The theory of colloidal dispersion stability
VT = VA + VR + VS
VT – Total energy potential between particles
- VA – Attraction between particles due to van der Waals, mostly London Forces
VR – Repulsion between particles due to the electrical double layer of co-ions and counter ions at the surface of a particle
VS – Repulsion between particles due to steric considerations, seen at shorter distances due to adsorbed polymers
In colloidal dispersions particles like to associate in order to reduce surface area.
Particles collide at random due to Brownian motion
- Primary Minimum – Attractive forces overpower the repulsive forces at low molecular distances. Particles coagulate and are difficult to redistribute.
- Primary Maximum – Repulsion is greater than attraction. Particles rebound and remain evenly dispersed throughout the medium. Primary Maximum needs to be greater than the thermal energy otherwise particles are close to VA and aggregate
- Secondary Minimum – Greater distances between particles but attraction causes flocculation. Particles form weak attractions but are easily redispersed.
ΔG = ΔH – TΔS
- ΔG – Free energy
- ΔH – Enthalpy
- T – Temperature
- ΔS – Entropy
If free energy has a positive value this indicates dispersion stability
If free energy has a negative value this indicates aggregation
If the ΔS and ΔH are both negative or both positive and the TΔS > ΔH then ΔG will be positive and the dispersion will be stable. These are known as entropic and enthalpic stabilisation.
Manipulating enthalpy and entropy can contribute to stability
Electrolytes compress the electronic double layer causing a decrease in the primary maximum. This deepens the secondary minimum increasing the tendency for flocculation.
Ionic surfactants also decrease the primary maximum increasing the ease of coagulation.