Colloids and Their Classification
The 1-1000 nm Particle, Lyophilic vs Lyophobic, and Micelles.
A colloidal state is not a specific class of substances, but a specific state of a substance. Whether a substance forms a true solution, a colloid, or a suspension depends entirely on the size of its particles. Thomas Graham (the father of colloid chemistry) originally classified substances based on their ability to pass through a parchment membrane, but modern chemistry defines them strictly by particle dimensions.
1. True Solutions vs. Colloids vs. Suspensions
A colloidal system is a heterogeneous mixture consisting of two phases: the Dispersed Phase (DP) (the particles being distributed, analogous to a solute) and the Dispersion Medium (DM) (the continuous medium, analogous to a solvent).
Figure 1: Comparison of particle sizes defining the three states of mixtures.
| Property | True Solution | Colloidal Solution | Suspension |
|---|---|---|---|
| Particle Size | $\lt 1\text{ nm}$ | $1\text{ nm}$ to $1000\text{ nm}$ | $\gt 1000\text{ nm}$ |
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Visibility | Invisible | Visible under ultramicroscope | Visible to naked eye |
| Settling | Do not settle | Do not settle (unless centrifuged) | Settle under gravity |
| Filterability | Passes filter paper & membranes | Passes filter paper, but NOT parchment membrane | Trapped by filter paper |
2. Classification Based on Physical State
Colloids are classified based on the physical state of the Dispersed Phase (DP) and the Dispersion Medium (DM). There are 8 possible combinations.
| Dispersed Phase (DP) | Dispersion Medium (DM) | Name of Colloid | Crucial Examples (JEE/NEET Targets) |
|---|---|---|---|
| Solid | Solid | Solid Sol | Colored glasses, Gemstones, Ruby glass |
| Solid | Liquid | Sol | Paints, Cell fluids, Gold sol, Muddy water |
| Solid | Gas | Aerosol | Smoke, Dust |
| Liquid | Solid | Gel | Cheese, Butter, Jellies, Boot polish |
| Liquid | Liquid | Emulsion | Milk, Hair cream, Mayonnaise |
| Liquid | Gas | Aerosol | Fog, Mist, Cloud, Insecticide sprays |
| Gas | Solid | Solid Sol / Foam | Pumice stone, Foam rubber |
| Gas | Liquid | Foam | Froth, Whipped cream, Soap lather |
3. Classification Based on DP-DM Interaction
Depending on the affinity (attraction) between the dispersed phase and the dispersion medium, colloidal sols are divided into two fundamental types: Lyophilic (liquid-loving) and Lyophobic (liquid-hating). If water is the medium, they are called Hydrophilic and Hydrophobic.
| Property | Lyophilic Sols (Liquid-Loving) | Lyophobic Sols (Liquid-Hating) |
|---|---|---|
| Preparation | Easily formed by directly mixing DP with DM (warm/shaking). | Cannot be formed by direct mixing. Require special chemical/physical methods. |
| Stability | Highly Stable. | Highly Unstable. Easily coagulated by heating or adding electrolytes. |
| Reversibility | Reversible. If evaporated, the residue can be mixed with DM to instantly reform the sol. | Irreversible. Once coagulated, they cannot be reformed by simple mixing. |
| Hydration / Solvation | Particles are heavily hydrated/solvated (surrounded by DM molecules). | Virtually no hydration. Stabilized only by a layer of electrical charge. |
| Viscosity & Surface Tension | Viscosity is much higher than DM. Surface tension is lower than DM. | Viscosity and Surface tension are nearly the same as the pure DM. |
| Examples | Starch, Rubber, Gelatin, Gum, Proteins in water. | Metal sols (Gold, Ag), Metal sulfides ($As_2S_3$), Metal hydroxides ($Fe(OH)_3$). |
4. Classification Based on Type of DP Particles
Depending on how the particles reach the $1-1000\text{ nm}$ size range, colloids are divided into three types:
A. Multimolecular Colloids
The individual atoms or molecules are smaller than $1\text{ nm}$ (true solution size). However, upon dissolution, a large number of these small species aggregate (bunch together) to form species that fall into the colloidal range. They are usually lyophobic.
- Examples: Gold sol (clusters of hundreds of Au atoms), Sulfur sol (clusters of many $S_8$ molecules).
B. Macromolecular Colloids
The individual molecules are so massive (polymers) that a single molecule natively falls within the colloidal size range ($1-1000\text{ nm}$). These are typically lyophilic and highly stable.
- Examples: Naturally occurring (Starch, Cellulose, Proteins, Enzymes), Synthetic (Polythene, Nylon, Polystyrene).
C. Associated Colloids (Micelles)
These are substances that behave as normal, strong electrolytes at low concentrations. But at higher concentrations, they exhibit colloidal behavior due to the formation of aggregated particles called Micelles.
- Example: Soaps (Sodium Stearate, $C_{17}H_{35}COO^-Na^+$) and synthetic detergents.
5. Deep Dive: Associated Colloids (Micelles)
Micelle formation is critical to the cleansing action of soaps. A soap molecule (like Sodium Stearate) has two distinct parts:
- Hydrophobic Tail: A long non-polar hydrocarbon chain ($C_{17}H_{35}-$). It repels water but mixes with oil/grease.
- Hydrophilic Head: A polar, ionic carboxylate group ($-COO^-$). It is strongly attracted to water.
Figure 2: Structure of a micelle trapping an oil droplet (Cleansing action of soap).
The Two Absolute Conditions for Micelle Formation:
Micelles will NOT form unless both of these conditions are met simultaneously:
- Critical Micelle Concentration (CMC): The concentration of the soap/detergent must be above a certain threshold. Below this concentration, they act as individual dissolved ions. (For soap, CMC is $\sim 10^{-4}$ to $10^{-3} \text{ mol/L}$).
- Kraft Temperature ($T_k$): The temperature of the solution must be above a specific minimum temperature. Below $T_k$, the surfactant precipitates out as a solid rather than forming micelles.
Mechanism of Cleansing: When dirty cloth is put in a soap solution, the hydrophobic tails plunge into the oil/dirt droplet, while the hydrophilic heads stay pointing outward into the water. This forms a Micelle. The entire oil droplet is ripped from the cloth and suspended in the water as a stable colloidal particle. The outward negative charges repel other micelles, preventing the dirt from clumping back together.
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