Ultralong-Chain Ionic Liquid Surfactants Derived from Natural Erucic Acid

Abstract

Ionic liquid surfactants (ILSs) have shown increasing promise for substituting the traditional amphiphiles because of their flexible tunability and advantageous physicochemical properties. However, the improvement of common ILSs in terms of self-assembly ability and thickening ability remains a significant challenge. Herein, two novel ultralong-chain ILSs (ULILSs), ErBCho (choline cis-4-erucylamidobenzoate) and ErBBTA (benzyltrimethylammonium cis-4-erucylamidobenzoate), were designed and synthesized from natural unsaturated fatty acid (erucic acid). Their structures were characterized via 1H NMR, 13C NMR, and electrospray ionization-mass spectrometry techniques, and physicochemical properties were evaluated by surface tension, polarized optical microscopy, and rheology. The insertion of a phenyl ring between the charged headgroup and unsaturated C22-tail significantly enhanced the hydrophobicity of ULILSs, but they retained the desirable characteristics of ILSs (low melting temperature) and excellent water-solubility (Krafft temperature <4 °C), for application in low-temperature water. Compared with counterparts without a phenyl ring, the critical micelle concentration (cmc) of the presented ULILSs was markedly decreased by at least an order of magnitude (particularly cmc of ErBBTA 3.5 μM), and interfacial activity was higher (larger pC20). In particular, the current ULILSs were able to form wormlike micelles on their own and then thicken the solution at relatively low overlapping concentrations (1.68–5.71 mM), without any additives and regardless of counterions. More importantly, viscoelastic fluids formed by the current ULILSs (especially ErBBTA) were not only resistant to high temperature and cycled shear but also exhibited excellent sand-carrying and oil-induced gel-breaking performances. Such ULILSs have shown promise to replace those common surfactants in hydro-fracturing and other applications.