Electric Double Layer and Zeta Potential

Electric Double Layer

• Charged particles in suspension form an electric double layer to neutralize their charge.
• The solvent directly surrounding the particle is tightly bound to the surface, forming the Stern layer, which contains mostly counter-ions (opposite charge to the particle).
• The diffuse layer surrounds the Stern layer and contains more counter-ions than co-ions (same charge as the particle)
• The diffuse layer ions are mobile due to thermal energy, constantly moving between the particle surface and the main body of the continuous phase.

Zeta Potential

• The electric potential difference between the particle surface and the electroneutral region is the Nernst potential.
• Zeta potential is the potential difference between the shear plane (boundary of the Stern layer) and the electroneutral region.
• Zeta potential is a key indicator of dispersion stability; a high zeta potential indicates strong electrostatic repulsion between particles, promoting stability.
• Low zeta potential leads to weaker repulsion, allowing attractive forces to dominate and causing flocculation (particle aggregation).
• Suspensions with high zeta potentials (positive or negative) are electrically stabilized, while those with low zeta potentials tend to coagulate or flocculate.

Formulation of Suspensions

Particle Size Control

• Decreasing particle size reduces sedimentation rate.
• Large particles (> 5µm) lead to gritty textures and potential irritation if injected or instilled into the eyes.
• Particles larger than 25µm can block hypodermic needles.
• Smaller particle size increases total surface area (A) and surface free energy (ΔF):
• ΔF = γ . ΔA
• Where γ is the interfacial tension between the liquid and solid phases.

Surface Potential

• Lyophobic colloidal systems stability depends on surface potential.
• Dispersed particles in suspension gain charge relative to their surrounding liquid medium.
• Causes of surface potential:
• Electrolyte adsorption: Selective adsorption of ions from the surrounding medium onto the particle surface creates a charged layer.
• Surface-active agent ionization: Some surfactants adsorbed at the solid-liquid interface ionize, giving particles a charge.
• Functional group ionization: Ionizable groups on the surface of particles can contribute to charge, with the degree of ionization influenced by pH.

Electric Double Layer Formation

• When dispersed particles are in contact with an electrolyte solution, they may selectively adsorb one type of ion.
• Potential-determining ions (or co-ions) are the ions adsorbed onto the particle surface, determining its overall charge.
• Counter-ions are oppositely charged ions in the solution, attracted to the charged surface by electric forces.
• Counter-ions repel the approach of other co-ions towards the particle surface, maintaining charge balance.
• Electric forces and thermal motion keep the ions distributed evenly in the solution.

Flocculation Control

• To convert a deflocculated suspension (stable, individual particles) to a flocculated suspension (aggregates), electrolytes, surfactants, and/or hydrophilic polymers can be added.

Electrolytes

• Electrolytes reduce zeta potential, decreasing repulsion between particles and promoting flocculation.
• Role:
• Lower the electric barrier between particles.
• Reduce the Zeta Potential.
• Form bridges between particles, creating a loosely associated structure.

Surfactants

• Ionic surfactants: can cause flocculation by neutralizing particle charge
• Nonionic surfactants: have minimal effect on particle charge density but can form a flocculated structure by adsorbing onto multiple particles due to their linear structure.
• Examples:
• Bismuth subnitrate in water has a large positive charge and high zeta potential, preventing flocculation.
• The addition of monobasic potassium phosphate (electrolyte) adsorbs negative charge onto the bismuth subnitrate particles, decreasing zeta potential.
• Continued addition of potassium phosphate further reduces zeta potential, leading to deflocculation at sufficiently negative values.
• Sedimentation volume changes indicate flocculation levels; maximum sedimentation volume reflects maximum flocculation and the absence of caking.

Polymeric flocculants

• Polymers like starch, alginates, cellulose derivatives, tragacanth, or silicates can control flocculation.
• Mechanism:
• These polymers adsorb onto particle surfaces due to their linear structure, forming a gel-like network that keeps particles flocculated.
• This results in a large sedimentation volume, although some settling can occur.
• Excessive polymer concentration can coat individual particles, resulting in a deflocculated system due to the formation of a hydrophilic barrier around each particle.
• Proper blending during manufacture is critical to prevent excessive polymer adsorption onto individual particles, ensuring the formation of a flocculated structure.

This quiz explores the concepts of electric double layers and zeta potential in colloidal systems. It covers the roles of the Stern layer and the diffuse layer, as well as the significance of zeta potential in dispersion stability. Test your understanding of these fundamental concepts in colloidal chemistry.