Dissolution Flashcards
Why are fewer new medicines increasingly available?
Increasing in phase 1 and 2 candidates is not always passed through to phase 3
What does dissolution refer to?
The process by which a drug molecule moves from solid phase to the liquid phase
If a drug has low solubility in the body dissolution may be the rate limiting step to absorption and thereafter, therapeutic effect. TRUE OR FALSE?
TRUE
What are solutions?
They are a homogenous mixture of two or more pure substances
In a solution, the solute is not dispersed uniformly thoughout the solvent. TRUE OR FALSE?
FALSE
How does a solid dissolve into a liquid?
Electrostatic in nature
What drives the dissolution process?
Water molecules are electrically neutral
What are the enegetics of this process?
Thermodynamics (heating)
If the intermolecular forces between solute and solvent are stronger it is more likely that the solute will dissolve in the solvent. TRUE OR FALSE?
TRUE
What type of intermolecular forces may occur between solute and solvent?
Hydrogen bonds
Dipole-dipole
Van der waals (for ionic species)
What are vitamins A,D, K and E soluble in?
Non polar solvents e.g oils
Vitamin B and C are water soluble. TRUE OR FALSE?
TRUE
How does a solution form?
- Solvent molecules are attracted to surface ions
- Each ion is surrounded by solvent molecules
- Energy results in solvation/ hydration (e.g an ionic solid dissolving in water)
What is a saturated solution?
- Solvent holds as much solute as possible at a particular temperature, the undissolved solid remians in the vessel as a sediment
- Dissolved solute is in dynamic equilbrium with the solid solute particles
What is an unsaturated solution (sub-saturated)?
- contains less than the maximum possible amount of solute at a specific temp
- There is no excess solid present in the vessel
Solvents do not hold more solvent than normally possible at a specific temp. TRUE OR FALSE?
FALSE
Supersaturated solutions are unstable and often result in crystallisation, the opposite of solvation. TRUE OR FALSE?
TRUE
Descibe the mechanism of dissolution?
- It is an interfacial reaction (which results in liberation of a solute molecule from solid phase/the solution in contact with the solid will become saturated)
- Solute molecules migrates through the boundary layers surrounding the solid to the bulk solution
- The boundary layer is predominately static and surrounds the wetted surface of the solid
- Mass transfer occurs slowly through the boundary layer to the bulk solution
- Reaction must be energetically favourable
What is the dissolution boundary layer?
-A thin layer of solution in contact with the solid (i.e dosage form)
What is dissolution depend on?
- The slowest or rate limiting step of the process governed by ficks first law
What does the Noyes and whitney equation describe?
The rate of dissolution of a solid in a liquid
If C is significantly smaller than Cs, the system is said to be at sink condition allowing the equation to be simplified. TRUE OR FALSE?
TRUE
What does the diffusion coefficient consider?
- How temp affects the diffusion coeffcient
- How Viscosity affects diffusion coefficient
What is dissolution rate affected by?
Temp and viscosity
What is sink conditions?
-usually when the bulk concentration is significantly lower than the concentration in the boundary layer
What are the factors affecting dissolution?
- Surface area - larger SA higher dissolution rate/particle size and porosity
- Solubility - particle size (solubility increases when particle size is reduced)/ temp solubility increases when temp is increased
- Solvent choice - co-solvents may alter solubility of drugs
- Structure of the drug molecule - weak acid/base/hydrophilic
- Polymorphism - different solid states
- Thickness of boundary layer
What is the common ion effect?
-the solubility of one salt is reduced by the presence of another having a common ion
What biological factors affect dissolution?
- Viscosity of fluid in the GI tract may be increased by food (this may reduce dissolution rate)
- Dissolution increases due to the presence and activity of surfactants and bile salts in the gastric fluid
- Gastric motility can reduce the size of the diffusion layer
- Gastric pH is acidic - dissolution of weak acids will increase dissolution in the stomach
Polar liquids will dissolve in polar solvents (like dissolves like) due to hydrogen and dipole-dipole interactions. TRUE OR FALSE?
TRUE
Increasing the pressure in a system will significanlty increase the solubility of dissolved gases. TRUE OR FALSE?
TRUE
What is henry’s law?
The amount of a gas dissolved in a solution is directly proportional to the pressure of the gas above the solution
