Colloids: Definition, Types, Function and Examples

Colloids

Edited By Shivani Poonia | Updated on Jul 02, 2025 06:05 PM IST

Colloids are one of those things that pervade our lives without us even realizing it, and they make a big difference. Think of a glass of milk. Picture the window with a streak-free shine from the new glass cleaner. Picture again that shampoo in your hand. They probably all have their unique properties because of the fascinating world of colloids that they belong to. A colloid is a mixture in which small particles of one substance are dispersed in, but do not chemically react with, a second substance to produce a homogeneous solution.

This Story also Contains

Types of Colloids
Some solved examples
Conclusion

Colloids

Colloids are heterogenous mixtures whereby one substance—the dispersed phase—is finely divided and uniformly dispersed within another substance, the continuous phase. The size of the particle in the colloid generally remains in the range of 1 to 1000 nm, larger than that found in solutions but smaller than in suspensions. This size range lends a few special characteristics to colloids, such as the Tyndall effect, whereby they scatter light and make the path of light visible through the colloidal dispersion.

Tyndal effect

Types of Colloids

Lyophilic and Lyophobic Colloids

Lyophilic Colloids: The solids that disperse in suitable liquids also show solubility. These are colloids, with a kind of affinity of the dispersing medium (solvent) towards a continuous phase. In simpler words, this could be gelatin and starch in water. These colloids are easily reversible in normal conditions and thus highly stable.

Lyophobic Colloids: There is no such good affinity of the dispersing medium (solvent). This means it has a bad affinity. For example, gold and silver in water. Such colloids are not easily reversible and are not very stable; therefore, they generally require stabilizing agents too.

Lyophobic and Lyophilic Colloids

Multimolecular and Macromolecular Colloids

Multimolecular Colloids

Definition: The colloids that are multimolecular have the dispersed molecules in the form of aggregates/clusters.

Particle Size: Size of the particle that is dispersed in multimolecular colloids is very small, generally less diameter less than 1 nm.

Examples

Gold sol: It contains the colloidal particles that are based on Au and stabilized by citrate ions and exists in water.

Hydrosols: These contain stabilized hydrophilic ions plus metallic oxides and sulfides that find existence as colloidal solutions.

Formation: These colloids are formed when the dispersed phase aggregates to the extent of forming particles of a very small size that gravity is overcome by the movement of the resultant molecules; and they always remain in suspension in the dispersion medium.

Stability: Such colloids are more stable than other colloids due to the presence of macromolecules in them. The macromolecular structure does not allow the particles to aggregate or coagulate.

Macromolecular Colloids:

Definition: Chemical compounds or mixtures in which large-sized molecules are dispersed are known as macromolecular colloids. These dispersed molecules may include polymers, proteins, synthetic polymers, etc. The substance in which the particles are dispersed is known as a solvent or dispersion medium.

Particle Size: The particle size of the substance, which is dispersed, is in the range of 1 to 1000nm and

Protein solutions: Albumin or gelatin in water are typical of this

Synthetic polymers: Polyvinyl alcohol (PVA) in a measured amount of water is in this class

Formation: The colloids result from the dispersion of large molecules or polymers in the medium, and such molecules remain in suspension due to their big sizes and the stabilizing influences of solvation with surface charge.

Stability: The macromolecular colloids are more stable than multimolecular colloids, with the large size of the dispersed phase and surface charges that might prevent rapid aggregation or precipitation.

Key differences

Size: Multimolecular colloids weigh very small, almost negligible quantities of even less than 1 nm. But macromolecular colloids are much in size, usually in the range 1 to 1000 nanometers.

Nature: One that is composed of small molecular aggregates; the other one that is composed of large molecules or polymers.

Colloidal stability: In comparison with the multimolecular colloids, the macromolecular colloids are relatively more stable because of the larger size of the particles, coupled with the possibility of their electrostatic repulsion or steric hindrance. The multimolecular colloids may easily combine into aggregates.

Associated Colloids

Associated colloids or micelles, form at concentrations above a certain level, known as CMC or critical micelle concentration. Soaps and detergents are familiar examples of associated colloids. At and above the CMC value, surfactants form agglomerates called micelles. The hydrophilic ends face the outer environment while the hydrophobic ends combine inside the micelle.

