Flashcards in Tutorial 2: Solubility Deck (10):
What is solubility?
The extent to which dissolution proceeds under a given set of environmental conditions
The maximum amount of solute that will dissolve in a given quantity of solvent at a given temperature
How does the solute interact with the solvent? (simple rule)
A solute dissolves best in a solvent with similar chemical properties
I.e. Like dissolves like
What are the solvation mechanisms for polar solvents?
Dielectric constant (this is essentially a measure of polarity)
Acid base reactions and breaking covalent bonds
Hydrogen bond formations
How is polarity assessed?
For solutes and solvents (solids, liquids) they can be assessed using a logp value.
Low log p = polar
For solvents (liquids) they can be assessed using the dielectric constant
High DC = polar
For surfactants polarity is assessed using an HLB value
High HLB = polar
What factors affect aqueous solubility?
Temperature (increased temp increases solubility in endothermic dissolution. This is the reverse for an exothermic dissolution)
Hydrophobicity (like dissolves like)
Acid base properties / pH
Crystalline form of the drug
Ionisation (ionised form have a higher solubility)
Surface area (smaller particle size = more SA:volume ratio = increased solubility)
cosolvents/ mixed solvent systems
Number of Carbon atoms (increased C decreases solubility)
Branching of structure (reduced molecular volume increases solubility)
Functional groups like CH, OH, NH
What factors are considered when choosing a cosolvent?
Log P value
High or low dielectric constant?
What is the relationship between solubility and ph of an acidic drug?
Solubility increases with pH
What is the relationship of solubility and pH of a basic drug.
Solubility decreases with pH
What is an elixir?
Sweetened aromatic solution of alcohol serving as a vehicle for medicines
Solvent frequently contains a high proportion of ethanol and syrup
Other solvents like glycerol are also sometimes used