Surface Chemistry (Colloids)

Colloids

Colloids, or colloidal solutions, are mixtures in which one substance of a finely divided state is dispersed in another substance. The dispersed particles are larger than molecules but too small to be seen with the naked eye. They are intermediate in size between true solutions and suspensions.

A colloidal system consists of two phases:

Dispersed Phase: The substance that is dispersed as fine particles.
Dispersion Medium: The substance in which the dispersed phase is dispersed.

The size of colloidal particles ranges from 1 nm to 1000 nm (or $10^{-9}$ m to $10^{-6}$ m).

Colloidal particles are generally electrically charged and exhibit Brownian motion.

Comparison of True Solution, Colloid, and Suspension

Classification Of Colloids

Classification Based On Physical State Of Dispersed Phase And Dispersion Medium:

This classification categorizes colloidal systems based on the physical states of the dispersed phase and the dispersion medium.

Dispersion Medium	Dispersed Phase	Type of Colloid	Examples
Solid	Solid	Solid Sol	Gemstones, coloured glass
Solid	Liquid	Sol	Paints, ink, muddy water
Solid	Gas	Solid Aerosol	Smoke, dust in air
Liquid	Solid	Gel	Cheese, butter, jams, jelly
Liquid	Liquid	Emulsion	Milk, hair cream, face lotion
Liquid	Gas	Aerosol (Liquid Aerosol)	Fog, mist, cloud, hair spray
Gas	Solid	Foam	Pumice stone, foam rubber, sponge
Gas	Liquid	Foam (Liquid Foam)	Whipped cream, shaving cream, soap lather

Classification Based On Nature Of Interaction Between Dispersed Phase And Dispersion Medium:

This classification is based on the affinity between the dispersed phase and the dispersion medium.

1. Lyophilic Colloids (Solvent-Loving):

These colloids are formed when there is a strong attraction between the dispersed phase and the dispersion medium.
They are readily formed by mixing directly (e.g., mixing gum, gelatin, starch, or albumin with water).
These colloidal solutions are quite stable and are not easily coagulated.
If the dispersion medium is water, they are called hydrophilic.
They can exist in a gel state.

2. Lyophobic Colloids (Solvent-Hating):

These colloids are formed when there is little or no attraction between the dispersed phase and the dispersion medium.
They cannot be readily formed by simple mixing and often require special methods for preparation.
These colloidal solutions are relatively less stable and can be easily coagulated by adding electrolytes.
They do not readily form gels.
If the dispersion medium is water, they are called hydrophobic.
Examples: Sols of metals (e.g., silver sol), metal sulphides ($As_2S_3$ sol).

Classification Based On Type Of Particles Of The Dispersed Phase, Multimolecular, Macromolecular And Associated Colloids:

1. Multimolecular Colloids:

In these colloids, a large number of atoms or smaller molecules (of low molecular weight) aggregate together to form particles of colloidal size.
The aggregated molecules are held together by van der Waals forces.
The particles in the dispersed phase are held together by intermolecular forces.
Example: Sols of gold, silver, sulphur.

2. Macromolecular Colloids:

These are large molecules (macromolecules) which have the size of colloidal particles, formed by polymerization.
These molecules are held together by covalent bonds.
They are generally stable and have high viscosity.
Examples: Proteins (e.g., enzymes, haemoglobin), synthetic polymers (e.g., nylon, polyethylene, polystyrene), natural polymers (e.g., starch, cellulose, proteins).

3. Associated Colloids (Aggregated Colloids):

These are substances that behave as electrolytes at low concentrations but behave as colloidal aggregates (micelles) at higher concentrations due to association of molecules.
The molecules in the associated colloid are called சமையல் சோப்புகள் (micelles).
Critical Micelle Concentration (CMC): The concentration above which micelle formation begins.
Kraft Temperature: The minimum temperature above which micelle formation occurs.
Example: Soaps and synthetic detergents.
Soap Micelle Formation:

In water, soap molecules (like Sodium Stearate, $C_{17}H_{35}COONa$) dissociate into ions ($C_{17}H_{35}COO^- + Na^+$).
Above CMC, the stearate ions ($C_{17}H_{35}COO^-$) aggregate to form a spherical micelle, with the hydrophobic hydrocarbon chains directed inwards and the hydrophilic carboxylate groups directed outwards towards water.

Preparation Of Colloids:

Colloids can be prepared by two main types of methods:

1. Chemical Methods:

These methods involve chemical reactions in which the products are formed in the colloidal state.

