Tutorial 2: Solubility

Question 0

How does the solute interact with the solvent? (simple rule)

Answer 0

A solute dissolves best in a solvent with similar chemical properties

I.e. Like dissolves like

Question 1

What is solubility?

Answer 1

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

Question 2

What are the solvation mechanisms for polar solvents?

Answer 2

Dielectric constant (this is essentially a measure of polarity)
Acid base reactions and breaking covalent bonds
Hydrogen bond formations
Dipole interactions

Question 3

How is polarity assessed?

Answer 3

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

Question 4

What factors affect aqueous solubility?

Answer 4

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

Question 5

What factors are considered when choosing a cosolvent?

Answer 5

Log P value
Aqueous solubility
High or low dielectric constant?

Question 6

What is the relationship between solubility and ph of an acidic drug?

Answer 6

Solubility increases with pH

Question 7

What is the relationship of solubility and pH of a basic drug.

Answer 7

Solubility decreases with pH

Question 8

What is an elixir?

Answer 8

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

Question 9

What is invert syrup?

Answer 9

Mixture of glucose and fructose prepared by hydrolysing 66.7%w/v in a solution with sucrose with suitable mineral acids like HCl

The solution is then neutralised using sodium bicarbonate or calcium

The degree ofinv ersion is usually 95% at least