1. SOLUBILITY OF DRUGS
SOLUBILITY EXPRESSION, MECHANISM OF SOLUTE-SOLVENT
INTERACTION
Presented by – Mr Shubhrat Maheshwari
Assistant Professor, Faculty of
Pharmaceutical Sciences, Rama University
Kanpur

2. Solubility of Drugs
• Solubility is defined as the concentration of a
substance (solute) that dissolves in a given volume of
solution (solvent) at a certain temperature to form a
spontaneous solution of solute in the solvent.
• Solubility can also be defined as the spontaneous
interaction of two or more substances to form a
homogenous molecular dispersion.(Qualitative)
• Solubility of a substance in a particular solvent is
defined as the concentration of the substance in
saturated solution at certain temperature.
(Quantitative)

3. Solubility of Drugs
• Solubility is an intrinsic material properties -> can be
altered By chemical modification of the molecules.
• Dissolution →→ is an extrinsic material property,
influenced by various chemical, physical, and
crystallographic means like complexations, particle
size. surface properties.
• Solubility of a compound depends on:
 Physical, Chemical properties of the solvent and
salute.
 Temperature, pressure and pH of solution.

4. SOLUBILITY OF DRUGS
Terms used in Solubility
• SOLUTION
It is mixture of components which are physically and chemically
homogeneous.
• SOLUTE
The components which are present in the solution is called as solute
that dissolves in solvent.
• SOLVENT
The medium in which components are dissolved is known as solvent.
• SOLUBILITY
The ability of substance to dissolve in a solvent is called solubility.

5. SOLUBILITY OF DRUGS
Terms used in Solubility
• SATURATED SOLUTION
The solution containing maximum number of solute at a constant
temperature is called saturated solution.
• SUPER SATURATED SOLUTION
A solution that contains more of the dissolved material then could
be dissolved by the solvent under normal circumstances.
• UNSATURATED SOLUTION
A solution where the solute concentration is lower than its
equilibrium solubility.

6. Solubility of Drugs
SOLUBILITY EXPRESSION- Descriptive forms
• Table-1: No. of parts of solvent requires to dissolve 1 part of the solute.

7. Solubility of Drugs
SOLUBILITY EXPRESSION- Quantitative expression
• PERCENTAGE TERM USED IN SOLUBILITY
1. % W/W = Percent Weight by Weight → No. of gram of solute
dissolved in 100 gram of solution.
2. % V/V = Percent volume by volume → No. of gram of solute
dissolved in 100 gram of solution.
3. % W/V = Percent Weight by volume → No. of gram of solute
dissolved in 100 ml of solution.

8. Solubility of Drugs
MOLARITY
• It is defined as the number of moles (or gram molecular weight) of solute
dissolved in 1 litre of solution.
MOLALITY
• It is defined as the moles of solute dissolved in 1000 g of solvent.
NORMALITY
• It is defined as the number of gram or mole equivalents of solute
present in one litre of a solution.
MOLE FRACTION
• Mole fraction is the number of moles of a specific component in the
solution divided by the total number of moles in the given solution.

9. Solvent-Solute Interactions
• Binding forces between molecules Cohesive
and adhesive forces are manifestation of
intermolecular force.
• When molecule interact both repulsion force
and attraction forces act.
• To cohere attraction forces are necessary and
to prevent interpenetration repulsion forces are
required.

10. Solvent-Solute Interactions
 Types of attraction forces
• Van der Waals Force
 Dipole-dipole interaction (Keesom interactions)
 Dipole-induced dipole interaction (Debye interactions)
 Induced-dipole -Induced-dipole interaction (London dispersion
forces)
• Ion-Dipole and ion-Induced Dipole Force
• Ion-ion Interaction
• Hydrogen Bonds
 Selection of solvent is based on 'like dissolve like’

11. Solvent Solutes Interactions
• Water is good solvent for –salt and Sugar&
similar compounds
• Mineral oils dissolve - normally only slightly
water soluble substances
• So “like dissolves like”

12. Solvent Solutes Interactions
Polar Solvents-
• Solubility of drug is due to polarity of solvent
that is dipole moment.
• Polar solvents dissolves
ionic solutes
Polar substances
• Water mixes in all proportions with
 Alcohol
 Sugars
 Other polyhydroxy compounds.

13. Solvent Solutes Interactions
• Hildebrand showed that a consideration of dipole
moments, alone is not adequate to explain solubility of,
polar substances in water
• The ability of the solute to form hydrogen bonds is
more significant factor.
• Water dissolves → phenols, alcohols. aldehydes,
ketones, amines, and other. oxygen and nitrogen
containing compounds that can form hydrogen bonds.
with water.
• Difference in acid- base character of constituents (in the
Lewis electron donor-acceptor sense) also contributes
to specific interaction in solutions.

14. Solvent Solutes Interactions
Nonpolar solvents
• Nonpolar solvents unable to reduce attraction
b/w ions of strong and weak electrolytes
because solvent low dielectric constants.
• Non polar solvent dissolve non polar solute
with similar internal pressure.

15. Solvent Solutes Interactions
Nonpolar solvents
• The solute molecules are kept in solution by
weak van der Waals, London types of forces.
• Oil and fats dissolved in carbon tetrachloride
benzene and mineral oils.
• Alkaloids bases and fatty acids dissolves in
non polar solvents.

16. Solvent Solutes Interactions
Semipolar Solvents-
• Semipolar solvents → ketones and alcohols.
induce certain degree of polarity in non polar
solvents.
• Example-benzene (readily polarizable) become
soluble in alcohol.
• Semipolar compounds can act as intermediate
solvents - bring miscibility of polar and
nonpolar liquids.

17. Mechanism of Solute-Solvent
Interaction
• If the solvent is A & the solute is B. and the
forces of attraction are represented by A-A, B-
B and A-B.
• If A-A >> A-B The solvent molecules will be
attracted to each other.
• Example: Benzene & water, where benzene
molecules are unable to penetrate the closely
bound water aggregates.

18. Mechanism of Solute-Solvent
Interaction
• If B-B >> A-A The solvent will not be able to break the
binding forces between solute molecules.
• Example NaCl in benzene, where the NaCl crystal is
held by strong electrovalent forces which cannot be
broken by benzene.
• If A-B >> A-A or B-B, or the three forces are equal.
Formation of solution takes place.
• Example: NaCl in water.
• Solution is formed when Solvent-solvent interaction <
Solvent -solute interaction > Solute -solute interaction.

19. Any query?

20. Thank you.