Colloids

Colloids

A colloid is a heterogeneous system in which one substance is dispersed (dispersed phase) as very fine particles in another substance called a dispersion medium.

The essential difference between a solution and a colloid is that of particle size. While in a solution, the constituent particles are ions or small molecules, in a colloid, the dispersed phase may consist of particles of a single macromolecule (such as protein or synthetic polymer) or an aggregate of many atoms, ions or molecules. Colloidal particles are larger than simple molecules but small enough to remain suspended. Their range of diameters is between 1 and 1000 nm (10–9 to 10–6 m).

Colloidal particles have an enormous surface area per unit mass as a result of their small size. Consider a cube with 1 cm side. It has a total surface area of 6 cm2. If it were divided equally into 1012 cubes, the cubes would be the size of large colloidal particles and have a total surface area of 60,000 cm2 or 6 m2

Classification of Colloids 

 Colloids are classified on the basis of the following criteria:

(i) Physical state of dispersed phase and dispersion medium

(ii) Nature of interaction between dispersed phase and dispersion medium

(iii) Type of particles of the dispersed phase.

 

Classification Based on Physical State of Dispersed Phase and Dispersion Medium

1.     Solid-liquid colloids: In this type of colloid, the dispersed phase is a solid and the dispersion medium is a liquid. Examples include milk, paint, and blood.

2.     Liquid-liquid colloids: In this type of colloid, both the dispersed phase and the dispersion medium are liquids. Examples include emulsions, such as oil in water and water in oil.

3.     Gas-liquid colloids: In this type of colloid, the dispersed phase is a gas and the dispersion medium is a liquid. Examples include foam, whipped cream, and aerosols.

4.     Solid-gas colloids: In this type of colloid, the dispersed phase is a solid and the dispersion medium is a gas. Examples include smoke, dust, and airborne pollen.

Dispersion medium

The dispersion medium is the substance in which the dispersed phase is distributed. It is also known as the continuous phase. In a colloid, the dispersion medium is usually a liquid, but it can also be a gas or a solid. The properties of the dispersion medium, such as viscosity and surface tension, can affect the behavior of the particles in the dispersed phase.

Dispersed phase

The dispersed phase is the substance that is distributed throughout the dispersion medium. It is also known as the discontinuous phase. In a colloid, the dispersed phase can be a solid, liquid, or gas. The properties of the dispersed phase, such as particle size and charge, can affect the stability and behavior of the colloid.

Classification Based on Nature of Interaction between Dispersed Phase and Dispersion Medium

Colloidal sols can be divided into two categories based on the nature of interaction between the dispersed phase and the dispersion medium: lyophilic and lyophobic.

 

Lyophilic colloids

  • Lyophilic colloids have a strong affinity for the dispersion medium and can be directly formed by mixing substances like gum, gelatine, starch, and rubber with a suitable liquid as the dispersion medium.
  • Lyophilic sols are also known as reversible sols because they can be reconstituted by simply remixing with the dispersion medium if separated.
  • Lyophilic sols are quite stable and cannot be easily coagulated.

 

Lyophobic colloids

  • Lyophobic colloids have little or no affinity for the dispersion medium and can be prepared only by special methods.
  • Lyophobic sols are readily precipitated or coagulated on the addition of small amounts of electrolytes, by heating, or by shaking, and are hence not stable.
  • Lyophobic sols are also known as irreversible sols because once precipitated, they do not give back the colloidal sol by simple addition of the dispersion medium.
  • Lyophobic sols need stabilizing agents for their preservation.

 

Classification Based on Type of Particles of the Dispersed Phase,

Multimolecular, Macromolecular and Associated Colloids

  • Colloids are classified into three types based on the type of particles present in the dispersed phase: multimolecular, macromolecular, and associated colloids.

Multimolecular colloids

Multimolecular colloids are formed when a large number of atoms or smaller molecules of a substance aggregate together to form species with sizes in the colloidal range (1-1000 nm).

Macromolecular colloids

Macromolecular colloids are formed when macromolecules in suitable solvents form solutions in which the size of the macromolecules is in the colloidal range.

