Flashcards in Green: Solubility Deck (24):
Give reasons why aqueous solubility is important?
1. Drugs are formulated as solution forms
2. Absorption and bioavailability need drugs in solution to cross membrane barrier
3. Low solubility drugs = poor bioavailability
What is solvation (hydration)?
1. Process of binding solvent to solute
2. Attractive forces that lead to coherence and different bonds such as dipole to dipole, hydrogen bonding, electrostatic, hydrophobic
What is drug dissolution and what is it effected by?
1. Drug dissolution: Spontaneous dissolving that's effected by:
1. Solute: Molecular structure of solid form
2. Solvent: pH, temperatures, co-solvents
3. Additives: Interaction with solute and solvent
What are the factors that influence solubility?
1. Surface area
2. Boiling and melting point: solubility is inversely related to melting point, removal of solute represents strength of interaction/bond
3. Functional groups of solutes: different functional groups can be more or less polar
4. Crystal properties: different electrolytes
5. Additives (water structure make or breakers, salting in or out)
What are the physical properties of drugs that impact on the rate of dissolution?
2. Particle size
3. Agitation: state of excitation
What's the difference between crystalline and amorphous state?
- Stable state
- Slower dissolution and lower solubility
- Less stability
- Faster dissolution and higher solubility
Describe what must be done when working out solubility and how it's done?
1. Preparing a saturated solution
2. Working out in terms of g per 1mL the amount of solvent that can dissolve 1g of solute
What happens when you increase number of polar groups and halogen atoms?
1. Increase polar groups: Increases solubility in organic compounds (Adding OH decreases amount of solvent needed to dissolve solute)
2. Halogen atoms: Decreases solubility
Describe solubility based on pH of drugs?
1. Acidic drugs are less soluble in acidic solutions since unionised form cannot interact with water (absorption in stomach)
2. Basic drugs are more soluble in acidic solutions where ionic form is predominant (absorption in intestines)
Describe unionised and ionised form in terms of solubility?
1. Solubility of ionised form is greater
2. Unionised form can cross the membrane much greater
3. Weak acids in equilibrium are both unionised and ionised in solution
4. Buffers stabilise pH and determine degree of dissociation- henderson hasselbalch equation
What is the equation for an dissociation at acidic pH?
HA + H2O A- + H3O+
What is the equation for an dissociation at alkaline pH?
B + H2O BH+ + OH-
What are the methods to improve solubility?
2. Salts forms and changing
3. Hydrotropic salts
4. Solubilisation: use of salt in surfactant micelles to improve solubility
5. Complexation or chemical modification
Describe what drug salts are?
1. Medicinal agents that are weakly soluble which can react with strong acids or bases to form highly soluble salts
- Sodium Salicylate dissolves in water
- Dissociates to form conjugate base
Give examples of weakly acidic and alkaline drug salts?
1. Weak bases: antihistamines and local anaesthetics- formulated as their acidic salts, increasing pH may lead to precipitation
2. Weak acids: barbituate drugs, sulphonamide
- formulated as basic salts
- reducing pH may lead to precipitation
Describe what co-solvency is, give examples?
1. Weak electrolytes and non polar compounds
- solvency improved by altering polarity of solution
2. Water miscible co-solvents: glycerol, propylene glycol and ethanol
3. Excipients influencing solute to solvent interactions
4. Excipients that influence solute to solvent interactions
What is a co-solvent?
Lowers the dielectric constant and increases the solubility of unionised form
What does the degree of dissociation decreasing lead to?
1. Increase in concentration of the unionised form (may precipitate)
2. pH balances to maintain solubility
3. Buffers can be used as the solubility of sparingly double electrolytes can be decreased by common ions
Describe cyclodextrins as solubilising agents?
1. Hydrophobic inside and hydrophillic outside
2. Forms complexes with hydrophobic molecules which improves their solubility and bioavailability
Describe how solubilisation is affected using surfactants?
1. Maximum additive concentration leads to amount of drug incorporated
2. Micellar solutions can affect drug activity and protect it against hydrolysis
3. Surface active drugs- Micellar solution of Penicillin G is 2.5 more stable than monomeric solutions
Describe the solubilisation of non ionic surfactants such as ethylene oxide?
1. Paraffin like central core (hydrophobic)
2. Solubilisation increases with:
- Temperature and micellar size
3. Solubilising capacity is influenced by solubilisation site
Describe the dissolution rates and the solubility of solid in dissolution (Cs) and how they may be affected by which factors in oral delivery?
1. Temperature- dissolution is exothermic or endothermic process
2. Nature of dissolution media- factors such as solubility, pH and presence of co-solvents will affect rate
3. Molecular Structure of solute- use of salts of either weakly acidic or basic drugs- esterification of neutral compounds, can influence solubility and dissolution rate
4. Crystalline form of solid- presence of polymorphs, hydrates, solvates or the amorphous form of the drug and can have an influence on dissolution rates
5. Presence of other compounds- common ion effect, complex formation and presence of solubilising agent
What does the pharmaceutical industry mainly focus on when to comes to dissolution and solubility?
1. Nature of dissolution medium and presence of other compounds
2. Use of salt forms
3. Crystalline form of solid