Emulsion 1 (Lec 5) - PDF

Coarse Dispersions Emulsions Lecture 5  When two immiscible liquids are mechanically agitated, both phases initially tend to form droplets. When the agitation is stopped, the droplets quickly coalesce, and the two liquids separate. The lifetime of the droplets is increased if an emulsifier...

Coarse Dispersions Emulsions Lecture 5  When two immiscible liquids are mechanically agitated, both phases initially tend to form droplets. When the agitation is stopped, the droplets quickly coalesce, and the two liquids separate. The lifetime of the droplets is increased if an emulsifier is added to the two immiscible liquids. Usually, only one phase persists in droplet form for a prolonged period of time. This phase is called the internal (disperse or discontinuous) phase, and it is surrounded by an external (continuous) phase. Emulsions can be defined as: “Thermodynamically unstable system consisting of two or more immiscible liquids, one of which is dispersed as globules of diameters between 1 and 25 µm (the dispersed or internal phase) in the dispersion media (the continuous or external phase) and stabilized by presence of an emulsifying agent.”  Types of Emulsions Oil-in-water (O/W) O/W emulsion is formed when the oil droplets are dispersed throughout the aqueous phase. It is used for oral or IV or IM administration (globule size less than 1µm to avoid emboli formation). Water-in-oil (W/O) A system in which the water is dispersed throughout the oil. It is used for intramuscular injections for a depot therapy and should not never be used for oral or IV therapy. Multiple emulsion. "water-in-oil-in-water" (W/O/W) emulsion: a small water droplet can be enclosed in a larger oil droplet which is itself dispersed in water. The alternative O/W/O is also possible. Microemulsion A quite transparent preparation in which the dispersed globules are of colloidal dimension (1 nm to 1000 nm diameter) Pharmaceutical application of emulsions: It is suitable for oral, rectal and topical administration of oils and oil-soluble drugs. Emulsions are used to enhance palatability of oils when given orally by disguising taste and oiliness (O/W emulsion). Theycan increase absorption of oils and oil-soluble drugs through intestinal walls. For intramuscular injections of some water-soluble drugs to provide slow release. Total parenteral nutrition makes use of an emulsion formulation. Sterile oil-in- water emulsions are used to deliver oily nutrients intravenously to patients, using non- toxic emulsifying agents, such as lecithin (O/W emulsifier).  If the emulsion is for oral or intravenous administration, it will always be O/W. Intramuscular injections are O/W and may be W/O for a depot therapy. Emulsions for external use are O/W or W/O, the therapeutic use, texture and patient acceptability will be taken into account. The O/W emulsions are less greasy, easily washed off the skin and more cosmetically acceptable than W/O emulsions. They have an emollient effect, which hydrates upper layers of skin.  Tests for identification of emulsion type Tests O/W W/O Are miscible with oil 1 Miscibility Are miscible with water (its continuous phase) (its continuous phase) tests but not with oil. but not with water. By incorporating of an oil-soluble dye e.g. scarlet The same test shows red and place a drop on a microscope slide, cover coloureless globules with a cover-slip, and examine under a microscope. against a red The dispersed globules will be red, and the background. 2 “ground” will appear colorless. Staining tests By incorporating of an water-soluble dye e.g. The same test shows methylene blue and examine under a microscope. blue globules on a The dispersed globules will be colorless against a colorless background. blue background O/W W/O  Water, being the continuous phase, will Oil is the conduct electricity throughout the system. continuous phase Electrical Two metal electrodes, when placed in will not conduct 3 conductivity such a preparation with a battery and suitable electricity. The lamp tests light source connected in series will cause will not glow. the lamp to glow.  Phase-volume ratio It is the volume of the internal phase compared with that of the external phase. The most stable emulsions have an internal phase occupying up to 40 to 60 % of the emulsion. The relative volume of internal and external phases of an emulsion is important, regardless of the type of emulsifier used. As the internal concentration of an emulsion increases, so does the viscosity of the emulsion to a certain point, after which the viscosity decreases sharply. At this point, the emulsion has undergone inversion, that is, it has changed from an o/w emulsion to a w/o or vice versa. In practice, emulsions may be prepared without inversion with as much as about 75% of the volume of the product being internal phase. mayonnaise Preparation of emulsions To prepare emulsions, using pestle and mortar a thick (primary) emulsion must be made first. The quantities of water, oil and emulsifying agent (usually gum acacia) for primary emulsions have been obtained by experience and are given in the following table: Quantities for primary Type of oil Examples emulsion (Parts) oil water gum Almond oil, arachis oil, castor oil, cod-liver Fixed 4 2 1 oil Mineral Liquid paraffin 3 2 1 Volatile Turpentine oil, cinnamon