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Industrial Chemistry Part VI Principles Emulsions 2011 Importance of Emulsions Many foods consist either partly or wholly as food emulsions, or have been in an emulsified state sometime during their manufacture. milk, cream, salad cream, mayonnaise, salad dressings, soups, sauces, butter, margarine, beverages, ice cream and coffee whitener. What is Emulsion? Emulsions are mixtures of two immiscible liquids; in which both the dispersed phase and the dispersion medium are liquids. Dispersed phase: is the liquid present in a small amount (small droplets) The dispersed liquid is known as the internal or discontinuous phase Continuous phase: is the liquid present in a large amount (medium) the continuous phase is known as the external or dispersion medium Classification (Types) of Emulsions Based on dispersed phase Oil in Water (O/W): Oil droplets dispersed in water Water in Oil (W/O): Water droplets dispersed in oil More complex types consist of three or more phases, which can be achieved by e.g. dispersing a w/o-emulsion into a second watery phase, leading to a water-in-oil-inwater-(w/o/w-)emulsion. In summary 1. oil-in-water (o/w) 2. water-in-oil (w/o) 3. water-in-oil-in-water (w/o/w) 4. oil-in-water-in-oil (o/w/o) Types Based on Size of Liquid Droplets 0.2-50 µm Macroemulsion 0.01-0.2 µm Microemulsion Mayonnaise: Oil in Water emulsion Milk: Oil in Water emulsion Water in oil emulsion Sodas: Oil in Water emulsion Emulsion is Thermodynamically Unstable W/O & O/W ONLY Most oils are less dense in water, and if oil and water are mixed then the oil will simply float to the surface. In emulsions, the oil is dispersed as liquid droplets through the continuous phase, usually but not necessarily water. Those droplets want to combine together again to form a single drop of oil this means that an emulsion is thermodynamically unstable. A difference in the densities of the two liquids may cause undesired creaming of the dispersed droplets !!! What we can do to prevent this unwanted physical process? Stability of emulsions may be engineered to vary from seconds to years depending on application. How we can do that? Compositions of Emulsion Internal/Discontinuous/Dispersed phase (liquid) External/Continuous phase (Liquid) Emulsifying agent Stabilizers Emulsifying Agents An emulsifier is a molecule with one oilfriendly and one water-friendly end i.e. hydrophilic tail and hydrophobic head. (Do you remember what is that?) These agents surrounds the oil droplets in water and reduces the tension between the two liquids thus impart stability. Emulsifying agents are often used to help the formation of emulsions Classification of Emulsifiers Chemical Structure Mechanism of Action Synthetic Natural Auxiliary agents dispersed solids Monomolecular Multimolecular Solid Particle Films Synthetic Anionics: alkali soaps (sodium or potassium oleate) detergents (sodium lauryl sulfate, sodium dioctyl sulfosuccinate, sodium docusate). Non-ionics: Sorbitan esters (Spans®), polyoxyethylene derivatives of sorbitan esters (Tweens®), or glyceryl esters Cationics: benzalkonium chloride, benzethonium chloride Amphoterics Natural Emulsifying Agents Natural emulsifying agents are derived from plant and animal tissues and mostly in the form of hydrated lypophilic colloids. These emulsifiers make the protective sheath around the droplets, give droplets a charge so that they repel each other and swell to step-up the viscosity of the liquid. Although natural agents are inexpensive, safe and non toxic but these are slow in action. So large quantity of emulsifier is required for proper action. Also the natural emulsifiers need preservatives as these are subjected to microbial growth. 1) Carbohydrate Materials: Acacia, Tragacanth, Agar, Pectin. o/w emulsion. 2) Protein Substances: -Gelatin, Egg yolk, Caesin o/w emulsion. 3) High Molecular Weight Alcohols: - Stearyl Alcohol, Cetyl Alcohol, Glyceryl Mono stearate o/w emulsion, cholesterol w/o emulsion. Finely divided solids: These agents form a particulate layer around dispersed particles. Most will swell in the dispersion medium to increase viscosity and reduce the interaction between dispersed droplets. Most commonly they support the formation of o/w emulsions, but some may support w/o emulsions. Examples Magnesium Hydroxide, Aluminum Hydroxide o/w emulsion magnesium trisilicate. Mechanism of action of emulsifying agents: Monomolecular - Coherent monomolecular film - flexible film formed by SAA, - depend on lower the d o/w , - can prepare o/w and w/o emulsion Examples: Synthetic SAA, K laurate Mechanism of action of emulsifying agents: Multimolecular Strong rigid film formed, mostly by the hydrocolloid, - which produce o/w