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Sodas: Oil in Water emulsion Milk: Oil in Water emulsion Balm: Water in oil emulsion Mayonnaise: Oil in Water emulsion Emulsions Emulsion suitable for intravenous injection. Emulsion : Emulsion Definition Applications Classification Theory of emulsification Stability of emulsion Preservation of emulsion Emulsion preparation Nascent method Dry gum Wet gum Incorporation of drugs into emulsion Microemulsion Emulsion : Emulsion A’. Two immiscible liquids, not emulsified; B’. An emulsion of Phase B dispersed in Phase A; C’. The unstable emulsion progressively separates; D’. The (purple) surfactant positions itself on the interfaces between Phase A and Phase B, stabilizing the emulsion An emulsion is a thermodynamically unstable system consisting of at least two immiscible liquid phases, one of which is dispersed as globules in the other liquid phase, stabilized by the presence of an emulsifying agent. Pharmaceutical application of emulsions : Pharmaceutical application of emulsions O/W emulsion is convenient for oral dosing To cover unpleasant taste To increase oral absorption I.V. O/W, if oral o/w not possible (RES uptake) External use (topical cream) A broad-spectrum antifungal agent administered orally to treat a variety of fungal infections. Emulsion types : Emulsion types Types Oil-in-water (o/w) Water-in-oil (w/o) Oil-in-water-in-oil (o/w/o) Water-in-oil-in-water (w/o/w) Determination of o/w or w/o Water soluble dye (e.g., methylene blue) Dilution of emulsions Conduction of current Slide 6: Emulsions encountered in everyday life! Stability of emulsions may be engineered to vary from seconds to years depending on application Theory of emulsification : Theory of emulsification Change from A to B will significantly increase of the surface area of phase.e.g., if 1 cm3 of mineral oil is dispersed into globules having diameter of 0.01 mm in 1 cm3 of water, how much will be the surface area increased. The surface area will become 600 m2 (greater than a basketball court); the surface free energy will increase by 8 calories. Therefore, emulsions are thermodynamically unstable, and the droplets have the tendency to coalesce. Emulsifying agents are needed to decrease the surface tension and to stabilize the droplets. Classification of emulsifying agents : Classification of emulsifying agents Surface active agents (monomolecular film) Hydrophilic colloids (multimolecular film) Finely divided solid particles (Particulate film) Slide 9: Stable suspensions of liquids constituting the dispersed phase, in an immiscible liquid constituting the continuous phase is brought about using emulsifying agents such as surfactants Surfactants must exhibit the following characteristics to be effective as emulsifiers - Good surface activity - Should be able to form a condensed interfacial film - Diffusion rates to interface comparable to emulsion forming time Emulsifying Agents Slide 10: Surfactants Anionic – Sodium stearate, Potassium laurate Sodium dodecyl sulfate, Sodium sulfosuccinate Nonionic – Polyglycol, Fatty acid esters, Lecithin Cationic – Quaternary ammonium salts, Amine hydrochlorides Solids Finely divided solids with amphiphilic properties such as soot, silica and clay, may also act as emulsifying agents (Pickering Emulsions: Attribute of high stability) Common Emulsifying Agents Surfactant Packing Parameter : Conceptual framework that relates molecular parameters (head group area, chain length and hydrophobic tail volume) and intensive variables (temperature, ionic strength etc.) to surfactant microstructures Critical Packing Parameter / Packing Parameter v: Volume of hydrocarbon core l: hydrocarbon chain length a0: Effective head group area Surfactant Packing Parameter Surfactant Packing Parameter : v: Volume of hydrocarbon chain= 0.027(nc + nMethyl) l: hydrocarbon chain length= 0.15 + 0.127nc Where nc = number of carbon atoms without the methyl group nMethyl = number of methyl groups ao: Effective head group area: difficult to calculate. Surfactant Packing Parameter Surfactant Packing Parameter : Surfactant Packing Parameter Packing Parameter is inversely related to HLB : Packing Parameter is inversely related to HLB Mid Point of Packing Parameter P = 1 analogous to