Question

Choose the correct reason(s) for the stability of the Iyophobic colloidal particles. (A) Preferential adsorption of ions on their surface from the solution (B) Preferential adsorption of solvent on their surface from the solution (C) Attraction between different particles having opposite charges on their surface (D) Potential difference between the fixed layer and the diffused layer of opposite charges around the colloidal particles

Short answer

The stability of the lyophobic colloidal particles is due to (A) the preferential adsorption of ions on their surface from the solution, and (D) the potential difference between the fixed layer and the diffused layer of opposite charges around the colloidal particles.

Step-by-step solution

Understanding the Concept

It is important to understand that the stability of lyophobic colloids (such as metals and their insoluble compounds) arises from the charges they acquire when dispersed in a particular medium. These charges prevent the particles from coalescing by offering a repulsive force, thus keeping them stable and in suspension.

Analyzing Each Option

Option (A) Preferential adsorption of ions on their surface from the solution can lead to the development of a charge on the surface of the colloidal particles, which can contribute to their stability. Option (B) Preferential adsorption of solvent is not related to charge stability and in the case of lyophobic colloids, the solvent molecules do not preferentially adsorb to the particle surface. Option (C) Attraction between particles with opposite charges would lead to coagulation, not stability. Option (D) is known as the zeta potential, which is the potential difference between the fixed layer and the diffused layer of opposite charges around the colloidal particles and is directly related to the stability of the colloids.

Choosing the Correct Option(s)

On analyzing the given options, we conclude that the correct reasons for stability of lyophobic colloidal particles are (A) and (D), as both are associated with the presence and maintenance of charge on colloidal particles, thereby contributing to their stability.

Key concepts

Charge on Colloidal Particles

In colloidal systems, particles are often dispersed throughout a medium and carry an electric charge. This charge typically originates from several sources, including the dissociation of surface molecules, ionization of groups on the particles, or preferential adsorption of ions from the surrounding medium.

For lyophobic colloids, such as those made of metals or their insoluble compounds, their stability is highly dependent on the charge they carry. When these particles have a charge, they repel each other due to electrostatic forces. If the particles were uncharged or had opposite charges, they would attract each other and possibly aggregate or coagulate, leading to a destabilized system. Hence, the charge on colloidal particles is a fundamental aspect of preventing aggregation and maintaining the colloid's stability.

The charge on colloidal particles can be manipulated by changing the pH or ionic strength of the solution, which can affect the degree of ionization or adsorption of ions. Therefore, understanding the charge dynamics is crucial for applications that rely on the controlled stability of colloids, such as in pharmaceuticals, food products, and paints.

Zeta Potential

Zeta potential is a key indicator of the stability of colloidal systems. It is defined as the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle.

The significance of zeta potential lies in its ability to provide insight into the strength of electrostatic repulsion or attraction between particles. A high zeta potential means there is strong repulsion among particles, which prevents them from coming close enough to coagulate. Conversely, a low zeta potential indicates that the attractive forces may overcome the repulsive forces, leading to instability and aggregation of particles.

Measuring the zeta potential allows researchers and manufacturers to predict and control the stability of colloidal systems. It also helps in understanding the effects of factors like pH, ionic strength, and addition of surfactants on the colloidal stability. Zeta potential can be influenced by the type and concentration of ions present in the solution, emphasizing the connection between zeta potential and the preferential adsorption of ions.

Preferential Adsorption of Ions

What is Preferential Adsorption?

Preferential adsorption is a phenomenon where certain ions are more likely to be adsorbed onto the surface of colloidal particles than others. This occurs because colloids can attract ions that are present in the solution, which can bind to their surface.

The choice of which ions are adsorbed depends on the specific interactions between the ions and the particle surface. For example, if a colloid has acidic groups on its surface, it will tend to adsorb positively charged ions (cations). On the other hand, if the surface has basic groups, negatively charged ions (anions) may be preferentially adsorbed.

Impact on Colloidal Stability

Preferential adsorption can have a direct impact on the charge of colloidal particles and, consequently, their stability. By adsorbing ions, colloids become charged and create a repulsive barrier that prevents the particles from coming together. This repulsion plays a crucial role in maintaining the suspension and stabilizing colloidal systems. When formulating colloids for industrial use, controlling the types and amounts of ions available for adsorption is a vital consideration in achieving the desired level of stability.

Repulsive Forces in Colloids

Repulsive forces in colloidal systems are the invisible shields that maintain the dispersion and prevent the tiny particles from aggregating. Two main types of repulsive forces are common in colloids: electrostatic repulsion and steric repulsion.

Electrostatic repulsion occurs when colloidal particles carry like charges; since similar charges repel, the particles push away from each other, maintaining a stable dispersion. This repulsion is enhanced by the presence of a surface charge and the subsequent formation of an electrical double layer around the particle.

Steric repulsion, on the other hand, involves layers of molecules adsorbed on the surfaces of colloidal particles that create a physical barrier to prevent the particles from getting close enough to coalesce. This is most often seen in polymeric colloids where the long chains of molecules extend out into the solution and resist compression.

Both electrostatic and steric repulsive forces play a role in the stability of lyophobic colloids. These forces, together with factors like zeta potential and preferential adsorption of ions, collectively contribute to the stability and longevity of colloidal suspensions across a wide array of applications, from medical formulations to industrial suspensions.