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Flexible films formed by surfactants (potassium laurate, polyoxyethylene sorbitan monooleate). These agents also lower surface tension markedly. They are widely used especially the non-ionic type. Can produce O/W and W/O emulsions depending on the agent chosen. Multimolecular Film: Strong rigid films formed mostly by hydrocolloid (acacia, gelatin) that produces o/w emulsions. Interfacial tension is not reduced to any extent; stability is due mainly to the interfacial film.maintains stability of the emulsionSolid Particles Film: Particles must be wetted by both phases to some extent in order to remain atthe interphase to form a stable film.Note: It is more than likely that even within a given emulsion system more than one of the theories is applicable and plays a part7
EMULSIFYING AGENTS Emulsions are not stable systems; coalescence of droplets will occur. The process of coalescence can be reduced to insignificant levels by the addition of a third component an emulsifying agent or emulsifierThe desirable properties of an emulsifying agent:Be surface active and reduce surface tension to below 10 dynes / cm Be adsorbed quickly around the dispersed drops as a condensed, non-adherent film which will prevent coalescence Impart to the droplets an adequate electric potential so that mutual repulsion occurs 1.depends on the ionic character of the emulsifierIncrease the viscosity of the emulsion Be effective in a reasonably low concentration Pharmaceutically acceptable emulsifiers must also Be chemically stable Be compatible with other ingredients Be non-toxic Possess little odor, taste or color Not interfere with the stability of efficacy of the active agentNot cost prohibitive Classification: Emulsifying agents can be classified according to(1)Chemical Structure (2)Mechanism of action.Classes according to chemical structure are(1) Synthetic (2) Natural (3) Finely dispersed solids (4) Auxiliary agents. Classes according to mechanism of action are8
(1)Interfacial Tension Theory (2) Oriented wedge Theory (3) Interfacial Film TheoryChemical StructuresSynthetic emulsifying agentsNonionic, e.g., sorbitan esters (Spans®), polyoxyethylene derivatives of sorbitan esters (Tweens®), or glyceryl esters Cationic and anionic surfactants are generally limited to usein topical, o/w emulsions stearate is hydrophobic part,stearate indicates emulsifier9