Cleaning Action of Soaps

Soaps are surfactants that consist of a long alkyl chain attached to a polar carboxylate group, which imparts hydrophilic properties to the compound. Much as birds realign themselves into an arrowhead formation based on hydrophobic and hydrophilic principles, soap will orient itself in such a way that the alkyl chain will immerse itself into an oil droplet and the carboxylate will align on the perimeter. This effectively turns an oil droplet into a hydrophilic entity surrounded by hydrophobic alkyl chains.

Micelle formation

Some solved examples

Example 1:
Dust storm is an example of a

(correct) dispersion of solid in gas
dispersion of gas in solid
dispersion of solid in solid
dispersion of solid in liquid

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Solution:
In a dust storm, solid particles (dust) are dispersed in the gas phase (air), making it an example of dispersion of solid in gas.

Example 2:
Among the following, the INCORRECT statement about colloids is:

They can scatter light.
They are larger than small molecules and have high molar mass.
(correct) The osmotic pressure of a colloidal solution is of a higher order than the true solution at the same concentration.
The range of diameters of colloidal particles is between 1 and 1000 nm.

Solution:
The incorrect statement is option (3). The osmotic pressure of a colloidal solution is of small order compared to a true solution at the same concentration.

Example 3:
The colloidal solution of two liquids is called

Gel
Aerosol
(correct) Emulsion
Liquid Sol

Solution:
A colloidal solution of two liquids is called an Emulsion. Emulsions are stabilized mixtures of immiscible liquids, like oil and water.

Example 4:
A colloidal system consisting of a gas dispersed in a solid is called a /an:

Gel
Aerosol
(correct) Solid sol
Foam

Solution:
A colloidal system consisting of a gas dispersed in a solid is called a Solid sol.

Conclusion

The paper has stepped into the broad scientific spectrum and involved several colloids under discussion. It has gone about investigation in terms of lyophilic and lyophobic colloids, multimolecular and macromolecular colloids, and colloids relevant to adsorption. It has further interwoven the application of soap in colloidal arrangement within the ambit of wet colloidal science. We have also taken up the practical application of the science of colloids using the example of how soaps help in the process of cleansing. All these types of colloids can throw more light not only on understanding some basic scientific principles but can also lead to developing a richness in the universal presence and critical applications in industrial processes as well as the products we use in our daily lives.

Frequently Asked Questions (FAQs)

1. What is a colloidal substance?

It is a mixture in which microscopic dispersed particle size ranges from 1 to 1000 nanometers.

2. What are lyophilic and lyophobic colloids?

Lyophilic colloids are those colloids that attract the dispersing medium (solvent) and remain stable, but lyophobic colloids are those which repel the dispersing medium, and unsteady and need stabilizing agents to prevent coagulation.

3. What are multimolecular colloids?

When small molecules combine and give a larger particle which is then suspended into a medium known as a multimolecular colloid.

4. How do associated colloids work?

Associated colloids, for example: micelles - are formed with the aggregation of surfactant molecules at a specific concentration of CMC, such that they trap oils, and dirt to be removed with water.

5. How do soaps clean?

Soaps clean by forming micelles around the oil and grease particles that disperse them in water to let them be rinsed easily

6. What are colloids and how do they differ from solutions and suspensions?

Colloids are mixtures where tiny particles of one substance are dispersed throughout another substance. Unlike solutions, colloids don't fully dissolve, and unlike suspensions, their particles don't settle out over time. Colloids have particles sized between 1-1000 nanometers, which is larger than dissolved particles in solutions but smaller than those in suspensions.

7. What is the Tyndall effect and how does it relate to colloids?

The Tyndall effect is the scattering of light by colloidal particles. When a beam of light passes through a colloid, the particles scatter the light, making the beam visible. This effect is not seen in true solutions. It's a key way to distinguish colloids from solutions and is why we can see car headlights in fog (a colloid of water droplets in air).

8. Why don't colloidal particles settle under the influence of gravity?

Colloidal particles don't settle because of their small size and constant Brownian motion. The random movement of particles due to collisions with molecules of the dispersing medium counteracts the force of gravity. Additionally, electrical charges on the particles can cause repulsion, further preventing settling.

9. What is Brownian motion and how does it affect colloids?

Brownian motion is the random, zigzag movement of particles in a fluid, caused by collisions with molecules of the fluid. In colloids, this constant motion helps keep the dispersed particles suspended and prevents them from settling, contributing to the stability of the colloidal system.

