What are the two layers in the Electrical Double Layer?
The Stern Layer, inner layer and the diffuse layer, outer layer.
What are the two characterisations of the Stern Layer?
- approximately equal to the hydration radius of the specifically adsorbed ions
- Ions whose centres are inside the layer form the Stern Layer
What are the electrical characteristics of the Stern Layer?
Electric fields are large closest and potential tends to decay at an exponential rate
What does the potential distribution look like?
Linearly step in the stern layer than decreases in the diffuse layer
What happens to Wd what specific co-ions are absorbed?
Peak up and exponential decline
What happends to W0 and Wd when sufficient surfactant is adsorped
They have different signs, big dip down past zero than exponential increase negatively to form an asymptote with 0.
What is measured when a voltage is applied to a colloid?
- Direction, depends on the charge sign
- Speed, depends on the charge magnitude
What is the Zeta potential?
It is the potential at the plane of shear
What is an Isoelectric Point (IEP)?
pH where the zeta potential is zero and te net surface charge is zero.
What is the Point of Zero Charge?
Usually the same as the IEP (pH where zeta potential is 0. Except it is when polyvalent ions are specifically adsorbed to the surface
What happens to the middle when two Double layers come together?
Double Layer Overlap, where ionic concentration is greater than the bulk
What causes the double layer force?
- The overlap of the electric potential distributions (Maxwell Stress)
- the overlap in ion concentration (Osmotic pressure)
What shape is the curve that forms from potential and distance between two surfaces?
A parabola. U shape one
What is the effect of salt to EDL's of the particles?
Reduction in thickness leading to colloid instability
What happens when two charged particles approach?
EDL overlap generating inter-particle repulsive forces and they will repel
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