Published on February 10, 2009
Slide 1: AMIT M. GUPTA LECTURER, AGNIHOTRI COLLEGE OF PHARMACY, WARDHA Colloids Slide 2: Colloids Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due to gravity. Particle size: 10 to 2000 Å. There are several types of colloid: Aerosol (gas + liquid or solid, e.g. fog and smoke), Foam (liquid + gas, e.g. whipped cream), Emulsion (liquid + liquid, e.g. milk), Sol (liquid + solid, e.g. paint), Solid foam (solid + gas, e.g. marshmallow), Solid emulsion (solid + liquid, e.g. butter), Solid sol (solid + solid, e.g. ruby glass). Slide 3: Colloidal Systems Particle Size Range10 A - 5000 A1 A = 10 cm = 10 m Small particle size means a large interfacial area and a system in which interfacial properties are important. o o o -8 -10 Slide 4: Definitions Sol general term used primarily for dispersions of solids in liquids, but also for dispersions in solid or gaseous media hydrosol - dispersion in water alcosol - dispersion in alcohol aerosol - dispersion in air Slide 5: Definitions Gel a colloidal system which under a set of conditions of concentration and temperature, "sets" into a solid or semisolid the rigidity of a gel is due to an intertwining network which traps the dispersion medium Colloids : Colloids Have medium size particles Cannot be filtered Separated with semipermeable membranes Scatter light (Tyndall effect) Types of Colloids : Types of Colloids Classification Based on Size : Classification Based on Size Sols….another definition : Sols….another definition Sol – refers to any colloidal system in which the dispersion medium is a liquid. Lyophilic sol – a sol consisting of a dispersed phase which has an affinity for the continuous phase. This means that the colloid is readily formed e.g. starch in water. Lyophobic sol – a sol which is solvent-repelling, such that the disperse phase has little or no attraction for the dispersion medium e.g. gold in water. Use of Colloidal Phenomena : Use of Colloidal Phenomena Detergency Dewatering of sludges via coagulation Emulsion polymerisation Natural phenomena i.e. milk (casein) Examples of Colloids : Examples of Colloids Fog Whipped cream Milk Cheese Blood plasma Pearls Osmosis : Osmosis In osmosis, the solvent water moves through a semipermeable membrane Water flows from the side with the lower solute concentration into the side with the higher solute concentration Eventually, the concentrations of the two solutions become equal. Osmosis : Osmosis semipermeable membrane 4% starch 10% starch H2O Slide 14: Colligative Properties Osmosis Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane. Osmosis: the movement of a solvent from low solute concentration to high solute concentration. There is movement in both directions across a semipermeable membrane. As solvent moves across the membrane, the fluid levels in the arms becomes uneven. Slide 15: Colligative Properties Osmosis Eventually the pressure difference between the arms stops osmosis. Slide 16: Colligative Properties Osmosis Osmotic pressure, ?, is the pressure required to stop osmosis: p = osmotic pressure M = Molarity (mol/L) R = Ideal Gas Constant T = Temperature (K) Pure solvent/solution : Pure solvent/solution Seawater 48 feet Sea water Pure Water Pure Water Slide 18: Colloids Hydrophilic and Hydrophobic Colloids Focus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water. Slide 19: Colloids Hydrophilic and Hydrophobic Colloids Slide 20: Colloids Hydrophilic and Hydrophobic Colloids Typical hydrophilic groups are polar (containing C-O, O-H, N-H bonds) or charged. Hydrophobic colloids need to be stabilized in water. Adsorption: when something sticks to a surface we say that it is adsorbed. If ions are adsorbed onto the surface of a colloid, the colloids appears hydrophilic and is stabilized in water. Consider a small drop of oil in water. Add to the water sodium stearate. Slide 21: Colloids Hydrophilic and Hydrophobic Colloids Sodium stearate has a long hydrophobic tail (CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+). The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface. The hydrophilic heads then interact with the water and the oil drop is stabilized in water. Slide 22: Colloids Hydrophilic and Hydrophobic Colloids Slide 23: Colloids Hydrophilic and Hydrophobic Colloids Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. Emulsifying agents help form an emulsion. Hydrophilic molecules : Hydrophilic molecules Amphipathic molecule : Amphipathic molecule Different Colloidal Systems : Different Colloidal Systems Hydrophilic – lyophobic Hydrophobic – lyophilic Generally composed of compounds of an inorganic nature In most cases are more difficult to prepare than hydrophilic colloids. Condensation, dispersion and washing AgNO3 + KI ? KNO3 + AgI ? Lyophilic Colloidal Systems : Lyophilic Colloidal Systems Dispersions that spontaneously form from macroscopic phases, and are thermodynamically stable with respect to both enlargement of particles through their aggregation and disintegration to individual molecules Property: equilibrium size distribution (do not change in time) Slide 28: DLVO theory Around 1942, Deijaguin-Landao-Verwey-Overbeek 1) The inter-particle attraction: long-range dispersion forces Hamaker constant 2) The inter-particle repulsion: Debye-Huckel constant The inter-particle potential DLVO Theory : DLVO Theory Distance van der Waal’s attraction Electrostatic repulsion Distance Energy Attraction Repulsion III II I Effect on stability : Effect on stability Distance Energy Electrolytes in solution Adsorption of counterions polymers, surfactants Slide 31: Valence: the higher the valence, the lower the precipitating value. = 100 : 1.6 : 0.3 MI : MII : MII = Hardy-Schulze rules Hardy-Schulze rule is only valid without specific adsorption. The precipitating efficiency of morphia (I) chloride is larger than Mg (II) and Ca (II) DLVO suggests Slide 32: Hardy and Schulze made systematic