Associated colloids

Associated colloids, also known as micelles, are formed when some substances at low concentrations behave as normal strong electrolytes but exhibit colloidal behavior at higher concentrations due to the formation of aggregates.

Micelles are formed above a particular temperature called Kraft temperature (Tk) and a particular concentration called critical micelle concentration (CMC).

 Surface-active agents, such as soaps and synthetic detergents, belong to the class of associated colloids.

 

Micelles have both lyophobic and lyophilic parts and may contain as many as 100 molecules or more.

 The mechanism of micelle formation involves the dissociation of soap into RCOO- and Na+ ions in water. The RCOO- ions consist of a long hydrocarbon chain R (non-polar tail) and a polar group COO- (polar-ionic head).

 In water, the hydrophilic heads are attracted to the water molecules, while the hydrophobic tails are repelled and tend to associate with each other to form the core of the micelle.

 The polar head groups of the micelles face outwards and interact with the water molecules, while the nonpolar tails face inwards and are shielded from the water molecules. This structure stabilizes the micelle in the solution.

 The RCOO ions are, therefore, present on the surface with their COO groups in water and the hydrocarbon chains R staying away from it and remain at the surface. But at critical micelle concentration, the anions are pulled into the bulk of the solution and aggregate to form a spherical shape with their hydrocarbon chains pointing towards the centre of the sphere with COO part remaining outward on the surface of the sphere. An aggregate thus formed is known as ‘ionic micelle’. These micelles may contain as many as 100 such ions.

Similarly, in the case of detergents, e.g., sodium laurylsulphate, CH3(CH2)11SO4Na+, the polar group is –SO4 along with the long hydrocarbon chain. Hence, the mechanism of micelle formation here also is same as that of soaps.

 Cleansing action of soap

Soaps are widely used as cleaning agents due to their unique properties. The cleansing action of soap is based on its ability to emulsify grease and dirt, allowing them to be easily removed from surfaces. The emulsifying action of soap arises from the structure of its molecules, which have both hydrophilic (water-loving) and hydrophobic (water-repelling) regions.

When soap is dissolved in water, it forms a micelle structure due to its amphiphilic nature. The hydrophilic head of the soap molecule orients towards water, while the hydrophobic tail orients towards the center of the micelle. The hydrophobic tail of the soap molecule can interact with the hydrophobic molecules in the dirt or grease, while the hydrophilic head can interact with water molecules.

When the soap solution comes into contact with a dirty surface, the hydrophobic tails of the soap molecules interact with the dirt or grease, forming a spherical structure called a micelle. The hydrophobic tail of the soap molecule surrounds the grease and dirt particles, while the hydrophilic head points outwards towards the water.

This makes the grease and dirt particles soluble in water and easy to rinse away. The micelle structure of soap also prevents the dirt from re-depositing on the surface being cleaned. The soap micelles remain suspended in the water and are easily rinsed away, taking the dirt with them.

Thus, the cleansing action of soap is based on the formation of micelles that emulsify the dirt and grease, allowing them to be easily removed from the surface being cleaned.

Preparation of Colloids

 Colloidal dispersions can be prepared by chemical reactions leading to the formation of molecules by double decomposition, oxidation, reduction or hydrolysis. These molecules then aggregate leading to the formation of sols.

As2O+ 3H2S  As2S3(sol) + 3H2O

SO+ 2H2S  3S(sol) + 2H2O

2 AuCl3 + 3 HCHO + 3H2O  2Au(sol) + 3HCOOH + 6HCl

 

FeCl+ 3H2O  Fe(OH)3 (sol) + 3HCl

Bredig’s Arc method

Bredig's arc method is a technique used to prepare colloidal solutions. It was developed by the German chemist Wolfgang Bredig in the late 19th century. The method involves the use of an electric arc between two metal electrodes (usually silver) in a liquid medium to produce extremely fine particles of the metal. The liquid medium used is generally water or an organic solvent. The arc vaporizes the metal electrodes, and the resulting vapor then condenses to form colloidal particles.

