Phase transition of a binary room-temperature ionic liquid composed of bis(pentafluoroethanesulfonyl)amide salts of tetraheptylammonium and N-tetradecylisoquinolinium and its surface properties at the ionic liquid|water interface

A binary room-temperature ionic liquid (RTIL) composed of bis(pentafluoroethanesulfonyl)amide (C₂C₂N^-) salts of tetraheptylammonium (THpA⁺) and N-tetradecylisoquinolinium (C₁₄Iq⁺) undergoes a phase transition upon increasing the mole fraction of C₁₄Iq⁺ (x) in the bulk RTIL. The initial decrease with x of the interfacial tension (γ) at the interface between water (W) and the binary RTIL reaches a break point at x = 0.2 irrespective of the values of the phase-boundary potential. The surface tension at RTIL|air interface and the conductivity of the binary RTIL support that the break point at x = 0.2 at the RTIL|W interface is attributable to the change of the bulk property. However, unlike the micelle formation of a surfactant solution, a further increase in x gives rise to a further change in γ. Whereas the phase transition at x = 0.2 does not depend on the applied potential (E) across the RTIL|W interface, the mode of the change in γ at x > 0.2 strongly depends on E and the apparent deficit of C₁₄Iq⁺ at the interface is more pronounced when E is closer to the point of zero charge.