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Liquid emulsion membrane also referred to as LEM, is a technique of downstream process (purification of biomolecules) used for applications involving selective and controlled transport of biochemicals. In a broad sense, LEM may be considered to be an extension of liquid-liquid extraction but they have many differences and advantages. The LEM technique was invented by Li N.N in 1968 for the fractionation of hydrocarbons.

Principle of LEM Liquid emulsion membranes are double emulsion, i.e., water-in-oil systems or oil-in-water systems depending on the method of emulsification. The structure of an LEM basically contains three phases namely, the interior phase which is usually aqueous is the innermost phase, the intermediate phase is an organic solvent which is the membrane phase and the outer or exterior phase is aqueous feed from which the molecule of interest is to be extracted, using LEM. LEM works on the principle of transport of solute from the feed phase to the inner aqueous phase via the organic or the membrane of the LEM, the membrane phase here is thin and permits selective transport of the molecules. The driving force for the movement of molecules in the concentration gradient. Depending upon the nature of the feed or type of species to be extracted, selectivity of extraction can be enhanced by incorporating suitable carriers in the membrane phase. An LEM should satisfy certain requirements to be practically useful. It should provide rapid and selective transport of the desired species, be able to concentrate the products, retain the activities of the encapsulated enzymes or cells close to their native activities, be stable under variations in process conditions and allow easy and inexpensive de-emulsification and recovery of the product.

Important components to form LEM Surfactant: Since the aqueous and organic phases are immiscible, a stable emulsion is possible only by decreasing the interracial tension between the liquids. This is achieved by using surface active agents. Commercial surfactants which are commonly used are complex compounds that have hydrophillic and lipophilic groups, the latter orienting the molecules towards the organic membrane phase. The ratio of the two groups, called the hydrophillic-lipophilic balance (HLB) determines the kind of emulsion obtained Carrier agent Organic membrane phase Inner aqueous or stripping phase

Advantage of LEM Some notable advantages of LEM is their robustness. Particulates and floes do not affect the mass transfer rates, and LEMs can function efficiently even in the presence of live bacterial cells or enzymes. Thus, LEMs can be used directly in fermentation broths and to obtain both separation and subsequent reaction in one stage. The major advantages of LEM are that the separation and concentration of the solute are achieved in a single stage.

Fields of extraction LEMs have diversified into the recovery of heavy metal ions, the removal of contaminants from wastewater, heterogeneous reactions replacing conventional catalytic processes, treatment of disorders in the blood stream and the extraction of products of fermentations. Extraction of organic acids, Extraction of alcohol, Extraction of dyes, Amino acids, Carboxylic acids, Extraction of enzymes