Pharmaceutics-Week4-Colligative properties.pdf

Colligative properties PY4030/PY5130 Dr Gianpiero Calabrese Room MB1037, [email protected] School of Pharmacy and Chemistry Properties of solutions -Learning OutcomesIdentify and describe colligative properties. Define ideal and real solutions using Raoult’s and Henry’s laws. Calculate v...

Colligative properties PY4030/PY5130 Dr Gianpiero Calabrese Room MB1037, [email protected] School of Pharmacy and Chemistry Properties of solutions -Learning OutcomesIdentify and describe colligative properties. Define ideal and real solutions using Raoult’s and Henry’s laws. Calculate vapour pressure lowering. Define/describe the phenomenon of osmosis, osmolarity and the concept of isotonicity. Appreciate the clinical relevance of these. Ideal solutions An ideal solution can be defined as one in which there is no change in the properties of the components, other than dilution, when they are mixed to form a solution. No heat is evolved or absorbed during the mixing process. In other words, no shrinkage or expansion occurs when the substances are mixed. Escaping tendency Two bodies are in thermal equilibrium when their T are the same. If T1>T2, heat will flow from the hotter (1) to the colder (2). The heat in 1 has a greater escaping tendency. T is a quantitative measure of the escaping tendency. Free energy is a quantitative measure of the escaping tendencies of material substances undergoing transformations. Pressure The vapour pressure of a solution serves as a quantitative expression of escaping tendency. The ability of water molecules to escape the surface into vapour would be diminished if you apply an external force that keeps the molecules down = Less molecules of water escaping the liquid and going into vapour. Molecules of water escaping the liquid and going into vapour. Pressure The opposite is also true: the lower the force (i.e. the pressure) applied on the surface, the easier it will be for molecules to pass into vapour. More pressure Less pressure H2O H2O Raoult’s Law The vapour pressure of a solution serves as a quantitative expression of escaping tendency. “The partial vapour pressure of each volatile constituent is equal to the vapour pressure of the pure constituent multiplied by its mole fraction in solution.” For two constituents A and B: PA = XAPA PB = XBPB where • X is the molar fraction of compound A or B • P is the normal vapor pressure of pure A or B at that temperature Example If the vapour pressure of pure ethylene chloride = 236 mmHg at 50 oC, then in a solution consisting of a mole fraction of 0.4 ethylene chloride and 0.6 benzene, the partial vapour pressure of ethylene chloride is 40% of 236 mmHg (i.e. 94.4mmHg). In an ideal solution, when A is mixed with B, the vapour pressure of A is reduced by dilution with B in a manner depending on the mole fractions of A and B. Example What is the partial vapour pressure of benzene and ethylene chloride in a solution at a mole fraction of benzene of 0.6? The vapour pressure of pure benzene at 50 oC is 268mmHg and the PA of ethylene chloride is 236 mmHg. PB = 268 x 0.6 = 160.8 mmHg PA = 236 x 0.4 = 94.4mmHg The total pressure is the sum of the partial pressures of all constituents. P = PA + PB = 160.8 + 94.4 = 255.2 mmHg CFCs Propellants - MDIs CFCs are perfectly miscible with each other Suitable blends give intermediate vapour pressure (200-450 KPa) Vapour pressure given by Raoult’s law Propellant Vapour Suspended Drug Liquefied Propellant Real solutions Ideality in solution presupposes complete uniformity of attractive forces. Though, many solutions are known where the attractive forces between A and B are greater than those between A and A or B and B (or viceversa). Such mixtures/solutions are real (or non-ideal) and Raoult’s law is not applicable. 2 possibilities: positive deviation or negative deviation from Raoult’s law. Adhesion between A and B < cohesion between A and A or B and B Adhesion between A and B > cohesion between A and A or B and B Positive Deviation Adhesion between A and B < cohesion between A and A or B and B Example: Chloroform and ethanol A maximum is often showed at a particular molar fraction Negative Deviation Adhesion between A and B > cohesion between A and A or B and B Interaction between A and B is strong Cl3C – H …. O=C(CH3)2 Henry’s Law When chloroform is a solute there are little molecules of this in a relatively uniform matrix of acetone. The partial pressure of chloroform is somewhat proportional