Tutorial 5: CMC Measurement

Question 0

What are a very important property of surfactant?

Answer 0

It can reduce the interfacial tension

Question 1

What are surfactants?

Answer 1

Amphiphillic molecules

Question 2

What happens to the SLS surfactant when no salt is added?

Answer 2

SLS is ionised in water
It has a negative head group
So when concentration of the SLS increases these will repel each other
This repulsion prevents the aggregation so the CMC value increases

Question 3

What happens to the SLS surfactant when salt is added?

Answer 3

The sodium ions interact with the negative head group to neutralise it
This reduces the magnitude of negative charge so they no longer repel
The SLS surfactants are able to aggregate better
So the CMC decreases

Question 4

What is hydrophilic-lipophilic balance?

Answer 4

The HLB is a way to measure the polarity of surfactants,

It describes the degree to which a surfactant water is soluble or oil soluble

Question 5

What is the relationship between HLB and CMC?

Answer 5

A high HLB surfactant has a high saturation solubility which causes an increases in the CMC

Whereas a low HLB surfactant has a low saturation solubility and therefore a decreased CMC

Question 6

What are some other factors which affect the CMC?

Answer 6

Length of hydrocarbon chain
Increased ethylene oxide chain length (head group)
Ionic surfactants
Electrolyte addition to solutions of ionic surfactants
Effect of temperature

Question 7

How can surface tension be measured?

Answer 7

Sessile drop in which you measure the degree of wetting

Capillary rise method in which you measure the surface tension by measuring the distance the liquid rises and using a formula (ST= 1/2rhpg)

Wilhelmy plate method
The plate is placed in a liquid and the force required to remove the
Plate is measured

Question 8

What are effects do micelles have that can be used to measure the CMC?

Answer 8

Level out the osmotic pressure
Increase the turbidity
Decrease the molar conductivity

Question 9

How can surfactants be classified?

Answer 9

Anionic
Cationic
Amphoteric
Non-ionic

Question 10

What are anionic surfactants like?

Answer 10

They dissociate in water to give negatively charged ions

E.g. SLS, salt of fatty acid, and di/trivalent soaps

Question 11

What are cationic surfactants?

Answer 11

They dissociate in water to give positively charged ions
E.g. Quaternary ammonium compounds like cetrimide

It is unstable at high pH and with anionic surfactants

Question 12

What are amphotoeric surfactants?

Answer 12

Have both positive and negative charge.
The net charge depends on the pH medium

E.g. Lecithin

Question 13

What is non ionic surfactant?

Answer 13

They have no net charge
E.g. Tweens, spans
These have low toxicity and are compatible with other material
These are less sensitive to pH change
However they are sensitive to temperatur change and are expensive

Question 14

Which surfactant is most toxic and why?

Answer 14

Cationic surfactants are most toxic because they dissociate to give positively charged ions.
The biological membrane is negatively charged due to phospholipids so will interact with cationic surfactants

This disrupts the cell membrane which increases leakage

Question 15

What are some applications of surfactants?

Answer 15

Emulsifying agents
wetting/suspending agents: This is because the contact angle provides an inverse measure of wettability. So addition of the surfactant decreases the contact angle to increase wettability of a substance. This helps it to dissolve

Solubilising agents due to micelle formation

Permeation enhancer - due to increasing drug absorption across biological membranes

Detergents - to remove substances from solid surfaces e.g. Soaps
Antibacterial- e.g. The destruction of cell wall of bacteria by insertion in to the cell membrane e.g. By quaternary ammonium compounds