Lecture 21: Pharmaceutics I Flashcards
What is unique about an interface?
Molecules forming that part of the system often have properties which render them significantly different from molecules in the bulk
What is an interface?
The boundary between the existence of two phases
What are the different types of interfaces?
G-L : liquid air surface e.g. Lake surface
G-S: solid surface e.g. Table top
L-L: liquid liquid interface e.g. Emulsion
L-S: liquid solid interface e.g. Suspension
S-S: solid solid interface e.g. Powder particles in contact
What is the difference between a surface and an interface?
Surface is used for gas-solid or gas-liquid. I.e. Restricted to a gas and other state of matter interface
What are some applications of interfaces in pharmacy?
Emulsion formation and stability Suspension formation and stability Penetration of molecules through biological membranes Absorption of lipids and fatty acids Efficient operation of lungs of alveoli
What are the forces present at a liquid interface?
Cohesive forces- attraction of molecules in the same phase
Adhesive forces- attraction of molecules in different phases
What are the forces in the vapour liquid surface?
Bulk liquid molecules have cohesive forces equal in all directions
Surface liquid molecules have cohesive forces with molecules adjacent and below. They also have weak adhesive forces with vapour molecules above
Net effect is some surface molecules of the liquid experiencing a net inward force towards the bulk
What is the rresult of the net inward force pulling molecules toward the bulk?
Liquid surface tends to contract, resulting in surface tension
What could surface tension be defined as?
The force per unit length that has to be applied parallel to the surface to counteract the inward pulling force
Unit: dynes/cm
What forces are in the liquid liquid interface?
Strong cohesive forces in the bulk between molecules of the same phase, weak adhesive forces between molecules of the two immiscible phases at the interface
Net outcome: inward pulling force at interface favouring contraction of interface
What is interfacial tension?
The same as surface tension, but involving two immiscible liquids
What is the interfacial tension between water and alcohol?
0 as the liquids are miscible
What methods could be used to measure surface and interfacial tensions?
Capillary rise method
Du nouy ring method
What does the method choice depend on? (Measuring surface and interfacial tensions)
Whether surface or interfacial tension is to be determined
The accuracy and convenience desired
The size of the sample
Whether effect of time is to be studied
What is the capillary rise method?
Liquid usually rises up in a capillary tube because of adhesion forces between liquid molecules and capillary walls exceeding the cohesion between liquid molecules.
The liquid then wets the capillary wall.
The liquid will rise due to ST until the upwards movement is balanced by the effect of gravity.
This upward force due to surface tension will be equal to the downward force due to effect of gravity on the mass of rising liquid.
We use the equation γ = 1/2rhρg to find out the surface tension
What is the capillary rise method limited to?
Surface measurements
What is the du nouy ring method?
Can be used for both surface and interfacial tension measurements
the force required to detach a platinum ring immersed at the surface or interface is proportional to the surface or interfacial tension.
This force is provided by a torsion wire and is recorded in dynes on a calibrated dial.
The ring is placed in the sample, and the dials turned. The instrument measures the weight of liquid pulled out of the plane of the interface before the ring becomes detached.
What must be considered when using the du nouy ring method?
The correction factor (B) as the theory does not take into account the radius of the ring, radius of wire forming the ring and volume of liquid raised out of the surface.
Errors as large as 25% may be possible
What causes adsorption?
Molecules at the vapour liquid or liquid liquid interface experience strong cohesion than adhesion forces.
Net effect is an inward pulling which favours the surface to contract.
This causes particles to adsorb to the surface
Why is the contraction of a surface spontaneous?
It is a thermodynamically favoured situation. Any attempt to expand the surface or interface must involve an increase in the surface free energy.
What is surface free energy?
This is the work done to increase the surface by unit area
Some molecules tend to favour redistribution from the bulk to the interface due to a strong adhesion than cohesion forces
What kinds of molecules favour redistribution to the surface?
Surfactants as they have dual character
What is the most important property of surfactants?
Their ability to adsorb at interfaces
In a water air surface, how would surfactants orient?
Hydrophilic heads towards the water, tails towards the air
