Surface Chemistry - Critical coagulation concentration and Hardy-Schulze rule

Surface Chemistry - Critical Coagulation Concentration and Hardy-Schulze Rule

Slide 1

  • Surface chemistry is the study of phenomena that occur at the interface of two phases, such as the interaction between a solid and a gas or liquid.
  • Critical coagulation concentration (CCC) refers to the minimum electrolyte concentration required to cause coagulation or precipitation of a colloid.
  • Hardy-Schulze rule predicts the order of coagulating power of various ions.

Slide 2: Colloids

  • Colloids are heterogeneous systems in which one substance is finely dispersed in another.
  • They consist of two phases: a dispersed phase and a dispersion medium.
  • The dispersed phase can be solid, liquid, or a gas, and the dispersion medium is usually a liquid.
  • Examples of colloids include milk, aerosols, sols, gels, and foams.

Slide 3: Critical Coagulation Concentration (CCC)

  • CCC is the minimum electrolyte concentration required for the coagulation or precipitation of a colloidal solution.
  • It represents the threshold value at which the repulsive forces between colloidal particles are overcome by attractive forces, leading to aggregation.
  • CCC depends on factors such as the nature of the dispersed phase, dispersion medium, and temperature.

Slide 4: Factors Affecting CCC

  • Nature of the colloidal particles: Different colloidal particles have different CCC values based on their surface charge and size.
  • Nature and concentration of electrolytes: Electrolytes with higher charges or higher concentrations have a greater coagulating power and lower CCC values.
  • Temperature: An increase in temperature generally decreases the CCC due to increased thermal motion.

Slide 5: Derivation of Hardy-Schulze Rule

The Hardy-Schulze rule helps predict the order of coagulating power of various ions. It is based on the following principles:

  1. Coagulating power is directly proportional to the valency of the coagulating ion.
  1. Coagulating power is inversely proportional to the size of the coagulating ion.

Slide 6: Example - Hardy-Schulze Rule

Consider the following ions and their valencies:

  • Al3+
  • Ba2+
  • Mg2+
  • Na+ According to the Hardy-Schulze rule, the order of coagulating power is as follows:
  • Al3+ > Ba2+ > Mg2+ > Na+ This means that Al3+ has the highest coagulating power, while Na+ has the lowest.

Slide 7: Application of Hardy-Schulze Rule

Hardy-Schulze rule is useful in determining the coagulating power of various ions in practical applications, such as:

  • Water treatment processes to remove colloidal impurities.
  • Formation of precipitates in analytical chemistry.
  • Industrial processes involving clarification and purification of solutions.

Slide 8: Equations for CCC Calculation

  1. For trivalent ions:
    • CCC = [(8πηK3) / (9kT)] × (1 / radius^3)
  1. For divalent ions:
    • CCC = [(8πηK2) / (27kT)] × (1 / radius^3)
  1. For monovalent ions:
    • CCC = [(8πηK) / (27kT)] × (1 / radius^3) Here, η is the viscosity of the medium, K is the Boltzmann constant, T is the absolute temperature, and radius is the radius of the colloidal particle.

Slide 9: Example Calculation

Given the following data:

  • η = 0.01 poise
  • K = 1.38 × 10^(-23) J/K
  • T = 298 K
  • radius = 10 nm (1 nm = 10^(-9) m) For trivalent ions (e.g., Al3+): CCC = [(8π × 0.01 × 1.38 × 10^(-23) × 3^2) / (9 × 298)] × (1 / (10^(-9))^3) = 1.39 × 10^(-6) M

