Chapter 4: Colloid Preparation

Chapter 4: Colloid Preparation

# 1. Colloid Definition

A colloid is a heterogeneous system consisting of a dispersed phase (colloidal particles) and a dispersion medium (solvent).

• Colloidal particles have sizes ranging from 10^-7 to 10^-5 cm.

• A stable colloid maintains its dispersed state for a reasonable period.

# 2. Methods of Colloid Preparation

There are two primary methods for preparing colloids: condensation and dispersion. Additionally, other methods include peptization, solvent exchange, and colloid purification.

2.1. Condensation Method

Condensation involves combining smaller molecules or ions to form colloidal particles.

a. Specific Condensation Methods:

• Simple Condensation: Metal vapor condenses in a suitable environment. Example: Sodium vapor condensing in benzene.

• Condensation by Chemical Reactions:

• Exchange Reactions: Example: AgNO₃ + KI ? AgI colloid + KNO₃

• Oxidation-Reduction Reactions: Example: H₂S + O₂ ? S (colloid) + H₂O

• Hydrolysis Reactions: Example: FeCl₃ + 3H₂O ? Fe(OH)₃ colloid + 3HCl

• Solvent Exchange: Example: Dissolving sulfur in absolute alcohol, followed by adding water to the mixture. Sulfur aggregates into small particles dispersed in diluted alcohol, creating a cloudy colloidal system.

2.2. Dispersion Method

Dispersion involves breaking down larger particles into colloidal size, typically requiring energy input.

a. Specific Dispersion Methods:

• Mechanical Dispersion:

• Manual: Grinding in a mortar and pestle.

• Machinery: Ball mills, colloid mills.

• Ultrasonic Dispersion: Using ultrasonic waves (high frequency 10-30 kHz) to generate energy for particle breakdown.

• Arc Dispersion: Employed for preparing metal colloids in organic solvents. Two metal electrodes create an arc, generating high temperatures that melt and vaporize the metal, which then cools and forms colloidal particles.

• Peptization: Transforming a precipitate into a colloidal state using chemical agents.

b. Peptization:

• Peptization converts a precipitate into a colloidal state by the action of a peptizing agent, typically a chemical.

• The peptization process is tailored to the cause of precipitation.

• Common Scenarios:

• Precipitation due to colloidal particle adsorption of electrolyte ions causing coagulation: The peptizing agent must remove these ions from the precipitate, usually by washing the precipitate with water.

• Precipitation due to lack of protective factors (insufficient potential-determining ions or solvation shell): Adding protective factors to the system.

• When adsorption forces are strong, extensive washing of the precipitate is necessary: This often occurs when the precipitate adsorbs ions with high valence and large radii.

• Example:

• FeCl₃ + K₄[Fe(CN)₆] ? KFe[Fe(CN)₆] (blue colloidal precipitate) + 3KCl

• Slowly adding oxalic acid to the above solution ? blue colloidal solution.

• Explanation: Oxalate ions adsorb onto the colloidal surface, providing it with a charge, causing repulsion between particles and forming a stable colloidal system.

# 3. Colloid Purification

Unpurified colloids often contain ions, low molecular weight compounds, and other impurities. Removal of these impurities is necessary to obtain pure colloids.

a. Purification Methods:

• Dialysis: Intermittent or continuous.

• Electrodialysis: Using an electric current to remove ions.

b. Intermittent Dialysis:

• The colloidal solution is placed in a dialysis bag and immersed in a container of water.

• Electrolyte ions diffuse through the semi-permeable membrane into the surrounding water, which is periodically replaced to remove the ions.

c. Gel:

• A gel is a solidified form of a high molecular weight compound when exposed to water.

• Gels are used for purifying colloidal solutions containing small, spherical particles.

• Procedure:

• Immerse the gel in water for swelling.

• Pack the gel into a column and pass the colloidal solution through it at a controlled rate.

• The colloidal particles and larger particles pass through the spaces between gel particles.

• Impurity ions are retained within the gel particles.

d. Ultrafiltration:

• Ultrafiltration membranes are thicker than dialysis membranes, often made from cellulose acetate derivatives, with a thickness of 1-2 mm.

• They allow the passage of molecules and ions while retaining colloidal particles.

• Ultrafiltration units are connected to a compressor to increase pressure or a vacuum pump.

• Applications: Purification of heat-sensitive products such as enzymes and hormones.

# 4. Applications of Colloids

Colloids have numerous applications in daily life and industrial processes:

• Industry: Paints, inks, adhesives, detergents, dyes, pharmaceuticals…

• Agriculture: Pesticides, fertilizers…

• Healthcare: Antibiotics, injections, intravenous fluids…

# Conclusion

Colloids are important heterogeneous systems with widespread applications across various fields. Understanding colloid preparation and purification methods is crucial for effective utilization in practical applications.