Molecular association in aqueous mixtures of a phosphonium-based biamphiphilic ionic liquid with ionic surfactants and its effect on foam stabilization

Biamphiphilic ionic liquids (BAILs) contain elongated alkyl chains in both ions, potentially resulting in enhanced surface activity and molecular organization in aqueous mixtures. The formation of mesoscopic structures allows combining the benefits of organic molten salts with their localized dispersion properties. The introduction of an additional component (e.g., a common ionic surfactant) can serve as a trigger for morphological changes in the mesoscopic and nanoscopic structures. This may offer possibilities for tailoring dispersion morphology based on particular applications. Considering the distinctive characteristics of BAILs as nanostructured solvents and their surface activity, the appropriate question is whether they can be integrated into foam. These foams might retain the properties of ionic liquids while benefiting from the significantly enhanced surface-to-volume ratio afforded by their foam structure. A BAIL including a bulky phosphonium cation and dodecylbenzene sulfonate anion ( [ P 4 , 4 , 4 , 14 ] [DBS]) is used. The surface activity, molecular interactions, mesoscopic organization, and foam stability of BAIL in water as well as in mixtures with CTAB and SDS are examined. The BAIL in water at concentrations as low as 0.01 wt% is organized in molecular bilayers, namely bilamellar vesicles, and presents a high surface activity at an air/water interface. Such activity, along with vesicles, enables the BAIL to effectively produce and stabilize foams. The foam stability increases with increasing BAIL content up to 100 wt%. The addition of an ionic surfactant modifies intermolecular interactions in the BAIL, which is reflected in the observed mesoscopic morphologies. CTAB molecules interact with the BAIL vesicles by either embedding within their bilayers or anchoring on them through the electrostatic interactions with the DBS head group. The stability of the foam is predominantly determined by the BAIL. SDS induces morphological vesicle-micelle transition by inserting its tails into the bilayers and altering their curvature. The micelles comprise all three components. The stability of the foam is controlled by the BAIL, with assistance from micelles. • A phosphonium-based biamphiphilic ionic liquid (BAIL) is capable of stabilizing wet foams in water. • Stabilization mechanisms include BAIL's surface-active nature and the formation of vesicles. • With CTAB, the BAIL governs foam stabilization due to the CTAB trapping in vesicles. • With SDS, vesicle dissolution and the formation of mixed micelles aid in foam stabilization.