Physical properties of drugs Flashcards
what is a solution?
single phase
what is the definition of a solution?
“a mixture of two or more components that form a homogeneous molecular dispersion”
what is a saturated solution?
‘Where the solute is at the limit of its solubility at a defined temp. and pressure’
how can a drug solubility be changed?
structure altered
what do substituents change to effect solubility of drug?
intermolecular forces
interactions between substituents and water
position of substituents on drug
what are the two kinds of substituents?
hydrophilic and hydrophobic
why is solubility important?
‘preparation of drugs in solution
drug dissolution to understand how drug behaves in the body
related to diffusion, partitioning, absorption’
what are the types of solution?
non-electrolytes (non-ionic)
electrolytes (ionic)
what is one way to predict solubility?
polarity
what are used to improve solubility of drugs in water?
cosolvents
why is cosolvent used?
less polar than water
what is water good for?
good solvent for ionic and polar substances
what is minerals and benzene good for?
good solvent for water insoluble substances
how are poorly soluble weak electrolytes improved in solubility?
cosolvent added which increases the relative solubility of the unionized species
also improves the stability of the drug in solution
how is solubility predicted?
‘like dissolved like’
e.g. polar dissolves polar
what are weak acids more soluble in?
alkaline solutions
what is the equation for solubility for weakly acidic drugs?
slide 1
SOM lecture 11 Physical properties of drugs part 3
what do S and So stand for?
S=total solubility at a given pH
So=solubility of unionised drug
what are basic drugs more soluble in?
acidic solutions
what is equation is used to calculate solubility of weak bases?
slide 2
SOM lecture 11 Physical properties of drugs part 3
what are Amphoteric (zwitterionic) drugs?
contain both acid and base groups and are ionised at all pH values
what equation is used for pH below isoelectric point?
base equation
what equation is used for pH above isoelectric point?
acid
draw the graph of Solubility of a weak acid, weak base, and amphoteric compound as a function of pH?
slide 4
SOM lecture 11 Physical properties of drugs part 3
what is solubility?
property of molecule that representing dissolution capacity
what is dissolution rate of a drug described by?
Whitney-Noyes equation
what is diffusion?
tendency of a substance to distribute uniformly over a space
what does diffusion include?
mass transfer brought about by random walk and associated with concentration gradient
what is the driving force of diffusion?
concentration gradient
what happens with dissolved solute?
‘molecules passively diffuse from areas of high concentration to areas of low concentrate
major role in passive transport of drugs across membranes’
what is Fick’s first law of diffusion?
slide 4
SOM lecture 11 Physical properties of drugs part 4
what is D?
constant depending on temperature, solute
conditions to use Flick first law equation?
only for steady-state flux
what does a negative flux mean?
decreasing concentration
what does a higher gradient mean?
the higher the gradient in Fick’s law the faster the rate of diffusion
what is partition?
‘the distribution of drug between two immiscible phases’
how is the partition coefficient defined as?
P= Co/Cw
Co= concentration in organic phase
Cw=concentration in aqueous phase
what is P a measure of?
‘relative affinity of the solute for an aqueous and a lipid phase’
when does the Partition coefficient apply?
when in dilute solutions
when is P constant?
‘only if a single molecular species is involved’
when is equilibrium obtained?
when solute soluble in both phases
