Disperse systems 7 Flashcards
Ion distribution in disperse systems
• A disperse system is overall electrically neutral,
but the surface charge of the particles influences
the distribution of ions in the rest of the liquid.
• There is a layer around each particle with a
different composition from the rest of system.
The electrical double layer can be split into:
– inner region: includes charged surface & adsorbed
ions
– diffuse region: beyond adsorbed ions and up to the
edge of the electrically neutral region
– electrically neutral region: outside the EDL
Electrical Double Layer
Imagine putting a particle into contact with a
solution containing ions. The surface of the
particle may become charged.
Counter (opposite) ions are then attracted to the
charged surface.
Further away from the surface, the effect is less
pronounced. There is still an excess of
counterions, but some co-ions are present.
Further still the distribution of ions is uniform and
the area is electrically neutral.
Electrical Potentials
The electrical double layer theory shows that there is an uneven distribution of ions in a disperse system.
Ions are charged particles so there will be some areas in a suspension that are more charged than others.
A difference in charge between two areas is called a
potential.
Changes in electrical potentials
The biggest difference in charge (i.e. highest
potential) is between ENR and particle surface.
Potential decreases with distance from the surface.
It drops rapidly and linearly to the Stern plane, then
has a more gradual exponential decrease to zero.
This is because the counterions close to the surface act as a screen that reduces the attraction between the charged surface and the counterions further away.
DeBye Huckel length
The distance to the ENR is called the DeBye
Huckel length (1/k).
It is the thickness of the electrical double layer.
It covers the exponential region of the graph.
k is a value which varies with the system. It is
dependent on the electrolyte concentration
of the liquid phase.
Adding electrolytes increases the k and therefore decreases 1/k and that means that the thickness of the EDL is decreased.
Nernst potential
The potential between the actual surface of the particle and the electroneutral region of the solution is the
Nernst potential (E).
The potential between the Stern plane and the ENR is the Stern potential (ψ).
The potential between the shear plane and the ENR is the zeta potential (ζ).
Zeta Potential (ζ)
Varies between formulations and can be used as a
measure of the forces acting in a system
Indicates the degree of repulsion between
adjacent, similarly charged, dispersed particles
The higher ζ, the more stable the system is likely
to be:
– for a dispersion to remain stable ζ should be either
greater than +30 mV or less than -30 mV
– as ζ approaches zero, attractive forces exceed
repulsive forces and particles stick together
Particle-particle interactions
Because particles in disperse systems have a large total surface area, they tend to associate to decrease their total surface energy
When particles meet they may
– rebound and remain separate
– become temporarily attached
– become permanently attached
The balance of attractive and repulsive forces between particles dictates what happens
Particle concentration also has a role
Attractive Interactions (VA)
These result from van der Waals forces between
molecules in the surface layers of the interacting
particles.
The attraction decreases as the distance increases.
Repulsive Interactions (VR)
These result from electrical charges on the surfaces of particles, due to:
– Adsorption of charged polymers or surfactants at the
interface
– Polarity differences between the solid and liquid
– Ionisation of chemical groups at the surface of the
particles
– Adsorption of small inorganic ions onto particle
surfaces
These act over approximately the thickness of the double layer.
• VR also decreases with distance between
particles, more sharply than VA
DLVO Theory
Derjaguin and Landau, and Verwey and Overbeek described the interaction between two particles in terms of van der Waals attractive forces (VA) and electrical repulsive forces (VR) :
VT = VA + VR
As particles approach each other they will come under the attractive influence of VdW forces, which will be opposed by the repulsive forces of overlapping diffuse layers.
DLVO maximums and minimums
VA predominates at very low H, i.e. when particles are close together. This is the “primary minimum”.
A “primary maximum” occurs at intermediate distances (within the thickness of the electrical double layer) due to a high VR. The height depends on the VR and therefore surface and ζ potential.
A “secondary minimum” occurs at very large H because, even though the VA decreases with distance, VR decreases even more sharply. The depth of the trough on the graph depends on particle size.
