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Classification of Colloids-Multimolecular, Macromolecular, Associated Colloids, Micelle, Practice Problems and FAQs
Have you thought about how soaps and detergents remove dirt from our clothes? As soon as soap or detergent comes in contact with water, the lather is formed. Is it a lather that removes dirt? After dipping the cloth in clean water, the lather gets washed away, taking off the dirt from the cloth. How actually this process of removal of dirt from the cloth is happening?
It all happens due to the formation of micelle (which is a form of a colloid).
Let us take a closer look at the process of formation of micelle and study under which category of colloids does micelle lies.
Table of contents
- Multimolecular colloids
- Macromolecular colloids
- Associated colloids
- Micelle
- Practice problems
- Frequently Asked Questions-FAQs
Based on the type of particles of the dispersed phase, colloids can be classified into three types:
1. Multimolecular colloids
2. Macromolecular colloids
3. Associated colloids
1. Multimolecular colloids
On dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having the size in the colloidal range (1- 1000 nm). Such species are called multimolecular colloids.
Example:
1. Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a dispersed medium, usually water. The colloid is usually either an intense red colour or blue/purple depending upon the aggregation and size of the particles. Gold sol is made by Bredig's arc method.
2. Macromolecular colloids
Macromolecules in suitable solvents form solutions in which the size of the macromolecules may be in the colloidal range. Such systems are called macromolecular colloids. These colloids are quite stable and resemble true solutions in many respects.
Example:
Starch, cellulose, proteins, enzymes, polythene, nylon, polystyrene, synthetic rubber, etc.
3. Associated colloids
At low concentrations some substances behave as normal strong electrolytes. On the other hand at high concentrations, some substances behave as colloidal particles. This behaviour is due to the formation of aggregates. These aggregated particles are called associated colloids or micelles.
Example: Soap, detergent.
Micelle
Micelles are relatively small, spherical structures composed of a few thousand molecules. They attract one another to form a larger aggregate which falls in the colloidal range.
Example: Soap solution. Soap is a sodium salt of a higher fatty acid which is represented as RCOO_NA+. R represents hydrocarbon chain (non-polar group) which is a hydrophobic part. COO-Na+ represents polar group, which is the hydrophilic part.
In sodium stearate CH3(CH2)16COO-Na+, CH3(CH2)16 represents hydrophobic tail and COO-Na+ represents the hydrophilic tail. When sodium stearate is dissolved in water, it dissociates into and Na+. RCOO- arranges itself in such as way that COO- can be close to water and hydrocarbon chain can stay away from it by staying at the surface.
- Hydrophilic part is below the surface and it interacts with water and the hydrophobic part is above the surface and it does not interact with water at low concentration.
- But at higher concentration, these are pulled into the bulk of the solution.and they aggregate in a spherical form. The concentration, above which micelle formation takes place is called critical micelle concentration (CMC).
- The formation of micelles takes place only above a particular temperature is called Kraft temperature, Tk Spherical form has the hydrocarbon chains pointing towards the centre, with COO- part remaining outward on the surface of the sphere.
- This aggregated form is known as micelle.
- At high concentration and above CMC and above kraft temperature, stearate ions with dirt particle form aggregates many such micelles are formed.
- It has been observed that as the temperature of the system rises, the CMC of each ionic surfactant decreases at first, then rises.
Practice problems
Q 1. Which of the following statement is correct?
(A) The cleaning action of soap is due to the formation of micelle.
(B) Micelle is an associated colloid.
(C) Both (A) and (B)
(D) None of the above
Answer: (C)
Associated colloids are chemicals that behave like strong electrolytes at low concentrations yet like colloidal particles at greater concentrations. Micelles are the collective term for such particles. Soap molecules when dissolved in water dissociate to form an aggregate, known as a micelle, which is responsible for removing dirt particles.
Q 2. Starch, cellulose, proteins, are examples of:
(A) Multimolecular colloid
(B) Associated colloid
(C) Macromolecular colloid
(D) None of the above
Answer: (C)
Starch, cellulose, and proteins are examples of macromolecular colloid.
Q 3. Size of multimolecular colloid ranges between
(A) 1 - 100 nm
(B) 1 - 1000 nm
(C) 1 - 10 nm
(D) None of the above
Answer: (B)
Multimolecular colloids are formed when on dissolution, a large number of atoms or smaller molecules of a substance aggregate together to form species having the size in the colloidal range (1 - 1000 nm).
Q 4. Critical micelle concentration(CMC) depends upon
(A) Nature of dispersion medium
(B) Length of hydrophobic chain
(C) Both (A) and (B)
(D) None of the above
Answer: (C)
Critical micelle concentration depends upon dispersion phase, dispersion medium and length of hydrophobic chain. Generally, longer the hydrophobic chain, smaller is its CMC. CMC increases with decreasing polarity of the dispersion medium.
Frequently Asked Questions-FAQs
Q 1. How can we distinguish multimolecular and macromolecular colloids based on forces?
Answer: Multimolecular colloids have weak van der Waals interactions between them, on the other hand macromolecular particles, have strong chemical interactions between it particles.
Q 2. What is the number of particles that should aggregate to form a micelle?
Answer: Around 100 ions forms an aggregate to form a micelle.
Q 3. Does detergent works in a similar manner as soap to form a micelle?
Answer: The mechanism of micelle formation is the same as that of soaps. Detergents like sodium lauryl sulphate, CH3(CH2)11SO4-Na+ where CH3(CH2)11SO4- represents hydrophobic tail and SO4-Na+ represents the hydrophilic tail forms micelle.
Q 4. What are surfactants?
Answer: Surfactants are surface-active substances containing both ionic and non-ionic part. Example- soaps and detergent. The surfactant forms a monomolecular layer at the interface between the dispersed droplets and the dispersion medium, lowering the interfacial tension between oil and water and facilitating the process of combining two liquids.
Q 5. What are ionic surfactants?
Answer: Charged surfactants carrying a positive and negative charge on the ionic head are referred to as ionic surfactants. Example: cationic surfactants -Sodium stearate [CH3(CH2)16COO-Na+]
Anionic surfactant- Laurylamine hydrochloride [CH3(CH2)11COO-Na-]
Q 6. What is Kraft temperature
Answer: The Krafft temperature (Kraft point) is the minimum temperature at which ionic surfactants, such as sodium lauryl sulphate (a component of household detergents), form micelles. It depends on the nature of hydrophobic groups and ionic composition of the detergent.
On the contrary, the solubility of nonionic surfactants decreases with increasing temperature. They are characterised by the cloud point, i.e. the maximum temperature at which nonionic surfactants form micelles and above which the mixture starts to phase separately in the surfactant- solvent
Related topics
| Preparation of Lyophobic colloids-Condensation methods,Dispersion methods | Osmosis and Osmotic pressure |
| Properties of colloids | Coagulation of colloids |
| Emulsion | Colloidal solutions |
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