The document discusses solvation and association as factors that influence drug solubility. It defines solvation as the interaction of a solute with solvent molecules that leads to stabilization. Solvation is quantified by its dissolution rate. Solubility depends on an equilibrium between dissolution and precipitation rates. Solvent-solute interactions like hydrogen bonding, ion-dipole interactions, and van der Waals forces affect solvation. Solvation energy is determined by a balance of enthalpy, entropy, and temperature factors. Association occurs via chemical reactions where oppositely charged ions or molecules come together to form distinct chemical entities in solution. The extent of association depends on the dielectric constant of the solvent.

1. IIIrd Semesester B. pharmacy
Physical Pharmaceutics-I
Unit-I
Solubility of drugs(Part-3)
(Solvation and Association)
Presented By :
Miss. Pooja D. Bhandare
(Assistant Professor)
Kandhar College of Pharmacy, Nanded

2. SOLVATION
• ACCORDING TO IUPAC DEFINITION, SOLVATION IS AN INTERACTION OF A SOLUTE WITH THE
SOLVENT, WHICH LEADS TO STABILIZATION OF THE SOLUTE SPECIES IN THE SOLUTION.
• SOLVATION OF SOLUTE BY WATER IS CALLED AS HYDRATION.
• IN THE SOLVATED STATE, AN ION OR UNCHARGED MOLECULE
IN A SOLUTION IS SURROUNDED OR COMPLEXED BY SOLVENT MOLECULE.
• THE STRENGTH AND NATURE OF THIS INTERACTION INFLUENCE MANY PROPERTIES OF THE
SOLUTE SOLVENT
Solubility Viscosity
Reactivity Density
Color

3. SOLVATION AND SOLUBILITY
• SOLVATION OR DISSOLUTION IS A KINETIC PROCESS AND IS A
QUANTIFIED BY ITS RATE AND DISSOLUTION RATE IS MOL/S
• SOLUBILITY QUANTIFIES THE DYNAMIC EQUILIBRIUM STATE ACHIEVED
WHEN THE RATE OF DISSOLUTION EQUALS THE RATE OF PRECIPITATION
SOLVENTS AND INTERMOLECULAR INTERACTIONS:
• HYDROGEN BONDING
• ION-DIPOLE INTERACTIONS
• VAN DER WAALS FORCES
 DIPOLE-DIPOLE
 DIPOLE-INDUCED DIPOLE
 INDUCED DIPOLE- INDUCED DIPOLE INTERACTIONS

4. • SOLVATION ENERGY AND THERMODYNAMIC CONSIDERATION:
Δ G= Δ H − T Δ S
WHERE
Δ G = GIBBS ENERGY
Δ H = CHANGE IN ENTHALPY
T = ABSOLUTE TEMPERATURE
Δ S = CHANGE IN ENTROPY /DEGREE OF RANDOMNESS

5. • SOLVATION INVOLVES MULTIPLE STEPS WITH DIFFERENT ENERGY CONSEQUENCES.
FIRST, A CAVITY MUST FROM IN THE SOLVENT TO MAKE SPACE FOR A SOLUTE.
 THIS IS BOTH ENTROPICALLY AND ENTHALPICALLY UNFAVORABLE.
A PARTICLE OF SOLUTE MUST SEPARATE FROM THE BULK. THIS IS ENTHALPICALLY
UNFAVORABLE.
THE RESULTING SOLVENT SOLUTE INTERACTIONS ARE ENTHALPICALLY FAVORABLE

6. ASSOCIATIONS
• ASSOCIATION OF THE ION OR MOLECULE IS THE CHEMICAL REACTION.
• OPPOSITE ELECTRIC CHARGE MOLECULE OR ION COME TOGETHER TO FORM NEW IN
SOLUTION TO FORM DISTINCT CHEMICAL ENTITY.
• THE EXTENT OF IT DEPENDS ON THE DIELECTRIC CONSTANT OF THE SOLVENT.
• IONS- ELECTROSTATIC FORCE- COLUMB’S LAW:
WHERE, Q1 AND Q2 ARE MAGNITUDE OF ELECTRIC CHARGES,
F IS THE FORCE OF ATTRACTION
Ε IS THE DIELECTRIC CONSTANT
R IS THE DISTANCE BETWEEN THE IONS

7. Example: Dimerisation of Benzoic acid

8. THANK YOU!