Lecture 20 - Drug solubility and dissolution rate 5 Flashcards
How are surfactants classified? Give the classifications.
Classified depending on their polar end.
- Anionic – the polar head is negatively charged so counterion is positive
- Cationic – the polar head is positively charged so counterion is negative
- Non-ionic
- Zwitterions – can have both negative and positive charges at the same time.
What makes up the non-ionic surfactant heads?
hydroxyl and ether groups make up the polar end of the molecule
non-ionic surfactants are less polar than ionized groups. How do they make up for this?
more units are therefore needed for it to be an effective surfactant – needs to be able to pull a non-polar chain
give the typical hydrophilic head of non-ionic surfactants
Polyoxymethylene chains with 20 or more ether groups = the typical hydrophilic head of non-ionic surfactants. These chains are linked to cyclic sugar and an alkyl group.
What is the POE number?
- The POE number = the number of monomeric polyoxymethylene groups in polar end of the surfactant molecule.
give applications of anionic surfactant molecules?
widely used, cheap, toxic so only for external applications, o/w emulsifiers – allow oil to mix in water
Give applications of cationic surfactants
disinfectant, preservative properties, toxic o/w emulsifiers
Give applications of non - ionic surfactants
o/w OR w/o emulsifiers. Low toxicity and irritancy. Oral and parenteral use
what is an O/W emulsifier?
surfactants are emulsifiers. oil suspended in water. Emulsifiers stabilise them to prevent them from separating.
What is an o/w emulsification?
water droplets dispersed in an oil phase
Some surfactants are actual drugs. Give their roles
Their hydrophobic portions are aromatic or heterocyclic rings.
- Tranquilizers – chlorpromazine
- Antidepressants – imipramine
- Antihistamine – diphenhydramine
- Antibiotic – penicillin G
surfactants are involved in solubilisation of non-polar drugs. What is solubilisation?
The process by which water insoluble or partly soluble substances are brought into aqueous solution by incorporation into micelles.
Explain how the site of solubilisation depends on the chemical nature of the solubilisation.
- Site of solubilisation depends on the chemical nature of the solubilisate
- The more polar the solute, the more likely to be solubilised close to the surface
Compare the size of the non-polar region of a non-ionic micelle with that of an ionic micelle.
- The polar region of a non-ionic micelle is significantly larger than that of an ionic micelle (large polar heads are needed in non-ionic surfactants because they are not as sufficient as ionic surfactants)
Give the most common term used to quantify solubilisation
solubilisation capacity, K
Solubilisation capacity, K measures…
the ability of a surfactant to solubilize a solute
Define molar solubilization capacity, K
the number of moles of solute that can be solubilised by 1 mole of micellar surfactant.
Give the equation for molar solubilization capacity, K
Solubilisation capacity can be calculated from a line plot
K = solubilisation capacity
What happens if you increase the carbon chain of a surfactant?
Increase the hydrocarbon chain = larger the non polar region, so you solubilize more solute = decreased CMC
What happens if you introduce a polar group or double bond to the hydrocarbon chain of a surfactant?
- Introduction of a polar group or double bond in the chain = decrease chain length = solubilize less of the solute = increased CMC
- Branched surfactants = smaller micelles
Surfactant selection. What things are considered? We need a balance of certain properties. Describe the ideal structure.
- Ability to solubilize a solute – very short chain, very high CMC so require a high concentration of surfactant to form the micelles
- Increase chain length – decrease CMC- more solutes can be solubilised but you decrease the solubility of the surfactant itself. Although the CMC is reduced the solubility of the surfactant is reduced which reduces the amount of surfactant that can be used
Surfactants selected usually have…12-16 carbons/18 with a double bond. Provides low CMC and sufficient water solubility. Therefore, a sufficient concentration of micelles forms in the water.
Key rule when understanding surfactants…
The rule is that any factor that decreases the solubility of the surfactant will promote surface activity (they sit at the interface, saturating the interface), so decrease surface tension- water molecules at the surface are replaced by the surfactant so less water-water molecule interactions.
The hydrophile must provide enough interaction with water to…
bring the insoluble lipophilic part of the molecule into solution
The HLB scale is used to categorise surfactants. A 100% lipophilic molecule= and a 100% hydrophilic molecule=
HLB 1
HLB 20
What does a high HLB number indicate?
- High HLB numbers indicate a surfactant exhibiting mainly polar or hydrophilic properties
What does a low HLB indicate?
represent lipophilic or non-polar properties.
Different functional groups in the molecules are given different group numbers. Therefore, to find the overall HLB number for the surfactant we use the equation…
Give applications of molecules who have these HLB values
3-6 =
7-9 =
8-15 =
13-15 =
15-18 =
W/O emulsions
wetting agents
O/W emulsions
detergents
solubiliser
Describe the dispersibility of molecules with these HLB values…
1-4 nil
3-6 poor
6-8 unstable milky dispersion
8-10 stable milky dispersion
10-13 translucent dispersion/solution
13+ clear solution
Sometimes mixtures of high HLB and low HBL surfactants give greater stability than single surfactants. Give the equation to calculate the HLB of a mixture of surfactants…
Calculate the HLB of a mixture of surfactants containing…
40% of Span 80, HLB 4.3
60% of Span 20, HLB = 8.6
Surfactants can stabilise emulsions. What do we call surfactants used to stabilise emulsions?
emulgent
How do surfactants stabilise emulsions?
They reduce the interfacial tension between oil and water
Different oils require an emulgent of particular ………….. in order to ………….. the emulsion
HLB
stsbilise
Draw a diagram to show how emulgents work in O/W emulsions
Find the composition of a mixture containing span 80, HLB=4.3 and tween 80, HLB=15 which provides a HLB of 12.1
What does PIT stand for?
phase inversion temperature
The HLB of an emulsifier will vary with temperature. Why? What type of surfactants/emulsifiers feel the effects of temperature more and why?
- the solubilities of the lipophile and hydrophile vary with temperature
this effect is more pronounced for non-ionic surfactants because their solubility in water depends on H bonding, which is affected by temperature
Common non-ionic emulsifiers at low temperatures…
are water soluble and stabilise o/w emulsions
common non-ionic emulsifiers
oil soluble at high temp, stabilise w/o emulsions
What is the PIT of an emulsifier?
the temperature at which a surfactant/ emulsifier changes from being O/W emulsifier to W/O emulsifier. At the PIT, the hydrophilic and lipophilic natures are balanced out