10. How does the size of colloidal particles compare to those in solutions and suspensions?

Colloidal particles are typically between 1-1000 nanometers in size. This is larger than the particles in solutions (which are less than 1 nm) but smaller than those in suspensions (which are larger than 1000 nm). This intermediate size is key to the unique properties of colloids.

11. How does the DLVO theory explain colloidal stability?

The DLVO theory (named after Derjaguin, Landau, Verwey, and Overbeek) explains colloidal stability by considering the balance between attractive van der Waals forces and repulsive electrostatic forces between particles. When repulsive forces dominate, the colloid remains stable. When attractive forces overcome repulsion, particles can aggregate, leading to coagulation.

12. What is the significance of the zeta potential in colloidal systems?

Zeta potential is the electrical potential difference between the bulk of the dispersing medium and the stationary layer of fluid attached to the dispersed particle. It's a key indicator of colloidal stability. A high absolute value of zeta potential (typically above 30 mV) indicates a stable colloid, as particles repel each other strongly. Low zeta potential can lead to aggregation and instability.

13. How does the Schulze-Hardy rule relate to colloidal stability?

The Schulze-Hardy rule states that the coagulating power of an electrolyte is determined by the valence of the ion with charge opposite to that of the colloidal particles. Higher valence ions are more effective at coagulating colloids. For example, Al³⁺ is more effective than Ca²⁺, which is more effective than Na⁺ in coagulating a negatively charged colloid.

14. What is meant by "gold number" in colloidal chemistry?

The gold number is a measure of the protective power of a colloid. It's defined as the number of milligrams of a protective colloid that will just prevent the color change of 10 mL of a gold sol when 1 mL of 10% NaCl solution is added to it. A lower gold number indicates a more effective protective colloid.

15. What is the role of the Hardy-Schulze rule in water treatment?

The Hardy-Schulze rule is crucial in water treatment processes, particularly in coagulation and flocculation. It guides the choice of coagulants based on their valence. Trivalent ions like Al³⁺ or Fe³⁺ are commonly used in water treatment plants because they are highly effective in neutralizing the charge on colloidal particles, allowing them to aggregate and be removed more easily.

16. How does the critical micelle concentration (CMC) relate to colloids?

The critical micelle concentration (CMC) is the concentration of surfactants above which micelles form and all additional surfactants added to the system go to micelles. This concept is important in colloid science, especially for emulsions and microemulsions. At concentrations below the CMC, surfactants exist as individual molecules; above CMC, they aggregate into micelles, which can solubilize otherwise insoluble substances.

17. What is the significance of the hydrophilic-lipophilic balance (HLB) in emulsions?

The hydrophilic-lipophilic balance (HLB) is a measure of the degree to which a surfactant is hydrophilic or lipophilic. It's crucial in formulating stable emulsions. Surfactants with low HLB values are more oil-soluble and tend to form water-in-oil emulsions, while those with high HLB values are more water-soluble and form oil-in-water emulsions. The HLB system helps in selecting appropriate emulsifiers for specific applications.

18. How does electrophoresis work in the study of colloids?

Electrophoresis is a technique used to study and separate colloidal particles based on their electrical charge. When an electric field is applied to a colloidal dispersion, charged particles move towards the electrode of opposite charge. The rate and direction of movement depend on the particle's charge, size, and shape. This technique is useful for determining the charge on colloidal particles and for separating different types of particles.

19. How does the depletion interaction affect colloidal stability?

The depletion interaction is a force that can cause colloidal particles to aggregate in the presence of smaller, non-adsorbing particles or polymers (depletants). When two large particles come close, the depletants are excluded from the gap between them, creating an osmotic pressure difference that pushes the larger particles together. This can lead to flocculation or phase separation in colloidal systems.

20. How does the DLVO theory account for the stability of hydrophobic colloids?

The DLVO theory explains the stability of hydrophobic colloids by considering the balance between attractive van der Waals forces and repulsive electrostatic forces. According to this theory, the total interaction energy between particles is the sum of these two forces. When the repulsive force dominates at a certain distance, it creates an energy barrier that prevents particles from coming close enough to aggregate, thus stabilizing the colloid.

