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Aqueous two-phase system

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Title: Aqueous two-phase system  
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Subject: Electroextraction, Perstraction, Unit operations, Separation processes, Depth filter
Collection: Separation Processes, Unit Operations
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Aqueous two-phase system

Aqueous biphasic systems (ABS) or aqueous two phase systems (ATPS) are clean alternatives for traditional water solvent extraction systems.

ABS are formed when 2 biotechnological applications as non-denaturing and benign separation media. Recently, they have been used for metal ion separations, environmental remediation, metallurgical applications and as a reaction media.


  • Introduction 1
  • The two phases 2
    • PEG–dextran system 2.1
  • Advantages 3
  • References 4
  • Bibliography 5
  • External links 6


In 1896, Beijerinck first noted an 'incompatibility' in solutions of agar, a water-soluble polymer, with soluble starch or gelatine.[1] Upon mixing, they separated into two immiscible phases. Subsequent investigation has led to the determination of many other aqueous biphasic systems, of which the polyethylene glycol (PEG) - dextran system is the most extensively studied. Other systems that form aqueous biphases are: PEG - sodium carbonate or PEG and phosphates, citrates or sulfates. Aqueous biphasic systems are used during downstream processing mainly in biotechnological and chemical industries.

The two phases

It is a common observation that when chloroform, toluene, hexane etc.) and form a two-phase system. However, in an ABS, both immiscible components are water-based.

The formation of the distinct phases is affected by the pH, temperature and ionic strength of the two components, and separation occurs when the amount of a polymer present exceeds a certain limiting concentration (which is determined by the above factors).

PEG–dextran system

The "upper phase" is formed by the more hydrophobic polyethylene glycol (PEG), which is of lower density than the "lower phase," consisting of the more hydrophilic and denser dextran solution.

Although PEG is inherently denser than water, it occupies the upper layer. This is believed to be due to its solvent 'ordering' properties, which excludes excess water, creating a low density water environment.[2] The degree of polymerisation of PEG also affects the phase separation and the partitioning of molecules during extraction.


ABS is an excellent method to employ for the extraction of proteins/enzymes and other labile biomolecules from crude cell extracts or other mixtures. Most often, this technique is employed in enzyme technology during industrial or laboratory production of enzymes.

  • They provide mild conditions that do not harm or denature unstable/labile biomolecules
  • The interfacial solvent extraction, causing less damage to the molecule to be extracted
  • The polymer layer stabilizes the extracted protein molecules, favouring a higher concentration of the desired protein in one of the layers, resulting in an effective extraction
  • Specialised systems may be developed (by varying factors such as temperature, degree of polymerisation, presence of certain ions etc. ) to favour the enrichment of a specific compound, or class of compounds, into one of the two phases. They are sometimes used simultaneously with ion-exchange resins for better extraction
  • Separation of the phases and the partitioning of the compounds occurs rapidly. This allows the extraction of the desired molecule before endogenous proteases can degrade them.
  • These systems are amenable to scale-ups, from laboratory-sized setups to those that can handle the requirements of industrial production. They may be employed in continuous protein-extraction processes.

Specificity may be further increased by tagging ligands specific to the desired enzyme, onto the polymer. This results in a preferential binding of the enzyme to the polymer, increasing the effectiveness of the extraction.

One major disadvantage, however, is the cost of materials involved, namely high-purity dextrans employed for the purpose. However, other low-cost alternatives such as less refined dextrans, hydroxypropyl starch derivatives and high-salt solutions are also available.


  1. ^ Beijerinck, M.W. (1896).  


  • Albertsson, P-A (1986). Partitioning of Cell Particles and Macromolecules. John Wiley & Sons. 
  • Zaslavsky, Boris (1995). Aqueous Two-Phase Partitioning: Physical Chemistry and Bioanalytical Applications. Marcel Dekker Inc.  

External links

  • London South Bank University on ABS
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