Electrical Properties of Colloidal Solution

Q1. How you will account for the presence of electrical charge on colloidal particles?

Ans:In a particular colloidal solution, all the colloidal particles carry the same type of charge, while the dispersion medium has an equal but opposite charge. Thus, the charge on colloidal particles is balanced by that of the dispersion medium and the colloidal solution as a whole is electrically neutral.

For example:  In a ferric hydroxide {Fe(OH)3} sol, the colloidal ferric hydroxide particles are positively charged, while the dispersion medium carries an equal and opposite negative charge.

Q2. How is the presence of charge on colloidal particles responsible for the stability of colloidal sols?

Ans: The stability of a colloidal solution is mainly due to the presence a particular type

of charge on all the colloidal present in it. Due to the presence of similar and equal charges, the colloidal particles repel one another and are thus unable to combine together to form larger particles or settle down . This keeps them dispersed in the dispersion medium and the colloidal remains stable. This is why sol particles do not settle down even on standing for a long time.

Q3. Explain the classification of colloidal solution depending upon the nature of charge on the particles of the dispersed phase?

Ans: the colloidal solutions can be classified into positively charged and negatively charged colloids. Some of the typical examples of it are as follows:

                             

# THEORIES OF ORIGIN OF CHARGE ON COLLOIDAL PARTICLES

Q4: How will you account for the origin of Charge on Colloidal Particles?

Ans: There are several views regarding the origin of charge on colloidal particles. According to these views, colloidal particles acquire charge due to the following reasons.

1.Due to dissociation of the adsorbed molecular electrolytes: (i) (i) Colloidal particles have a strong tendency to adsorb reactant or product molecules. The molecules thus adsorbed on the surface of colloidal particles may undergo dissociation/ionization and may impart charge to them.

For example: During the preparation of sulphide sols (e.g., As2S3 sol), H2S molecules get adsorbed on colloidal particles. H2S molecules thus adsorbed undergo ionization and release H+ ions into the medium. Consequently, colloidal particles are left with negative charge.

(ii) Due to the dissociation of molecules forming colloidal aggregates: The molecules responsible for the formation of aggregates of colloidal dimensions may themselves undergo dissociation/ionisation resulting in the development of charge on the colloidal particles formed by their aggregation.

For example: The soap molecules (RCOONa) dissociate to give RCOO- and Na+ ions. RCOO- ions aggregate due to weak attractive forces present in the hydrocarbon chains together to form micelles which carry negative charge as explained earlier.

(2) Due to frictional electrification

(i) It is supposed that the frictional electrification due to the rubbing of the dispersed phase particles with that of dispersion medium results in some charge on the colloidal particles.

(ii) The dispersion medium must also get some charge, due to the friction. As it does not carry any charge, theory does not appear to be correct.

(3) Due to selective adsorption of ions / Due to preferential adsorption of ions from solutions: The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution. A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice.

For example: When a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a tendency to adsorb preferentially the Fe3+ ions present in the solution. This is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+ ions thus adsorbed impart positive charge to the colloidal particles present in the sol.

        

   

(ii) For example: When a small quantity of silver nitrate (AgNO₃) solution is added to a large quantity of potassium iodide KI solution, colloidal particles of the silver iodide adsorb I^- from the solution to become negatively charged, (at this stage KI is in excess, and I^-being common to AgI)   

         

But, when a small quantity of potassium iodide (KI) solution is added to a large quantity of silver nitrate solution (AgNO₃); the colloidal silver iodide particles adsorb Ag⁺ from the solution to become positively charged, (at this stage AgNO₃ is in excess and Ag⁺ is common to AgI),

          

1. Electrophoresis: Due to the presence of a particular type of electrical charge, the colloidal particles present in a colloidal dispersion move towards a particular electrode under the influence of an electric field.

The direction of movement of the colloidal particles is decided by the nature of charge present on them. If the colloidal particles carry positive charge, they move towards cathode when subjected to an electric field and vice versa. The phenomenon is called electrophoresis.

 It  may be defined as “ the movement of colloidal particles towards a particular electrode under the influence of an electric field”.

Electrophoresis is an important phenomenon and finds several applications in industry.

