(393p) 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.