Aqueous two-phase system

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

ABS are formed when either two

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

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.[6] The degree of polymerization of PEG also affects the phase separation and the partitioning of molecules during extraction.^{[citation needed]}

Advantages

ABS is an excellent method to employ for the extraction of

• 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
  proteases
  can degrade them.
• These systems are amenable to scale-ups, from laboratory-sized set-ups 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

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.

Thermodynamic Modeling

Besides the experimental study, it is important to have a good thermodynamic model to describe and predict liquid-liquid equilibrium conditions in engineering and design. To obtain global and reliable parameters for thermodynamic models usually, phase equilibrium data is suitable for this purpose. As there are polymer, electrolyte and water in polymer/salt systems, all different types of interactions should be taken into account. Up to now, several models have been used such as NRTL, Chen-NRTL, Wilson, UNIQUAC, NRTL-NRF and UNIFAC-NRF. It has been shown that, in all cases, the mentioned models were successful in reproducing tie-line data of polymer/salt aqueous two-phase systems. In most of the previous works, excess Gibbs functions have been used for modeling. ^[7]

References