Physical Pharmacy (II) - Solutions, Solubility, and Distribution Phenomena PDF

# **Physical Pharmacy (II)** ## **Solutions, Solubility and Distribution Phenomena** By **Prof. Mokhtar M. El-Baseir** ## **Solutions** ### **Definitions** **Solutions**: A homogeneous mixture of two or more pure substances with a uniform composition throughout. ### **Types** * **True soluti...

# **Physical Pharmacy (II)** ## **Solutions, Solubility and Distribution Phenomena** By **Prof. Mokhtar M. El-Baseir** ## **Solutions** ### **Definitions** **Solutions**: A homogeneous mixture of two or more pure substances with a uniform composition throughout. ### **Types** * **True solution**: A homogeneous molecular dispersion (one phase system) of two or more components. The composition can vary over a wide range. * **Colloidal solution**: A system having a particle size intermediate between that of a true solution and a coarse dispersion, roughly 10 A to 5000 A. * **Coarse dispersion**: The diameter of the particles in emulsion and suspension for the most part being larger than 0.1 µm (1000 A or 10-5 cm). ### **Examples** **Droplets in emulsion.** **Particles in Pharmaceutical suspension.** ### **Binary Solutions** A solution composed of only two substances (Solvent and solute). * The solvent is usually present in a greater amount, the Solute in a lesser amount. **NB.** 1. When a solid is dissolved in a liquid, the liquid is usually taken as the solvent and the solid as the solute irrespective of their relative amount. 2. When water is present in a liquid mixture, water is usually the considered solvent. 3. When dealing with mixtures of liquids that are miscible in all proportions such as alcohol and water, it is difficult to clarify the constituents as solute and solvent. ### **Types** * **Two-phase system (Heterogeneous)** * **Example**: Silver proteinate in water has distinct particles constituting a separate phase. * **Two-phase system (Homogenous)** * **Examples:** * Acacia in water. * Sodium carboxymethylcellulose in water. * These solutions do not differ from a solution of sucrose and may be considered as a single-phase system or true solution ### **Properties of Solutions** * **Colligative Properties**: Depend mainly on the number of particles in the solution. * Examples: Osmotic pressure, vapor pressure lowering, freezing point depression and Boiling point elevation. * **Additive Properties**: Depend on the total contribution of atoms in molecular or on the sum of the properties of the constituents in the solution. * Examples: Molecular weight (the sum of the constituent atoms) and mass of the components of the solution (the sum of the mass of individual components). * **Constitutive Properties**: Depend on the arrangement and to a lesser extent on the number and kinds of atoms within the molecules. * Examples: Refraction of light, electric properties, surface and interfacial characteristics and solubility. ## **Solution Mixture** A solution may be classified according to the states in which the solute and solvent occur: gas, liquid and crystalline solid. | Solute | Solvent | Examples | |:---------:|:--------:|:----------:| | Gas | Gas | Air | | Liquid | Gas | Water in oxygen | | Solid | Gas | Iodine vapor in air | | Gas | Liquid | Carbonated water | | Liquid | Liquid | Alcohol in water | | Solid | Liquid | Aqueous sodium chloride solution | | Gas | Solid | Hydrogen in palladium | | Liquid | Solid | Mineral oil in paraffin | | Solid | Solid | Gold-Silver mixture, mixture of alums | ## **Ideal and Real Solutions** ### **Ideal Solutions** Ideal solutions are formed by mixing substances with similar properties. * **Examples:** 100 mL of methanol mixed with 100 mL of ethanol to yield a 200 mL mixture with no heat evolved or absorbed (ideal solution). **Properties**: * **No change in the properties of the components other than dilution when they are mixed.** * **No heat evolved or absorbed during the mixing process.** * **The final volume of the solution represents an additive property of the individual constituents.** * **There is no shrinkage or expansion when the substances are mixed.** * **The constitutive properties e.g. the vapor pressure, refractive index, surface tension and viscosity of the solution are weighted averages of the properties of the pure individual constituents.** ### **Real Solutions (Non-Ideal solutions)** * **Ideality in solutions assumes complete uniformity of attractive forces** * Many examples of solution pairs (A and B) are known: * The "cohesive" attraction at A exceeds the "adhesive" attraction existing between A and B * The attraction forces between A and B may be greater than those between A and A or B and B * This occurs even when the liquid are miscible in all proportions. * They do not adhere to Raoult's Law throughout the entire range of composition * **Example**: 100 mL of sulfuric acid + 100 mL water yields 180 mL at room temperature (non-ideal solution). ## **Solution Concentration Expression** * We sometimes speak of dilute or concentrated solution terms. These terms have no precise meaning. * **Dilute solution:** A dilute solution is one which contains relatively small amount of solute per unit volume of the solution. * **Concentrated solution:** A concentrated solution is one which contains relatively greater amount of solute per unit volume of the solution. * We need to define solution comparison more precisely in order to perform calculations. * The amount of solute, solvent and solutions may be expressed by volume, mass or number of moles. ## **Percent Expression** * Defined as the mass of solute present in 100 g of solution. It is also referred to as percent weight/weight * **Mass percent** = g amount solute x 100 / g solution * **Example**: Calculating the mass percent of a solution containing 10 g sugar dissolved in 100 g water: * **Mass percent** = grams of sugar x 100 / g of solution * **Mass percent** = (10 g sugar) / (10 g sugar + 100 g water) x100 * **Mass percent** = 9.09% * **Percent weight by volume (w/v)**: Amount of solute dissolved per 100 parts by volume of solution (the volume of the solvent is usually not known). * **Percent w/v** = mass of solute (g) / Volume of solutions (cm3) x 100 * **Example**: A 10% (w/v) NaCl solution contains 10 g NaCl dissolved in 100 cm³ of solution. * **Volume by volume (v/v):** Volume of solute dissolved per 100 parts by volume of solution. The volumes of solute and solvent may be necessary equal to the volume of solution. * **Percent v/v** = cm³ of solute / Cm³ of solutions x 100 * **Example**: A 5% (v/v) ethanol solution means 5 cm³ ethanol dissolved per 100 cm³ of solution. ## **Molarity (M)** Defined as the number of moles of solute dissolved per dm³ of solution. * **Note**: the reference volume is that of solution and not pure solvent. * **M** = moles of solute / dm³ of solution * **M** = grams of solute / Molar mass of solute x dm³ of solution * **Example**: If 0.5 mole of NaOH (20g) is dissolved in enough water to make 1 dm³ of solution, a 0.5 molar or 0.5 M NaOH solution is obtained. ## **Molality (m)** Defined as the number of moles of solute dissolved per kilogram of solvent. * **m** = moles of solute / Kg of solvent * **Example**: 58.5 g of NaCl (1 mole) in one kilogram of water results in one molal or 1m NaCl solution. ## **Normality (N)** Gram equivalent weights of solute in one liter of solution. ## **Mole Fraction** Defined as the ratio of the number of moles of one component to the total number of moles in the solution. * **X1** = n1 / n1 + n2 * **X2** = n2 / n1+ n2 * **Where**: X1 is the mole fraction of solvent, X2 is the mole fraction of constituent 2 (usually the solute), n1 is the number of moles of the solvent, and n2 is the number of moles of the solute.