Molecular origins of nonideality in surface properties of Surfactant-Ionic liquid mixed monolayers

Experimental surface pressure -area (pi-A) isotherms revealed partial miscibility between the cationic surfactant dioctadecyldimethylammonium bromide, DODAB, and the ionic liquid trihexyl(tetradecyl)phosphonium dicya-namide, [P6 6 6 14][N(CN)2]. To address the origins of this nonideal behavior, the interaction of 3:1 mixture of DODAB and [P6 6 6 14][N(CN)2] was studied at the water-vacuum interface at different pressure -area regimes using molecular dynamics simulations. At low pressure regime there is indication of the existence of surfactant -rich and IL-rich regions at the water surface. Further compression results in increment of the vertical distance between cation headgroups and a mismatch between tilt angles of the octadecyl chains of [DODA]+ and [P6 6 6 14]+. The cohesion forces between neighboring hydrocarbon chains of the surfactant displaces the ionic liquid out from the contact with water surface, inducing phase separation. On the other hand, the anion exchange between the bulk water subphase and the interface with vacuum closely follows the ideal behavior, indicating no preferential interaction of bromide or dicyanamide anions with cation headgroups. Also, the surface tensions of the pure and mixed-composition monolayers were successfully evaluated from the density profiles by a group contribution method.