Formation of aqueous two-phase systems based on p-sulfonatocalix[4]arene and imidazolium bromide and their selectivity toward vanillin and phenylalanine

An aqueous two-phase system (ATPS) has been considered a green separation system. How to improve the separation efficiency and selectivity of ATPS has always been a widely concerned issue. However, so far, the construction of selective ATPS still remains a challenging task. Calixarenes are the third generation of supramolecular compounds. They exhibit significant advantages in recognizing small molecules. In the present study, calixarene was used for the first time to prepare selective ATPSs, which are sulfonatocalix[4]arene (SC[4])/1-decyl (or hexyl)-3-methylimidazolium bromide ([C n mim]Br)/trisodium citrate (Na 3 C 6 H 5 O 7 ) ATPS and SC[4]/1-decyl-3-methylimidazolium bromide ([C 10 mim]Br) ATPS. Ten aromatic molecules, including theobromine, syringic acid, vanillin, vanillic acid, theophylline, cephalexin, L-tyrosine, L-phenylalanine, S-mandelic acid, and L-tryptophan, were selected as model extracts to evaluate the selectivity of these SC[4]-based ATPSs. The results showed that among these 10 extracts, the SC[4]/[C n mim]Br/Na 3 C 6 H 5 O 7 ATPS has high selectivity toward vanillin, and the SC[4]/[C 10 mim]Br ATPS has high selectivity toward L-phenylalanine. Specifically, in the SC[4]/[C n mim]Br/Na 3 C 6 H 5 O 7 ATPS, the partition coefficient of vanillin can reach 315.7, while the partition coefficients of the other 9 extracts are around 2.4–23.1 In the SC[4]/[C 10 mim]Br ATPS, the partition coefficient of L-phenylalanine is much higher than 1 (i.e., 4.38–5.55), while the partition coefficients of the other extracts are below or around 1. The 1 H NMR spectra and response surface plots based on the Box-Behnken design further demonstrate the role of [C n mim]Br-SC[4] complex in the selectivity of these ATPSs.