Interaction of Hydrophobic Ionic Liquids with Lipids in Langmuir Monolayers

The interaction of two ionic liquids, trihexyl­(tetradecyl)­phosphonium bis­(trifluoromethylsulfonyl)-imide and trihexyl­(tetradecyl)­phosphonium dicyanamide, [P6 6 6 14]­[Ntf2] and [P6 6 6 14]/[N­(CN)2], with several long-chained lipids with a different net charge at the hydrophilic group, a cationic surfactant, dioctadecyldimethylammonium bromide (DODAB), a zwitterionic phospholipid (DPPC), an anionic phospholipid (DPPG), and the neutral stearic acid (SA), was investigated at the air–water interface using the Langmuir trough technique. The experimental surface pressure–area (π–A) isotherms obtained for selected compositions of each binary system reveal distinct interfacial behavior. The degree and the nature of the IL–lipid interaction strongly depend on the charge distribution in the lipid polar group. Miscibility, or immiscibility, at the monolayer was inferred from the comparison of the experimental π–A isotherm with the theoretical curve calculated for the corresponding ideal mixture based on the π–A isotherms of the pure components. Phase separation and partial miscibility occurred in IL/DODAB and IL/DPPC systems, respectively. In both the IL/DPPG and the IL/SA systems, a new catanionic complex was found. For the IL/SA system, the catanionic complex formation varies with the subphase pH.