to its mole fraction but the proportionality constant is not Po Psolute = ksolute Xsolute where Ksolute < Posolute Colligative properties Greek: “collected together” Colligative properties depend only on the number of solute particles present, rather than on the nature of the constituents. Among colligative properties are: Vapor pressure lowering Boiling point elevation Freezing point depression – covered in injections lectures Osmotic pressure Vapour Pressure Lowering As solute molecules are added to a solution, the solvent become less volatile (=decreased vapor pressure). Solute-solvent interactions contribute to this effect. Therefore, the vapor pressure of a solution is lower than that of the pure solvent. Example Calculate the relative vapour pressure lowering at 20 oC for a solution containing 171.2 g of sucrose (w2) in 1000 g (w1) of water. The MW of sucrose (M2) is 342.3 and the one of water (M1) is 18.02 g/mole. Moles of sucrose = n2 = w2 / M2 = 171.2/342.3 = 0.500 moles Moles of water = n1 = w1 / M1 = 1000/ 18.02 = 55.5 moles Δp = p1o X2 Δp / p1o = X2 = n2 / (n1 + n2) Δp / p1o = 0.500 / (55.5 + 0.50) = 0.0089 Δp / p1o can also be expressed as 0.89% Boiling Point Elevation The normal boiling point is the T at which the vapour pressure of the liquid equals the external pressure (760 mmHg). A solution will boil to a higher T than will the pure solvent, hence addition of solute will lower the vapour pressure of the solvent. The vapour pressure curve for the solution lies below that of the pure solvent. The T must be increased to achieve the boiling point. ΔTb = T - To The ratio between ΔTb and the vapour pressure lowering, Δp, is constant: ΔTb / Δp = k’ Because Δpo is constant and by applying the Raoult’s law: ΔTb = k X2 In diluted aqueous solutions, X2 is equal approximately to m / (1000/M1) hence ΔTb = kb m Kb is called molal elevation constant or ebullioscopic constant. Osmosis If a semi-permeable membrane separates a solution from a solvent, solvent passes through the membrane into the solution. (diluting the solution) Semi-permeable membranes only allow passage of solvent molecules, NOT solute. membrane solvent solution Osmotic Pressure, p The osmotic pressure is the pressure that has to be applied to stop the osmosis pressure, p solvent solution For non electrolytes it can be shown pV = nRT where n is the number of moles of solute, V the volume of solution. If a solute dissociates into ions (electrolytes) then each ion contributes to the osmotic pressure Eg. 1.0 M NaCl solution will have an osmotic pressure twice that of a 1.0 M solution of CH3OH because we have both 1.0 M Na+ AND 1.0 M Cl- in the solution For real solutions we must use the activity rather than the concentration. Real solutions have a lower osmotic pressure than their concentration would lead us to expect. For electrolytes pV =  nRT - ν is the number of moles of ions that form when one mole of the electrolyte dissolves in solution - Φ is the practical osmotic coefficient which is the deviation of the solution from ideality (activity coefficient) Osmotic pressure and sterility Bacteria respond rather slowly to changes in osmotic pressure, but they are plasmolysed by strongly hypertonic solutions and they swell and may burst when in hypotonic medium. Moulds and yeasts are more tolerant of high osmotic pressure than bacteria and are often found as contaminants of unpreserved syrups, semisolid creams and ointments. Osmolarity & Osmolality The amount in moles of osmotically active species in solution is expressed as a number of osmoles. The concentration can therefore be expressed in osmolarity or osmolality. Symbol ξM and ξm respectively. Iso-Osmotic Solutions Two solutions that have the same osmotic pressure are isoosmotic, or isotonic. This is important in medicine because osmosis can cause cell damage or discomfort. e.g. eye medication must be isotonic so that osmotic pressure does not damage the delicate membranes in the eye. i.v. injections or infusions must be of similar tonicity to blood plasma to avoid damage to blood cells or capillary walls. Importance of osmotic effects Hypotonic Hemolysis Isotonic Hypertonic Crenated cells Questions? • Aulton's Pharmaceutics: The Design and Manufacture of Medicines. M. E. Aulton, 6th edition, Churchill Livingstone • Physicochemical Principles of Pharmacy. A. Florence & D. Attwood, 5th edition, Pharmaceutical Press (2011). • Martin’s physical pharmacy and pharmaceutical sciences. 6th ed. by Patrick J. Sinko.