Disperse Systems

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  • Created by: SamDavies
  • Created on: 11-04-18 18:19
Disperse system
The presence of two distinct phases in which one substance (the dispersed phase) is distributed throughout the second substance (the continuous phase)
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Surface tension
This arises due to a difference in attractive forces between the bulk and the surface. Molecules at the surface experience an inward attraction, causing the surface to contract
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LaPlace pressure
The difference in pressure across a curved surface/interface where the pressure inside is greater than the pressure outside, e.g. in a bubble
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Cohesion
Affinity for molecules/phases that are similar
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Adhesion
Affinity for molecules/phases that are different
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Spreading
Phenomenon at liquid-liquid interface
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Wetting
Phenomenon at solid-liquid interface
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What happens when adhesion > cohesion?
There is enough affinity for both phases to interact with one another. This creates a film which results in one interface (e.g. milk in a cup of tea) and one value of interfacial tension. Contact angle is smaller
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What happens when cohesion > adhesion?
There is low affinity between the two interfaces, and a lot more work needs to be done to overcome the tension so the system is very unstable. This results in one interface but two surfaces. Contact angle is larger
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Lyophobic colloids
Low affinity for the continuous phase and form thermodynamically unstable dispersions, i.e. they require energy to form
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Lyophilic colloids
High affinity for the continuous phase and form thermodynamically stable dispersions, i.e. their formation is spontaneous
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Critical micellar concentration
The concentration at which the surface is saturated and micelles begin to form
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Krafft point
The temperature above which the solubility of a surfactant INCREASES sharply. (At temperatures below this point, surfactant molecules precipitate instead of forming micelles)
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Cloud point
The temperature above which the solubility of a surfactant DECREASES sharply; this results in a cloudy solution
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Dialysis
This allows for the separation of colloidal particles from small molecules/ions. The semi-permeable membrane allows for the diffusion of ions and molecules but not of colloids
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Brownian motion
The random movement of particles under thermal agitation (has been observed for particles up to 5 micrometers, anything larger is impacted by gravity so the randomness of movement is then reduced).
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Sedimentation
The downward motion under gravitational forces. This is not common in colloids due to their small size unless the gravitational force is increased by ultracentrifugation
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Light scattering
A colloid in a dispersion is able to scatter light so if you have a completely clear liquid, shining a light into the solution will scatter the light if there is a colloid present
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Ultramicroscopy
Scattered light can be detected by this method. Colloidal particles appear as spots against a dark background
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Zeta potential
The potential difference existing between the surface of a solid particle immersed in a conducting liquid (e.g. water) and the bulk of the liquid. It is measured in the "slipping plane"
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Coagulation
This is permanent aggregation - a lot of energy is needed to separate the colloids (shaking will not be enough)
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Flocculation
Re-dispersion of aggregates is easily achieved by shaking
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Suspension
A coarse dispersion in which the internal phase (therapeutically active ingredient) is dispersed uniformly throughout the external phase
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Stoke's law
Equation that measures sedimentation rate. It is only valid when the concentration of the dispersed phase is < 2% solids, the dispersed phase is composed of spherical particles of which there is a narrow distribution of size and the solvent is polar
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Ostwald ripening
The recrystallisation of smaller particles disappears due to the temperature fluctuations during storage
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Diffusible solid
Disperses very easily by shaking the vehicle in a calibrated bottle or by making a paste then increasing the water content. Sedimentation is slow which improves dose uniformity. The dispersed phase is still insoluble in H2O but is easily wetted
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Indiffusible solid
Insoluble in water and easily wetted but the suspensions formed are unstable. There is quick sedimentation of the dispersed phase which is a risk for uniformity - requires suspending agent
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Emulsion
Requires a dispersed phase, a continuous phase and an emulsifier. The dispersed and continuous phases are always immiscible with one another; they are usually an oil and water phase
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Microemulsion
Colloidal solutions that contain oil, water and a very high concentration of surfactant
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Emulsifier
They form a film on the surface of oil droplets to increase the affinity between the dispersed phase and the continuous phase. If the film is broken, the contents will "crack" - this is the separation of the two phases
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Surfactant
These reduce interfacial tension through electrostatic repulsion if they have charged polar heads
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Saponification
The result of the reaction between a triglyceride and a strong base. The product is amphiphilic so it can be used to stabilise and emulsion
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Hydrocolloid
They can be used as emulsifiers by forming a multimolecular layer on the dispersed droplets; working by steric repulsion. They have little effect on interfacial tension. They stabilise oil in water emulsions and can also increase viscosity
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Bancroft's Rule
The phase in which an emulsifier is most soluble becomes the continuous phase
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Hydrophilic Lipophilic Balance
Numerical value of an amphiphilic molecule - if it is too high, it is too soluble in water and if it is too low it is too soluble in oil. Should be between 1 and 50
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Sedimentation volume
Volume of the sediment (Vu) divided by volume of the suspension (Vo). Value will be higher for a flocculated suspension
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Degree of flocculation
Volume of the sediment (Vu) divided by the volume of the sediment in a deflocculated suspension (V infinity. If the answer is 1, the flocculating agent has not worked
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Other cards in this set

Card 2

Front

This arises due to a difference in attractive forces between the bulk and the surface. Molecules at the surface experience an inward attraction, causing the surface to contract

Back

Surface tension

Card 3

Front

The difference in pressure across a curved surface/interface where the pressure inside is greater than the pressure outside, e.g. in a bubble

Back

Preview of the back of card 3

Card 4

Front

Affinity for molecules/phases that are similar

Back

Preview of the back of card 4

Card 5

Front

Affinity for molecules/phases that are different

Back

Preview of the back of card 5
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