HLB 10 At P = 1/ HLB = 10, surfactant has equal affinity for oil and water Monomolecular adsorption : Monomolecular adsorption Rule of Bancroft: The type of the emulsion is a function of the relative solubility of the surfactant, the phase in which it is more soluble being the continuous phase. Multimolecular adsorption and film formation : Multimolecular adsorption and film formation 1. Hydrated lyophilic colloids (hydrocolloids) providing a protective sheath around the droplets imparting a charge to the dispersed droplets (so that they repel each other) swelling to increase the viscosity of the system (so that droplets are less likely to merge) 2. Classification of hydrocolloids vegetable derivatives, e.g., acacia, tragacanth, agar, pectin, lecithin animal derivatives, e.g., gelatin, lanolin, cholesterol Semi-synthetic agents, e.g., methylcellulose, carboxymethylcellulose Synthetic agents, e.g., carbomers (PEG and acrylic acid) Solid particle adsorption : Solid particle adsorption Description: Finely divided solid particles that are wetted to some degree by both oil and water can act as emulsifying agents. This results from their being concentrated at the interface, where they produce a particulate film around the dispersed droplets to prevent coalescence. Example of agents: bentonite (Al2O3.4SiO2.H2O), veegum (Magnesium Aluminum Silicate), hectorite, magnesium hydroxide, aluminum hydroxide and magnesium trisilicate 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. Auxiliary emulsifying agents : Auxiliary emulsifying agents Auxiliary (secondary) emulsifying agents include those compounds that are normally incapable themselves of forming stable emulsion. Their main values lies in their ability to function as thickening agents and thereby help stabilize the emulsion. Physical stability of emulsion : Physical stability of emulsion Creaming Creaming is the upward movement of dispersed droplets of emulsion relative to the continuous phase (due to the density difference between two phases) Stoke’s law: dx/dt = d2 (i-e)g/18h dx/dt = rate of setting D = diameter of particles = density of particles and medium g = gravitational constant h = viscosity of medium Physical stability of emulsion : Physical stability of emulsion Breaking, coalescence, aggregation Breaking is the destroying of the film surrounding the particles. Coalescence is the process by which emulsified particles merge with each to form large particles. Aggregation: dispersed particles come together but do not fuse. The major fact preventing coalescence is the mechanical strength of the interfacial film. Physical stability of emulsion : Physical stability of emulsion Phase inversion An emulsion is said to invert when it changes from an o/w to w/o or vice versa. Addition of electrolyte Addition of CaCl2 into o/w emulsion formed by sodium stearate can be inverted to w/o. Changing the phase:volume ratio Preservation of emulsions : Preservation of emulsions Growth of microorganisms in emulsions Preservatives should be in aqueous phase. Preservatives should be in unionized state to penetrate the bacteria Preservatives must not bind to other components of the emulsion Methods of emulsion preparation : Methods of emulsion preparation Continental or dry gum method English of wet gum method Bottle or Forbes bottle method Auxiliary method In situ soap method Calcium soaps: w/o emulsions contain oils such as oleic acid, in combination with lime water (calcium hydroxide solution, USP). Prepared by mixing equal volumes of oil and lime water. Nascent soap : Nascent soap Oil phase: olive oil/oleic acid; olive oil may be replaced by other oils, but oleic acid must be added Lime water: Ca(OH)2 should be freshly prepared. Equal volume of oil and lime water The emulsion formed is w/o or o/w? Method of preparation: Bottle method: Mortar method: when the formulation contains solid insoluble such as zinc oxide and calamine. Dry gum method (4:2:1 method) : Dry gum method (4:2:1 method) The continental method is used to prepare the initial or primary emulsion from oil, water, and a hydrocolloid or "gum" type emulsifier (usually acacia). The primary emulsion, or emulsion nucleus, is formed from 4 parts oil, 2 parts water, and 1 part emulsifier. The 4 parts oil and 1 part