The process of Bredig's arc method involves the following steps:

1.     Two silver electrodes are immersed in water or an organic solvent such as alcohol.

2.     An electric arc is passed between the two electrodes, which vaporizes the silver.

3.     The vapor then condenses into tiny particles, which remain suspended in the liquid medium as a colloidal solution.

4.     The colloidal solution is then stabilized by adding an appropriate stabilizing agent to prevent the particles from coagulating.

The Bredig's arc method is a simple and effective technique for the preparation of colloidal solutions of metals. It is widely used in the production of colloidal silver, which has several industrial and medicinal applications. However, the method has certain limitations, such as the high cost of the equipment and the difficulty in controlling the particle size and distribution.

Peptization

Peptization is the process of converting a precipitate into a colloidal sol by shaking it with the dispersion medium (solvent) in the presence of a small amount of an electrolyte. In this process, the precipitate is broken down into smaller particles and dispersed uniformly throughout the solvent to form a stable colloid.

The process of peptization involves the following steps:

1.     Formation of primary particles: When a salt is added to a solution containing ions of a different element, it may react to form a precipitate. The precipitate consists of large particles, which are not stable and tend to settle down.

2.     Adsorption of ions: The addition of an electrolyte results in the adsorption of ions on the surface of the precipitate. The adsorbed ions carry opposite charges to that of the precipitate, which results in the formation of an electrical double layer around the particles.

3.     Repulsion between like charges: Due to the electrical double layer, the particles acquire a like charge, which results in their mutual repulsion. This repulsion prevents the particles from aggregating and settling down.

4.     Formation of colloidal particles: The shaking action of the solvent provides energy to the particles, which breaks them down into smaller particles. These smaller particles are stabilized by the adsorbed ions and form a stable colloidal sol.

Peptization is used in the preparation of many colloidal sols, including gold sol, arsenious sulphide sol, and ferric hydroxide sol.

Purification of Colloidal Solutions

Dialysis

Dialysis is a process of purification of colloidal solutions based on the principle of diffusion of solutes through a semi-permeable membrane. A semi-permeable membrane is a membrane that allows the passage of solvent molecules but blocks the passage of solute molecules. The membrane used for dialysis can be made of cellulose, cellophane, or any other suitable material.

In the process of dialysis, the colloidal solution is placed in a bag made of semi-permeable membrane and the bag is suspended in a large volume of water. Due to the concentration gradient, the smaller ions or molecules present in the colloidal solution diffuse through the membrane and move into the surrounding water. However, the larger colloidal particles are unable to pass through the membrane and remain inside the bag.

This process continues until a state of equilibrium is reached, with the concentration of the smaller ions or molecules inside the bag becoming equal to the concentration of these species in the surrounding water. The resulting solution inside the bag is a purified form of the original colloidal solution.

Dialysis is a widely used method for the purification of colloidal solutions, especially biological macromolecules such as proteins and enzymes. The purified solutions obtained by dialysis can be used for further experiments or for various applications.

 

Electro-dialysis

Electrodialysis is a process used for the separation of ions in a solution using an applied electric field. It is a type of membrane-based separation process, which involves the use of selective membranes to separate ions based on their charge.

In electrodialysis, the solution to be separated is placed between two electrodes, which are separated by ion-selective membranes. These membranes allow only certain ions to pass through them based on their charge. When an electric potential is applied across the electrodes, the positively charged ions are attracted to the negatively charged electrode and the negatively charged ions are attracted to the positively charged electrode. As a result, the solution is separated into two streams, one containing the positively charged ions and the other containing the negatively charged ions.

The process of electrodialysis is used for desalination of water, purification of food products, and recovery of valuable materials from industrial wastewater. It is also used for the removal of harmful ions from wastewater before it is discharged into the environment.

One of the major advantages of electrodialysis is that it is a continuous process, which means that the solution can be continuously separated into two streams without the need for any additional steps. However, the process is limited by the type of membranes used, the concentration of ions in the solution, and the applied electric potential.

Ultrafiltration

Ultrafiltration is a process of purifying colloidal solutions based on the principle of size exclusion. It is a type of membrane filtration in which a semipermeable membrane is used to separate particles based on their size. The membrane used in ultrafiltration has very small pores, typically in the range of 10-100 nm. This means that particles larger than the pore size are retained by the membrane, while smaller particles and solvent molecules pass through the membrane.