       2. Electro-Osmosis:

It may be defined as the  movement of dispersion medium under the influence of an electric field in the situation when the movement of colloidal particles is prevented with the help of a suitable membrane.

The colloidal solution is placed between two partitions made by semi permeable membranes.The outer compartments consisting of platinum electrodes and side tubes are filled with water. On passing electric current, water level begins to rise in one of the side tubs and falls in the other.

The phenomenon can be explained as follows :

We have already seen that the colloidal particles and dispersion medium carry charges which are equal but opposite in nature. Under the influence of an electric field, both have a tendency to move towards the oppositely charged electrodes. Semi permeable membranes do not allow the passage of colloidal particles. However, dispersion medium can pass through them. Therefore during electro-osmosis, colloidal particles are checked and it is the dispersion medium that moves towards the oppositely charged electrode.

3. Electrokinectic Potential OR Zeta Potential

Having acquired a positive or a negative charge by selective adsorption on the surface of a colloidal particle as stated above, this layer attracts counter ions from the medium forming a second layer, as shown below. AgI/I- K+ AgI/Ag+ I – .

The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer. According to modern views, the first layer of ions is firmly held and is termed fixed layer while the second layer is mobile which is termed diffused layer.

Since separation of charge is a seat of potential, the charges of opposite signs on the fixed and diffused parts of the double layer results in a difference in potential between these layers. This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

                         

This potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

Noted Credit: Mr Pradeep Sharma

Mechanical Properties of Colloidal Solution (Brownian Movement)

Colloidal particles present in a colloidal solution exhibit a very important property called Brownian movement.

Q1. WHAT IS BROWNIAN MOVEMENT ?

Answer: “The continuous zig-zag movement of the colloidal particles in a colloidal solution is called Brownian movement. “

NOTE 1: The property was discovered by a botanist Robert Brown in 1827, when he observed that pollen grains suspended in water exhibit random zig-zag motion. After the name of the discoverer, the property was named as Brownian movement.

NOTE 2: More significantly, Albert Einstein’s later explained the Brownian motion more clearly in his paper stating that the pollen was moved by water molecules. This discovery served as great evidence of the existence of atoms and molecules.

Understanding Brownian movement is crucial as it forms a base for the modern atomic theory. The kinetic theory of gases is also based on the Brownian motion model of particles.

Q2. WHAT IS THE  CAUSE OF BROWNIAN MOVEMENT?

Ans: Brownian movement is due to the unequal bombardments                                  

of the moving molecules of dispersion medium on colloidal particles. 

The moving molecules of the dispersion medium continuously attack on colloidal particles from all sides and impart momentum to them.

Since the chances of their collisions are unequal, the net driving force on a colloidal particle forces it to move a particular direction.

As the particle moves in that direction, other molecules of the medium again collide with it and the particle changes its direction. The process continues. This results in a random zigzag movement of the colloidal particle.

Q3. WHAT ARE THE EXAMPLES OF BROWNIAN MOTION?

Ans: 1. Dust and smoke particles show continuous motion in beam of light in cinema hall.

2. Brownian motion is also observed in the plasma of cells in which the particles in the cell are also in random motion without making plasma in the cell dry.

3. Pollen grains suspended in water exhibit random zig-zag motion.

Q4. WHAT ARE THE IMPORTANCE OF BROWNIAN MOVEMENT?

                                                                 OR

HOW IS BROWNIAN MOTION IS RESPONSIBLE FOR STABILITY OF SOL?

Ans: Brownian movement plays an important role in imparting stability to a sol. This is because Brownian movement opposes the gravitational forces acting on colloidal particles and prevents them from getting settled down.

Q5. HOW DOES THE  SIZE OF COLLOIDS PARTICLES AFFECT  BROWNIAN MOTION?

Ans: The size of the particles is inversely proportional to the speed of the motion, i.e. Small particles exhibit faster movements. This is because the transfer of momentum is inversely proportional to the mass of the particles. Lighter particles obtain greater speeds from collisions.

Q6. HOW DOES THE  VISCOSITY OF DISPERSION MEDIUM AFFECT  BROWNIAN MOTION?

 Ans:The speed of the Brownian motion is inversely proportional to the viscosity of the fluid. The lower the viscosity of the fluid, the faster the Brownian movement.

Notes Credit: Pradeep Sharma