Solutions Flashcards
What are advantages of liquids?
*ease of administration
*increased drug absorption
*high bioavailability if administered IV
*taste masking
What are disadvantages of liquids?
*hard to formulate drugs with poor chemical stability-specialist techniques needed
*need special formulation strategies I,e taste masking, appearance, viscosity
What factors affect drug solubility?
*hydration
*structural features - substituents
*molecular size
*pH of solvent
*crystal/ liquid properties
*solvent properties
How do substituents influence solubility of molecules in water?
*its effect on solid/liquid properties- molecular cohesion
*effect of its interaction with water molecules
Substituents classified as either hydrophobic or hydrophilic
What are examples of hydrophilic substituents?
OH
NH2
NO2
COOH
Why would a para compound possess low solubility properties?
Para compounds have greater stability in their crystalline states
What is hydration?
Binding of water to solute
What conditions are acids more soluble in?
Basic
Dissociated species can interact
What is the equation for the dissociation constant of an acid? (Ka)
Ka= [H3O+][A-]/ [HA]
What is the equation of an acid dissociating in water?
HA + H2O >< H3O+ + A-
What is the equation of a base dissociating in water?
RNH2 + H20>< RNH3+ + OH-
What is the equation for the dissociation constant of a base (Kb)?
Kb= [RNH3+][OH-]/ [RNH2]
What is an amphoteric drug?
Drug posses acidic and basic properties.
What are the two dissociation constant equations for amphoteric drugs?
Ka1= [HA+-][H+]/[HAH+]
Ka2= [A-][H+]/[HA+-]
What is the equation for % ionisation?
[H+]/ [HA] x100
What is a co- solvent?
Substance added to primary solvent (water) to increase solubility of poorly water soluble drug
Eg glycerol , ethanol
What two methods used to predict solubility of a solution?
*solubility parameter- thermodynamic measure of cohesive forces in a substance
Max solubility- parameters of solute and solvent identical
*dielectric constant- max solubility at a defined dielectric constant
What is a surfactant?
Substance that reduces surface tension between two liquids or solid and liquid
Increase solubility
Combinations- tweens, spans, phospholipids
Used for vitamins and steroids
What is a hydrotrope?
Substances that solubilise compounds by means other than surfactant/micellar mechanism
(Salts with large anions or cations)
Eg sodium benzoate
What is a disadvantage of using a hydrotrope?
Large amounts required (20-50%)
What are required properties that a preservative should have?
*broad spectrum efficacy
*physically, chemically and biologically stable over shelf life
*non toxic, physiologically and physio chemically inert
What are examples of preservatives?
*parabens
*benzoic/sorbic acid
What are examples of sweetening agents used in solutions?
*Sucrose- can be combined with other polyols (polyols reduce cap locking (sugar crystals on cap)).
*liquid glucose
*saccharin- sugar alternative
*aspartame- sugar alternative (200x sweeter than sucrose)
What are examples of flavours used in solutions?
*menthol/chloroform- mild anaesthetic effect on sensory receptor organs
*monosodium glutamate- reduces bitterness
How is viscosity control achieved?
Modification of sugar concentration or hydrophilic polymers
Eg hydroxypropylcellulose
How do you calculate % ionisation of an acid?
Given PKA and pH
Start with Equation: pH=PKA + log[A-/HA]
NOTE: [A-] and [H+] are same thing
rearrange:
pH-pKa (X) =log[A-/HA]
Do shift log^X= [A-] aka H+
Add 1=[HA]
So end equation is [H+]/[HA] x100 = % ionisation
NOTE if given pH and PKA the same, % ionisation is 50%
