From emulsion to gel: Self-assembly behavior of ionic liquid surfactants in the water–oil interface

Abstract

Oleic acid (OA), behenic acid (BA) and N, N-dimethylcyclohexylamine (DMCHA) are used to synthesize two ionic liquid surfactants (ILSs, D-OA and D-BA) by acid-base reaction. ILSs greatly reduce the surface tension of water, and the critical micelle concentrations (CMC) are 5.86 × 10-4 mol/L and 2.57 × 10-6 mol/L, respectively. Different concentrations of D-OA can form a stable emulsion between water and oil, and the droplet size of emulsion with 3 wt% D-OA is mainly concentrated in 3 ∼ 6 μm, while D-BA is difficult to form a stable emulsion. ILSs with concentration greater than 4 wt% can form interfacial gels at the interface of water and oil, and the interfacial gel is analyzed by Polarizing Microscope (POM), Small Angle X-ray Scattering (SAXS), and Scanning Electron Microscope (SEM), confirming it as a layered liquid crystal, and the interlayer spacing is 120.83 Å, indicating that the interlayer contains a large amount of water and oil. Rheological behavior verifies its layered structure with low stability. The interfacial gel can support water for 2,000 times of ILSs weight for about 20 days, verifying its static loading capacity can reach 3.98 × 104 Pa/m2. The interfacial gel also has a certain temperature response with losing the adhesion and stability at high temperature and recovering the illiquidity at low temperature. Salt makes the interfacial gel compressed and its stability reduced. The ILSs realize the transformation from emulsion to gel by self-assembly behavior, that is, the low-concentration ILSs in water are as a single ion or low-dispersed micelle and oil is dissolved into water to form an emulsion, while the high-concentration ILSs are formed wormlike micelles through self-assembly, with oil and water being dissolved to form an interfacial gel to be used as oil–water partitions or channels.