Surface and interfacial phenomenon: Electrical properties of interfaces and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector exam
ELECTRICAL PROPERTIES OF INTERFACES:
The existence of difference in electrical potential across a solid–liquid interface is demonstrated by the following phenomena:
Electrophoresis: Movement of dispersed particles through a liquid medium under the influence of an electric field.
Electro-osmosis: Movement of a liquid relative to a fixed solid under the influence of an electric field.
Streaming potential: Potential difference set up across a fixed porous plug of solid when a liquid is forced through it.
Sedimentation potential: Potential difference set up between the top and bottom of dispersion of solid particles in a liquid when particles settle under the influence of gravity.
The above-mentioned electrokinetic phenomenon across an interface indicates that there must be a particular distribution of charge near the interface. This distribution is referred to as the electrical double layer.
Electrical Double Layer:
Let us consider solid particles carrying positive charge in contact with an aqueous solution containing positive and negative ions. The positively charged solid surface will influence the distribution of ions in the nearby layers of the solution. Thus, negative ions will be attracted towards the solid surface and negative ions repelled away from it. The resulting effects create a diffuse layer of solution in which negative ions gradually decrease on moving away from the interface and positive ions gradually increase. This type of distribution is referred to as the electrical double layer.
Stern layer: Strong adsorption of oppositely charged ions to the surface of particle.
Gouy layer: Distribution of oppositely charged ions in the diffuse layer.
The distribution of ions will affect the potential at varying distances. Potential decreases linearly across the Stern layer (stern potential; potential at the boundary between Stern and Gouy layer) from the surface potential and then decreases comparatively slowly until it is zero at the edge of the Gouy layer.
A layer of liquid will also be adsorbed onto the solid particle (solvating layer).
Fig 1 – Idealized representation of the electrical double layer(taken from conceptual representation of the electric double layer Research gate)
This solvating layer is strongly held to the surface and its outer surface represents the boundary of relative movement between the solid and the liquid. The potential at this point is termed as Zeta potential.
- Increasing the amount of electrolytes or increasing the valency of the counterion (keeping the total concentration of the electrolyte constant) decreases the Stern and Zeta potentials owing to the decrease in thickness of the double layer.
- Zeta potential acts as an energy barrier for the stability of colloids and suspensions.
- Zeta potential determines the degree of repulsion between adjacent, similarly charged dispersed particles and therefore has practical applications in the stability of systems containing dispersed particles.
Multiple choice questions (MCQs)
1.Movement of dispersed particles through a liquid medium under the influence of an electric field is called as
a)Electrophoresis
b)Electro-osmosis
c)Streaming potential
d)Sedimentation potential
2.Movement of a liquid relative to a fixed solid under the influence of an electric field is known as
a)Electrophoresis
b)Electro-osmosis
c)Streaming potential
d)Sedimentation potential
3.Potential difference set up across a fixed porous plug of solid when a liquid is forced through it is known as
a)Electrophoresis
b)Electro-osmosis
c)Streaming potential
d)Sedimentation potential
4.Potential difference set up between the top and bottom of dispersion of solid particles in a liquid when particles settle under the influence of gravity is called
a)Electrophoresis
b)Electro-osmosis
c)Streaming potential
d)Sedimentation potential
5.The existence of difference in electrical potential across a solid–liquid interface is demonstrated by which of the following phenomena?
a)Electro-osmosis
b)Streaming potential
c)Sedimentation potential
d)All of the above
6.Which of the following is strong adsorption of oppositely charged ions to the surface of particle?
a)Stern layer
b)Gouy layer
c)Both of these
d)None of these
7.Which of the following is distribution of oppositely charged ions in the diffuse layer?
a)Stern layer
b)Gouy layer
c)Both of these
d)None of these
8.The distribution of ions will affect the potential at varying distances.
a)True
b)False
9.Potential decreases linearly across the Stern layer (stern potential; potential at the boundary between Stern and Gouy layer) from the surface potential and then decreases comparatively slowly until it is zero at the edge of the Gouy layer.
a)True
b)False
10.The solvating layer is strongly held to the surface and its outer surface represents the boundary of relative movement between the solid and the liquid. The potential at this point is termed as
a)Zeta potential
b)Nernst potential
c)Stern layer
d)All of these
11.Which of the following is correct statement?
a)Increasing the amount of electrolytes or increasing the valency of the counterion (keeping the total concentration of the electrolyte constant) decreases the Stern and Zeta potentials owing to the decrease in thickness of the double layer.
b)Zeta potential acts as an energy barrier for the stability of colloids and suspensions.
c)Zeta potential determines the degree of repulsion between adjacent, similarly charged dispersed particles and therefore has practical applications in the stability of systems containing dispersed particles.
d)All of the above
12.Immediately adjacent to the interface in electric double layer model is a region called
a)Tightly bound layer
b)Diffuse second layer
c)Nernst potential
d)Zeta potential
13.Excess negative ions are present in this region in electric double layer model
a)Tightly bound layer
b)Diffuse second layer
c)Nernst potential
d)Zeta potential
14.It is the potential of solid surface itself owing to the presence of potential determining ions
a)Tightly bound layer
b)Diffuse second layer
c)Nernst potential
d)Zeta potential
15.It is the potential observed at shear plane.
a)Tightly bound layer
b)Diffuse second layer
c)Nernst potential
d)Zeta potential
Solutions:
- a)Electrophoresis
- b)Electro-osmosis
- c)Streaming potential
- d)Sedimentation potential
- d)all of the above
- a)Stern layer
- b)Gouy layer
- a)True
- a)True
- a)Zeta potential
- d)All of the above
- a)Tightly bound layer
- b)Diffuse second layer
- c)Nernst potential
- d)Zeta potential
References:
1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 136-137.
2. Martins Physical Pharmacy, 6th edition 2011, page no. 705-707.
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