COLLOIDAL DISPERSION

Shape of Colloidal Particles

• More extended shape

• Greater Specific Surface Area

• Greater interaction between dispersed phase and dispersion medium

Faraday-Tyndall Effect

• Scattering of light by colloidal particles

• Widely used for determining molecular weight (MW) of colloids

Electron Microscope

• Used to observe the size, shape, and structure of colloidal particles

• Has higher resolution power than optical microscope

10x

Small clusters or faces (if for noise colored OR mild instability begins)

Homogeneous phase. Early signs of instability may begin.

40X

Aggregated droplets OR small spherical droplets

More detailed structure visible, showing either aggregation or discrete droplets.

Brownian Movement

• Random movement of colloidal particles

• Velocity increases with decreasing particle size

Diffusion

• Spontaneous movement from high to low concentration until uniform system is achieved

• A direct result of Brownian movement

• Can be expressed by Fick's First Law

Osmotic Pressure

• Can be described by van't Hoff equation: π=cRTπ=cRT

Sedimentation (Settling)

• Velocity of sedimentation is given by Stoke's law

Viscosity

• Resistance to flow of a system under applied stress

• More viscous → greater force to make it flow

• Affected by shape of particles
  e.g., SPHERICAL → less viscous
  LINEAR → more viscous

Lyophilic Colloids

• "Solvent-loving"

• High affinity to dispersion medium → forms sols spontaneously

• e.g., Acacia in water, Gelatin in water, Celluloid in amyl acetate

Lyophobic Colloids

• "Solvent-hating"

• Little attraction to dispersion medium → requires stabilization

• Composed of inorganic materials in water (e.g., Gold (Au), Silver (Ag), Arsenic trisulfide (As₂S₃), Sulfur (S), Silver iodide (AgI))

Amphiphiles

have two distinct regions of opposing solution affinities within the same molecule or ion.

Polar Head

• Hydrophilic (water-loving)

Non-polar Tail(s)

Hydrophobic (water-hating)

Monomer

single amphiphile molecule

Critical Micelle Concentration (CMC)

The concentration of monomers at which micelle formation begins

Aggregation Number

The number of monomer molecules that aggregate to form a micelle

Stability of Colloidal Systems

• The presence and magnitude of a charge on colloidal particles is important.

Providing particles with electric charge

• Adding a small amount of electrolyte to lyophobic sols...

Surrounding particles with protective solvent sheath

• Adding a hydrophilic sol (protective colloid) to a hydrophobic colloid
  → Protective property is expressed in Gold Number

Lyophilic and association colloids

thermodynamically stable.

Coacervation

A separation into two liquid layers when a negatively charged (e.g., Acacia) and a positively charged (e.g., Gelatin) hydrophilic colloid are mixed

Syneresis Test

• Used to evaluate the stability of colloidal gels or emulsions, particularly to detect the separation of liquid (serum) from the gel network over time.

Syneresis

The expulsion of liquid from a gel, leading to shrinkage of the gel network.