Surfactants

Question 1

What is a surface? What is an interface? What are the types of interfaces?

Answer 1

surface
- outer boundary of a material

interface

• each surface is an interface
• molecules at the interface have different properties compared to molecules in the bulk of the phases on both sides

gas-liquid = solution 
gas-solid = tablet 
liquid-liquid = emulsion 
liquid-solid = suspension 
solid-solid = layered tablets

Question 2

What is surface tension?

Answer 2

surface tension

• unbalanced forces on liquid surface which result in a state of tension
• force that must be applied parallel to the surface so as to counterbalance the net inward pull

Question 3

What are surfactants?

Answer 3

surface active agents
- surfactants are compounds of amphiphilic nature, i.e. their chemical structure displays two distinct regions = hydrophobic non-polar regions and hydrophilic polar regions

Question 4

What are the types of surfactants?

Answer 4

anionic surfactants
cationic surfactant
non-ionic surfactant
amphoteric surfactants = both positive and negative charge

Question 5

How do surfactants work?

Answer 5

surfactants adsorb at the interface
the hydrophilic region (head) is dissolves well in the water
- remains in contact with aqueous solution
the hydrophobic region is not soluble in the hostile aqueous medium
- ‘escapes’ from the hostile aqueous environment and expands/dissolves in the air/oil

adsorbed surfactant molecule replace water molecules at the surface

• net inward pulling force (water become concave to try to reduce SA and minimise surface tension) reduces which reduces surface tension
• surface tension become stable when the interface is saturated with surfactants

Question 6

When do micelles form?

Answer 6

micelles form when the interface is saturated with surfactants
- surfactants are forced to submerge in water due to saturation

micelles form at critical micelle concentration

• when increasing surfactant has not effect on surface tension
• consists of 50 monomers of surfactants

Question 7

What can micelles be used for?

Answer 7

micelles can be used to increase the solubility of hydrophobic drugs

• micelle centre resembles a separate organic phase
• drug can be held in the centre of a reverse micelle = increases solubility as the hydrophobic tails are facing outward. these can pass through the lipid bilayer

reverse micelles

• hydrophilic heads face inwards
• hydrophobic tails face outwards

Question 8

what are emulsions?

Answer 8

an emulsion is a disperse system in which an insoluble (immiscible) liquid phase is dispersed in a second liquid phase

• oil in water
• water in oil

Question 9

What are the types of emulsions? What are their properties?

Answer 9

depends on droplet size

• coarse emulsion = 10 micrometers to 100 micrometers
• fine emusions - 0. 1 to 5 micrometers
• microemulsions (colloidal) = 10 nanometers - 100 nanometers

Question 10

What are the advantages of emulsions?

Answer 10

deliver poorly soluble drugs
mask the taste of therapeutic drugs
oral administration of therapeutic drugs
reduce drug irritancy of topical application
emulsions for total parenteral nutrition

Question 11

What are the disadvantages of emulsions?

Answer 11

poor physical stability

• difficult to disperse immiscible liquids
• separate easily
• coalesce at the top

Question 12

What is the required HLB?

Answer 12

rHLB is the HLB value required to most effectively form an emulsion for a given oil

Question 13

How do surfactants increase the stability of emulsions?

Answer 13

when a hydrophilic and hydrophobic surfactants are both employed, they interact on the interface

this interaction results in a mechanically robust interfacial film
- prevents disruption and coalescence of droplets and gives greater lowering of the interfacial tension.

hydrophilic colloidal polymers
- starch, cellulose

Question 14

Why are emulsions unstable? How can it be stabilised?

Answer 14

they have a high gibbs free energy = unstable

• internal area cannot be reduced = droplets would coalesce
• surface tension must be reduced = surfactant

natural tendency is to separate to revert to state of lowest energy
- not suitable as emulsion will not be dispersed

Question 15

What are the factors affecting stability of emulsions?

Answer 15

flocculation 
- reversible 
- droplets aggregate towards each other 
creaming 
- droplets combine and rise to the top
coalescence and breaking
- irreversible 
- droplets joining to form bigger droplets and a layer at the top
miscellaneous physical and chemical changes
phase inversion

Question 16

How do hydrophilic colloidal polymers stabilise an emulsion?

Answer 16

polymers macromolecules adsorb at water/oil interface

• form multilayer viscoelastic film - mechanical barrier
• steric repulsion = as polymers approach each, repulsion arise and they repel from each other (no coalesce)
• increase the viscosity of dispersion medium = thickening the continuous phase makes it harder for the disperse phase to move
• electrostatic repulsion if they have ionisable groups = particles have mutual repulsive forces that push them apart

Question 17

What are the factors preventing coalesce?

Answer 17

surfactant
- reduce surface tension
hydrocolloid
- presence of mechanical barriers = polymers
repulsion
- droplets with high charge density = push apart

Question 18

How can emulsions be tested?

Answer 18

dilution test
- oil in water = will not be affected by adding water, no immiscible layers

conductivity test
- water is a good conductor of electricity but oil is not

dye solubility test