21. What is the significance of the isoelectric point in colloidal systems?

The isoelectric point is the pH at which a colloidal particle carries no net electrical charge. At this point, the colloid is least stable because there's no electrostatic repulsion between particles. Understanding the isoelectric point is crucial in many applications, such as protein purification, as it helps predict colloidal behavior under different pH conditions and can be used to induce aggregation or maintain stability.

22. How does the concept of fractal dimension apply to colloidal aggregates?

The fractal dimension is a measure of the complexity and space-filling capacity of colloidal aggregates. Unlike simple spherical particles, colloidal aggregates often have a complex, branched structure that can be described using fractal geometry. The fractal dimension provides information about the growth mechanism and structure of these aggregates, which in turn affects properties like density, strength, and optical characteristics of the colloidal system.

23. How does the concept of critical coagulation concentration (CCC) relate to colloidal stability?

The critical coagulation concentration (CCC) is the minimum concentration of electrolyte required to cause rapid coagulation of a colloidal dispersion. It's a key parameter in understanding colloidal stability. Below the CCC, the colloid remains stable; above it, rapid coagulation occurs. The CCC depends on factors like the valence of the counterions (as described by the Schulze-Hardy rule) and the nature of the colloidal particles.

24. What is the significance of the Hofmeister series in colloidal systems?

The Hofmeister series ranks ions based on their ability to salt out or salt in proteins and other colloids. It affects various colloidal properties including stability, solubility, and viscosity. The series goes beyond simple electrostatic effects, involving ion-specific interactions with water and colloidal surfaces. Understanding the Hofmeister series is crucial in fields like protein chemistry, where it can predict how different salts will affect protein solubility and stability.

25. What is meant by the term "colloidal dispersion"?

A colloidal dispersion refers to the even distribution of colloidal particles throughout a dispersing medium. It's a stable system where the dispersed phase (colloidal particles) remains uniformly distributed in the continuous phase (dispersing medium) due to their size, Brownian motion, and electrical charges.

26. How do emulsions differ from other types of colloids?

Emulsions are a type of colloid where both the dispersed phase and dispersing medium are liquids that don't mix, like oil and water. They differ from other colloids in that they often require an emulsifying agent to stabilize them and prevent the liquids from separating. Examples include milk and mayonnaise.

27. What role do surfactants play in colloidal systems?

Surfactants, or surface-active agents, play a crucial role in stabilizing many colloids, especially emulsions. They reduce surface tension between the dispersed and continuous phases, preventing coalescence of particles. Surfactants have a hydrophilic head and a hydrophobic tail, allowing them to bridge between water-loving and oil-loving substances.

28. How does the electrical double layer contribute to colloidal stability?

The electrical double layer is a structure that forms around colloidal particles in a liquid. It consists of a fixed layer of ions of one charge tightly bound to the particle surface, and a diffuse layer of oppositely charged ions. This double layer creates electrostatic repulsion between particles, preventing them from coming close enough to aggregate, thus stabilizing the colloid.

29. What is coagulation in the context of colloids?

Coagulation is the process by which colloidal particles come together to form larger aggregates. This can lead to the destabilization of the colloid and eventual separation of the dispersed phase from the dispersing medium. Coagulation can be induced by adding electrolytes, changing pH, or applying heat, among other methods.

30. What is meant by the term "lyophilic colloid"?

A lyophilic colloid is one where the dispersed particles have a strong affinity for the dispersing medium. "Lyophilic" means "solvent-loving." These colloids form spontaneously and are very stable. Examples include proteins in water or certain polymer solutions. They're less sensitive to electrolyte addition compared to lyophobic colloids.

31. How do lyophobic colloids differ from lyophilic colloids?

Lyophobic colloids, unlike lyophilic ones, have dispersed particles that do not have an affinity for the dispersing medium. "Lyophobic" means "solvent-fearing." These colloids are less stable and often require stabilizing agents. They're more sensitive to electrolyte addition and can easily undergo coagulation. Examples include gold sols in water.

32. How does dialysis work in purifying colloids?

Dialysis is a method used to purify colloids by removing small, unwanted molecules or ions. It involves placing the colloidal mixture in a semi-permeable membrane and immersing it in pure solvent. Small molecules and ions can pass through the membrane, while larger colloidal particles remain inside. This process is repeated until the desired purity is achieved.