emulsifier represent their total amounts for the final emulsion. In a mortar, the 1 part gum (e.g., acacia) is levigated with the 4 parts oil until the powder is thoroughly wetted; then the 2 parts water are added all at once, and the mixture is vigorously and continually triturated until the primary emulsion formed is creamy white. Additional water or aqueous solutions may be incorporated after the primary emulsion is formed. Solid substances (e.g., active ingredients, preservatives, color, flavors) are generally dissolved and added as a solution to the primary emulsion. Oil soluble substance, in small amounts, may be incorporated directly into the primary emulsion. Any substance which might reduce the physical stability of the emulsion, such as alcohol (which may precipitate the gum) should be added as near to the end of the process as possible to avoid breaking the emulsion. When all agents have been incorporated, the emulsion should be transferred to a calibrated vessel, brought to final volume with water, then homogenized or blended to ensure uniform distribution of ingredients. Preparing emulsion by dry gum method : Preparing emulsion by dry gum method Cod liver oil 50 mL Acacia 12.5 g Syrup 10 mL Flavor oil 0.4 mL Purified water, qs ad 100 mL Accurately weigh or measure each ingredient Place cod liver oil in dry mortar Add acacia and give it a very quick mix Add 25 mL of water and immediately triturate to form the thick, white, homogenous primary emulsion Add the flavor and mix Add syrup and mix Add sufficient water to total 100 mL Wet gum method : Wet gum method In this method, the proportions of oil, water, and emulsifier are the same (4:2:1), but the order and techniques of mixing are different. The 1 part gum is triturated with 2 parts water to form a mucilage; then the 4 parts oil is added slowly, in portions, while triturating. After all the oil is added, the mixture is triturated for several minutes to form the primary emulsion. Then other ingredients may be added as in the continental method. Generally speaking, the English method is more difficult to perform successfully, especially with more viscous oils, but may result in a more stable emulsion. Bottle method : Bottle method This method may be used to prepare emulsions of volatile oils, or oleaginous substances of very low viscosities. This method is a variation of the dry gum method. One part powdered acacia (or other gum) is placed in a dry bottle and four parts oil are added. The bottle is capped and thoroughly shaken. To this, the required volume of water is added all at once, and the mixture is shaken thoroughly until the primary emulsion forms. It is important to minimize the initial amount of time the gum and oil are mixed. The gum will tend to imbibe the oil, and will become more waterproof. Auxiliary method : Auxiliary method An emulsion prepared by other methods can also usually be improved by passing it through a hand homogenizer, which forces the emulsion through a very small orifice, reducing the dispersed droplet size to about 5 microns or less. Incorporation of medicinal agents : Incorporation of medicinal agents Addition of drug during emulsion formation Addition of drugs to a preformed emulsion 1. Addition of oleaginous materials into a w/o emulsion 2. Addition of oleaginous materials to an o/w emulsion 3. Addition of water soluble materials to a w/o emulsion 4. Addition of water soluble materials to an o/w emulsion Microemulsion : Microemulsion Microemulsions are thermodynamically stable, optically transparent, isotropic mixtures of a biophasic oil-water system stabilized with surfactants. Pharmaceutical applications of microemulsions : Pharmaceutical applications of microemulsions Increase bioavailability of drugs poorly soluble in water Topical drug delivery systems Preparation of nanoparticles from microemulsion precursors : Preparation of nanoparticles from microemulsion precursors Slide 37: Bancroft's rule Emulsion type depends more on the nature of the emulsifying agent than on the relative proportions of oil or water present or the methodology of preparing emulsion. The phase in which an emulsifier is more soluble constitutes the continuous phase In O/W emulsions – emulsifying agents are more soluble in water than