269510 Molecular Dynamics Simulations of Nanoparticle Self-Assembly At Ionic Liquid-Based Interfaces
Molecular Dynamics Simulations of Nanoparticle Self-Assembly At Ionic Liquid-Based Interfaces
Denzil S. Frost and Lenore L. Dai
Liquid-liquid interfaces are able to provide 2D templates for the self-assembly of small particles. By the virtue of inter-particle interactions alone, intricate patterns and structures spontaneously form and may be fused together to maintain their shape in the absence of the interfacial template. So far, particle self-assembly has been studied primarily at oil/water interfaces. Ionic liquids (ILs) are a new class of liquids with an unfathomed wealth of useful properties that may revolutionize this field. Particle behavior at IL-based liquid-liquid interfaces, however, has not yet been investigated. It is the objective of this study to pioneer a fundamental understanding of this phenomenon in comparison to oil/water interfaces and provide reference examples of particle self-assembly using this new class of liquids.
We have studied the self-assembly of hydrophobic nanoparticles at ionic liquid (IL)-water and IL-oil (hexane) interfaces using molecular dynamics (MD) simulations. For the 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6])/water system, the nanoparticles rapidly approached the IL-water interface and equilibrated more into the IL phase although they were initially in the water phase. In contrast, when the nanoparticles were dispersed in the hexane phase, they slowly approached the IL-hexane interface and remained primarily in the hexane phase. Consequently, the IL-hexane interface was rather undisturbed by the nanoparticles whereas the IL-water interface changed significantly in width and morphology to accommodate the presence of the nanoparticles. Potential of mean force (PMF) calculations supported the equilibrium positions of the nanoparticles.
The effect of particle charge on self-assembly was also investigated. In the IL/water system, nanoparticles equilibrated at the interface, somewhat favoring the IL, but this preference for the IL diminished with increased nanoparticle charge. In the IL/hexane system, all charged nanoparticles interacted with the IL to some degree, whereas the uncharged nanoparticles remained primarily in the hexane phase. PMF calculations provided insight into the particle-interface interaction. In particular, these calculations may suggest that macroscopic theories underestimated the range and magnitude of the particle-interface interaction due to low interfacial tension, thus large capillary waves, of the ionic liquid based interfaces (See Figure 1).
Figure 1 Interfacial deformations in the (a) IL/water and (b) IL/hexane systems with +4 charged particles. Solvent molecules were removed so as to create a clear view of the particles and the deforming interface.
Interesting ordering and charge distributions were observed at the IL-liquid interfaces. At the IL-hexane interface, the [BMIM] cations preferentially oriented themselves so that they immersed more into the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributes to a heightened IL density and slightly positive charge at the IL-hexane interface. In contrast, the cations at the IL-water interface were oriented isotropically unless in the presence of nanoparticles, where the cations aligned across the nanoparticle surfaces.