Interfacial phenomena Flashcards
What is an interface?
The boundary between two phases
At the interface, the characteristics are the same as those at the bulk. True or false?
False - they differ
What is surface tension?
The property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water molecules
When equilibrium is reached, forces are equal on each side and there is an ________ in surface area of the interface
increase
Surface area depends on the area of the film. True or false?
False
What factors does surface tension depend on?
chemical nature of film
change in area (initial compared to at equilibrium)
length of film
Does temperature influence surface tension?
Yes
As temperature increases, the majority of liquids see a _______ in surface tension
decrease
Why does surface tension decrease as temperature increases?
Because there is less cohesion between liquid molecules
In liquid/liquid systems, when there is no spreading - it shows there is ____ interfacial tension between the two liquids
high
In liquid/liquid systems, when there is spreading - it shows there is a ____ interfacial tension between two liquids
low
What is used to determine whether spreading will occur?
Spreading coefficient
The spreading coefficient is the difference between the works of _______ and _______
adhesion
cohesion
What is the works of cohesion Wa/a?
The work necessary to pull apart a column of liquid
What is the works of adhesion Wa/b?
The work necessary to separate two immiscible liquids to form two liquid/gas interfaces
Spreading only occurs when the spreading coefficient is negative. True or false?
False - only when it is positive
Behaviour of a liquid in contact with a solid is important in the _______ of a tablet
disintegration
What is the contact angle used to determine?
the spreading coefficient (S) adhesion tension (AT)
Spreading takes place when the contact angle is ____ than 90 degress
less
What happens if the contact angle is more than 90 degrees?
no spreading
How does the contact angle help in formulations?
Because if you have a contact angle of more than 90 degrees, you’ll know you need to add something to the solution so the solid can be in contact with water i.e. a surfactant
What is wetting?
The ability for a liquid to maintain contact with solid surface
What is used to measure the wetting of a solid?
Spreading coefficient
Why do hydrophobic drugs require addition of wetting agents such as surfactants?
Because they are needed to lower the surface tension between the solid drug and the vehicle to favour the dispersion of the solid.
Polymers’ surface tension is related to their_________
molecular weight
_______ can be used as binding agents to increase the tablets compaction, contribute to the formation of a hydrated layer over the solid
Polymers
In terms of moisture penetrability, what does it mean if a polymer has a high interfacial tension?
It will results in hard and thick polymer layers and therefore poor moisture penetrability
Why is it important to control he surface tension of a tablet product?
Because you need to achieve a presentable product for the patient - a high surface tension will result in a hindered wetting process and a low surface tension will result in an “orange peel”
Emulsions have a vast interfacial area between dispersed phase and continuous phase. True or false?
True
Microemulsions have little/no surface tension. True or false?
True
In emulsions, surfactants are used to control/reduce surface tension. True or false?
True
What is drug absorption in suppositories influenced by?
particle size
aqueous solubility
interfacial tension
The distribution behaviour in a suppository base is dependent on the _________ of the drug
surface tension
Drugs with a ____ surface tension have lower affinity to the lipophilic suppository base and will partition into the rectal fluid easily
high
All parts of the skin show the same spreading/wetting. True or false?
False
The skin has a high surface tension. True or false?
True
Surfactants in transdermal delivery systems are used to interact with ________, ________ and its extracellular matrix
keratin
stratum corneum
Reducing surface tension in transdermal delivery systems helps to drug to penetrate through ________
stratum corneum
Ibuprofen is a self penetration enhancer. It behaves as an ______ surfactant
ionic
How is adsorption different to absorption?
Adsorption is the accumulation of molecules at the interface whereas absorption is the penetration of one compound through the body of a second
What are the two types of adsorption?
Physical
Chemical/chemisorption
In physical adsorption, the adsorbate is bound to the surface through weak _____ forces
vdW
In chemisorption, the adsorbate is bound through _______ valence forces and involves an ion exchange process
stronger
You can calculate the surface occupied by each molecule just after cmc has been reached. True or false?
False - just before cmc
What does the Freundlich model describe?
adsorption of molecule at relatively high concentration of solute and the adsorbate forms multilayers
What does the Languir model describe?
The adsorption of molecules at a relatively low concentration of solute and the adsorbate forms a monolayer
What are the four factors that affect the adsorption in S/L systems?
Solubility of the adsorbate
pH of solution
Temperature
Surface area of adsorbant
Adsorption is proportional to the solubility. True or false?
False - inversely proportional as bonds are necessary for solubility
Is adsorption an exothermic or endothermic reaction?
Exothermic
An increase of temperature limits the extent of adsorption. True or false?
True
A large surface area leads to a large extent of adsorption. True or false?
True
What are the applications of adsorption?
Adsorption of poisons/toxins - activated charcoal
Taste masking
Haemoperfusion - to treat severe overdose
Analysis - TLC uses adsorption
