CHEE2945 – Lecture 7
Surface charge:
- If a particle has a weakly acidic group, it can lose a hydrogen and gain a
negative charge. Most mineral oxides are weakly acid (depending on pH
(according to Le Chatelier’s Principle), so the magnitude of the charge can be
changed by changing the pH).
- Similarly, particles can be weakly basic (also pH dependant) by gaining
protons on basic groups, or losing hydroxyl groups.
- Differential dissolution is when either a cation or anion dissolves preferentially,
so the particle becomes oppositely charged. Often AgI is used for this.
- If charged surfactants adsorb to non-polar particle, the particle will have a
surface charge. If the surfactant was instead just highly polar, not charged,
the particle may act as if it had a surface charge.
Electric double layer (EDL) Structure:
- Consider a flat, infinite, atomically smooth, and uniform surface, which is
positively charged. The surface is in water. The surface will repel the protons
and attract the hydroxide ions. Hence, close to the interface, there are more
hydroxide ions, and less protons, and so there is a change in the distribution
of the ions due to the interface.
- In the bulk, the concentration of protons and hydroxide ions are equal.
- Very close to the surface, the hydroxide ions adsorb to the surface, creating a
layer known as the stern layer.
- After the stern layer, there is a region where counter and co-ions are less
strongly associated, known as the diffuse region. The concentration of ions in
the diffuse region is impacted by electrostatic interactions, and thermal
energy.
, - The shape of a concentration-distance curve is dependant on a number of
factors:
o Surface charge,
o Bulk concentration,
o Ion valency.
- Electrostatic charges decay over distance. The way the surface charge
decays is dependent on ions in the diffuse region.
- The above situation describes an electrical double layer (EDL).
- For the above situation, it is assumed that:
o The surface is flat, infinite, and uniformly charged,
o Ions in the diffuse layer are point charges distributed according to the
Boltzmann distribution.
o The solvent only affects the EDL through the dielectric constant, which
is assumed to be constant.
o There is only a single symmetrical electrolyte of charge number z .
- The Boltzmann distribution, which describes the distribution of ions:
Surface charge:
- If a particle has a weakly acidic group, it can lose a hydrogen and gain a
negative charge. Most mineral oxides are weakly acid (depending on pH
(according to Le Chatelier’s Principle), so the magnitude of the charge can be
changed by changing the pH).
- Similarly, particles can be weakly basic (also pH dependant) by gaining
protons on basic groups, or losing hydroxyl groups.
- Differential dissolution is when either a cation or anion dissolves preferentially,
so the particle becomes oppositely charged. Often AgI is used for this.
- If charged surfactants adsorb to non-polar particle, the particle will have a
surface charge. If the surfactant was instead just highly polar, not charged,
the particle may act as if it had a surface charge.
Electric double layer (EDL) Structure:
- Consider a flat, infinite, atomically smooth, and uniform surface, which is
positively charged. The surface is in water. The surface will repel the protons
and attract the hydroxide ions. Hence, close to the interface, there are more
hydroxide ions, and less protons, and so there is a change in the distribution
of the ions due to the interface.
- In the bulk, the concentration of protons and hydroxide ions are equal.
- Very close to the surface, the hydroxide ions adsorb to the surface, creating a
layer known as the stern layer.
- After the stern layer, there is a region where counter and co-ions are less
strongly associated, known as the diffuse region. The concentration of ions in
the diffuse region is impacted by electrostatic interactions, and thermal
energy.
, - The shape of a concentration-distance curve is dependant on a number of
factors:
o Surface charge,
o Bulk concentration,
o Ion valency.
- Electrostatic charges decay over distance. The way the surface charge
decays is dependent on ions in the diffuse region.
- The above situation describes an electrical double layer (EDL).
- For the above situation, it is assumed that:
o The surface is flat, infinite, and uniformly charged,
o Ions in the diffuse layer are point charges distributed according to the
Boltzmann distribution.
o The solvent only affects the EDL through the dielectric constant, which
is assumed to be constant.
o There is only a single symmetrical electrolyte of charge number z .
- The Boltzmann distribution, which describes the distribution of ions:
