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Questions and Answers
What occurs to anions near a positively charged surface after initial adsorption is complete?
What occurs to anions near a positively charged surface after initial adsorption is complete?
What characterizes the first layer (aa₁bb₁) in the diffuse double layer?
What characterizes the first layer (aa₁bb₁) in the diffuse double layer?
What determines the net charge at the shear plane (bb₁)?
What determines the net charge at the shear plane (bb₁)?
What does the zeta potential represent in the context of the diffuse double layer?
What does the zeta potential represent in the context of the diffuse double layer?
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What happens if the number of counter ions in the tightly bound layer is equal to the number of potential determining ions?
What happens if the number of counter ions in the tightly bound layer is equal to the number of potential determining ions?
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Which of the following is true regarding the electro-thermodynamic potential (Nernst potential)?
Which of the following is true regarding the electro-thermodynamic potential (Nernst potential)?
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What is the relationship between zeta potential and repulsion between dispersed particles?
What is the relationship between zeta potential and repulsion between dispersed particles?
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If the total charge of counter ions in the region aa₁bb₁ is greater than that of the potential determining ions, what will be the net charge at bb₁?
If the total charge of counter ions in the region aa₁bb₁ is greater than that of the potential determining ions, what will be the net charge at bb₁?
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What type of charge do colloidal AgI particles acquire when an equimolar solution of KI is added to AgNO₃?
What type of charge do colloidal AgI particles acquire when an equimolar solution of KI is added to AgNO₃?
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What happens to protein molecules in an alkaline solution according to pH dependence?
What happens to protein molecules in an alkaline solution according to pH dependence?
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At the isoelectric point (IEP), what is the charge status of protein molecules?
At the isoelectric point (IEP), what is the charge status of protein molecules?
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How does the dielectric constant (DEC) of a dispersing medium relate to its polarity?
How does the dielectric constant (DEC) of a dispersing medium relate to its polarity?
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When a particle has a higher dielectric constant than its dispersion medium, what type of charge does it typically acquire?
When a particle has a higher dielectric constant than its dispersion medium, what type of charge does it typically acquire?
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Which of the following ions predominantly causes the majority of particles in water to acquire a negative charge?
Which of the following ions predominantly causes the majority of particles in water to acquire a negative charge?
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What effect does increasing the dielectric constant have on the solubilization of drugs?
What effect does increasing the dielectric constant have on the solubilization of drugs?
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What happens to the solubility of proteins at their isoelectric point (IEP)?
What happens to the solubility of proteins at their isoelectric point (IEP)?
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Study Notes
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.
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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.
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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.
- Applications of DEC measurements include:
The Electrical Double Layer
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A solid surface with adsorbed cations in a solution with ions. Cations are attached to surface. Remaining cations and anions remain in the solution.
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Anions are attracted to the positive surface by electrical forces, also repelling further cation approach after initial adsorption.
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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.
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The diffuse double layer consists of two layers:
- A first tightly bound layer (aa₁bb₁).
- A second layer (bb₁cc₁) that is more diffuse.
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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.
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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.
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Description
This quiz explores the electrical properties of interfaces, focusing on the charge behavior of dispersed particles in liquid media. Key concepts include ion adsorption, charge dependence on pH, and the behavior of proteins in different pH environments. Test your knowledge on how these factors influence the properties of colloids.
