7 Interfacial Phenomena Flashcards
Define ‘interface’
an interface is a boundary between two immiscible phases
What are cohesive and adhesive forces? Include theire relative strengths
Cohesive forces exist between molecules in the same phase. These tend to be stronger than adhesive forces.
Adhesive forces exist between molecules of differing phases. These are relatively weaker than cohesive forces.
What forces exist in bulk liquid molecules and surface liquid molecules?
Bulk = cohesive forces formed on all sides, and are equal in all directions. Surface = cohesive forces formed below and adjacent. Adhesive forces form with the above molecules but are relatively weaker. This results in a net inward force.
What is the result of a net inward force between a gas phase and liquid phase?
Surface tension
what is surface tension? what does it measure?
Force per unit length that has to be applied parallel to a surface to counterbalance the inward pulling force.
Measures resistance of a fluid surface to deformation/breakge.
What is interfacial tension?
Same as surface tension but exists between two liquid interfaces.
Describe the Du Nouy ring method and what it is used for
Finds the force necessary to detach a platnium-iridium ring immersed at the surfac/interface of a substance. This is proportional to surface/interfacial tension.
What are surfactants
Molecules possessing both hydrophilic and hydrophobic properties and will absorb at a surface/interface.
What do surfactants do?
Used to reduce surface/interfacial tension by interact with both phases in that system.
What are micelles and how do they form?
Micelles are round structures composed of surfactant molecules existing within systems. These are formed at the critical micellar concentration, where surfactants have saturates an interface beyond their solubility in a liquid
What are micelles used for? give an example
Materials i.e. drugs can be contained within micelles, being able to exist within a system they are otherwise immsicible in.
Can suspend oil within and aqueous environment
How does surface tension change with the addition of surfactant
Initially, ST will not change.
ST decreases as the surface becomes more saturated with surfactant.
At full saturation, ST drops and micelles form (CMC). Surface tension does not decrease past this point.
what is the orientation of surfactants of micells in oily/aqueous environments
oily: non-polar tails face outward and polar heads face inward
water: polar heads face outward and non-polar tails face inward.
Describe wetting and when this is applied
Lipophilic drugs tend to form air pockets with water. Wetting is applied in making suspensions to reduce ST between lipophilic drugs and aquous oral liquids.
How are surfactants used in emulsions?
Emulsifying agents allows oil droplets to stay seperate from eachother without phase seperation (e.g. oil in water)