Dissolution is the movement of molecules or
ions from a solid state to a solution state
Solubility on the other hand is the amount
of solute that dissolves in a solvent
Why is dissolution important?
Many drugs are in the
form of solutions
Drugs must be in solution to be
absorbed across biological
membranes
Drugs of low solubility provide
bioavailability challenges
Process
The first thing is the
removal of the solute
molecule from the
solid
Second the solvent
molecules create a cavity
The individual solute
molecules fit inside these
cavities
Energy Change
Necessary to overcome
cohesive forces (Wa +Wb)
Replaced by adhesive forces
An Ideal solution has an
energy change of zero.
Noyes Whitney Equation
How is it used?
The Noyes Whitney equation is used
to calculate the rate of dissolution i.e
the rate at which a molecules in a
solid state move into a solution phase
The Noyes Whitney equations simply states that the
diffusion co-efficient, the surface area of the solute
particles, the concentration of the solute particles at
the boundary layer and the height of the boundary
layer all effect the rate of dissolution
The diffusion co-effeicient, the SA od the
solute particles and the the concentration of
solute particles at the boundary layer are all
directly proportional to the rate of dissolution
Whereas the height of the boundary
layer is inversely proportional to the
rate of dissolution
Dissolution Mechanism
Overall the rate of dissolution depends on the
rate determining step, this is the diffusion of
the solute molecules across the static
boundary layer
Sink Conditions
If c is less than 10% of Cs
then sink conditions apply
Assumptions
The Noyes Whitney equation assumes that there is no
change in the surface area of the dissolving particles.
Therefore if a change of surface area occurs it cannot
be used
SOLUTIONS
What is a solution?
A mixture of two or more
components to form a single.
molecularly homogeneous phase
In pharmacy we are interested in
how a solid or liquid mixed within
another liquid