Microemulsion system formed with new piperazinium-based surface-active ionic liquid

The intensive use of ionic liquid (IL) in various researches and applications induces the imperative need of specific functional ILs. Present work introduces a novel piperazinium-based surface-active IL (SAIL) which shows great potential for the preparation of microemulsion (ME), namely 1-(2-hydroxyethyl) piperazinium laurate ([HEP][C11CO2]). Important fundamental physicochemical properties have been investigated, revealing higher surface activity for the SAIL than common surfactants. [HEP][C11CO2] is able to solubilize different oil components and water with short-chain alcohol as cosurfactant, forming stable ME in a wide range of ratio of water to oil, and a broad microemulsion region covering over 50 % of the pseudo-ternary phase diagram can be found. The structural transition of water-in-oil to bicontinuous occurs at the water content of 0.60 to 0.65 for the SAIL/1-butanol/n-heptane/water systems at the ratios of SAIL to 1-butanol of 1:1 and 1:2, as confirmed by electrical conductivity measurements. Moreover, the solubilization of oil and water was investigated in the presence of NaCl, KCl or MgCl2, and the optimal salinity was determined. While Mg2+ induces formation of emulsion probably due to the poor solubility of Mg2+/[C11CO2]- complexes, the ME system is much more stable with the presence of monovalent ions like Na+ and K+. For the NaCl-contained system, the salinity window of the bicontinuous phase changes from 0.030 to 0.025 and 0.020 when the temperature rises from 15 degrees C to 25 degrees C and 35 degrees C, which mainly results from the enhanced hydrophobic interactions and the weakened electrostatic interactions. Present studies elucidate that [HEP][C11CO2] is feasible to form various microemulsion systems which may have extensive potential in further applications like size-controlled syntheses and oil recovery. (c) 2022 Elsevier B.V. All rights reserved.