INTERFACES AND SURFACTANTS Flashcards
1
Q
What is an interface?
A
- When to insoluble phases meet they form an interface
- E.g. air and liquid, water and oil
2
Q
Define surface tension
A
- Unbalanced forces at the surface lead to a net inward force of attraction on surface molecules that tends to minimise the area of the surface (surface tension)
3
Q
What is surface free energy?
A
- Contraction of the surface therefore leads to a minimum free energy state and work is required to increase the surface area.
- The surface free energy is the ‘work’ required to increase the surface area by 1 m2
- Free energy and surface tension are numerically equal with units N m-1 (usually quoted as mN m-1)
4
Q
Describe the methods of measuring surface tension
A
- Whilmey plate method: Thin plate can be wetted by liquid. Force acting on the plate is is proportional to surface tension
- Capillary rise: Measure how far up the tube liquid rises, with knowledge of its density, radius of capillary and gravity
5
Q
When a substance dissolves does surface tension increase or decrease and WHY?
A
- When a substance dissolves in a liquid the surface tension (water = 72 mN m-1) decreases but never significantly increases
6
Q
Draw out a surfactant
A
7
Q
What is the surface activity of an surfactant dependent on?
A
- The surface activity of a surfactant depends on the balance between its hydrophobic and hydrophilic properties
8
Q
What does it mean if a drug is amphilic
A
- The drug has both hydrophobic and lipophilic properties
- So acts as a surfactant
9
Q
What is a wetting agent?
A
- They are commonly used to reduce surface tension of water and thus help water based solutions to spread
10
Q
Surfactants adsorb at interfaces to form a layer one molecule thick, called a ….
A
Surfactants adsorb at interfaces to form a layer one molecule thick - a molecular monolayer
11
Q
Why is the surface tension of water high?
A
- Therefore, water molecules are cohesive because they are strongly attracted to each other.
- Water has a high surface tension because hydrogen bonds among water molecules resist stretching or breaking the surface.
- Essentially due to strong intermolecular bonds
12
Q
What is traubes law
A
- It states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples.
- So, shorter chain length molecules have less surface activity than higher chain length molecules.
13
Q
Q.
A
14
Q
What is the critical micelle concentration
A
15
Q
How to calculate surface excess and what is it
A
- Surface excess is the conc of surfactants at the surface (mol/m2)
- Area per molecule is the space each molecule has at its surface
- where n is number of moles and A is surface area