investigation on the precipitation of sols by adding electrolytes and summarized rules latterly named as Hardy-Schulze rules. Precipitating value of different electrolytes towards the same colloids The ion which is effective in causing precipitation of a sol is the one whose charge is of opposite sign to that of the colloidal particles, i.e., counterions Slide 33: 2) Radius Hofmeister / lyotropic series H+ > Cs+ > Rb+ > NH4+ > K+ > Na+ > Li+ F- > Cl- > Br- > NO3- > I- 3) Co-ions When counterion is the same, the higher the valence of the co-ions, the higher the precipitating value. Precipitating values for As2S3 colloids Properties of Colloidal Suspensions : Properties of Colloidal Suspensions Tyndall Effect Brownian movement Filtration Adsorption Electrical properties Precipitation Gels Slide 35: Optical Properties Tyndall Effect light may be absorbed, scattered, polarized or reflected by the dispersed phase of a colloid Properties of Colloids beam of light solution beam of light colloid Slide 36: Colloids Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid. Slide 37: Properties of Colloids Brownian Movement particles are generally small enough to be influenced by the collision with molecules of the dispersion medium when particles are observed, they are seen to move in a random, erratic manner Slide 38: Properties of Colloids Consequences of Brownian movement Stable colloids are systems in which the dispersed particles do not settle, because the force of gravity is counteracted by Brownian movement Colloidal sols will diffuse from a region of high concentration to a region of low concentration Colloidal sols show colligative properties Slide 39: Colloidal Diffusion Fick's Law-dM/dt = D A [dc/dx]D = R T 6 ??r N Slide 40: Osmotic Pressure van't Hoff equation??= c R T c = molar concentrationc = g/M/literM = molecular weight Slide 41: Amphiphilic Molecules Polar region Nonpolar region Slide 42: Micelles aggregates of amphiphilic molecules Critical Micelle Concentration (cmc) concentration above which micelles begin to form Micelle Formation Slide 43: Solution of Amphiphiles Concentration of surfactant below the critical micelle concentration. A B C Slide 44: Association Colloids Concentration above the critical micelle concentration. Slide 45: Solubility of non-polar solutes Concentration of surfactant below the critical micelle concentration. nonpolar solute Slide 46: Micellar Solubilization The interior of the micelle represents a hydrocarbon (non-polar) reservoir. nonpolar solute Slide 47: Properties of Association Colloids Magnitude of Property Concentration of Surfactant Surface Tension critical micelle concentration Solubility of nonpolar solute Slide 48: Micellar Kinetics Slide 49: In a typical surfactant system, ? bulk concentration, ? surface concentration -- until cmc is reached. cmc (critical micelle concentration) – surfactant conc. where micellization occurs. Micelles CMC Surface excess Slide 50: Micellar Kinetics Micelles are NOT static structures. Micelles are unstable structures with two characteristic relaxation times – fast relaxation time (t1) and slow relaxation time (t2) Slide 51: Techniques Used to Measure Micellar Kinetics Pressure-Jump (conductivity or optical detection) Temperature-Jump (optical detection) Stopped-Flow (conductivity, optical detection and fluorescence) Ultrasonic Absorption Fluorescence Shock-Tube Slide 52: Effect of Surfactant Conc. on Micelle Lifetime It has been shown that micelle ‘slow’ relaxation time, t2, is a function of surfactant concentration For all surfactants that form micelles, t2 increases to a certain maximum value For ionic surfactants, t2 begins to decrease from the maximum value Slide 53: Foaming (foamability & foam stability) Fabric Wetting Solubilization Emulsification Importance of Micellar Kinetics in Technological Processes The Importance of Micelle Break-up in Foaming : The Importance of Micelle Break-up in Foaming Thin Liquid Film Air Air Air Surfactant solution Slide 55: Micellar Solubilization Surfactant molecules accumulate in the interfaces between water and water insoluble compound. Their hydrocarbon chains penetrate the outermost layer of insoluble compound which combine with the waterinsoluble molecules. Micelles form around the molecules of the waterinsoluble compound inside the micelles’ cores and bring them into solution in an aqueous medium. This phenomenon is called micellar solubilization. The inverted micelles formed by oilsoluble surfactant which dissolves in a hydrocarbon solvent can solubilize water-soluble compound which is located in the center of the micelle, out of contact with the solvent. Slide 56: Micelles of nonionic surfactants consist of an outer shell containing their polyethylene glycol moieties mixed with water and an inner core formed by their hydrocarbon moieties. Some compounds like phenols and benzoic acid form complexes with polyethylene glycols by hydrogen bonding and/or are more soluble in liquids of intermediate polarity like ethanol or ethyl ether than in liquids of low polarity like aliphatic hydrocarbons. These compounds locate in the aqueous polyethylene glycol outer shell of nonionic micelles on solubilization. Drugs which are soluble in oils and lipids can be solubilized by micellar solubilization. Slide 57: As Micellar solubilization depends on the existence of micelles; it does not take place below the CMC. So dissolution begins at the CMC. Above the CMC, the amount solubilized is directly proportional to the surfactant concentration because all surfactant added to the solution in excess of the CMC exists in micellar form, and as the number of micelles increases the extent of solubilization increases . Compounds that are extensively solubilized increase the size of micelles in two ways: The micelles swell because their core volume is augmented by the volume of the solubilizate. The number of surfactant molecules per micelle increases.