oil, peppermint oil 2 2 1 Oleo-resin Male fern extract 1 2 1 Methods of emulsion preparation 1- dry gum method Emulsifier is triturated with the oil in perfectly dry porcelain mortar (complete distribution) 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 Methods of emulsion preparation 2. Wet Gum Method Once the primary After adding all Oil is added emulsion has Triturate gum of the oil, been formed slowly in thoroughly mixed with water in a portions the remaining mortar to form a for several quantity of water mixture is minute to form mucilage triturated is added to make the primary the final emulsion emulsion. Methods of emulsion preparation 3.Bottle or Forbes Bottle Method It is used in preparation for volatile oils or oil with low viscosity. This method is not suited for viscous oils because they cannot be thoroughly agitated in the bottle when mixed with the emulsifying agent. gum + oil (dry bottle) Shake Water remaining quantity of water is added to (volume equal to oil) is make the final added in portions with emulsion vigorous shaking to form primary emulsion Additional and special cases in formulation of emulsions Emulsions containing more than one oily liquid: When two or more oily liquids are present, the quantity of acacia required for each is calculated, and the sum of these quantities is used for the emulsion. Alternatively, each oil may be emulsified separately before mixing. Emulsions containing water-soluble substances: These substances such as salts, syrups, glycerin. They are added in a dilute state as possible because some substances have "de-emulsifying" properties, i.e. they might destabilize the emulsion if added in concentrated solution. Emulsions containing oil-soluble substances: Substances such as salol and naphthol are oil-soluble substances commonly required in emulsions. There should be prepared with 50 % more gum than required for other emulsions. The oil-soluble substances are dissolved in the oil before preparing the primary emulsion. Emulsions containing substances insoluble in either oil or water: These substances must be finely powdered in a mortar, and mixed with the acacia required for the primary emulsion. The oil is then added, and the primary emulsion prepared as usual.  Theories of emulsification the plastic or the surface the oriented- interfacial film tension theory wedge theory theory The surface tension theory: In which, all liquids have a tendency to assume a shape having the minimal surface area exposed. For a drop of a liquid, that shape is the sphere. A liquid – spherical in shape – possesses internal forces that tend to promote association of the molecules to resist distortion of the sphere. If two or more drops of the same liquid come into contact with one another, the tendency is for them to coalesce, making one larger drop having a smaller surface area than the individual drops.  When the liquid is in contact with a second liquid in which it is insoluble and immiscible, the force causing each liquid to resist breaking up into smaller particles is called interfacial tension. Substances that reduce this resistance encourage a liquid to break up into smaller drops or particles. These tension-lowering substances are surface- active (surfactant) or wetting agents. According to the surface tension theory of emulsification: the use of these substances as emulsifiers and stabilizers lowers the interfacial tension of the two immiscible liquids, reducing the repellent force between the liquids and diminishing each liquid’s attraction for its own molecules.  Thus, the surface-active agents facilitate the breaking up of large globules into smaller ones, which then have a lesser tendency to reunite or coalesce. The oriented-wedge theory: It assumes mono-molecular layers of emulsifying agent curved around a droplet of the internal phase of the emulsion. The theory is based on the presumption that certain emulsifying agents orient themselves about and within a liquid in a manner reflective of their solubility in that particular liquid. In a system containing two immiscible liquids, the emulsifying agent is soluble in one of the phases and is embedded more deeply in that phase than the other.  Generally, an emulsifying agent having a greater hydrophilic than hydrophobic character will promote an O/W emulsion, and a W/O emulsion results from use of an emulsifying agent that is more hydrophobic than hydrophilic. The phase in which the emulsifying agent is more soluble will become the continuous or external phase of the emulsion. The plastic or interfacial film theory: It places the emulsifying agent at the interface between the oil and water, surrounding the droplets of the internal phase as a thin layer of film adsorbed on the surface of the drops.  The film prevents contact and coalescing of the dispersed phase. Enough of the film-forming material must be available to coat the entire surface of each drop of the internal phase. Here again, the formation of an o/w or a w/o emulsion depends on the degree of solubility of the agent in the two phases. It is unlikely that a single theory of emulsification can explain the means by which the many and varied emulsifiers promote emulsion formation and stability. It is more than likely that even within a given emulsion system more than one of the above theories play a part.