2What is a colloid? A colloid is a suspension of particles in a medium The particles may be solid, liquid or gas and are called the disperse phaseThe medium may also be solid liquid or gas and is called the continuous phase
4Why don’t colloids break down? They do. Colloids are inherently unstableGravitational forces and attractive forces on the surface of the particles will cause the two phases to separateThis is prevented by Brownian motion and charges on the surface of the particles which keep the particles suspended.
5Stokes’ Law A particle in a fluid may descend or rise The rate at which it does depends on a balance between gravity, buoyancy and frictionThis results in a constant terminal velocity, the value of which is given by Stokes Law
6Brownian MotionBrownian is the result of the continuous buffeting of the colloidal particles by the molecules of the continuous phase.It gives rise to the diffusion where the particles will migrate from a region of high concentration to one of low concentrationDiffusion is defined by Fick’s law which is on the right
7Kinetic Stabilisation Kinetic stabilisation is the result of a combination of Stokes law and Fick’s law effectsParticles will tend to settle as a result of Stokes’ lawThis sets up a concentration gradient which causes diffusion in the opposite direction to settlingIf the two are in equilibrium the colloid will be stable.
8Molecular attractionColloidal particles tend to be attracted to one another as a result of Van der Waal’s forces between the molecules on the particle surfaceThis results in agglomeration of particles and the break up of the colloid.Van der Waal’s forces are inversely proportional to the square of the distance between colloidal particles.
9Electric double layerSome colloidal particle have charges on their surfaceIf the continuous phase is an electrolyte, ions are attracted to the particle surfaceThis results in a charged layer near the particle surface which decays exponentially from the surfaceThis is known as the electrical double layer
10Surface Tension The molecules of a liquid will attract each other In the body of the liquid this attraction is equal all roundAt the surface, the attraction is unbalancedThis imbalance of attractive forces is called surface tension
11Emulsifiers An emulsifier molecule comprises two sections A hydrophyllic (Water loving head) and a hydrophobic (Water hating) tailSuch molecules are called surfactantsThey form a layer on the droplet surface
12Emulsifiers - continued The tail of the molecule is in the oil and the head in the water“Spare” molecules cluster in the form of micellesThis aids the break up of droplets into smaller onesCharges on the surface of the emulsifier keep the droplets apart.
13Stearic repulsionThis is a result of macromolecules adhering to the particle surfaceThe shape and conformation of the molecule prevents aggregation of particles
14Breakdown of colloidsThe breakdown of a colloid is the result of particles coming together to form larger particles. There are three basic formsFlocculationCoagulationCoalescence
15Breakdown of electrostatically stabilised colloids The DVLO theory explains the breakdown of such colloidsIt is based on a balance between the Van der Waal’s forces and the repulsive forces. ThusEint = Eatt + ErepAdding electrolyte tends to reduce double layer thickness, such that Eatt becomes dominant
16Bridging and Depletion flocculation Bridging flocculation is the result of macromolecules becoming attached to two particlesDepletion flocculation is the result of an osmotic effect where macromolecules act as a semi-permeable membrane
17CoalescenceCoalesence is the merging of two liquid droplets into a single dropletThe process involves expulsion of the continuous phase from between the dropletsThe ease with which this occurs depends on surface tension and continuous phase viscosity.
18For more information This lecture has only been an overview. More information is available via the Module website.Or go directly to