Ionic liquid vs tri-block copolymer in a new aqueous biphasic system for extraction of Zn-cholesterol complex

A new aqueous biphasic system (ABS) was designed by mixing aqueous solutions of tri-block copolymer polypropylene glycol-block-polyethylene glycol-block-polypropylene glycol (PPG-PEG-PPG) with a nontoxic ionic liquid tris (2-hydroxyethyl) methyl-ammonium methyl sulfate. Well defined phase diagram validated with Merchuk equation along with suitable tie-lines was obtained by turbidometric titration method at 298 K. The tie-line lengths and slope of tie lines were calculated and the results were validated using Othmer-Tobias and Bancroft equations. The newly designed ABS has been used to perform the extraction studies of cholesterol, zinc and Zn-cholesterol complex at different pHs of the solution. Zn-cholesterol complex was found to be formed at an acidic pH (1.5). The stability constant value of Zn-cholesterol complex was obtained in ethanolic medium, pristine polymer medium and in the ABS extracted polymer rich phase using Benesi-Hildebrand equation. The aggregation number of PPG-PEG-PPG solution obtained after extraction of cholesterol and Zn-cholesterol complex in ABS was found to be greater in comparison to the pristine copolymer solution. Kinetic spectrophotometric studies were also supporting higher stability of Zn-cholesterol complex in the ABS extracted polymer rich phase. Zn-cholesterol complex formation in acidic micellar medium (pH 1.5) also falls in the pH range of normal human stomach and may offer a suitable method of zinc supplementation by absorption via cell membrane that resembles with the polymeric micellar aggregation.