Science of Medicines Week 7 Flashcards
define interfacial area
the total area of contact between two liquids in a liquid-liquid operation –> large one will prevent dissolving
How do inclusion compounds work?
by incorporating the non-polar portion of one molecule into the non-polar part of another molecule that is water soluble –> you reduce the non-polar water interfacial area by inserting the solute into the complexing agent
What is the most commonly used inclusion compouns?
cyclodextrins
What is a cyclodextrin?
an enzymatically modified starch, and their units form a cylindrical ring
What is the structure of a cyclodextrin?
the outer surface of the ring is hydrophilic and the internal surface of the cavity is non-polar
define surface tension
adhesive forces between the liquid phase of one substance and either a solid, liquid or gas phase of another surface at the interface
How do surfactants work?
they reduce the surface tension at an interface without needing large concentrations of them
To be a surfactant, what properties are needed?
- one element must have a high affinity for the solvent (hydrophilic or polar head, non-ionic or ionic)
- one element must have a minimal affinity for the solvent (lipophilic or nonpolar chain)
What must the polar region on a surfactant be able to do?
- have an affinity for water
- must be capable of pulling long-chain hydrocarbons into water
- must be polar enough to hold the nonpolar region of the surfactant in solution
How are surfactants classified?
by the charge carried by polar part: anionic, cationic, zwitterionic, non-ionic
define critical micelle concentration
the concentration of monomer (surfactant) at which micelles form
define aggregation number
the number of monomers that aggregate to form a micelle
At the critical micelle concentration, which physical properties of surfactants change?
- osmotic pressure
- turbidity
- electrical conductance
- surface tension
What factors of surfactants may increase the critical micelle concentration?
decrease the carbon chain length, increase the polarity of the head
What factors of surfactants may decrease the critical micelle concentration?
temperature, pH, a second surfactant, addition of electrolytes, longer carbon chain length
What are the 4 critical values for micelles?
- critical micelle concentration
- Kraft point (critical micelle temperature)
- cloud point
- critical micelle pH
define the Kraft point
the temperature at which the solubility becomes equal to the critical micelle concentration
What happens when temperature is less than the Kraft point?
the critical micelle concentration is greater than the solubility, so micelles can’t form
What happens when temperature is greater than the critical micelle concentration?
the surfactant forms micelles
What happens to micelles at the cloud point?
an increase in temperature leads to dehydration of POE chains, decreased water solubility and the formation of very large micelles making the solution cloudy
What happens at the critical micelle pH?
if the ionised form of a compound is surface active and the unionised form is surface inactive (or has a lower CMC than the ionised form) then a change in pH can induce micellisation
Can the polar part of a surfactant be non-ionic?
yes, for example hydroxyl and ether groups
What do surfactants do?
decrease the surface tension at the interface
What is the downside to non-ionic polar heads?
they are less polar than ionised groups, so we need more ‘units’ to produce an effective polar head
What is polyoxyethylene (POE)?
a chain with 20 or more ether groups linked to the nonpolar part
What does POE-23 lauryl ether mean?
there are 23 monomeric POE groups in this molecule
What are the applications of anionic surfactants?
oil/water emulsifiers
What are the applications of cationic surfactants?
disinfectants, oil/water emulsifers
What are the applications of non-ionic surfactants?
oil/water and water/oil emulsifiers, also have low toxicity unlike cationic and anionic
Can surfactants be used as drugs?
yes
When does micellisation occur?
when micelle concentration exceeds the CMC
define solubilisation
the process by which water-insoluble or partly soluble substances are brought into aqueous solution by incorporation into micelles
What is the solubilisation capacity (k)?
a measure of the ability of a surfactant to solubilise a solute
What is the molar solubilisation capacity (k)?
the number of moles of solute that can be solubilised by 1 mole of micellar surfactant
What are the equations for solubilisation capacity?
k = molar solubility of the solute in the micelle / molar concentration of micellar surfactants
Why is a low CMC for surfactant drugs preferred?
low CMC means that the surfactant can form micelles at a lower concentration, so when micelles are formed they can solubilise hydrophobic drugs
How can we increase the solubilisation capacity of a low polarity solute?
- increase hydrocarbon chain -> larger nonpolar regions will solubilise more solute and decreases CMC
- introduce polar group
- used branched surfactants as they form smaller micelles
What 4 factors need to be considered when selecting a surfactant?
- amount of surfactant that can be placed in water
- ability of it to solubilise the solute
- the chain length -> influences CMC
- finding a balance
What is Lundelius’ rule?
any factor that decrease the solubility of the surfactant promotes surface activity
What does a high HLB surfactant value indicate?
a surfactant with mainly polar or hydrophilic properties
What does a low HLB surfactant value indicate?
surfactant with mainly lipophilic or non-polar properties
How do you calculate the HLB of a mixture of surfactants?
HLB = x HLB(A) + (1-x) HLB(B) where x are fractions of each surfactant
define required HLB
the particular HLB of a surfactant needed to form a stable w/o or o/w emulsion
Why does HLB of an emulsifier vary with temperature?
temperature affects the relative solubilities of the lipophilic and hydrophilic parts
What happens to non-ionic surfactants at higher temperature?
hydrogen bonds are weakened by thermal forces and the emulsifier is less soluble in water
define the Phase Inversion Temperature (PIT)
the T at which an emulsifier changes from being an O/W emulsifier to a W/O emulsifier
If a non-ionic emulsifier is water soluble at low temperature, what kind is it?
it stabilises O/W emulsions
If a non-ionic emulsifier is oil soluble at high temperature, what kind is it?
it stabilises W/O emulsions
