solutions and their properties

Solutions and Their Properties

Dr. Basavaraj K. Nanjwade M. Pharm., Ph. DDepartment of Pharmaceutics

Faculty of PharmacyOmer Al-Mukhtar University

Tobruk, Libya.E-mail: [email protected]

29/04/2015 1Faculty of Pharmacy, Omer Al-Mukhtar University, Tobruk, Libya.

CONTENTS1. Definition, preparation of solutions, types of solutions.2. Solvent-solute interaction and thermodynamics of

solvents.3. Solubility of gases in liquids and liquids in liquids.4. Solubility of solids in liquids (electrolytes: soluble,

slightly soluble, Weak) with calculations.5. Factors affecting solubility.6. Distribution phenomenon and application.7. Colligative properties and lowering of vapor pressure.8. Depression of freezing point, osmotic pressure.

29/04/2015 2Faculty of Pharmacy, Omer Al-Mukhtar University, Tobruk, Libya.

1. Definition of Solutions• A solutions may be defined as a mixture of two or

more components that form a single phase which is homogeneous down to the molecular level.

• The component that determines the phase of the solution is termed the solvent and usually constitutes the largest proportion of the system.

• The other components are termed solutes, and these are dispersed as molecules or ions throughout the solvent, i.e. they are said to be dissolved in the solvent.

29/04/2015 Faculty of Pharmacy, Omer Al-Mukhtar University, Tobruk, Libya. 3

Preparation of Solutions• Write out the formula either from the prescription

(unofficial) or from an official text (official).• Calculate the quantities required for each ingredient in

the formula to produce the required final volume. • Remember, it is not usual to calculate for an overage of

product in the case of solutions as it is relatively easy to transfer the entire final contents of the conical measure.

• Also, as far as is practically possible, the product will be assembled in the final measure, thus reducing any transference losses.


Types of solutions


State of Solvent

State of Solute

State of Solution

Examples

Gas Gas Gas Air, natural gasLiquid Liquid Liquid Alcoholic beverages,

Antifreeze solution;Liquid Solid Liquid Seawater, sugar solutionLiquid Gas Liquid Carbonated water (soda)

Ammonia solution;Solid Solid Solid Metal alloys: brass, bronze,..Solid Gas Solid Hydrogen in platinum

2. Solvent-solute interaction

• Solubility depends on chemical, electrical & structural effects that lead to mutual interactions between the solute and the solvent.

• In pre or early formulation, selection of the most suitable solvent is based on the principle of “like dissolves like”.

• Polar solutes dissolve in polar solvents. Eg. Salts & sugar dissolve in water .

• Non polar solutes dissolve in non polar solvents. Eg. Naphtalene dissolves in benzene.


Solvent- solute interaction


Hsoln = H1 + H2 + H3

Thermodynamics of solvents

• The thermodynamic solubility of a drug in a solvent is the maximum amount of the most stable crystalline form that remains in solution in a given volume of the solvent at a given temperature and pressure under equilibrium conditions.

• The equilibrium involves a balance of the energy of three interactions against each other: (1) solvent with solvent, (2) solute with solute, and (3) solvent and solute.


3. Solubility of gases in liquids

• The solubility of a gas in a liquid is the concentration of the dissolved gas when it is in equilibrium with some of the pure gas above the solution.

• Gases like hydrochloric acid, ammonia water, and effervescent preparations containing carbon dioxide that are dissolved and maintained in solution under positive pressure.

• The solubility depends on the pressure, temperature, presence of salts and chemical reactions.


Solubility of liquids in liquids

• The components of an ideal solutions are miscible in all proportions.

• Such complete miscibility is also observed in some real binary systems, e.g. ethanol and water, under normal conditions.

• In cases where partial miscibility occurs under normal conditions the degree of miscibility is usually dependent on the temperature.


4. Solubility of solids in liquids

• If two solids are either melted together and then cooled or dissolved in a suitable solvents, which is then removed by evaporation, the solid that is redeposited from the melt or the solution will either be a one-phase solid solution or a two-phase eutectic mixture.

• In a solid solution, as in other types of solution, the molecules of one component (the solute) are dispersed molecularly throughout the other component (the solvent).


5. Factors affecting solubility• Temperature• Molecular structure of solute• Nature of solvent: cosolvents• Crystal characteristics: polymorphism and solvation• Particle size of the solid• Common ion effect• Effect of indifferent electrolytes on the solubility product• Effect of non-electrolytes on the solubility of electrolytes• Effect of electrolytes on the solubility of non-electrolytes• Complex formation• Solubilizing agents


6. Distribution phenomenon and application

• If the substance is added to the immiscible solvents in an amount insufficient to saturate the solutions, it will still become distributed between the two layers in a definite concentration ratio.

• If C1 and C2 are the equilibrium concentrations of the substance in solvent 1 and solvent 2, the equilibrium expression becomes

C1/C2 = K


Applications• Effect of partition of ionic dissociation and molecular

association.• Extraction.• Solubility and partition coefficients.• Preservative action of weak acids in oil-water

systems.• Drug action and partition coefficients.


7. Colligative properties and lowering of vapor pressure

• When a non-volatile solute is dissolved in a solvent, certain properties of the resultant solution are largely independent of the nature of the solute and are determined by the concentration of solute particles.

• These properties are known as colligative properties (Geek: collected together).

• The freezing point, boiling point, and osmotic pressure of a solution also depend on the relative proportion of the molecules of the solute and the solvent.


Lowering of vapor pressure• It is more convenient to express the vapor pressure of

the solution in terms of the concentration of the solute, rather than the mole fraction of the solvent.

• A solute lowers the vapor pressure of a solution in an amount dependent on the mole fraction of the solute.

X1 = 1 - X2

• In which X1 is the mole fraction of the solvent and X2 is the mole fraction of the solute.


8. Depression of freezing point

• The freezing point (or melting point) of a pure compound is the temperature at which the solid and liquid phases are in equilibrium under a pressure of 1 atmosphere (atm).

• The freezing point of a solution is the temperature at which the solid phase of the pure solvent and the liquid phase of the solution are in equilibrium under a pressure of 1 atm.

• The amount of depression of the freezing point depends on the molality of the solution.



Depression of freezing point

Osmotic pressure• Osmosis is the process by which solvent molecules pass

through a semipermeable membrane (a barrier through which only solvent molecules may pass) from a region of dilute solution to one or more concentrated solution.

• The pressure that must be applied to the more concentrated solution to prevent the flow of pure solvent into the solution is known as the osmotic pressure.

• Solvent molecules move from a region where their escape tendency is high to one where their escape tendency is low.



Osmotic pressure

THANK YOUE-mail: [email protected]


solutions and their properties

Health & Medicine