Double Decomposition:
- Example: Arsenic(III) sulphide ($As_2S_3$) sol is prepared by reacting arsenous acid ($H_3AsO_3$) with hydrogen sulphide ($H_2S$).
- $2H_3AsO_3(aq) + 3H_2S(g) \rightarrow As_2S_3(\text{sol}) + 6H_2O(l)$
Oxidation:
- Example: Sulphur sol is prepared by oxidizing $H_2S$ with $SO_2$.
- $2H_2S(aq) + SO_2(g) \rightarrow 3S(\text{sol}) + 2H_2O(l)$
Reduction:
- Example: Gold sol is prepared by reducing a solution of gold salt (e.g., $AuCl_3$) with a reducing agent like stannous chloride ($SnCl_2$) or formaldehyde ($HCHO$).
- $2AuCl_3 + SnCl_2 + H_2O \rightarrow 2Au(\text{sol}) + SnCl_4 + 2HCl$
Exchange of Solvent:

Example: If a solution of nitrocellulose in acetone is poured into water, nitrocellulose precipitates in the colloidal form.

2. Electrical Methods (Bredig's Arc Method):

This method is used for preparing colloidal sols of metals like gold, silver, platinum.

Process: An electric arc is struck between electrodes of the metal in a finely divided state, immersed in the cold dispersion medium (often water containing a stabilizer). The heat from the arc vaporizes the metal, which then condenses to form colloidal particles.

3. Mechanical Methods:

Process: Suspending the substance in the dispersion medium with the help of grinding and mixing appliances.
Example: Ball mill is used to prepare sulphur sol.

4. Methods involving condensation of ionic compounds:

Used for lyophobic sols.

Example: $FeCl_3$ solution is added to boiling water to prepare $Fe(OH)_3$ sol.
$FeCl_3(aq) + 3H_2O(boiling) \rightarrow Fe(OH)_3(\text{sol}) + 3HCl(aq)$

Purification Of Colloidal Solutions:

The colloidal solutions prepared by chemical or electrical methods often contain soluble electrolytes. These electrolytes can coagulate the colloid. Therefore, purification is necessary.

1. Dialysis:

Principle: Based on the difference in the diffusion rates of ions (small) and colloidal particles (large) through a semipermeable membrane.
Process: The colloidal solution is placed in a bag made of a semipermeable membrane (like cellophane or parchment paper), which is then immersed in pure water. Small ions and molecules diffuse out into the pure water, while the colloidal particles remain inside the bag. The pure water is continuously replaced.

2. Electrodyalsis:

Principle: Speeds up dialysis by applying an electric field across the semipermeable membrane.
Process: Involves dialysis with electrodes inserted into the dialysis chamber. Ions move faster towards their respective electrodes under the influence of the electric field, leading to more efficient removal of electrolytes.

3. Ultrafiltration:

Principle: Uses filter membranes with pore sizes small enough to retain colloidal particles but allow the dispersion medium and dissolved electrolytes to pass through.
Process: Colloidal solution is passed through an ultrafilter. The dispersion medium and electrolytes pass through, leaving behind the purified colloidal solution.

Properties Of Colloidal Solutions:

1. Tyndall Effect:

Phenomenon: The scattering of light by colloidal particles, making the path of light visible.
Explanation: When a beam of light is passed through a colloidal solution, the colloidal particles scatter the light, and the path of light becomes visible as a cone (Tyndall cone). This is due to the larger size of colloidal particles compared to molecules in a true solution.
Conditions: The Tyndall effect is observed only when the wavelength of light used is not smaller than the size of the dispersed particles, and when the refractive indices of the dispersed phase and dispersion medium differ significantly.
Applications: Used to distinguish between true solutions and colloids.

2. Brownian Movement:

Phenomenon: The random, erratic movement of colloidal particles suspended in the dispersion medium.
Cause: The continuous, unbalanced bombardment of the colloidal particles by the molecules of the dispersion medium.
Effect: It is responsible for the stability of colloidal solutions, preventing the particles from settling down.

3. Electrical Properties:

Charge on Colloidal Particles: Colloidal particles carry a positive or negative electric charge. This charge arises from:
- Electron capture or loss during colloidal particle formation.
- Preferential adsorption of ions from the solution (most common).
- Dissociation of macromolecules.
Electrophoresis: The movement of charged colloidal particles under the influence of an electric field towards the electrode of opposite charge. This phenomenon can be used to determine the sign of charge on the colloidal particles.
Electroosmosis: The movement of the dispersion medium towards the electrode under the influence of an electric field, while the colloidal particles remain stationary.
Coagulation or Flocculation: The process by which colloidal particles aggregate together to form larger particles that settle down, leading to the destruction of the colloidal system. This is usually caused by the addition of electrolytes.

4. Coagulation and Coagulation Factor:

Coagulation is the process of settling of colloidal particles. It is caused by the addition of electrolytes, heating, or mixing of oppositely charged sols.

Coagulation Factor: The minimum amount of an electrolyte (in millimoles per litre of the solution) required to cause precipitation of a colloid.
Hardy-Schulze Rule: This rule states that for coagulating a negatively charged sol, the coagulating power of an electrolyte increases with the magnitude of the charge of the anion. For coagulating a positively charged sol, the coagulating power of an electrolyte increases with the magnitude of the charge of the cation.

