What are a very important property of surfactant?
It can reduce the interfacial tension
What are surfactants?
What happens to the SLS surfactant when no salt is added?
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
What happens to the SLS surfactant when salt is added?
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
What is hydrophilic-lipophilic balance?
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
What is the relationship between HLB and CMC?
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
What are some other factors which affect the CMC?
Length of hydrocarbon chain
Increased ethylene oxide chain length (head group)
Electrolyte addition to solutions of ionic surfactants
Effect of temperature
How can surface tension be measured?
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
What are effects do micelles have that can be used to measure the CMC?
Level out the osmotic pressure
Increase the turbidity
Decrease the molar conductivity
How can surfactants be classified?
What are anionic surfactants like?
They dissociate in water to give negatively charged ions
E.g. SLS, salt of fatty acid, and di/trivalent soaps
What are cationic surfactants?
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
What are amphotoeric surfactants?
Have both positive and negative charge.
The net charge depends on the pH medium
What is non ionic surfactant?
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
Which surfactant is most toxic and why?
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
What are some applications of surfactants?
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