CHEE2945 – Lecture 5
Adsorption:
- Consider a layer of oil on top of water. This has a much lower surface area
than numerous oil droplets in water. This is energetically unfavourable, so the
oil droplets will coalesce.
- Surface tension is a force that acts to decrease surface area. Surface tension
is surface energy per unit area.
γ=γ 0−π where:
γ=¿surface tension with surfactant,
γ 0=¿surface tension without surfactant,
π=¿ surface pressure.
- If surfactants are present in a water/oil mixture, the surfactant molecules will
adsorb at the interface, where the tail group enters the oil phase, and the
head stays in the water phase.
- The hydrocarbon tails occupy volume, and hydrogen and carbon atoms can
rotate, however, their rotation is impeded by surrounding hydrocarbons, so
there is a force pushing the tails away from each other, creating a force that
increases surface area.
- In addition, if the head group is charged, they will repel each other, adding to
the force that acts to increase surface area.
- For surfactants adsorbed to an oil droplet, if the heads are charged, then the
oil droplet itself will be charged, slowing coalescence.
Air/water interface:
- The air water interface has a very high interfacial tension. If surfactant is
placed into the water, the interfacial tension is greatly reduced, and so stable
air bubbles can be created in the water, creating a foam.
- The water can drain out of a water/foam mixture, leaving very little water in
the mixture. However, the surfactant remains. This leaves a stable foam
without much water present.
Solids:
- Negatively charged solids include:
o SiO2,
o TiO2,
Adsorption:
- Consider a layer of oil on top of water. This has a much lower surface area
than numerous oil droplets in water. This is energetically unfavourable, so the
oil droplets will coalesce.
- Surface tension is a force that acts to decrease surface area. Surface tension
is surface energy per unit area.
γ=γ 0−π where:
γ=¿surface tension with surfactant,
γ 0=¿surface tension without surfactant,
π=¿ surface pressure.
- If surfactants are present in a water/oil mixture, the surfactant molecules will
adsorb at the interface, where the tail group enters the oil phase, and the
head stays in the water phase.
- The hydrocarbon tails occupy volume, and hydrogen and carbon atoms can
rotate, however, their rotation is impeded by surrounding hydrocarbons, so
there is a force pushing the tails away from each other, creating a force that
increases surface area.
- In addition, if the head group is charged, they will repel each other, adding to
the force that acts to increase surface area.
- For surfactants adsorbed to an oil droplet, if the heads are charged, then the
oil droplet itself will be charged, slowing coalescence.
Air/water interface:
- The air water interface has a very high interfacial tension. If surfactant is
placed into the water, the interfacial tension is greatly reduced, and so stable
air bubbles can be created in the water, creating a foam.
- The water can drain out of a water/foam mixture, leaving very little water in
the mixture. However, the surfactant remains. This leaves a stable foam
without much water present.
Solids:
- Negatively charged solids include:
o SiO2,
o TiO2,
