PHYSICAL PHARMACEUTICS I

SOLUBILITY AND DISTRIBUTION PHENOMENA

CLASS I

Importance of solubility

• Select best solvent for the drugs
• Solubility used to categorize drug substances
• Can overcome the problems arising during preparation of pharmaceutical solutions
• Solubility is important to understand to measure, predict, alter and model the solubility of a particular substance and mixture
• SOLUTION: system in which solute molecules dissolved in solvent

     Saturated  Solution:is one in which the solute in solution is in equilibrium with the solid phase.

• Unsaturated or subsaturated solution is one containing the dissolved solute in a concentration below that necessary for complete saturation at a definite temperature.
• Supersaturated solution is one that contains more of the dissolved solute than it would normally contain at a definite temperature, were the undissolved solute present.

• Solubility is defined in
• quantitative terms as
• the concentration of solute in a saturated solution at a certain temperature,
• qualitative way, it can be defined as the spontaneous interaction of two or more substances to form a homogeneous molecular dispersion.
• Solubility is an intrinsic material property that can be altered only by chemical modification of the molecule.
• In contrast to this, dissolution is an extrinsic material property that can be influenced by various chemical, physical, or crystallographic means such as complexation, particle size, surface properties, solid-state modification, or solubilization enhancing formulation

SOLUBILITY EXPRESSIONS

• SOLUBILITY-expressed in a number of ways
• United States Pharmacopeia (USP) describes the solubility of drugs as parts of solvent required for one part solute.
• Solubility is also quantitatively expressed in terms of molality, molarity, and percentage.
• The European Pharmacopoeia lists six categories (it does not use the practically insoluble grouping).