Formation of Novel Aqueous Two-Phase Systems with Piperazinium-Based Ionic Liquids and Anionic Surfactants: Phase Behavior and Microstructure

Two novel aqueous two-phase systems (ATPSs) involving protic piperazinium-based ionic liquids (ILs) and anionic surfactants were found in the 1-ethylpiperazinium tetrafluoroborate ([C(2)pi][BF4]) + sodium dodecyl sulfate (SDS) + H2O system and the 1-phenylpiperazinium tetrafluoroborate ([Phpi][BF4]) + sodium dodecyl benzenesulfonate (SDBS) + H2O system. The ATPS regions in the ternary phase diagrams were determined, and the compositions and the microstructures of the conjugated phases were analyzed by UV-vis, H-1 NMR, DLS, and cryogenic TEM measurements. The results demonstrate size-enhanced micelles for both ATPSs. The strong electrostatic interactions between the cationic moiety of IL and the anionic surfactant play a very important role in the assembly of the large aggregates, and the cation-pi interactions are involved in the [Phpi] [BF4] + SDBS + H2O ATPS. In addition, the small cationic moiety of [C(2)pi][BF4] can be packed in the micelles, while the larger hydrophilic cationic moiety of [Phpi][BF4] makes it difficult to get into the micelles, leading to the different size enhancement effects. The driving force of phase separation is the formation and distribution of the large aggregates in the aqueous solutions. This work presents a novel nonaromatic ATPS formed by a piperazinium-based IL and an anionic surfactant, in which considerable size enhancement of aggregates takes place without the assistance of aromaticity in contrast to the other aromatic ATPSs.