in oil (High HLB surfactants). In W/O emulsions – emulsifying agents are more soluble in oil than in water (Low HLB surfactants). W/O vs. O/W emulsions Bancroft’s Rule: Relation to HLB & CPP of Surfactant : Bancroft’s Rule: Relation to HLB & CPP of Surfactant Surfactant more soluble in water (CPP < 1, HLB > 10) O/W emulsion Surfactant more soluble in oil (CPP > 1, HLB < 10) W/O emulsion Bancroft’s Rule: Relation to HLB & CPP of Surfactant : Bancroft’s Rule: Relation to HLB & CPP of Surfactant Surfactant more soluble in water (CPP < 1, HLB > 10) O/W emulsion Surfactant more soluble in oil (CPP > 1, HLB < 10) W/O emulsion Slide 40: Based on the Bancroft’s rule, many emulsion properties are governed by the properties of the continuous phase Dye test Dilution test Electrical conductivity measurements Refractive index measurement Filter paper test Tests for Emulsion Type (W/O or O/W emulsions) Slide 41: Rate of coalescence – measure of emulsion stability. It depends on: Physical nature of the interfacial surfactant film For Mechanical stability, surfactant films are characterized by strong lateral intermolecular forces and high elasticity (Analogous to stable foam bubbles) Mixed surfactant system preferred over single surfactant. (Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions) NaCl added to increase stability (electrostatic screening) Emulsions are Kinetically Stable! Slide 42: (b) Electrical or steric barrier Significant only in O/W emulsions. In case of non-ionic emulsifying agents, charge may arise due to (i) adsorption of ions from the aqueous phase or (ii) contact charging (phase with higher dielectric constant is charged positively) No correlation between droplet charge and emulsion stability in W/O emulsions Steric barrier – dehydration and change in hydrocarbon chain conformation. Emulsions are Kinetically Stable! Slide 43: (c) Viscosity of the continuous phase Higher viscosity reduces the diffusion coefficient Stoke-Einstein’s Equation This results in reduced frequency of collision and therefore lower coalescence. Viscosity may be increased by adding natural or synthetic thickening agents. Further, as the no. of droplets (many emulsion are more stable in concentrated form than when diluted.) Emulsions are Kinetically Stable! Slide 44: (d) Size distribution of droplets Emulsion with a fairly uniform size distribution is more stable than with the same average droplet size but having a wider size distribution (e) Phase volume ratio As volume of dispersed phase stability of emulsion (eventually phase inversion can occur) (f) Temperature Temperature , usually emulsion stability Temp affects – Interfacial tension, D, solubility of surfactant, Brownian motion, viscosity of liquid, phases of interfacial film. Emulsions are Kinetically Stable! Phase Inversion in Emulsions : Phase Inversion in Emulsions Bancroft's rule Emulsion type depends more on the nature of the emulsifying agent than on the relative proportions of oil or water present or the methodology of preparing emulsion. Based on the Bancroft’s rule, it is possible to change an emulsion from O/W type to W/O type by inducing changes in surfactant HLB / CPP. In other words... Phase Inversion May be Induced. Consider systems of 2 immiscible and 1 miscible pairs of liquids : Consider systems of 2 immiscible and 1 miscible pairs of liquids Acetic acid & water are miscible in all proportions Benzene & water - partly miscible, acetic acid & water - partly miscible Acetic acid added to a mixture of benzene & water, preferentially partitions into water (slope of tie line) Surfactant and water are miscible in all proportions Oil and water - partly miscible, surfactant and oil - partly miscible Tie line Surfactant added to a mixture of oil & water, preferentially partitions into water (slope of tie line) Increase T: At a specific temperature, surfactant becomes Oil Soluble across all proportions, Acetic Acid does not! : Increase T: At a specific temperature, surfactant becomes Oil Soluble across all proportions, Acetic Acid does not! Increase in T, P Increase in T, Electrolyte Why does Phase Inversion Take Place for system with Surfactants? : Why does Phase Inversion Take Place for system with Surfactants? O/W emulsion W/O emulsion Temperature for