Electrical Properties of Interfaces

Electrical Properties of Interfaces

• Charges on dispersed particles in liquid media can arise from selective adsorption of specific ions from solution.
• The majority of particles dispersed in water acquire a negative charge due to preferential adsorption of OH⁻ ions. Ions from an electrolyte or, in pure water, H⁺ or OH⁻ ions are involved.
• Example: Adding KI to an equimolar solution of AgNO₃ produces positively charged AgI colloid due to excess Ag⁺ ions. Conversely, adding AgNO₃ to KI produces negatively charged AgI colloid due to excess I⁻ ions.
• Ionization of Groups: The total charge is pH dependent.
  • In alkaline solutions, protein molecules' carboxylic acid groups ionize into carboxylate anions (NH₂-R-COO⁻).
  • In acidic solutions, amino groups protonate (NH₃⁺-R-COOH).
  • Proteins are negatively charged in alkaline and positively charged in acidic solutions. They exist as zwitterions (electrically neutral) at a specific pH (isoelectric point, IEP), where both groups are ionized. Solubility is minimum at IEP, leading to precipitation at this pH.
• Charge due to Dielectric Constant (DEC) difference: Another way particles can acquire charge is due to differences in DEC between the particle and the dispersion medium. A higher DEC particle compared to the medium acquires a positive charge (and vice versa). This happens through electron transfer from higher to lower DEC substances.
• DEC: A physicochemical property of a solvent, indicating the energy required to separate opposite charges in the solvent compared to vacuum. It's a measure of a solvent's ability to induce dipoles in other molecules.
  • Applications of DEC measurements include:
    • Determining the polarity of solvents (higher DEC = higher polarity).
    • Assessing drug solubilization in solvents (higher DEC = higher solubilization).
    • Choosing solvents or solvent mixtures for drug applications.

The Electrical Double Layer

• A solid surface with adsorbed cations in a solution with ions. Cations are attached to surface. Remaining cations and anions remain in the solution.

• Anions are attracted to the positive surface by electrical forces, also repelling further cation approach after initial adsorption.

• At equilibrium, some excess anions drawn to surface, while others are distributed less densely further away from the charged surface. Anions and cations concentrations are equal resulting in an electrically neutral system.

• The diffuse double layer consists of two layers:

  • A first tightly bound layer (aa₁bb₁).
  • A second layer (bb₁cc₁) that is more diffuse.

• A region of tightly bound solvent molecules and some counter ions adjacent to the surface, are restricted to a shear plane (bb₁). The attraction between solvent molecules and counter-ions is such that if the surface moves compared to the liquid, the shear plane moves rather than the surface.

• Three other electrostatic conditions are possible.

  • If counter ions in the tightly bound layer are fewer than the potential determining ions on the solid surface, the net charge at the shear plane is positive.
  • If the counter ions are equal to the potential determining ions, the electric neutrality in the tightly bound region occurs.
  • If total charge of counter ions is greater than potential determining ions, the net charge at the shear plane is negative.

Electrostatic Potentials

• Electro-Thermodynamic potential (Nernst potential): The potential at the solid surface (aa₁) due to potential-determining ions. Difference in potential between the surface and the electro-neutral solution region.
• Electro-Kinetic potential (zeta potential): The potential at the shear plane (bb₁). Difference in potential between the tightly bound layer and the electro-neutral region of solution.
• Zeta potential, rather than Nernst potential, governs the repulsion between similarly charged dispersed particles.
• Flocculation: If zeta potential falls below a threshold, attractive forces triumph over repulsive forces, and particles aggregate.