Phase separation in aqueous systems for realizing virtually significant extractions† Arabinda Chakraborty and Kamalika Sen * Polyvinylpyrrolidone (PVP) forms aqueous biphasic systems with tri-block co-polymers, surfactants and salts. The phase diagrams of PVP–salt and PVP–surfactant systems were constructed by a turbidometric titration method at 296 K. Density measurements of the phase forming solutions at this temperature also led to an attractive outcome of the formation of two tri-phasic systems among the components of our study (Na-tartrate + PVP + Triton-X-100 and Na-tartrate + PVP + PPG–PEG–PPG). We also constructed a spreadsheet reflecting the relationship between miscibility and the possibility of phase separation between the components, consisting of the seven phase forming solutions to open up different possibilities of ABS formation. Some completely new biphasic systems were obtained as well as the process of gel formation of a PVA solution in the presence of salt and polymer solutions was also observed. The applications of some of the developed ABSs are described for the recovery of an antibiotic drug, amoxicillin and a catalyst, molybdenum disulphide. Introduction Environmental concerns and sustainability requirements have led to a demanding research eld with unconventional solvents and extraction techniques. Aqueous biphasic systems (ABSs) provide an alternative which is an efficient and clean technique and has a wide spectrum of applications. The mixing of certain mutually incompatible aqueous solutions of polymers, or of a polymer and a salt or two solutions of salts above certain critical thermodynamic conditions leads to the formation of distinctly separable phases. 1–3 Both phases are composed predominantly of water, and each is richer in one of the components over the other. As both the phases are aqueous solutions, they may form a single phase initially upon mixing, which becomes more and more impossible as the concentrations of both the solutes increase and nally result in a separation of the two phases. Depending on the components of the biphase and their compositions several types of mechanisms have been suggested for this in the literature, like salting out, hydrophilic and hydrophobic interactions, etc. 4 In case of ionic liquid–salt and polymer–salt biphasic systems, salting out phenomenon in assistance with hydrophobic interactions have been demon- strated. 5,6 Whilst for a polymer–polymer ABS, the hydrophobic interactions, difference in polarity of the components and hydrogen bond orientation in the two phases are described as the main reasons behind biphase formation. 7 Moreover, if a solute is added to this biphasic system it will distribute unevenly between the two phases depending upon various hydrophobic or electrostatic interactions of the solute with the solvent. Conventional polymer-based ABSs have been largely explored since the 1980's. These are mainly composed of two incompatible polymers or a polymer and a salt having a salt-out inducing effect. In 2003, Rogers and co-workers reported a new alternative, pointing towards the possible creation of ABS by the addition of inorganic salts to aqueous solutions of ionic liquids 5 and since then considerable effort has been given towards the development of hydrophilic ionic liquids for replacing the polymer-rich phases. 8–10 ABS formed from ionic liquids in conjunction with different kosmotropic salts, carbohydrates, amino acids and different polymers along with their applica- tions have been discussed in a very recent review. 4 Due to the inherent aqueous environment, ABSs are recognized as biocompatible systems for cells, cell organelles and biologically active substances in downstream processing. 11,12 It is suitable for the separation and purication of a broad array of biomol- ecules, metal species, dyes, drug molecules and small organic moeities. 13,14 Furthermore, the low cost, high capacity, ease to scale-up, possibility of direct application to fermentation broths and use of environmentally benign green solvents are the obvious advantages. Phase separation in solutions containing polymer mixtures is very familiar phenomenon. 7,15 In aqueous solutions most hydrophilic polymer pairs can form a biphase and the driving force for the demixing process is the enthalpy associated with the interactions of the components, which is opposed by the loss in entropy associated with the segregation of the compo- nents during phase separation. 16 Water, as a universal solvent, Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata 700 009, India. E-mail: kamalchem.roy@gmail.com † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06798c Cite this: RSC Adv. , 2014, 4, 64328 Received 8th July 2014 Accepted 19th November 2014 DOI: 10.1039/c4ra06798c www.rsc.org/advances 64328 | RSC Adv. , 2014, 4, 64328–64335 This journal is © The Royal Society of Chemistry 2014 RSC Advances PAPER