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Colloids

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Colloids

Two phase system: one phase (disperse phase A) is dispersed in a second phase (continuous phase B)
- Systems containing large molecules and/or small particles in range 1 nm - 10 μm
Main types of Colloids
- Lyophobic
  - Liquid hating and are irreversible sols
    - Thermodynamically unstable
  - Lyophobic Colloids
    - Colloid instability
      - metastable / kinetically stable
      - Tendency to aggregate
      - Main aggregation mechanisms
        Flocculation
        Particles form in clusters and agglomerate together by weak forces
        Losee, open structures
        Reversible (thermodynamically) - with heating or agitation, original system can be restored
        Coagulation
        Strong interactions between colloidal surfaces
        Compact, dense structure of particles form
        Irreversible - original system cannot be restored
    - Van der Waals forces
      - Attractive interactions between molecules in vaccuum
      - Colloids are unstable due to Van der Waals forces of attraction
    - Stability - DLVO theory
      - Balance between two forces: electrostatic repulsion and Van der Waals attaction
        Colloidal repulsion mechanisms:
        Electrostatic forces: from net surface charge
        Steric forces: created by adsorbed polymer molecules
        Origins of surface charge
        1. Ionisation of surface groups
        2. Adsorptions of specific ions
        3. Dissolution of ions from surfaces
        4. Isomorphous replacement
        Example of importance of colloid stability: Water treatment
        Colloidal particles removed in water treatment plants by coagulation, flocculation, sedimentation and filtration
        Use of precipitated metal hydroxides for water treatment to flocculate and remove organic matter
        Flocculants function by adsorbing onto surfaces and forming bridges between particles
      - The electric double layer
        Coutercharges form electric double layers
        Most surfaces are charged in aqueous systems
        Counter ions are attracted to the surface
        Charge of surface is balanced by equal charge of the ions
        Due to electrostatic forces
      - Stability is promoted by repulsive forces
- Lyophibic
  - Liquid loving and are reversible sols
    - Thermodynamically stable
- Association
  - Thermodynamically stable
  - Built up from amphiphilic molecules
    - Contain distinct functional groups of different affinities for water and oil
Properties of Colloids
- Factors contributing to the nature of colloidal systems
  - Particle size
  - Particle shape and flexibility
  - Surface properties
  - Particle - particle interactions
- Colloidal can be in many forms
  - Regular/ irregular
  - Network
  - Aggregated
  - Concentrated/ dilute
  - Natural/ synthetic
- Brownian motion
  - When colloid is examined, particles appear as tiny points of light in constant motion, helping to keep particles in suspension
- Tyndall effect
  - Scattering of light: when beam of light is passed through a colloid, light is reflected by particles and path of light can be observed
- Adsorption
  - the finely divided colloidal particles have a large surface area exposed, and so can selectively absorb ions and acquire an electric charge
Colloidal System examples
- Aerosols
- Foams
- Agrochemicals
- Cosmetics
- Foodstuffs
- Pharmaceuticals
- Dyestuffs
Formation of Colloidal systems
- Condensation of small dissolved molecules into larger colloidal particles
  - Done by condensation, redox reactions, precipitation or crystallisation from saturated solutions containing precursor compounds
- Dispersion: reducing large particles to fine particles in colloidal dimensions
  - Done by grinding solids, atomising liquids in air, milling, spraying or applying shear (shaking, mixing)
Surfactants
- Amphiphiles that exhibit high level of surface actiity
- Surface active agents
- Properties:
  - Able to locate at interfaces (adsorption) and alter properties like surface and interfacial tensions
  - Aggregate in solution
  - Controls processes like wetting, spreading, adhesions, emulsification, foaming..
  - Ability to lower surface energy
- Classes of surfactants
  - Non-ionic
  - Ionic (cationic, anionic)
  - Zwitterionic/ amphoteric/ ampholytic
- Surface tension:
  - Molecular interactions at interfaces
    - Bulk molecule: no imbalance of forces with the bulk
    - Surface molecule: unequal short-range attraction forces results in net inward pull of surface molecules into the bulk phase
    - Different types of intermolecular attraction contribute to surface tension
  - Interfacial tension
    - Depends largely on polar and dispersion interactions
- Applications
  - Household detergents
  - Personal care products
  - Industrial cleaning
  - Food
  - Oilfield (drilling fluids, oil recovery, emulsion treatment)
  - Emulsion polymerisation