Properties of Colloids

Brownian movement may be used to distinguish between solutions and colloids. Brownian motion is the random movement of colloidal particles suspended in a liquid or gas, caused by collisions with molecules of the surrounding medium. The particles in solutions and colloids are in constant motion. However colloid particles are large enough to be observed and are small enough to still be affect by the random molecular collisions. Colloid particles resist settling rapidly to the bottom of a vessel due to Brownian motion.
Optical properties –Tyndall effect – light scattering – Colloid shows visible cone due to light scattering. Hydrophilic colloids have less pronounced Tyndall effect than hydrophobic colloids.

Diffusion –Movement of particles from areas of high concentration to areas of low concentration to establish equilibrium – Fick’s First Law dQ = - DA (dc/dx) dt. Amount (dQ) of substance diffusing in time (dt) across plane of area (A) is directly proportional to change in concentration (dc) with distance traveled (dx). D influenced by diffusant properties, solvent properties, temperature – is not constant but depends on conditions.
Sedimentation –Stoke’s Law:
v = 2r2 g (ρ –ρo) / 9η
where: v is rate of sedimentation,
radius of particle, η is viscosity of dispersion medium, g gravitation constant, ρ is particle density and ρo is density of dispersion medium.
Viscosity – Measure of resistance of a liquid to flow, the more viscous the liquid the greater force required to make it flow at a particular rate –Liquid composed of parallel “layers” or plates: – F/A =η dv/dx . F/A is shear stress (F applied to area A), dv/dx is shear rate (velocity of shear ; distance between plates) often plotted: dv/dx = 1/η (F/A).
Shearing force required to produce velocity gradient between parallel plates of a block material. Top plane moves at constant velocity (dv) – each lower layer moves at velocity proportional to its distance from the fixed bottom layer (dx)
Unit of viscosity – centipoise (cPs).
φ =1/η(fluidity).