Non Ionics, Salting out electrolytes for ionics Bancroft’s Rule: Manifested in Response of Surfactant Solubility : Bancroft’s Rule: Manifested in Response of Surfactant Solubility O/W emulsion W/O emulsion Temperature for Non Ionics, Salting out electrolytes for ionics Temperature and electrolytes disrupt the water molecules around non-ionic and ionic surfactants respectively, altering surfactant solubility in the process – Also reflected by change in curvature of the interface Slide 50: O/W W/O The order of addition of the phases W O + emulsifier W/O O W + emulsifier O/W Nature of emulsifier Making the emulsifier more oil soluble tends to produce a W/O emulsion and vice versa. Phase volume ratio Oil/Water ratio W/O emulsion and vice versa Inversion of Emulsions (Phase inversion) Slide 51: 4. Temperature of the system Temperature of O/W (polyoxyethylenated nonionic surfactant) makes the emulsifier more hydrophobic and the emulsion may invert to W/O. 5. Addition of electrolytes and other additives. Strong electrolytes to O/W (stabilized by ionic surfactants) may invert to W/O Example. Inversion of O/W emulsion (stabilized by sodium cetyl sulfate and cholesterol) to a W/O type upon addition of polyvalent Ca. Inversion of Emulsions (Phase inversion) Slide 52: Droplets larger than 1 mm may settle preferentially to the top or the bottom under gravitational forces. Creaming is an instability but not as serious as coalescence or breaking of emulsion Probability of creaming can be reduced if a - droplet radius, Δρ - density difference, g - gravitational constant, H - height of the vessel, Creaming can be prevented by homogenization. Also by reducing Δρ, creaming may be prevented. This is achieved by producing a polyphase emulsion Creaming of Emulsions Methods of Destabilizing Emulsions : Methods of Destabilizing Emulsions 1. Physical methods Centrifuging Filtration – media pores preferentially wetted by the continuous phase Gently shaking or stirring Low intensity ultrasonic vibrations 2. Heating Heating to ~ 700C will rapidly break most emulsions. Methods of Destabilizing Emulsions : Electrical methods Most widely used on large scale 20 kV results in coalescence of entrained water droplets (W/O) e.g. in oil field emulsions and jet fuels. (mechanism – deformation of water drops into long streamers) For O/W, electrophoretic migration of charged groups to one of the electrodes. Ex. Removing traces of lubricating oil emulsified in condensed water. Methods of Destabilizing Emulsions Selection of Emulsifiers : Selection of Emulsifiers Correlation between chemical structure of surfactants and their emulsifying power is complicated because (i) Both phases oil and water are of variable compositions. (ii) Surfactant conc. determines emulsifier power as well as the type of emulsion. Basic requirements: Good surface activity Ability to form a condensed interfacial film Appropriate diffusion rate (to interface) Slide 56: Type of emulsion determined by the phase in which emulsifier is placed. Emulsifying agents that are preferentially oil soluble form W/O emulsions and vice versa. More polar the oil phase, the more hydrophilic the emulsifier should be. More non-polar the oil phase more lipophilic the emulsifier should be. General Guidelines: Slide 57: HLB method – HLB indicative of emulsification behavior. HLB 3-6 for W/O 8-18 for O/W HLB no. of a surfactant depend on which phase of the final emulsion it will become. Limitation – does not take into account the effect of temperature. General Guidelines Slide 58: 2. PIT method – At phase inversion temperature, the hydrophilic and lipophilic tendencies are balanced. Phase inversion temperature of an emulsion is determined using equal amounts of oil and aqueous phase + 3-5% surfactant. For O/W emulsion, emulsifier should yield PIT of 20-600C higher than the storage temperature. For W/O emulsion, PIT of 10-400C lower than the storage temperature is desired. General Guidelines Slide 59: Cohesive energy ratio (CER) method Involves matching HLB’s of oil and emulsifying agents; also molecular volumes, shapes and chemical nature. Limitation – necessary information is available only for a limited no. of compounds. General Guidelines You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.