Emulsions Flashcards
What are the different types of emulsions?
Coarse emulsions (includes o/w, w/o, multiple emulsions)
Microemulsions (o/w, w/o, microemulsions)
Two immiscible liquids (oil and water) as well as a surfactant/emulsifier
What is an emulsion?
2 phased system in which
Internal or dispersed phase is fine droplets of liquid and is dispersed in the external or continuos phase
What is a common example of o/w emulsion?
Milk
What is an common example of w/o emulsion?
Butter
What are surfactants?
Surface active agents with 2 distinct regions (polar head group and hydrophobic tail)
They orient themselves preferentially at surfaces or interfaces
What effect do surfactants have on surface tension?
Surface tension of a surfactant solution decreases with increasing amounts of surfactant present at the surface
What happens if the surface layer of a solution becomes saturated with surfactants?
Micelles form to hide the hydrophobic tables of surfactants from the aqueous medium
What is the name of the concentration at which micelles first form?
Critical micellular concentration
What is the preferable dispersion medium for micelles and reverse micelles?
Micelles prefer to be in aqueous medium as their polar heads are I pointing outwards and they hide their hydrophobic tails
Reverse micelles prefer to be in hydrophobic medium for the opposite reason
What are examples of ionic surfactants ?
Sodium lauryl sulfate
Cetrimide (aka alkyltrimethylammonium bromide)
What are examples of non ionic surfactants?
Tween 20 (polyoxyethylene 20 sorbitan monolaurate) which has a higher CMC Tween 80 (polyoxyethylene 80 sorbitan monoleate) which has a lower CMC
What factors affect CMC and micelle size?
Structure of hydrophobic tail group Nature of hydrophilic head group Nature of counterion Effect of temperature Addition of electrolytes
How does the structure of hydrophobic tail group affect CMC and micelle size?
Increased length of carbon chain means a lower HLB value (higher lipophilicity)
This causes a decrease in CMC as a smaller concentration of surfactants is required for saturation
This also results in an increase in micelle size as the hydrocarbon portion is bigger
How does the nature of the hydrophilic head group affect CMC and micelle size?
Non ionic surfactants have lower CMC than ionic surfactants as there is no repulsion between the head groups
For Non ionic surfactants the longer oxyethylene chain results in increased CMC
How does the nature of the counterion affect CMC and micelle size?
For cationic surfactants Cl-<Cs+
The more weakly hydrated the counterion the larger the micelle as the counterions can be absorbed more readily onto the micelle surface and so decreases charge repulsion between polar head groups
How does temperature affect CMC and micelle size?
For non ionic surfactants, an increased temperature increases mi cellular size and decreases CMC until cloud point which is where separation into two phases occurs.
For ionic surfactants, temperature has a much smaller effect on the micellular properties
How does the addition of electrolytes affect CMC and micelle size?
Addition of electrolytes to ionic surfactants decreases CMC and increases micelle size
This results in reduction in magnitude of forces of repulsion between charged head groups in micelle and a consequent decrease in the electrical work required for micelle formation
How many counter ions are attracted close to the micelle?
70-80%
What forms the stern layer of an ionic micelle?
Head groups plus bound counter ions
What is the outer surface of higher stern layer in an ionic micelle?
The shear surface of the micelle
What is the kinetic micelle comprised of?
The core, and the stern layer
What is the gouy chapman electrical double layer?
Aka diffuse layer
This contains the remaining counter ions which neutralise charge,
The thickness of this double layer depends on the ionic strength of the solution.
More electrolytes = more thickness
What is the hydrophobic core surrounded by in a non ionic micelle?
The palisade layer which is essentially a shell of POE chains
What is trapped inside the palisade layer in a non ionic micelle?
Water molecules.
These are trapped mechanically and via hydrogen bonding with POE.
Hence non ionic micelles tend to be highly hydrated, and are often less symmetrical and larger than ionic micelles