In the case of colloidal solutions, the ultrafiltration process is used to remove impurities such as ions, small molecules, and soluble salts, which can destabilize the colloidal particles and cause them to aggregate. Ultrafiltration is also used to concentrate dilute colloidal solutions by selectively removing the solvent from the solution.

The ultrafiltration process involves passing the colloidal solution through a semipermeable membrane under pressure. The pressure applied is typically in the range of 1-10 atm. The colloidal solution is fed to one side of the membrane, and the purified solution or concentrated colloidal solution is collected from the other side of the membrane.

Ultrafiltration is a widely used technique for the purification and concentration of colloidal solutions in various fields, including pharmaceuticals, food processing, and biotechnology. It is a highly efficient and cost-effective method for the separation of colloidal particles from impurities.

Properties of Colloidal Solutions

Colligative properties of colloids

Colligative properties of solutions are dependent on the concentration of solute particles, regardless of the nature of the solute. However, in the case of colloidal solutions, the concentration of dispersed phase particles is very low, and hence the colligative properties of colloidal solutions are negligible.

Tyndall effect

The Tyndall effect is the scattering of light by colloidal particles present in a solution. When a beam of light is passed through a colloidal solution, the light is scattered by the dispersed phase particles, and the path of the beam becomes visible. This effect can be observed as a bright cone of light visible when a flashlight is shone through a foggy room. The extent of scattering depends on the size, shape, and concentration of the colloidal particles.

The Tyndall effect can be used to distinguish between a true solution and a colloidal solution. In a true solution, the particles are very small, and the light passes straight through the solution without any scattering, making the beam invisible. However, in a colloidal solution, the particles are large enough to scatter light, making the path of the beam visible.

 

Colour of colloidal solution

Color of colloidal solutions: Colloidal solutions often exhibit colors due to the selective absorption of light by the dispersed phase. The colors observed depend on the size of the particles, their shape, and the nature of the dispersed phase. For example, gold sols are red, while silver sols are yellow.

Brownian movement

Brownian movement: Brownian movement is the random, zigzag movement exhibited by colloidal particles when suspended in a liquid medium. This movement is caused by the collisions of the particles with the molecules of the dispersion medium. Brownian movement provides evidence for the existence of colloidal particles, and can be observed through a microscope as a continuous, irregular motion of the particles.

Charge on colloidal particles

Charge on colloidal particles: Colloidal particles usually carry an electric charge due to the preferential adsorption of ions from the dispersion medium onto their surface. The charge on the particles can be either positive or negative, depending on the nature of the particle and the medium. The presence of an electric charge is an important factor in the stability of colloidal solutions, as like charges repel each other and opposite charges attract.

Electrophoresis

Electrophoresis: Electrophoresis is a technique used to separate charged particles in a colloidal solution based on their charge and size. When an electric field is applied to a colloidal solution, charged particles migrate towards the electrode of opposite charge at a rate proportional to their charge and size. Electrophoresis is commonly used in the purification and characterization of colloidal particles.

Electroosmosis

Electroosmosis: Electroosmosis is the movement of a liquid medium in a porous material in response to an electric field. In the case of a colloidal solution, the movement of the liquid medium can cause the particles to move along with it, resulting in a separation of the particles. Electroosmosis can be used as a technique for the purification and separation of colloidal particles.

 

Coagulation

Coagulation or precipitation is the process of settling down or aggregation of colloidal particles, leading to the formation of a precipitate. This can occur due to various reasons like addition of an electrolyte, change in pH, heating, etc.

The process of settling of colloidal particles is called coagulation or precipitation of the sol.

 The coagulation of the lyophobic sols can be carried out in the following ways:

(i) By electrophoresis: The colloidal particles move towards oppositely charged electrodes, get discharged and precipitated.

(ii) By mixing two oppositely charged sols: Oppositely charged sols when mixed in almost equal proportions, neutralise their charges and get partially or completely precipitated. Mixing of hydrated ferric oxide (+ve sol) and arsenious sulphide (–ve sol) bring them in the precipitated forms. This type of coagulation is called mutual coagulation.

