Colloids.pdf

THE NATIONAL RIBAT UNIVERSITY FACULTY OF PHARMACY DEPARTMENT OF PHARMACEUTICS PHYSICAL PHARMACY  ❑ Dispersed systems: ▪ Dispersed systems consist of particulate matter (dispersed phase), distributed throughout a continuous phase (dispersion medium). ▪ They are classified...

THE NATIONAL RIBAT UNIVERSITY FACULTY OF PHARMACY DEPARTMENT OF PHARMACEUTICS PHYSICAL PHARMACY  ❑ Dispersed systems: ▪ Dispersed systems consist of particulate matter (dispersed phase), distributed throughout a continuous phase (dispersion medium). ▪ They are classified according to the particle size of the dispersed material: 1. Molecular dispersions 2. Coarse dispersions 3. Colloidal dispersions 1. Molecular dispersions: Molecular dispersions are homogeneous in character and form true solutions. ✓ Characterized by: - Particle size less than 1nm. - Particles are invisible by electron microscope. - Do not scatter light. - Pass through filter paper and semipermeable membranes. - Do not settle even on ultracentrifugation. - Show rapid diffusion. - E.g. ordinary ions, glucose. 2. Coarse dispersions: Coarse dispersions are heterogeneous dispersion and form suspensions. ▪ Characterized by: - Greater than 0.5 µm. - Particles are visible under ordinary microscope - Do not pass through filter paper or semipermeable membrane. - Particles settle down under gravity. - Do not diffuse. - E.g. emulsions, suspensions, red blood cells. 3-Colloidal dispersions: Colloidal dispersions are heterogeneous dispersion in which the size of the dispersed particles is intermediate between true solutions and coarse dispersions. ✓ Characterized by: - Particle size from 1nm to 0.5 µm. - Particles are invisible by ordinary microscope but are visible by electron microscope. - Scatter light and show Tyndall effect. - Pass through filter paper but not pass through semipermeable membrane. - Particles settle by centrifugation. - Diffuse slowly. - E.g. colloidal silver sols. ❑ Pharmaceutical applications of colloids are as follows: 1. Therapy: Colloidal system are used as therapeutic agents in different areas. E.g. Silver colloid which is used as germicidal. Copper colloid which is used as anticancer. Mercury colloid which is used as Antisyphilis. 2. Enhanced absorption: Colloidal sulfur shows better and faster absorption compared to a coarse dispersion of sulfur. 3. Solubility: For example solubilization of salicylic acid, sulfonamides and Phenobarbital was achieved in micellar solutions. 4. Stability: E.g. Colloidal dispersion of gelatin is used in coating over tablets and granules 5. Targeting: Colloids are used in targeting the drugs to specific body organs. For example, liposomes are of colloidal dimensions and are being preferentially taken up by liver and spleen. ❑ Classification Of Colloidal Dispersions ✓Classification is based on following criteria: A. Physical state of dispersed phase and dispersion medium. Dispersed Phase Dispersion Medium Name Example Liquid Liquid Emulsion Milk, mayonnaise Solid Liquid Sol Paints, toothpaste Gas Liquid Foam Froth Solid Solid Solid sol Some coloured glasses Liquid Solid Gel Cheese, butter Gas Solid Solid foam Sponge Liquid Gas Aerosol Fog, liquid sprays Solid Gas Solid aerosol Dust, smoke B. Nature of interaction between dispersed phase and dispersion medium. 1. Lyophilic colloids. 2. Lyophobic colloids. 3. Association or amphiphilic colloids. 1. Lyophilic Colloids (solvent-loving or solvent attracting) ✓ The particles in a lyophilic system have a great affinity for the solvent. a. Their preparation is relatively easy. b. Thermodynamically stable. c. The sols are quite stable as the solute particle surrounded by two stability factors: - Charge (negative or positive) - Layer of solvent d. The dispersion is reversible. e. If water is the dispersing medium, it is often known as a hydrosol or hydrophilic. Examples of hydrophilic colloids are acacia, gelatin and starch dispersions in water. 2. Lyophobic Colloids (solvent-hating) ✓Very little attraction is possible between the dispersed phase and dispersion medium. a. Thermodynamically unstable. b. Less stable. c. These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking. d. Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols. e. Examples of lyophobic sols: gold, silver and sulfur in water. 3. Association/ amphiphilic Colloids 3. ✓In water: the hydrocarbon chains face inwards into the micelle forming hydrocarbon core and surrounded by the polar portions of the amphiphile associated with water molecules. ✓In non-polar liquid: the polar heads facing inward and the hydrocarbon chains are associated with non-polar liquid. ❖Micellar solubilization ✓Hydrophobic pockets inside micelles offer an environment for solubilization of poorly water soluble drugs. ✓The hydrolytic or oxidative decomposition of drugs are prevented or reduced by using solubilised products. ✓Examples are solubilised systems containing, Salicylic acid, coal tar materials and vitamins.  Formulation factors ✓ While formulating solubilised systems, following factors are to be considered: (a) Type of surfactants: o Normally nonionic surfactants are used for internal use. o Ionic surfactants are meant for external use. Also nonionic type is suitable for external use. (b) Concentration of surfactant: Optimum concentration should be selected. ✓ If the concentration is insufficient, micelles do not form, and drugs get precipitated. ✓ If the concentration of surfactant is higher, release of the drug may be delayed leading to reduced absorption and activity.  Purification of colloidal systems Colloidal systems can be separated from subcolloidal systems by: A. Dialysis In this process, ions or molecules are removed from a colloidal dispersion by diffusion through a semipermeable membrane. The semipermeable membrane is fixed tightly to one end of a hollow glass tube. The colloidal dispersion is placed in the glass tube and the glass tube is suspended in a vessel through which fresh water is continuously passed. Ions and other molecules diffuse out of the bag into water rapidly. B. Electrodyalysis ✓The principle of electrodialysis is similar to that of dialysis. ✓Diffusion of ions or molecules is enhanced by applying a potential difference across the membrane. ✓Nonionic impurities cannot be separated. ✓The principle is illustrated below. C. Ultrfiltration: ✓Using a modified filter papers are called ultrafilters. ✓Colloidal particles are retained on the filter paper. Then collected and dispersed in a pure dispersion medium to get a sol. ✓Ultrafiltrationproceeds very slowly, so pressure or suction is applied to increase the rate of filtration. ✓The principle is illustrated below