33. What is the difference between peptization and peptization?

Peptization is the process of converting a precipitate into a colloidal sol by adding a suitable electrolyte or peptizing agent. It's essentially the reverse of coagulation. Peptization often involves the adsorption of ions onto the precipitate particles, creating a charge that leads to repulsion and dispersion. This process is important in creating stable colloidal systems from precipitates.

34. How do aerosols differ from other types of colloids?

Aerosols are colloids where the dispersing medium is a gas and the dispersed phase is either a liquid (mist, fog) or a solid (smoke, dust). They differ from other colloids in that their dispersing medium is much less dense than the dispersed phase, leading to unique behaviors. Aerosols can remain suspended for long periods due to the low density of the gas medium.

35. What is the difference between a sol and a gel in colloidal systems?

A sol is a colloidal dispersion of solid particles in a liquid medium, where the particles remain dispersed. A gel, on the other hand, is a colloidal system where the dispersed phase has combined with the continuous phase to produce a semisolid material. Gels have a 3D network structure that gives them their characteristic properties. The transition from sol to gel is called gelation.

36. How does the Ostwald ripening process affect colloidal stability?

Ostwald ripening is a phenomenon in colloids where smaller particles dissolve and redeposit onto larger particles over time. This occurs because larger particles are more energetically favorable due to lower surface energy. Ostwald ripening can lead to the growth of larger particles at the expense of smaller ones, potentially destabilizing the colloidal system, especially in emulsions and foams.

37. What is the role of steric stabilization in colloidal systems?

Steric stabilization is a method of preventing colloidal particles from aggregating by attaching large molecules (often polymers) to their surfaces. These attached molecules create a physical barrier that prevents particles from getting close enough for van der Waals attractions to cause aggregation. This method is particularly useful in non-aqueous systems where electrostatic stabilization is less effective.

38. What is the difference between flocculation and coagulation in colloids?

While both flocculation and coagulation lead to the aggregation of colloidal particles, they differ in mechanism and result. Coagulation involves the neutralization of particle charges, allowing them to come together and form compact aggregates. Flocculation, on the other hand, involves the formation of loose, fluffy aggregates often through the addition of polymers that bridge between particles. Flocs are generally larger and less dense than coagulated particles.

39. How do associative colloids differ from other types of colloids?

Associative colloids, also known as association colloids, are formed when surfactant molecules aggregate into structures like micelles or vesicles. Unlike typical colloids where distinct particles are dispersed in a medium, associative colloids form spontaneously when the surfactant concentration exceeds the critical micelle concentration (CMC). These systems are dynamic, with molecules constantly exchanging between aggregates and the bulk solution.

40. What is the role of the Gibbs-Marangoni effect in foam stability?

The Gibbs-Marangoni effect plays a crucial role in stabilizing foams, which are colloidal dispersions of gas in liquid. When a foam film is stretched, it creates areas of higher surface tension. This gradient in surface tension causes surfactant molecules to flow from regions of lower surface tension to higher surface tension, dragging liquid along with them. This flow helps to thicken and stabilize the foam film, counteracting drainage and rupture.

41. What is meant by "salting out" in colloidal chemistry?

Salting out is a process where the solubility of a substance in a solution is decreased by adding electrolytes. In colloidal systems, this can lead to the precipitation or coagulation of colloidal particles. The added salt ions compete with the colloidal particles for water molecules, reducing the solvation layer around the particles. This weakens the electrostatic repulsion between particles, allowing them to come closer and aggregate.

42. How do magnetic colloids behave differently from non-magnetic colloids?

Magnetic colloids, also known as ferrofluids, contain magnetic nanoparticles dispersed in a carrier fluid. Unlike non-magnetic colloids, they respond to external magnetic fields, allowing their properties and behavior to be controlled magnetically. This responsiveness leads to unique phenomena like the formation of spike-like structures in strong magnetic fields. Magnetic colloids find applications in areas like magnetic sealing, damping, and biomedical imaging.

43. What is the role of the Pickering effect in stabilizing emulsions?

The Pickering effect refers to the stabilization of emulsions by solid particles rather than traditional surfactants. In Pickering emulsions, solid particles adsorb at the interface between two immiscible liquids, forming a mechanical barrier that prevents droplet coalescence. This can lead to extremely stable emulsions. The effectiveness depends on factors like particle size, shape, and wett