Solutions Part 1

Question 1

What are the similarities and differences between a solution and a suspension?

Answer 1

• solution is a homogeneous phase while suspension is heterogeneous.
• both solutions and suspensions comprise two or more components.
• continuous phase can be aqueous or non-aqueous.

Question 2

What are the various methods that can be employed to increase apparent solubility of compounds in an aqueous medium?

Answer 2

• cosolvency
• pH control
• solubilisation
• complexation
• chemical modification
• particle size control

Question 3

What type of drug particles is used to increase apparent solubility of compounds in an aqueous medium?

Answer 3

• cosolvency
• pH control
• solubilisation
• complexation
• chemical modification
• particle size control

Question 4

Describe cosolvency.

Answer 4

• cosolvency is used for drugs that are weak electrolytes or that are non-polar, to increase their apparent solubility in water.
• cosolvents selected must be miscible in water and something that the drug can dissolve in as well.
• solubility of drug is affected by the dielectric constant of the solvent system, therefore the blends should be between 25 and 80.

Question 5

Describe pH control.

Answer 5

• used for weakly basic and acidic drugs, whose extent of ionisation will ultimately affect its solubility.
• pH control must be in line with product requirements as well.

Question 6

Describe solubilisation

Answer 6

• surfactants can be added to increase the solubility of a drug in a particular solvent system.
• the concentration of these surfactants must be above their critical micellar concentration (CMC) –>micellar solubilisation.
• large excess of surfactant is undesirable.
• hydrophilic surfactants with HLB values above 15 are useful solubilising agents.

Question 7

What are some factors that affect the choice of solubilising agents?

Answer 7

• toxicity and irritancy
• miscibility with solvent system
• compatibility with other components
• odour and taste

Question 8

It doesn’t matter how much surfactant is added because the excess surfactants are harmless. (T/F)?

Answer 8

false, excess surfactants can have harmful effects.

Question 9

Describe complexation.

Answer 9

• interaction of a poorly soluble drug with a soluble material to form a soluble complex.
• process is easily reversible, as the complex is likely to be a macromolecule that is inactive.

Question 10

Describe chemical modification.

Answer 10

• involves the synthesis of soluble salts of the drug

- soluble salt may not have any activity and is converted back to the active base in the biological system.

Question 11

Describe particle size reduction.

Answer 11

• use of technologies like milling to reduce the particle size of drug molecules and improve their solubility in water.

Question 12

How does one determine the minimum concentration of surfactant for the solubilisation of the drug?

Answer 12

• use a series of vials, each with a constant concentration and proportion of solvent and surfactant.
• add different concentrations of drugs into the vials.
• calculate their optical density, and plot a graph of optical density against drug concentration.
• the last drug concentration where the solution remains clear is the maximum drug concentration.
• using the same procedure, calculate the MDC values of different concentrations of the surfactant (expand scope).
• plot graph of MDC against concentration of surfactant.
• point at which the drug starts to solubilise is the CMC.