CHEE2945 – Lecture 6
Surface excess:
- Consider a water/oil interface with surfactant molecules in the aqueous phase.
There would be a region around the interface which has a concentration
gradient. Everything below/above that region would have a constant
composition.
- Surfactant molecules accumulate in this region.
- This region would have a volume. If you took a sample of this volume from
anywhere else in the sample, the number of surfactant molecules in would be
lower than the interface region.
- The maximum surfactant concentration would be at the hypothetical infinitely
thin interface.
- This means that the amount of surfactant in this region is larger than it would
be if there was an infinitely thin interface. This extra surfactant is known as
surface excess.
- Surface excess varies with surfactant, and it affects surface tension.
Surface energy:
- Surface energy is surface tension multiplied by surface area.
- As surfactant concentration is increased, surface energy decreases.
- As the hydrocarbon tail length of a surfactant increases, the surface energy
decreases.
Negative absorption of salts:
- Solute-solvent attractive forces are stronger than solvent-solvent forces.
- The solute molecules migrate away from the surface into the bulk solution and
disturb solvent structure at the interface.
- Therefore, the concentration of salt at the interface will decrease in the
interface region. This means the surface excess is negative.
- Hence, salts can increase surface tension, and can be used to break
emulsions
Thermodynamics of adsorption:
- Adsorption at an interface is quantified by surface excess concentration, Γ .
- The interface region, or surface phase, is denoted by σ .
Surface excess:
- Consider a water/oil interface with surfactant molecules in the aqueous phase.
There would be a region around the interface which has a concentration
gradient. Everything below/above that region would have a constant
composition.
- Surfactant molecules accumulate in this region.
- This region would have a volume. If you took a sample of this volume from
anywhere else in the sample, the number of surfactant molecules in would be
lower than the interface region.
- The maximum surfactant concentration would be at the hypothetical infinitely
thin interface.
- This means that the amount of surfactant in this region is larger than it would
be if there was an infinitely thin interface. This extra surfactant is known as
surface excess.
- Surface excess varies with surfactant, and it affects surface tension.
Surface energy:
- Surface energy is surface tension multiplied by surface area.
- As surfactant concentration is increased, surface energy decreases.
- As the hydrocarbon tail length of a surfactant increases, the surface energy
decreases.
Negative absorption of salts:
- Solute-solvent attractive forces are stronger than solvent-solvent forces.
- The solute molecules migrate away from the surface into the bulk solution and
disturb solvent structure at the interface.
- Therefore, the concentration of salt at the interface will decrease in the
interface region. This means the surface excess is negative.
- Hence, salts can increase surface tension, and can be used to break
emulsions
Thermodynamics of adsorption:
- Adsorption at an interface is quantified by surface excess concentration, Γ .
- The interface region, or surface phase, is denoted by σ .