For negative sols (e.g., $As_2S_3$ sol): $Al^{3+} > Ba^{2+} > Na^+$
For positive sols (e.g., $Fe(OH)_3$ sol): $[Fe(CN)_6]^{4-} > PO_4^{3-} > SO_4^{2-} > Cl^-$

Peptization: The process of converting a precipitate into a colloidal solution by shaking it with a small amount of electrolyte or by adding a peptizing agent.

5. Protective Colloids:

Lyophilic colloids that are added to lyophobic sols to protect them from coagulation by electrolytes are called protective colloids.
They form a protective layer around the lyophobic particles, preventing their aggregation.
Example: Gelatin is used as a protective colloid for photographic emulsions.

Emulsions

Emulsions are a type of colloid in which one liquid is dispersed in another liquid in the form of fine droplets. Both dispersed phase and dispersion medium are liquids.

Types of Emulsions:

There are two types of emulsions:

Oil-in-Water ($O/W$) Emulsion: In this type, oil is dispersed in water (dispersed phase is oil, dispersion medium is water).

Examples: Milk (fat dispersed in water), hair cream, vanishing cream.

Water-in-Oil ($W/O$) Emulsion: In this type, water is dispersed in oil (dispersed phase is water, dispersion medium is oil).

Examples: Butter, margarine, cold cream, lubricating oils.

Emulsifying Agent (Emulsifier):

Emulsions are thermodynamically unstable and tend to separate into two liquid layers. An emulsifying agent is added to stabilize them. The emulsifier reduces the interfacial tension between the two liquids and forms a protective layer around the dispersed droplets, preventing them from coalescing.

For $O/W$ emulsions: Lyophilic colloids like gum arabic, gelatin, casein (proteins), soaps (like sodium stearate) act as emulsifiers. The hydrophilic part of the emulsifier interacts with water (dispersion medium).
For $W/O$ emulsions: Higher fatty acid salts of calcium, magnesium, and long chain alcohols act as emulsifiers. The hydrophobic part of the emulsifier interacts with oil (dispersion medium).

Tests for Emulsions:

Dilution Test: An $O/W$ emulsion can be diluted with water, but a $W/O$ emulsion cannot be diluted with water. A $W/O$ emulsion can be diluted with oil.
Electrical Conductivity Test: An $O/W$ emulsion conducts electricity because the dispersion medium is water, whereas a $W/O$ emulsion does not conduct electricity because the dispersion medium is oil.

Colloids Around Us

Colloids are not just a topic in chemistry; they are present all around us in our daily lives. Here are some examples:

1. In Food:

Milk: An emulsion of fat in water ($O/W$).
Butter and Cream: $W/O$ emulsions.
Mayonnaise: An emulsion of oil in vinegar ($O/W$).
Jams, Jellies, Marmalades: Gels (solid dispersed in liquid).
Whipped Cream, Ice Cream: Foams (gas dispersed in liquid).
Cheese: Gel.

2. In Nature:

Sky is Blue: Scattering of sunlight by tiny dust particles and gas molecules in the atmosphere (Tyndall effect).
Reddish Sunrise/Sunset: Scattering of sunlight by larger colloidal particles in the atmosphere causes the longer wavelengths (red) to be scattered more towards the observer.
Fog, Mist, Clouds: Aerosols of liquid droplets dispersed in gas (air).
Rain: Coagulation of water droplets in clouds.
Formation of Deltas: Rivers carry clay and silt (colloidal particles) which are coagulated by the ions present in seawater, leading to the formation of deltas at the river mouth.
Blood: Contains colloidal particles like proteins.
Soil: Humus particles dispersed in water form colloidal sols, contributing to soil fertility.

3. In Household Products:

Soaps and Detergents: Form colloidal micelles in water, enabling cleaning action.
Glues and Adhesives: Often colloidal in nature.
Paints: Sols of pigments dispersed in a liquid medium.
Face Creams, Lotions: Emulsions ($O/W$ or $W/O$).
Insecticides: Sprayed as fine colloidal sprays (aerosols).

4. In Medicine:

Medicinal Uses of Colloids: Many medicines are administered in colloidal form for better absorption and efficacy.

Colloidal Silver: Antiseptic.
Colloidal Gold: Used in diagnostic tests.
Antacids: Some antacids contain aluminium hydroxide gel, which is a colloid and neutralizes stomach acid effectively.
Activated Charcoal: Used in gas masks and for treating poisoning due to its adsorptive properties.

5. In Industry:

Artificial Rain: Scattering of clouds with dust or sand particles by aero planes.
Purification of Water: Alum ($K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O$) is added to water, which coagulates the suspended impurities, making them settle down.
Leather Tanning: Leather is prepared by tanning chrome-containing chemicals, which act as negatively charged colloids and adsorb onto positively charged collagen fibres of the hide.