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. Oil soluble substances in small amounts may be incorporated directly into the primary emulsion. When all agents have been incorporated, the emulsion should be transferred to a calibrated vessel, brought to final volume with water. Active ingredients, preservatives, color, flavors are added as a solution to the primary emulsion. Procedure : Take mortar, 1 part gum 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 triturated until the primary emulsion formed is creamy white and produces a "crackling" sound as it is triturated. This film is strong and resists the coalescence. Their main function as emulsion stabilizers is by making coherent multi-molecular film.They have low effect on the surface tension. e.g., methyl cellulose, hydroxypropyl cellulose and sodium carboxy methyl cellulose Hydrophilic colloids form multimolecular adsorption at the oil/ water interface.They primarily act by increasing the viscosity of the system. They are used for formulating o/w type of emulsions. Includes mainly cellulose derivatives like sodium carboxy methyl cellulose, hydroxyl propyl cellulose and methyl cellulose.At intermediate pH these behave as zwitterions. Below a certain pH, these are cationic while above a defined pH, these are cationic. Ampholytic surfactants: These are the substances whose ionic charge depends on the pH of the system.They are used in the formulation of i/v emulsions and can impart structures to vehicles and interfacial films. The molecules are synthesized as long segments of hydrophilic portions combined with long segments of the hydrophilic portions, with each portion referred to as block. Polyoxyethylene/polyoxypropylene block poymers, also known as poloxamers consist of combined chains of oxyethylene with oxypropylene where the oxyethylene portions imparts hydrophilicity and oxypropylene portion imparts lipophilicity.Polyoxyethylene derivatives of sorbitans fatty acids : They are hydrophilic and give o/w emulsion.They are oil soluble nonionic surfactants.and give w/o emulsions.
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