydrophilic Synthetic polymers like polyvinyl alcohol form lyophilic colloids.

• · Mecellar colloids: Surfactant molecules self-assembling from micelles. Hydrophilic heads (hydrophilic heads) interact with water molecules in the dispersion phase while hydrophobic tells cluster in the core of the micelles.

Application: Lyophilic colloids are used in pharmaceuticals, food, and cosmetics. They are used to stabilize emulsions, in the drug delivery systems, and as thickening or gelling agents in the food products.

Lyophobic colloids

The term lyophobic is derived from the two Greek words “lyo” meaning “to dissolve” and “Phobos” meaning “fear” or “repulsion”. In lyophobic colloids, dispersed colloidal particles have little or no affinity with the dispersion medium. If water is a dispersion medium and colloidal particles as dispersed phase is hydrophobic, it is also called hydrophobic colloid. Normally dispersed phase is an inorganic compound as colloidal particles and the dispersion medium is water. The following are the characteristic features of the lyophobic colloids.

• 1. Stability: Lyophobic colloids are less stable than lyophilic colloids. The continuous sheath of dispersion medium surrounding colloidal particles is absent in lyophobic colloids. Thus, Lyophobic colloids are thermodynamically unstable. Colloidal particles tend to aggregate or coalesce due to a lack of attractive force between colloidal particles and the dispersion medium.

• Lyophobic colloids are stable in the presence of charge on colloidal particles. Similar charges on colloidal particles keep them away from each other. They remain dispersed due to repulsive forces. They require the addition of stabilizing agents to prevent coagulation aggregation or precipitation.

• 2. Particle Size: The colloidal particle sizes of lyophobic colloids are larger than lyophilic colloids. They range from 100 nanometers to several micrometers.

• 3. Preparation: Lyophobic colloids are prepared by using finely divided solids in a liquid medium using stabilizing agents like surfactants or protective colloids.

• 4. Stabilizing agents: They enhance the stability of the lyophobic colloid. These agents are adsorbed at the surface of colloidal particles. They act as a barrier between colloidal particles and dispersed medium.

Types of Lyophobic Colloids:

• · Metal Sols: Colloidal suspension of metals like gold, silver, etc in water. They are stabilized by adsorbed ions or molecules on the metal surface.

• · Metal oxide Sols: Metal oxides are also used to prepare lyophobic colloids in water like silica or aluminia. These colloids require stabilizing agents to prevent particle aggregation.

• · Organic Colloids: Organic compounds like polymers are used to prepare lyophobic colloids in a solvent. It requires stabilizing agents to maintain colloidal stability.

Applications: Lyophobic colloids are used in paints, inks, and some pharmaceutical formulations. They are also used in water treatment processes.

Association Colloids

Amphiphiles have an affinity with both polar and non-polar liquids. An example is surface active agents. Surface active agents at low concentrations in water exist as monomers. Monomers are subcolloidal in size. Monomers aggregate upon an increase in their concentration in water. This aggregation forms micelles. One micelle contains approximately 50 monomers. Its size is about 50 Angstrom. Water containing micelles is colloid called association colloid or association dispersion. Surfactants form micelles under a crtain concentration range. Concentration range of surfactants to form micelles is called critical micelles concentration (CMC).

COMPARATIVE ACCOUNT (Distinguishing Features) OF COLLOIDS GENERAL PROPERTIES

1. Particles Size:

• · Sol: Submicron to colloid range.

• · Gel: Solid network with larger particles.

• · Emulsion: Liquid droplets in the colloidal range.

• · Foam: Gas bubbles in the colloidal range.

• · Solid Foam: Gas bubbles in a solid matrix.

• · Aerosol: Solid or liquid particles in the colloidal range

2. Stability:

• · Sol: Stable, may settle over time.

• · Gel: Stable due to the solid network

• · Emulsion: This may separate over time but can be stabilized.

• · Foam: Generally unstable and tends to collapse.

• · Solid Foam: Stable due to the solid matrix.

• · Aerosol: Stable, but sedimentation can occur.

3. Appearance:

• · Sol: Appearance homogeneous.

• · Gel: Semisolid

• · Emulsion: Homogeneous but may separate.

• · Foam: Frothy

• · Solid Foam: Solid with the gas-filled region.

• · Aerosol: Dispersed in a gas.

4. Preparation:

• · Sol: Dispersion or condensation.

• · Gel: Cooling or chemical reaction

• · Emulsion: Immiscible liquid dispersion

• · Foam: Mechanical Agitation

• · Solid Foam: Gas trapping in a solid matrix.

• · Aerosol: Dispersion or condensation

5. Examples

• · Sol: Paints, blood, ink.

• · Gel: Gelatin, cheese, butter.

• · Emulsion: Milk, mayonnaise.

• · Foam: Whipped cream, shaving cream

• · Solid Foam: Fog, smoke, cloud

• · Aerosol: Pumice stone, sponge

COMPARATIVE ACCOUNT (Distinguishing Features) OF COLLOIDS formed by Interaction between Dispersed Phase and Dispersion Medium

1. Dispersed phase:

• · Lyophilic Colloids: Organic molecules

• · Lyophobic Colloids: Inorganic particles.

• · Association Colloids: Surface active agents aggregates

2. Interaction

• · Lyophilic Colloids: Strong interaction between dispersed particles and dispersion medium,

• · Lyophobic Colloids: Little interaction between dispersion medium and dispersed particles

• · Association Colloids: Solvated aggregates

3. Charge

• · Lyophilic Colloids: Less or no charge on dispersed particles

• · Lyophobic Colloids: Highly charged dispersed particles.

• · Association Colloids: Charged micelles but solvated.

4. Preparation:

• · Lyophilic Colloids: Easy to form

• · Lyophobic Colloids: Require special methods

• · Association Colloids: Easy to form at CMC.

5. Viscosity

• · Lyophilic Colloids: Higher than dispersion medium alone.

• · Lyophobic Colloids: Same as dispersion medium

• · Association Colloids: Increase in viscosity but no appreciable increase.

6. Electrolytes

• · Lyophilic Colloids: At low concentration stable, at high concentration precipitation.

• · Lyophobic Colloids: Unstable

• · Association Colloids: Stable

7. Stability:

• · Lyophilic Colloids: Reversible

• · Lyophobic Colloids: Irreversible

• · Association Colloids: Reversible

Alok Bains