Colloidal Systems Overview

Study Notes

A dispersed system refers to a system where one substance (dispersed phase) is distributed through a second substance (continuous phase).
The dispersed phase can be an insoluble solid, liquid, or gas.
Colloids are disperse systems (colloidal dispersions) where the dispersed phase consists of particles with sizes ranging from 1 nm to 1000 nm.

Classified based on the interaction between particles of the dispersed phase and the dispersion medium.
Lyophilic Colloids: Dispersed phase has a high affinity for the dispersion medium. They are formed spontaneously by dissolving the material in the solvent.
- Hydrophilic colloids: Solvent is water. For example, acacia and insulin in water.
- Lipophilic colloids: Solvent is a non-aqueous, organic solvent. For example, rubber and polystyrene.

Stability: Dispersed phase does not easily precipitate. It requires neutralization of the charge or removal of the water layer. Addition of less polar liquids (acetone or alcohol) or small amounts of electrolytes can facilitate coagulation due to particle dehydration.
Reversible: Sols are stable due to two factors:
- Negative or positive charge on the particles.
- A layer of solvent surrounding the particles. They can be reconstituted by simple remixing with the dispersion medium.
Denaturation: Heat, acids, alkalies, and some salts can cause denaturation of lyophilic colloidal materials. This is not reversible.
Mixing: Similarly charged hydrophilic colloids mix easily with no precipitation, while those with different charges interact and may precipitate (e.g., antigen-antibody reaction).

Low Affinity: Dispersed phase has very little or no attraction for the dispersion medium.
Preparation: Require special methods. Typically, inorganic particles (e.g., gold, silver, sulfur) are dispersed in a solvent like water.
Instability: Easily precipitated by adding small amounts of electrolytes, heating, or shaking.
Irreversible: Once precipitated, it is difficult to reconstitute the sol by simple mixing.

Amphiphiles: Molecules or ions with two distinct regions with opposing solution affinities (surface active agent, SAA).
Micelle Formation: At low concentrations, amphiphiles exist separately (subcolloidal size). At high concentrations, they form aggregates or micelles (50 or more monomers) with colloidal size.
Spontaneous Formation: Formation is spontaneous if the amphiphile concentration exceeds the critical micelle concentration (CMC).

Feature	Lyophilic	Associated	Lyophobic
Dispersed Phase	Large organic molecules	Micelles of organic molecules or ions	Inorganic particles (e.g., gold)
Solvation	Dispersed phase molecules are solvated	Hydrophilic and lyophilic portions are solvated	Not formed spontaneously
Formation	Spontaneous	Formed at concentrations above CMC	Not formed spontaneously
Viscosity	Increases with increase in dispersed phase concentration	Increases with increase in micelle concentration	Not greatly increased
Stability	Stable in presence of electrolytes	CMC decreases with electrolytes	Unstable in presence of electrolytes

Lyophilic and Association Colloidal Sols: Usually prepared by dissolving/dispersing the dispersed phase in water.
Hydrophobic Colloids: Prepared by:
Condensation Processes:*
- Change of solvent.
- Double Condensation.
- Reduction.
- Oxidation.
Dispersion Processes:* Coarse particles are reduced in size by:
- Ultra generator.
- Electric Arc (Bridge's arc method).
- Colloid mill.
- Peptization.

Purpose: Removes impurities that can cause flocculation or other unwanted effects.
Methods:
- Dialysis: Uses a semipermeable membrane to separate colloidal particles from smaller molecules & ions (impurities).
- Electrodialysis: Uses an electric potential to increase the rate of movement of ionic impurities through the membrane.
- Ultrafiltration: Applies pressure or suction to force solvent and small particles across a membrane, retaining colloidal particles.

Two main mechanisms:
- Steric Stabilization: Surrounding each particle with a protective solvent sheath prevents adherence due to Brownian movement.
- Electrostatic Stabilization: Providing particles with an electric charge. Like charges on particles cause repulsion, preventing aggregation and precipitation.

Instability: Lyophobic sols are unstable.
Electrostatic Stabilization: Particles are stabilized by electrical charges on their surfaces, typically through the addition of small amounts of electrolytes.
Coagulation: Adding electrolytes beyond the necessary amount for stability results in the accumulation of opposite ions, decreasing zeta potential, and coagulation.