emulsion, - d is not reduced to any extent , - the stability due to strength of the formed interfacial film Examples: Hydrophilic colloid ( acacia, gelatin) Mechanism of action of emulsifying agents: Solid particles Film formed by solid particles that are small in size compared to the droplet of the dispersed phase. - Particles must be wetted by both phases in order to remain at the interface and form stable film, - can form o/w and w/o Examples: Colloid clays(bentonite, Mg(oH)2) Tests Used To Identify Emulsion Type: Dilution test: based on the solubility of external phase of emulsion. - o/w emulsion can be diluted with water. - w/o emulsion can be diluted with oil. Conductivity Test: water is good conductor of electricity whereas oil is non-conductor. Therefore, continuous phase of water runs electricity more than continuous phase of oil. Dye-Solubility Test: when an emulsion is mixed with a water soluble dye such as amaranth and observed under the microscope. ¡ if the continuous phase appears red, then it means that the emulsion is o/w type as water is the external phase ¡ if the scattered globules appear red and continuous phase colorless, then it is w/o type. Fluorescence test: Fluorescence test: oils give fluorescence under UV light, while water doesn’t. Therefore, O/W emulsion shows spotty pattern while W/O emulsion fluoresces. Theory of emulsification Droplets can be stabilized by three methods i. By reducing interfacial tension ii. By preventing the coalescence of droplets. a. By formation of rigid interfacial film b. By forming electrical double layer. Theory of emulsification –reduction of interfacial tension Phase A Phase B A B Change from A to B increases surface area of phase A, hence the Due to increased surface energy, the system is thermodynamically unstable. Emulsifying agents are needed to decrease the interfacial tension and to24stabilize the emulsion. Oriented-Wedge Theory: - mono molecular layers of emulsifying agents are curved around a droplet of the internal phase of the emulsion. Multimolecular films Theory of emulsification -Formation of electrical double layer + - + Oi l - - + - + -+ - - + + Water + + Emulsion made with sodium soap. - Electrical double layer at oil-water interface Interfacial films Solid particle film: These agents form a particulate layer around dispersed particles. Most will swell in the dispersion medium to increase viscosity and reduce the interaction between dispersed droplets. Most commonly they support the formation of o/w emulsions, but some may support w/o emulsions. Methods of emulsion preparation: On small scale: ¡ Porcelain mortar and pestle ¡ On large scale: Mechanical stirrer Methods of emulsion preparation: Emulsifier is triturated with the oil in perfectly dry porcelain mortar water is added at once triturate immediately, rapidly and continuously (until get a clicking sound and thick white cream is formed, this is primary emulsion) the remaining quantity of water is slowly added to form the final emulsion English or Wet Gum Method triturate gum with water in a mortar to form a mucilage oil is added slowly in portions the mixture is triturated after adding all of the oil, thoroughly mixed for several minute to form the primary emulsion Once the primary emulsion has been formed remaining quantity of water is added to make the final emulsion. Bottle or Forbes Bottle Method It is extemporaneous preparation for volatile oils or oil with low viscosity. gum + oil (dry bottle) Shake water (volume equal to oil) is added in portions with vigorous shaking to form primary emulsion remaining quantity of water is added to make the final emulsion Auxiliary Emulsifying Agents A variety of fatty acids (e.g., stearic acid), fatty alcohols (e.g., stearyl or cetyl alcohol), and fatty esters (e.g., glyceryl monostearate) serve to stabilize emulsions through their ability to thicken the emulsion. Because these agents have only weak emulsifying properties, they are always use in combination with other emulsifiers Be stable . Be compatible with other ingredients . Be non – toxic . Bossess little odor , taste , or color . Not interfere with the stability of efficacy of the active agent . Emulsion Stability: The instability of pharmaceutical emulsions may be classified as the following: a) Flocculation and creaming b) coalescence and breaking c) Phase inversion d) Miscellaneous physical and chemical change Emulsion Stability Flocculation and creaming: ¡ Flocculation - The small spheres of oil join together to form clumps or flocs which rise or settle in the emulsion more rapidly than individual particles. ¡ Creaming - it is a concentration of the floccules of the internal phase formed upward or downward layer according to the density of internal phase.