Colloids Flashcards
What are colloids?
Is a dispersion of one phase to another continious phase where the dispersed phase is made from particles that are small
What is an example of a colloid?
e.g. gas and solid would be pollution and gas and liquid would be aerosols
What is solution?
Is a dispersion of molecules if a material in a liquid medium and is clear
What are the size of molecules in colliods?
0.1 to 10 micrometres
What theory governs the interaction of particles and let us understand the behaviour
The DLVO theory
What happens when something is suspended into an aqueuous medium?
A particle will almost certainly have a charge in its surface
What happens to a charged particle?
- particle will have charge on surface eg negative.
- this negative charge attracts positive held close to the surface as fixed layer called stern layer
- now on the outside of particle most of the charge on particle has been screened so still negative charge.
- so outside stern layer there is a diffuse layer, an in-balance of ions that balances out charge on the particle so more positive
what happens to the electrical potential as the distance from the the particle surface increases?
it decreases
what are the 2 ways particles are stabilised?
- steric stabilisation
- elecrostatic stabilisation
what does steric stabilisation mean?
the coating of the particle in an polymer which is inert so they don’t interact
what is electro static stabilisation (zeta-potential)?
involving the surface charge on the surface of the particles. if zeta potential is great than + or - 30mV means its stable state.
what does the zeta potential govern?
the electrokinetic behaviour of the particles such as electrophoresis and sedimentation
what is the zeta potential dependant on?
-stern potential and ionic strength medium
what is zeta potential?
is the voltage at the edge of the diffuse layer where it meets the surrounding liquid
what is the stern potential?
the potential with the fixed layer
what is the stern potential dependant of?
particle chemistry and surface charge (pH) and presence of absorbed material
what will happen to the rate of sedimentation with a flocculated system?
settle faster
what will happen to the rate of sedimentation with a deflocculated system?
settle slower
what is the effect of reducing particle size?
- will increase diffusion
- reduce sedimentation
- effect dissolution behaviour
what is the effect of increasing viscosity?
- reduce diffusion
- reduce sedimentation
- overall effects are dependant on particle size
what is the effect of increasing density of the medium?
- no effect
- reduce sedimentation
- beneficial for most pharmaceutical suspensions
Is deflocculation reversible or irreversible?
irreversible
Is flocculation reversible or irreversible?
reversible
What is a surfactant?
Are surface active agents
What is an amphiphile?
Something that is happy to be in both water and oil
What is the structure of a surfactant?
Tail: lipophilic and head: hydrophilic
Where do you find surfactants?
Chocolate, washing up liquid ,ice cream, beer
What happens to surface tension when surfactants are added?
They are lowered
What do surfactants do to a liquid?
Interfere with intermolecular bonding of the liquid thats why they are used fo wetting and emulsifying?
What is surface tension?
An interface between two things like water and air/oil has cohesive forces act on molecules in bulk of liquid
What happens at the surface during surface tension?
Cohesive forces acts to pull molecules into bulk this acts to reduce surface area
What is waters surface tension?
72mN/m which is high most others will be lower
What happens when the temperature is increased how does this effect surface tension?
Decreases surface tension
When do micelles form?
Once added surfactant molecules have formed a complete monolayer at the liquid-air interface, any additional surfactant must remain in solution. These extra surfactant molecules then self-assemble so that their hydrophobic regions are kept from water forming micelles
What is the name of the concentratin at which micelles form?
Critical micelle concentration this depends on both the surfactant and medium
What factors affects Critical micelle concentration (CMC)?
- increasing in hydrocarbon chain length increases hydrophobicity, decreasing CMC and increasing micellar size for both ionic and non ionic surfactnats
- non ionic surfactants have much lower CMCs and higher aggregation numbers
- effect of counterion
- temp chsnge
- effect of addition of electrolytes
What are the pharmaceutical uses for surfactants?
- to aid solubilisation
- as emulsifers
- as antibacterial cleaning agents
- as penetration enhancers
- as penetration enhancers
- as drug delivery vehicles
What is an emulsion?
2 immiscible liquids one finely subdivided in the other 2 phases continous and disperse
How do you distinguise between 2 main liquids in an emulsion?
By doing a conductivity test
Why are emulsions inherently unstable ?
Due to high interfacial tension
What are the size of emulsion droplets?
Not unifrom, relatively low number of large droplets and lots of small ones
How does the conductivity test work?
If water is the conrinuous phase it will conduct elevtricity through it so the light bulb will light up
How does the dye solublity test work?
If the dye is water solube then it will partition into the water phase as either dropelets of change the full solution around the droplets the dye colour
What does a high interfacial/surface tension do to the stability?
Reduce it so its unstable
What are some types of emulsifying agents?
- surfancants
- interfacial complexes
- hydrophilic colloids
- solid particles
What may happen when surfactants accumulate at an interfeace to do with their film formed?
- sufactans such as sodium dodeyl (lauryl) sulphate it has a very high charge on its head group so there will be replusion between neighbouring surfactants. The interface will not be rigid
- sodium oleate forms a condensed film and will stay in play as the place and the head group isnt as strong the long tail creates condesed film
Why are all surfactnats not good emulsifyers?
Headgroup is critical
Why do you use a mixer of surfancants as emulsifyers rather than just one?
Inorder to make interfacial complexes which could be 2 surfactants or a surfactant and a long alcohol to make a better film, film formed makes them more viscous, high flexibility and lower interfacial tension
How may instablitiy manifest itself in an emulsion?
- flocculation
- coalescene
- phase inversion
- creaming
- cracking
