Physicochemical properties of drug molecules

Refractive index, Optical rotation, Dielectric constant, Dipole moment, Dissociation constant

1. PHYSICOCHEMICAL PROPERTIES OF DRUG MOLECULES: DETERMINATIONS AND APPLICATIONS Presented By: Mahewash Sana A. Pathan

2. CONTENTS: Refractive index Optical rotation Dielectric constant Dipole moment Dissociation constant

3. INTRODUCTION P hysicochemical properties play an important role in product development including studies on biological performance of drugs. A study of the physical properties of drug molecules is a prerequisite for product Preformulation, formulation development and optimizing storage and usage conditions. Physical properties of substances may be classified in to three types; Additive Properties: e.g. molecular weight, volume, etc. Constitutive Properties: These properties are depending on the structural arrangement of atoms and functional groups as well as bond structure that exists within the molecules. The examples of this type are optical activity, surface tension, viscosity etc. Combined Additive-Constitutive Properties : e.g. molar refraction

4. REFRACTIVE INDEX 1621, a Dutch physicist named Willebrord Snell

5. TABLE: RI OF SOME MATERIALS

6. Specific refraction : In 1880, scientist Lorentz showed the property specific refraction. Rs = [( n2 -1) / n2 + 2)] 1/d Molar Refraction : molecular weight times the specific refraction of substance.

7. MEASUREMENT OF REFRACTIVE INDEX: Abbe’s refractometer Pulfrich refractometer Immersion refractometer V block refractometer

8. ABBE’S REFRACTOMETER This was the first refractometer commercially available. Principle: based on measurement of critical angle . Used to determine RI of liquids.

9. Working: Sample is placed in a thin layer ( ̴3.1 mm) between an illuminating prism & refracting prism. Sodium lamp is projected through the illuminating prism, whose lower surface is ground or roughened. The radiation is refracted at the interface of the sample & the smooth surface of upper prism, it passes into fixed telescope. To avoid dispersion, Abbe added two compensating amiciprisms . Two amiciprisms collect the divergent critical angle rays of different colors into into a single white beam. T he prism lens can be rotated by means of arm which can be moved by a button. The RI is read through the eyepiece at the upper end.

10. Procedure: Turn on monochromatic lamp ‘Na’ & wait for 5 min to warm up the lamp. Put 1-2 drops of liquid whose RI is to be checked between illuminating prism & measuring prism. Close the lower prism case. Use the rotating knob to align with mark in eyepiece with shadow boundary seperating dark & bright areas in the field of view. Sharpness can be adjusted by moving the lens forward & backward. Read the RI from the scale. Clean the prism with filter paper.

11. PULFRICH REFRACTOMETER A type of critical angle refractometer. Contain two planar polar faces which are perpendicular to one other, one is vertical & another is horizontal. Sample is placed on horizontal surface. A beam of monochromatic light is incident at the grazing incidence(at right angle). Angle of refraction is measured with the help of telescope. If RI of glass ‘ nG ’ is known & angle ‘ i ’ is measured, then RI of liquid sample is calculated by- n = n G – sin I

12. DIP OR IMMERSION REFRACTOMETER Similar to Abbe’s refractometer but lacks illuminating prism. Lower surface is dipped into the sample solution. Always operate at the temperature of the liquid.

13. V-BLOCK REFRACTOMETER The design has been published in 1941.

14. Applications of RI To measure the concentration of solute in an aqueous solution. To identify a particular substance, conform its purity. To calculate the focal power of the lenses & dispersive power of prisms. Estimation of thermo-physical properties of hydrocarbons & petroleum mixtures. In examination of organic compounds, solutions, food products, serum protein concentration. Determine amount of sugar in sugar solution.

15. OPTICAL ROTATION Optical activity is a property of substances to rotate the plane of incident polarized light. Optical rotation is the angle at which the plane of polarization is rotated when the polarized light passes through a layer of one liquid . Optically active substances are of; Dextro rotatory (right handed) Leavo rotatory (left handed) In the International Pharmacopoeia, it is expressed in angular degrees. In SI, represented as Radians ( rad )

16. Specific rotation: It is rotation measured at the temperature t & at the wavelength λ given by a 1 m thickness of liquid or solution containing 1 kg/m3 of optically active substance.

17. Measurement of optical rotation Polarimeter measure the degree of rotation of polarized light.

18. Applications of optical rotation Identification & determination of optically active compound. In sugar industry to determine quality of juice from sugarcane & refined sucrose. In distinguishing chemicals. Concentration & purity of steroids, diuretics, antibiotics, narcotics, vitamins, analgesics, etc. Polarimetry is used in remote sensing applications.

19. DIELECTRIC CONSTANT Dielectric constant ( ɛ r ) is the ratio of permittivity of the medium( ɛ) and permittivity of free space ( ɛ ) . Ɛ r = ɛ / ɛ Permittivity- Ability of a substance to store electrical energy in an electric field. It is measure of polarity of a solvent.

20. Measurement of dielectric constant It is measured by measuring effect of intervening solvent on the electric field between two oppositely charged particles. Capacitance of plates holding sample solution is measured, capacitance of plates holding vacuum is measured. Then, dielectric constant is obtained by ratio. ɛ = C / C0 And C0 = ɛ0 A / t Where, C0 = capacitance using vacuum Ɛ0 = permittivity of free space A = area of plate t = thickness of sample

21. Applications of dielectric constant: In manufacture of capacitors. Used in insulation of the wires, cables, etc. Used in detection devices.

22. DIPOLE MOMENT SI unit: Coulombs meter (C-m) Debye

23. Measurement of dipole moment: When a solution of polar molecules is placed between two plates having opposite charge, they align themselves along the direction of field. This process consumes energy that is returned to the electrical circuit when the field is switched off, an effect known as electrical capacitance. Measurement of capacitance of gas or solution serves to determine magnitude of dipole moment of substance.

24. Applications of dipole moment: For predicting nature of molecules (polar or non polar) Degree of polarity Shape of molecule Predict nature of bond

25. DISSOCIATION CONSTANT Def: tendency of particular substance in solution is dissociated into ions.

26. Applications of dissociation constant: It is the key physicochemical parameter influencing many biopharmaceutical characters. Knowledge of pKa values is important for the quantitative evaluation of systems involving acid-base equilibria in solution. It is essential for working with buffers. In pH indicators.