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).
Expression |
symbol |
definition |
Molarity |
M |
Moles of solute(gram molecular weight)in one liter(1000ml) of solution |
Normality |
N |
Gram equivalent weights of solute in one liter of solution |
Molality |
m |
No of moles of solute in 1000gm of solvent |
Mole fraction |
x |
Ratio of no of moles of solute to the total moles of solute and solvent |
Percent by weight |
%w/w |
No of grams of solute in 100grams of solution |
Percent by volume |
%v/v |
No of ml of solute in one 100ml of solution |
Percent weight by volume |
%w/v |
No of grams of solute in 100ml of solution |
Very soluble |
Less than 1 |
Freely soluble |
From 1to 10 |
soluble |
From 10to 30 |
Sparingly soluble |
From 30-100 |
Slightly soluble |
From 100-1000 |
Very Slightly soluble |
From 100-10000 |
insoluble |
More than 10000 |