Surface Tension Flashcards
Explain how micellization, solubilisation and physical removal of greasy materials is done using soap and water
Micellisation: When soap is mixed with water at a concentations > CMC, micelles will form, with a hydrophobic core and a hydrophilic shell. Each micelle consists of 50- 100 surfactants.
Solubilisation: The greasy materials are hydrophobic and would normally be insoluble in water. The hydrophobic tails of the surfactants ‘dissolve’ into the grease, allowing them to be “solubilised” in water later on due to the hydrophilic heads on the surface.
Physical removal:
Through the physical action of mopping, the greasy surfactants can become completely solubilized by the surfactants and be removed easily from the surface with water.
For effective dispersion of an oil slick, should the dispersant be acting on the oil-air interface, or the oil-seawater interface?
The dispersant should be acting on the oil-seawater interface
How does temperature affect the efficiency of the surfactants?
As temperature increases, the CMC initially decreases and then increases, owing to the smaller probability of hydrogen bond formation at higher temperatures. The onset of micellization tends to occur at higher concentrations as the temperature increases
What is surface pressure, and how does it relate to surface expansion?
Surface pressure is the expanding pressure of an adsorbed layer of surfactant.
Surface pressure describes surface expansion. Surface tension describes surface contraction.
Net surface tension is original surface tension, minus surface pressure.
What is surface pressure (pi), and how does it relate to surface expansion?
Surface pressure is the expanding pressure of an adsorbed layer of surfactant.
Surface pressure describes surface expansion. Surface tension describes surface contraction.
Net surface tension is original surface tension, minus surface pressure.
Explain the process of micellisation
When the surface is tightly packed with a monolayer of surfactant molecules, further dissolution stops.
At the CMC, micelles begin to aggregate into micelles containing 50- 100 surfactants, forming an association colloid.
What is one scenario for adhesive failure to occur?
A liquid spreading over the adherent can easily trap air in the irregularities on the surface, leading to the formation of composite surface.
When a portion of the composite involves air-adhesive interface, the actual area of adhesive contact is greatly reduced.
The 3-phase boundaries formed represent excellent sites for the initiation of cracks and flaws in system.
The net result is significantly poorer adhesion.
Similar effects can be seen if the entrapped material is water, oil or other materials with poorer interactions with the adhesive
How does molecular weight affect molecular entanglement
At lower molecular weight, there is no entanglement, with relatively weak interaction among adjacent chains. The movement of one chain past one another is easy, and the tensile/shear strength is low. Cohesive failure will occur easily.
At higher molecular weight, entanglement of molecular chains is greatly increased, and the material exhibits a greater tensile/shear strength. Plastic deformation and necking will occur well before final failure.
Suggest 3 ways to improve entanglement between 2 solid surfaces
- Use solvent that swells the adherend surface, allowing interpenetration of adhesive and adherend
- Apply heat to increase mobility of polymer chains
- Use monomer system that polymerizes after applications on the surface
What is the relationship between contact angle, surface energy and interfacial energy?
A high wetting surface has a surface energy that can overcome the surface tension of the liquid’s molecules that would normally keep it in droplet form.
Spread out well = Low contact angle = high surface energy and high interfacial tension
Balls up = High contact angle = low surface energy and low interfacial tension.
Why is the octane-water interfacial tension greater than the octane-air surface tension?
The surface tension of octane-air is lower than that of water because octane is non-polar and dipole-dipole interaction and van der Waals forces (unlike water) amongst octane molecules are very much weaker.
In addition, at the octane-air interface, some of the octane molecules are able to stick its hydrocarbon chain in air above the surface.
Therefore, the forces pulling down the octane molecules on the octane-air interface are much weaker. The interaction between octane and air is easier than the one between octane and water.
The octane-water interfacial tension is higher than the octane-air surface tension because the octane molecules do not like to interact with water molecules.
This hydrophobicity increases the thermodynamic work needed to create an interface between octane and water.
