Colloids: Properties, Types, and Applications in Industry
Colloids: Systems with Dispersed Particles (1-100 nm)
Colloids represent an intermediate state between solutions and suspensions. They consist of a dispersed phase (like micelles) within a dispersant medium. Key characteristics include particle size, coating type, interface properties, settling behavior, and separation methods.
Comparison of Dispersions
- Solutions (<1 nm): Homogeneous, no settling, not separable by filtration.
- Colloids (1-100 nm): Heterogeneous, high surface area, no settling, not separable by filtration.
- Suspensions (>100 nm): Heterogeneous, larger particles, settle over time, separable by filtration.
Types of Colloids
Dispersed Phase / Dispersant Medium:
- Sol: Solid/Liquid (e.g., paint), Solid/Solid (e.g., porcelain)
- Aerosol: Solid/Gas (e.g., smoke), Liquid/Gas (e.g., fog)
- Emulsion: Liquid/Liquid (e.g., mayonnaise), Liquid/Solid (e.g., cheese)
- Foam: Gas/Liquid (e.g., shaving foam), Gas/Solid (e.g., pumice)
- Gel: Solidified sol (liquid)
Advantages of Emulsions
- High penetration and reactivity due to large surface area.
- Easy application, even on wet surfaces.
- Applications: Paints, lacquers, varnishes, coatings, asphalt emulsions (roads, roofing, fabric impregnation).
Hydrophilic Colloids
Organic molecules with long hydrocarbon chains (hydrophobic inward) and polar groups (hydrophilic outward, e.g., -COO-, -OH). Soaps form micelles with an interior chain. Gels form by swelling of colloidal particles, trapping liquid. Types include:
- Rigid Gels (e.g., silica gel): Irreversible, high mechanical resistance.
- Elastic Gels (e.g., rubber, gelatin): Reversible, weak Van der Waals forces.
- Thixotropic Gels: Become fluid (sol) when shaken, return to gel state at rest.
Hydrophobic Colloids
Attractive forces between like molecules are stronger than between different molecules, requiring a stabilizer for dispersion. Stabilization methods include:
- Protective Colloids: Hydrophilic colloids form a layer around hydrophobic particles (e.g., soap’s cleansing action).
- Ion Adsorption
Properties of Colloids
- High surface area.
- Two-phase systems that do not separate by filtration.
- Brownian motion (chaotic particle movement).
- Density variation with height.
- Electrically charged particles.
- Dialysis: Membrane separation of dispersed phase from dispersant.
- Tyndall Effect: Light scattering by colloidal particles, used for size detection.
- Electrophoresis: Separation of dispersed phase by applying an electric field (e.g., soaps, detergents).
Soaps and Detergents
Soaps
Sodium or potassium salts of fatty acids, produced by saponification. They act as protective colloids, coating dust and grease for easy removal by water. Disadvantages include insoluble calcium/magnesium salt formation and alkaline pH.
Detergents
Synthetic cleaning agents with similar action to soaps. Advantages include neutral pH and soluble calcium/magnesium salts. Disadvantages include environmental concerns (eutrophication, non-biodegradable substances, foaming).
Coagulation and Stabilization
Colloidal particle charge prevents aggregation. Coagulation can be induced by:
- Hydrophobic Colloid Coagulation: Adding electrolytes.
- Hydrophilic Colloid Stabilization: Removing excess ions by dialysis.
- Other Stabilization Methods: Heating, mixing oppositely charged colloids, Cottrell method (aerosol precipitation using electrodes).