Lecture 4 Colloidal Dispersion PDF

Lecture 4  Colloidal Dispersions  The term “dispersed system” refers to a system in which one substance (dispersed phase) which insoluble solid, liquid, gas is distributed through a second substance (the continuous phase)  Colloids are disperse system (colloidal dispersion)...

Lecture 4  Colloidal Dispersions  The term “dispersed system” refers to a system in which one substance (dispersed phase) which insoluble solid, liquid, gas is distributed through a second substance (the continuous phase)  Colloids are disperse system (colloidal dispersion)  Dispersion systems consist of a) Particulate matter (Dispersed phase, internal phase, discontinuous phase) b) Dispersion medium (continuous, external phase)  SOL. = COLLOIDAL SOLUTION. DISPERSION MEDIUM = SOLVENT. DISPERSED PHASE = MATERIAL = COLLOIDAL PARTICLES. Classification of colloidal system according to particle size Particle size 1nm- Particle size >1000 nm 1000 nm  Types of the Colloidal System According to the interaction between particles of dispersed phase and those of dispersion medium 1. Lyophilic colloids (Solvent Loving): the dispersed phase has high affinity for the dispersion medium 2. Lyophobic colloids (Solvent Hating): the dispersed phase has low affinity for the dispersion medium 3. Association (Amphiphilic)  1) Lyophilic colloids Colloidal particles interact to an appreciable extent with the molecules of the dispersion medium (solvent loving). Obtained simply by dissolving the material in the solvent ( due to the high affinity). Types of lyophilic colloids; (According to type of solvent) Hydrophilic colloids; Lipophilic colloids; Solvent: water. Solvent: non- aqueous, organic solvent. Example: acacia, insulin…. in water. Example: rubber &polystyrene. SO; material that form lyophilic colloid in a certain solvent may not do so in another solvent, e.g.; acacia + water lyophilic colloid (hydrophilic type). acacia + benzene NO lyophilic colloid formed. 1) Lyophilic colloids  the dispersed phase does not precipitate easily (they need neutralization of the charge or removal of the water layer)  Addition of liquid less polar than water ( as acetone or alcohol) or by addition of small amount of electrolyte will facilitate coagulation due to dehydration of the particles  the sols are quite stable as the solute particle surrounded by two stability factors: a- negative or positive charge b- layer of solvent  If the dispersion medium is separated from the dispersed phase, the sol can be reconstituted by simply remixing with the dispersion medium. Hence, these sols are called reversible sols 1) Lyophilic colloids  Ammonium sulphate the most widely used electrolyte suitable for fractional precipitation of serum protein  Heat, acids, alkalies and some salts cause denaturation of lyophilic colloidal material (Not Reversible)  Similarly charged hydrophilic colloids easily mixed with no precipitation while that of different charge interact together with precipitation ( e.g. antigen antibody reaction)  2) Lyophobic colloids  Colloidal particles have very little or no attraction for the dispersion medium (solvent hating).  Colloidal particles: inorganic particles (e.g. gold, silver, sulfur….) Dispersion medium: water. - These colloids are easily precipitated on the addition of small amounts of electrolytes, by heating or by shaking - Less stable as the particles surrounded only with a layer of positive or negative charge - Once precipitated, it is not easy to reconstitute the sol by simple mixing with the dispersion medium. Hence, these sols are called irreversible sols.  Not obtained simply i.e need special method for preparation 3. Association colloids:  Certain molecules or ions termed amphiphile (surface active agent SAA) are characterized by two distinct regions of opposing solution affinities within the same molecules or ions.  3. Association colloids: - At low concentration: amphiphiles exist separately (subcolloidal size) - At high concentration: form aggregates or micelles (50 or more monomers) (colloidal size) As with lyophilic sols, formation of association colloids is spontaneous, provided that the concentration of the amphiphile in solution exceeds the cmc. Amphiphiles may be 1. Anionic (e.g., Na. lauryl sulfate) 2. Cationic (e.g., cetyl triethylammonium bromide) 3. Nonionic (e.g., polyoxyethylene lauryl ether) 4. Ampholytic (zwitterionic) e.g., dimethyl dodecyl ammonio propane sulfonate.  