Colliods 8 W4 Flashcards
What do emulsifying agents do?
They lower the energy cost of increased surface area
They create repulsive interactions between droplets
They make the interface more rigid
What are molecules called that accumulate a interfaces and lower the surface tension?
Surface active agents or surfactants
Allow a lower surface tension allows what?
Allows sites with greater surface areas (like small droplets) to remain stable for longer
Emulsifying agents are…..
Surfactants with particular properties
Surfactant molecules have two parts…..
A hydrophilic head group and a hydrophobic tail group
Different surfactants have…
Have Alkane tail groups that vary in length (C8->C22)and number (1,3)
But the greatest variation is in the head group
The hedge group can be ionic (an ionic ionic or zitterionic) or non-ionic
What happens with the addition of a small amount of a surfactant?
Surface tension at an air: water interface is lowered dramatically
How do you surfactants lower surface tension?
The surfactant preferentially accumulates at the surface, creating two new interfaces:
- water: surfactant
- air:surfactant
Both interfaces have much lower surface tensions than the air : water they replace
So the energy cost of maintaining the interface is lower
How do they make interface more rigid?
Single town groups can only create a rigid interface if the head groups are sufficiently small enough to allow close packing with no gaps in between the heads
Adding co-surfactants with smaller head groups in between remove the gaps in between the regular heads
What is the measure of the different properties of surfactants?
HLB value (hydrophilic – lipophilic balance)
Hydrophilic groups in the molecule have high values and lipophilic groups have low values giving an overall number for the surfactant molecule
Low – HLB surfactants are suited to water in oil emulsions
High – HLB surfactants are suited to oil in water emulsions
Sometimes we need a mixture of surfactants to make the best emulsifier
What is a cream?
Highly ordered
Creams are semi-solid emulsions of oil and water.
They are divided into two types: oil-in-water (O/W) creams - small droplets of oil dispersed in a continuous water phase, and water-in-oil (W/O) creams - small droplets of water dispersed in a continuous oily phase.
What does the semisolid nature creams do?
Inherently increase the stability by making interfaces more rigid but the emulsifying agents are still needed too
