2.) Pharmaceutical Formulations 1

2.) Pharmaceutical Formulations 1

1. DISSOLUTION

   • What is dissolution?
     • 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

2. 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
   • What are the advantages and disadvantages?
     • Advantages
       • Easy to swallow
       • Rapid Absorption
       • Onset of action faster
       • Can be flavoured
     • Disadvantages
       • Hard to transport
       • Less stable than solid dosage forms
       • Can be easily contaminated
       • Dependant on patients ability
       • Can require taste masking
   • Choice of Solvent
     • Aqueous
     • Non- Aqueous
   • Types of Solutions
     • Local Use
     • Systematic Use
     • Intermediate Products

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