(260f) Molecular Dynamics Studies of the Effects of Ionic Liquid Molecular Properties and Particle Concentration on the Behavior of Nanoparticles at the Ionic Liquid/Water Interface

Ionic liquid (IL)/liquid interfaces have exhibited unique phenomena when interacting with hard particles--including particle "bridging" between emulsion droplets and, perhaps most interestingly, the spontaneous transport of microparticles across the liquid/liquid interface, something which acts against the accepted thermodynamics of such systems. The current theory behind this phenomena is only thinly supported. Here, extensive molecular dynamics simulations of hydrophobic nanoparticles at the IL/water interface are described. These simulations are the first of their kind to vary both the concentration of nanoparticles and the ionic liquid involved, allowing more general insights into the molecular-scale behavior of IL/water/particle systems. This study compares the interfaces of water with four ILs--1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF₆]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF₆]), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonate) ([BMIM][Tf₂N]). In addition, three levels of particle concentration are simulated for a total of twelve simulation systems. The three different imidazolium cations increase in hydrophobicity with the increasing length of the carbon chain, and so give insight into the effects of IL hydrophobicity on the studied systems and behaviors. Meanwhile, the two anions have markedly different shapes. Tools including density profiles and interaction-energy comparisons are used to analyze these simulations. It is found that the presence of hydrophobic nanoparticles profoundly affects the interface itself by moderating the repulsive forces between hydrophobic ILs and water. This leads to the creation of a "pseudo-phase" of intermingled water, IL, and particles. This effect is more prominent in systems with less hydrophobic ILs, smaller and more symmetrical ILs, and higher particle concentrations. This It is postulated that this pseudophase explains some of the unusual behavior of particles observed experimentally at the IL/liquid interface.