Classification of Dispersed Systems

Study Notes

Dispersions contain at least one internal phase (dispersed phase) within a dispersion medium (continuous phase).

Classified based on the size of dispersed particles.
Colloidal dispersions: particle size 1 nm - 1 μm, visible only with an electron microscope, particles diffuse but slower than solutions. Examples include milk, paint, and shaving cream.
Coarse dispersions: particle size greater than 1 μm, visible with a light microscope, particles do not diffuse. Examples include suspensions and emulsions.

One nanometer is very small, roughly the size of a tennis ball compared to the Earth.

Blood is a complex dispersed system with more than one dispersed phase.
The dispersion medium in blood is plasma, mostly water.
Serum albumin (particles larger than 1 nm) forms a colloidal dispersion in blood
Red blood cells (RBCs) with ~6 µm diameter and ~2 µm width form a coarse dispersion in blood.

Colloidal particles have a massive surface area relative to their volume.
Specific surface area describes the surface area per unit weight or volume of material.
Large surface area results in unique properties (e.g., catalytic, dissolution, electrical, kinetic, optical).

A technique used to separate colloidal particles from smaller molecules.
A semipermeable membrane (e.g., cellophane) is used.
The membrane permits smaller molecules/ions to pass but not larger colloidal particles.
At equilibrium, the colloidal material remains in one compartment, whilst subcolloidal material is distributed equally between the two parts.

Lyophilic (solvent-loving) colloids:
- Particles have a strong attraction to the dispersion medium, usually formed by spontaneous dispersion.
- Examples include acacia, gelatin, insulin, and albumin in water (hydrophilic colloids), and rubber/polystyrene in organic solvents (lipophilic colloids).
Lyophobic (solvent-hating) colloids:
- Particles have weak attraction to the dispersion medium.
- Examples include inorganic particles (gold, silver, sulfur, silver iodide) in water.
- Prepared by dispersion (breaking down larger particles) or condensation (aggregating smaller particles).
Association (amphiphilic) colloids:
- Particles are formed from amphiphiles (surfactants), when the concentration reaches a critical value.
- The formed structures are called micelles, which are nanoscale in size.
- The concentration at which micelles form is the critical micelle concentration (CMC).
- Below CMC, amphiphiles adsorb at the interface.
- At CMC, surfactant monomers form micelles in the bulk phase.

Micelle shapes (in water):
- Hydrocarbon chains face inward.
- Polar heads face outward.
Micelle shapes (nonpolar liquids; reverse micelles):
- Polar heads face inward.
- Hydrocarbon chains face outward.
Micelle shapes (higher amphiphile concentration; laminar micelles):
- Spherical micelles exist close to CMC values, changing into laminar micelles at higher concentrations in equilibrium.
Amphiphiles can be anionic, cationic, nonionic, or ampholytic, with associated ions termed counter ions.

Association colloids (e.g., micelles) increase the solubility of insoluble/slightly soluble materials in the dispersion medium.

Liposomes:
- Consist of membranes (unilamellar or multilamellar) and an inner liquid core.
- Formed using phospholipids.
- Used to deliver drugs.
Nanoparticles:
- Small (1nm to 1µm) spheres, made from natural/synthetic polymers.
- Enhanced drug delivery efficiencies, profiles, and targeting.
Hydrogels:
- Colloidal gels, with water as the dispersion medium.
- Used for wound healing and sustained-release delivery systems.

Colloidal particles' size, shape, and structure are observable using electron microscopy, with higher resolving power compared to optical microscopes.

A measure of the scattering of light by colloidal particles.
It's used to determine the molecular weight, size, and shape of colloidal particles.
Turbidity (τ) is the fractional decrease in light intensity as it passes through a solution.

Colloidal particles exhibit Brownian motion due to random collisions with dispersion medium molecules.
Velocity of particles increases with decreasing particle size.
Viscosity of the dispersion medium affects Brownian motion.

The velocity of sedimentation (v) of spherical particles in a liquid is given by Stokes' law, where particle size, density difference between the particle & medium, and viscosity of the medium are factors.
Brownian motion affects smaller particles, opposing gravitational sedimentation.

Colloidal stability depends on frequent encounters between particles (Brownian motion).
These encounters can lead to permanent aggregation/coagulation or temporary aggregation (flocculation).
Lyophilic dispersion stability depends on the electric charge of each particle, leading to repulsion between particles, and a protective sheath of solvent. Coacervation can occur upon mixing, whilst salting out happens when added excessive salt to a colloidal dispersion.
Lyophobic dispersions are less stable, frequently subject to aggregation and sedimentation due to attractive forces between the particles. Instability is increased when charges are not sufficient.

Lyophilic sols are thermodynamically stable, requiring an electric charge and/or protective sheath of solvent to prevent aggregation.
Stability can be compromised through mixing different charged colloids, or by high salt concentrations leading to agglomeration/salting out.

Lyophobic sols are thermodynamically unstable, requiring charges to maintain stability, and repulsion between particles.
Absence of sufficient charge can lead to particle aggregation and sedimentation.