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Thermodynamics of solubility. BY N.V.N.JYOTHI & DR. SN SAKARKAR M.PHARMACY PHARMACEUTICS. CONTENTS. INTRODUCTION. SOLUBILITY: Spontaneous interaction of two or more substances to form a homogenous molecular dispersion
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Thermodynamics of solubility BY N.V.N.JYOTHI & DR. SN SAKARKAR M.PHARMACY PHARMACEUTICS
INTRODUCTION • SOLUBILITY: Spontaneous interaction of two or more substances to form a homogenous molecular dispersion • SOLUBILITY PRODUCT: Product of the concentration of ions in a saturated solution and defines the degree of solubility of the substance. • SOLUBILIZATION: Enhancement of solubility of the substance by another agent i.e. solubilizing agent or solubilizer.
Ideal solution Ideal solution is one which obeys Raoult’s law. Roults law: The partial pressure of each constituent is equal to the vapor pressure of pure constituent multiplied by its mole fraction of the solution. pA = pA0 XA pB = pB0 XB where pA and pB are partial pressures of the constituents , XA and XB are the mole fraction concentrations
The total pressure is the sum of the partial pressures of all constituents. P = pA + pB
Real solutions The cohesive attraction of A for A exceeds the adhesive attraction between A & B. The adhesive attraction between A & B is greater than those between A & A or B & B. These type of system do not obey Raoult’s law.
Deviations from raoult’s law • Negative Deviations: Adhesive forces exceeds cohesive forces. • The vapor pressure of the solution less than the expected.
Raoult’s law deviations contd… When the cohesive forces exceeds attraction forces positive deviation occur. This will decrease the solubility of the compound. The vapor pressure of the system increases.
Thermodynamics of solution process A solid ↔ Aliquid ↔ Asolution • For a physical or chemical reaction to occur spontaneously at a constant temperature and pressure, the net free-energy change ∆G, for the reaction should be negative. Free-energy change depends on heat related enthalpy ∆H1, and order-related entropy ∆S. • ∆G = ∆H1 - T∆S
Thermodynamics of solution process • Involves three steps • The separation of molecules of solvent to create a space in the solvent. • The separation of solute molecules • Mixing of separated solvent and solute
Thermodynamics of solution process Hess law is used. The energy of solution formation, the enthalpy of solution, equals the sum of three steps: ∆Hsol. = ∆H1 + ∆H2 + ∆H3 ∆H1 and ∆H2 are positive it requires energy to pull the molecules away from each other.
Thermodynamics of solution process contd… The amount of energy required is based on the nature of the material. Solution will formed the energy of interaction b/n solvent and solute is greater than the sum of solvent – solvent and solute - solute interactions.
Thermodynamics of solution process contd… Solubility of strong Electrolytes: • The heat of solution of a crystalline substance is the sum of the heat of sublimation of the solid, as given by the crystal lattice energy, and the heat of hydration of the ions in solution. • ΔHsoln = ΔHsubl + ΔHhyd • The lattice energy is the energy required to separate 1 mole of a crystal into its ions in the gaseous state or to vaporize the solid. • NaClsolid Na+gas+ Cl-gas
temperature Temperature Solubility For Gases, Temperature Solubility Follows Le Chatelier’s principle.
Molecular structure of the solute • Molecules with polar functional groups Dissolve in water very fastly. • Molecule with nonpolar functional groups Dissolve in Nonpolar solvents. • Any modification in the functional groups will change the solubility of the molecule. • Eg. The introduction of hydrophilic hydroxyl group can produce a large improvement in water solubility of phenol and benzene.
Nature of solvents • “Like Dissolves Like” • Mixture of solvents to improve the solubility of drugs in water. • E.g.. Co-solvents like glycerin, ethanol, propylene glycol etc.
Crystal characteristics The change in the crystal lattice enthalpy is determined by the strength of interactions between adjacent molecules in a crystal lattice. The different crystalline forms of the same substance, which are known as polymorphs, consequently possess different lattice energies and this difference is reflected by changes in other properties. eg. The polymorphic form with lowest free energy will be the most stable which possess the highest melting point. Other less stable (meta stable) forms will tend to transform into most stable one at rates that depend on the energy differences between the metastable and stable forms.
Particle size of solute • As the particle size Solubility decreases • It is given by Logs/s0 = 2γM/2.303Rtρr • Where s is the solubility of small particles of radius ‘r’, so is the normal solubility, γ is the interfacial energy, M is the molecular weight of the solid, ρ is the density of bulk solid, R is the gas constant and T is thermodynamic temperature.
Complex formation Complex formation may the solubility of the solute present. E.g. The formation of the complexes between 3-amino benzoic acid and various di-carboxylic acids has been shown to increase the apparent water solubility of the former compound.
pressure • If pressure increases the solubility of the gas in liquid also increases. • Henry’s law, which states that ‘the amount of gas dissolved in a solution is directly proportional to the gas above the solution. • P = kH X • Where P is pressure of the gas, χ is concentration of gas in the solution ( mole fraction ), kH is Henry’s law constant for a given gas/solution
Solubilizing agents These agents are capable of forming large aggregates or micelles in solution when their concentration exceed certain values. In aqueous solutions the centre of these aggregates resembles a separate organic phase and organic solutes may be taken up by the aggregates thus producing an apparent increase in their solubilities in water. This phenomenon is known as solubilization.
REFERENCES • Martin, Physical Pharmacy and Pharmaceutical Sciences, 5th Edi., 2006. • Van Nostrand’s Encyclopedia of Chemistry, 5th Edi. • Aulton, Pharmaceutics: The Science of Dosage form Design. • Sparknotes,http:/www.sparknotes.com/chemistry/solutions/solubility/section1.html . • The Science and Practice of Pharmacy, Remington,20th Edi. Vol.I, 2000, Pradeep K Guptha, Solutions and Phase Equilibria,