Comparison of colloidal sols Lyophilic Associated Lyophobic Dispersed phase Dispersed phase Dispersed phase (large organic mole. (micelles of organic (Inorganic particles as With colloidal size) molec. Or ion –size gold) below the colloidal range) Molec. of dispersed Hydrophilic and lyophilic Not formed phase are solvated portion are solvated , spontaneously Formed spontaneously Formed at conc. above CMC The viscosity ↑ with ↑ The viscosity ↑ with ↑ the Not greatly increase the dispersed phase micelles conc. conc. Stable dispersion in CMC↓ with electrolytes Unstable dispersion in presence of presence of electrolytes electrolytes  Preparation of Colloids  Lyophilic sol. And association colloidal Sol. Are usually prepared by Dissolving/Dispersing the dispersed phase in water  While hydrophobic colloids were prepared by A. Condensation process: 1)Change of solvent 2)Double Condensation 3) Reduction 4) Oxidation B. Dispersion Coarse particles reduced in size by : 1)Ultra generator 2) Electric Arc (Bridge‘s arc method) 3) Colloid mill 4) Peptization Purification of colloids  1) Why? Many lyophobic sols contain more or less material in true solution. which may be undesirable for any number of reasons; e.g.,  electrolyte impurities : cause the flocculation of the sol. 2) How? a) Dialysis. b) Electro dialysis. c) Ultra filtration.  a)- Dialysis:  Depend on difference in size between colloidal particles & molecular particles (impurities).  Technique; 1) use semi permeable membrane (e.g. cellophane). 2) pore size of used semi permeable membrane prevent passage of colloidal particles & permit passage of small molecules & ions (impurities) such as urea, glucose, and sodium chloride, to pass through.  A type of dialysis equipment; “Neidle dialyzer”  At equilibrium, the colloidal material is retained in compartment A, while the subcolloidal material is distributed equally on both sides of the membrane. By continually removing the liquid in compartment B, it is possible to obtain colloidal material in A that is free from subcolloidal contaminants  b)- Electro dialysis:  Technique;  An electric potential may be used to increase the rate of movement of ionic impurities through a dialyzing membrane and so provide rapid purification.  Electrodialysis is carried out in a three- compartment vessel with electrodes in the outer compartments containing water and the sol in the center compartment. A typical apparatus is shown in the figure. Application of electrical potential causes cations to migrate to the negative electrode compartment and anions to move to the positive electrode compartment, in both of which running water ultimately removes the electrolyte.  c) Ultra filtration: Technique; Apply pressure (or suction) Solvent & small particles forced across a membrane while colloidal particles are retained. N.B. The membrane must be supported on a sintered glass plate to prevent rupture due to high pressure. Pore size of the membrane can be increased by soaking in a solvent that cause swelling e.g. cellophane swell in zinc chloride solution. e.g. collodion (nitrocellulose) swell in alcohol.  Stabilization of Colloid Electrostatic repulsion Electrostatic repulsion increase colloid stability either positive or negative charge The charged colloid particles repel other colloid particles and so are unable to aggregate to precipitate out 23  Stability of colloids  Two main mechanisms for colloid stabilization: 1-Steric stabilization i.e. surrounding each particle with a protective solvent sheath which prevent adherence due to Brownian movement 2-electrostatic stabilization i.e. providing the particles with electric charge  Stability of colloids A- Lyophobic sols: - Unstable. - The particles stabilized only by the presence of electrical charges on their surfaces through the addition of small amount of electrolytes. - The like charges produce repulsion which prevent coagulation of the particles and subsequent settling. 1. Addition of electrolytes beyond necessary for maximum stability results in accumulation of opposite ions and decrease zeta potential coagulation precipitation of colloids. 2. Coagulation also result from mixing of oppositely charged colloids.  Stability of colloids B- Lyophilic sols and association colloids: - Stable - Present as true solution - Addition of moderate amounts of electrolytes not cause coagulation (opposite lyophobic) ** Salting out: Definition: agglomeration and precipitation of lyophilic colloids.  Stability of colloids  This is obtained by: 1- Addition of large amounts of electrolytes - Anions arranged in a decreasing order of precipitating power: citrate > tartrate > sulfate > acetate > chloride> nitrate > bromide > iodide - The precipitation power is directly related to the hydration of the ion and its ability to separate water molecules from colloidal particles 2- addition of less polar solvent The addition of less polar solvent renders the solvent mixture unfavourable for the colloids solubility e.g. alcohol, acetone