(iii) By boiling: When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with the molecules of dispersion medium. This reduces the charge on the particles and ultimately leads to settling down in the form of a precipitate.

(iv) By persistent dialysis: On prolonged dialysis, traces of the electrolyte present in the sol are removed almost completely and the colloids become unstable and ultimately coagulate.

(v) By addition of electrolytes: When excess of an electrolyte is added, the colloidal particles are precipitated. The reason is that colloids interact with ions carrying charge opposite to that present on themselves. This causes neutralisation leading to their coagulation. The ion responsible for neutralisation of charge on the particles is called the coagulating ion. A negative ion causes the precipitation of positively charged sol and vice versa.

coagulation of the lyophobic sols

Coagulation of lyophobic sols: Lyophobic sols are colloidal solutions where the dispersed phase particles are not attracted to the dispersion medium. These sols are less stable than lyophilic sols and can be easily coagulated. The addition of an electrolyte to a lyophobic sol causes the ions of the electrolyte to neutralize the charges on the colloidal particles. This neutralizes the charges and reduces the electrostatic repulsion between the particles, leading to coagulation. The charge on the particles can also be neutralized by adding an opposite charge or a substance with opposite charge called a coagulating agent. This can lead to the formation of bigger particles which settle down due to gravity, forming a precipitate.

flocculating power

Flocculating power: The ability of an ion to coagulate a given sol is called its flocculating power. The flocculating power of an ion is determined by its valency and the nature of the ion. Higher the valency of the ion, greater is its flocculating power. For example, Al3+ and Fe3+ ions have higher flocculating power than Na+ and K+ ions. The nature of the ion also plays a role. For example, Cl- ion has lower flocculating power than SO42- ion.

The flocculating power of various ions can be compared by their flocculation values. The flocculation value is the minimum concentration of an electrolyte required to cause the coagulation of a given sol.

In summary, coagulation of colloidal particles leads to the formation of a precipitate. This can be caused by various factors like addition of electrolytes, coagulating agents, change in pH, heating, etc. The ability of an ion to coagulate a given sol is called its flocculating power, which is determined by its valency and nature.

 

Hardy-Schulze rule

Hardy-Schulze rule: It states that the greater the valency of the ion added, the greater is its power to cause precipitation (coagulation) of the oppositely charged sols. This rule is applicable only to the coagulation of electrolyte sols.

Coagulation of lyophilic sols

Coagulation of lyophilic sols: Lyophilic sols are not easily coagulated because of the strong interaction between the dispersed phase and the dispersion medium. However, coagulation can be induced by heating, addition of electrolytes or by mixing with another lyophilic sol which is not miscible with the first.

Flocculating power: It is the minimum concentration of an electrolyte required to cause the coagulation of a sol. The flocculating power of an ion decreases as its size increases and its valency decreases.

Protection of colloids

Protection of colloids: Colloids can be protected from coagulation by adding certain substances called protective colloids or stabilizers. These substances get adsorbed on the surface of the colloidal particles and prevent the coagulating ions from approaching them. Examples of protective colloids are gelatin, starch, gum, etc.

Applications of colloids

Applications of colloids: Colloids have a wide range of applications in various fields, some of which are:

1.     Medicine: Colloidal solutions are used in the treatment of various diseases. For example, colloidal gold is used in the treatment of rheumatoid arthritis.

2.     Photography: Colloidal silver is used in black and white photography to produce photographic prints.

3.     Food industry: Colloidal solutions are used as emulsifiers, stabilizers and thickeners in the food industry. For example, gelatin is used in the production of jellies and ice cream.

4.     Textile industry: Colloidal solutions are used in the textile industry for dyeing and printing fabrics.

5.     Paints: Colloidal solutions are used in the production of paints and pigments.

6.     Cosmetics: Colloidal solutions are used in the production of cosmetics like lotions and creams.

7.     Pollution control: Colloidal solutions are used in the treatment of waste water and air pollution control.

8.     Nanotechnology: Colloidal solutions are used in the production of nanomaterials for various applications.