Interfacial Phenomena 2+3 Flashcards
What is work of adhesion?
Work required to separate the two phases (unlike molecules) at their interface.
The larger the work of adhesion (Wa), the stronger the attraction between the unlike surface
What is work of cohesion?
Work required to produce two new surfaces between the like molecules.
The larger the work of cohesion (Wc), the stronger the attraction between the like molecules.
What is Wa and Wc?
Wa - An indication of the attractive strength between unlike surfaces
Wc - An indication of the attractive strength between like surfaces.
What is the equation for spreading?
S = Wa - Wc
When does spreading occur?
if a liquid adheres to the substrate more strongly than adhering to itself
If S >= 0
or
Wa >= Wc
Explain the results of having a greater Wa?
Having a greater Wa, means that the liquid is more strongly attracted to another liquid, than it is to itself. Causing spreading, than staying as a droplet.
Explain the results of having a greater Wc?
Having a greater Wc, means that the liquid is more strongly attracted to itself than it is to other liquids. So it stays as a droplet, rather than spreading.
Explain contact angle?
When the contact angle is small, the liquid covers a large area of the solid, but when the contact angle increases, the liquid covers a smaller area.
The lower the contact angle, the better the solid can be wetted by the liquid.
The larger the contact angle, the more difficult it is for the liquid to wet the solid.
Explain how to measure the contact angle for powders?
You make them into plates – compress the powder into a compact to achieve a flat surface
Then add liquid to form a drop on the surface
Then measure the contact angle
What is Contact angle affected by?
Pressure
Explain another way to measure the contact angle for powders?
If you compress the powder into a rectangle shaped plate, then insert the plate into liquid.
When you pull the plate out, the surface tension will form a curved surface.
Then record the net detachment force, F
Explains values for Cosθ?
Larger Cosθ, means smaller contact angle and shorter disintegration time.
What do surface active agents have (amphiphiles)?
have both a hydrophobic section and a hydrophilic section in the same molecule.
What is Physisorption (Physical adsorption)?
The adsorption of one material to the surface of another due to Vander Waals forces, hydrogen bonds, etc. It is a reversible process.
What is Chemisorption due to?
Involves strong chemical bonds and tends to be irreversible (unless bond is broken)