Slide 10: Summary

  • Surface chemistry involves phenomena at the interfaces of two phases.
  • CCC is the minimum electrolyte concentration for colloid coagulation.
  • Hardy-Schulze rule predicts the order of coagulating power of ions.
  • CCC depends on factors like particle charge, electrolyte concentration, and temperature.
  • Equations can be used to calculate CCC values for different ions and particles.
  1. Types of Colloids
  • Sol: Solid dispersed in a liquid medium.
  • Emulsion: Liquid dispersed in a liquid medium.
  • Foam: Gas dispersed in a liquid or solid medium.
  • Aerosol: Liquid or solid dispersed in a gas medium.
  • Gel: Liquid dispersed in a solid medium.
  1. Coagulation and Flocculation
  • Coagulation is the destabilization and aggregation of colloidal particles to form larger particles.
  • Flocculation is the process of forming aggregates or flocs, which then settle or can be easily removed.
  1. Electrostatic Stabilization
  • Charged particles repel each other, preventing coagulation.
  • Surface charge is influenced by ion adsorption and ionization at the interface.
  • Electrolytes neutralize the charges and reduce the electrostatic repulsion, leading to coagulation.
  1. Steric Stabilization
  • Polymers adsorbed on the surface create a protective layer preventing particle aggregation.
  • The polymer chains create a repulsive barrier, hindering particle contact.
  • Adding electrolytes disrupts the protective layer and allows coagulation.
  1. Application of Colloids
  • Colloids find applications in various fields:
    • Food industry (emulsions, gels, foams).
    • Pharmaceuticals (drug delivery systems).
    • Paints and coatings (dispersion).
    • Environmental remediation (colloid-based filtration).
  1. Ostwald Ripening
  • Ostwald ripening is the spontaneous growth of larger particles at the expense of smaller ones.
  • It occurs due to the difference in solubility of smaller and larger particles.
  • This process leads to the formation of equilibrium-sized particles.
  1. Coagulation Agents
  • Coagulation agents are substances used to induce coagulation or flocculation.
  • Examples include alum (Al2(SO4)3), ferric chloride (FeCl3), and polymeric coagulants.
  • These agents neutralize the charges on colloidal particles and promote aggregation.
  1. Application of CCC in Water Treatment
  • CCC is crucial in water treatment to remove suspended colloidal particles.
  • Coagulation is achieved by adding appropriate electrolytes at concentrations above the CCC.
  • The coagulated particles are then easily removable by sedimentation and filtration.
  1. Practical Importance of Hardy-Schulze Rule
  • Hardy-Schulze rule helps choose suitable coagulation agents based on their valencies and sizes.
  • It aids in controlling and optimizing coagulation and flocculation processes.
  • The rule provides a theoretical framework to understand and predict coagulation behavior.
  1. Limitations of Hardy-Schulze Rule
  • The rule is based on ideal assumptions and may not accurately predict coagulation in all cases.
  • Other factors, such as pH, temperature, and nature of the dispersing medium, can influence coagulation.
  • Experimental data and empirical observations are also crucial for successful coagulation processes.

Slide 21:

  • Factors influencing CCC:
    • Charge on colloidal particles
    • Charge on coagulating ions
    • Presence of surfactants or stabilizers
    • pH of the medium
    • Temperature

Slide 22:

  • Coagulation mechanisms:
    • Charge neutralization
    • Adsorption or bridging
    • Sweep flocculation

Slide 23:

  • Charge neutralization:
    • Coagulating ions neutralize the charge on colloidal particles.
    • This reduces the repulsion between particles and leads to aggregation.
    • Example: Addition of Al3+ ions to a negatively charged colloid.

Slide 24:

  • Adsorption or bridging:
    • Coagulating ions adsorb on the surface of colloidal particles.
    • This creates bridges between particles, causing flocculation.
    • Example: Addition of a polymer that adsorbs on the particle surface.

Slide 25:

  • Sweep flocculation:
    • Polymers or other large molecules sweep across the solution, capturing particles.
    • The captured particles form flocs that can be easily removed.
    • Example: Addition of polyacrylamide to wastewater for flocculation.

Slide 26:

  • Importance of CCC in industrial processes:
    • Production of pharmaceuticals
    • Manufacturing of paints and coatings
    • Water treatment and purification
    • Processing of food and beverages

Slide 27:

  • Significance of Hardy-Schulze rule in water treatment:
    • Helps select the most effective coagulant for removing colloidal impurities.
    • Saves cost and improves the efficiency of water treatment plants.
    • Reduces the formation of harmful byproducts during coagulation.

Slide 28:

  • Examples of precipitation reactions based on Hardy-Schulze rule:
    • AgNO3 + NaCl → AgCl(s) + NaNO3
    • Al2(SO4)3 + 6NaOH → 2Al(OH)3(s) + 3Na2SO4

Slide 29:

  • Limitations of Hardy-Schulze rule:
    • It assumes ideal conditions that may not be prevalent in all systems.
    • Other factors such as pH, temperature, and presence of other ions can influence coagulation.
    • Experimental data and empirical observations are still crucial in actual applications.

Slide 30:

  • Summary:
    • CCC represents the minimum electrolyte concentration required for colloid coagulation.
    • Hardy-Schulze rule predicts the order of coagulating power of ions based on their valency and size.
    • CCC depends on factors such as particle charge, electrolyte concentration, and temperature.
    • Coagulation mechanisms include charge neutralization, adsorption or bridging, and sweep flocculation.
    • Practical applications include water treatment, industrial processes, and precipitation reactions.