Design principles from multiscale simulations to predict nanostructure in self-assembling ionic liquids.

Molecular dynamics simulations (up to the nanoscale) were performed on the 3-methyl-1-pentylimidazolium ionic liquid cation paired with three anions; chloride, nitrate, and thiocyanate as aqueous mixtures, using the effective fragment potential (EFP) method, a computationally inexpensive way of modeling intermolecular interactions. The simulations provided insight (preferred geometries, radial distribution functions and theoretical proton NMR resonances) into the interactions within the ionic domain and are validated against 1H NMR spectroscopy and small- and wide-angle X-ray scattering experiments on 1-decyl-3-methylimidazolium. Ionic liquids containing thiocyanate typically resist gelation and form poorly ordered lamellar structures upon mixing with water. Conversely, chloride, a strongly coordinating anion, normally forms strong physical gels and produces well-ordered nanostructures adopting a variety of structural motifs over a very wide range of water compositions. Nitrate is intermediate in character, whereby upon dispersal in water it displays a range of viscosities and self-assembles into nanostructures with considerable variability in the fidelity of ordering and symmetry, as a function of water content in the binary mixtures. The observed changes in the macro and nanoscale characteristics were directly correlated to ionic domain structures and intermolecular interactions as theoretically predicted by the analysis of MD trajectories and calculated RDFs. Specifically, both chloride and nitrate are positioned in the plane of the cation. Anion to cation proximity is dependent on water content. Thiocyanate is more susceptible to water insertion into the second solvent shell. Experimental 1H NMR chemical shifts monitor the site-specific competition dependence with water content in the binary mixtures. Thiocyanate preferentially sits above and below the aromatic ring plane, a state disallowing interaction with the protons on the imidazolium ring.

Self-Assembly Directed Organization of Nanodiamond During Ionic Liquid Crystalline Polymer Formation.

The UV-initiated free radical polymerization of a lyotropic mesophase prepared by co-assembly of an aqueous mixture of an ionic liquid (IL) monomer, 3-decyl-1-vinylimidazolium chloride, in a dimethyl sulfoxide dispersion of an IL-monomer nanodiamond conjugate yields a well-ordered 2D hexagonally structured network-polymer composite. The IL monomer is covalently bound to carboxylated detonation diamond via ester-linked 3-decyl-1-vinylimidazolium bromide. Successful preparation of the amphiphile-functionalized nanodiamond is determined by ATR/FT-IR, thermogravimetric analysis, and small-angle X-ray scattering (SAXS). Mesophase and composite structure are evaluated by SAXS, revealing a columnar architecture composed of amphiphilic ionic liquid cylinders containing solvent-rich cores. Self-assembly directed site localization of the nanodiamond positions the particles in the alkyl chain continuum upon polymerization. The composite reversibly swells in ethanol allowing structural variation and modulation of the nanoparticle internal packing arrangement. This work demonstrates that through careful molecular design, self-organization and site-directed assembly of nanodiamond into chemically distinct regions of a nanostructured organogel can be achieved.

Cascade synthesis of a gold nanoparticle-network polymer composite.

The multi-step, cascade synthesis of a self-supporting, hierarchically-structured gold nanoparticle hydrogel composite is described. The composite is spontaneously prepared from a non-covalent, lamellar lyotropic mesophase composed of amphiphiles that support the reactive constituents, a mixture of hydroxyl- and acrylate-end-derivatized PEO117-PPO47-PEO117 and [AuCl4](-). The reaction sequence begins with the auto-reduction of aqueous [AuCl4](-) by PEO117-PPO47-PEO117 which leads to both the production of Au NPs and the free radical initiated polymerization and crosslinking of the acrylate end-derivatized PEO117-PPO47-PEO117 to yield a network polymer. Optical spectroscopy and TEM monitored the reduction of [AuCl4](-), formation of large aggregated Au NPs and oxidative etching into a final state of dispersed, spherical Au NPs. ATR/FT-IR spectroscopy and thermal analysis confirms acrylate crosslinking to yield the polymer network. X-ray scattering (SAXS and WAXS) monitored the evolution of the multi-lamellar structured mesophase and revealed the presence of semi-crystalline PEO confined within the water layers. The hydrogel could be reversibly swollen without loss of the well-entrained Au NPs with full recovery of composite structure. Optical spectroscopy shows a notable red shift (Δλ ∼ 45 nm) in the surface plasmon resonance between swollen and contracted states, demonstrating solvent-mediated modulation of the internal NP packing arrangement.

Electrochemical activity of glucose oxidase on a poly(ionic liquid)-Au nanoparticle composite.

Glucose oxidase (GOx) adsorbed on an ionic liquid-derived polymer containing internally organized columns of Au nanoparticles exhibits direct electron transfer and bioelectrocatalytic properties towards the oxidation of glucose. The cationic poly(ionic liquid) provides an ideal substrate for the electrostatic immobilization of GOx. The encapsulated Au nanoparticles serve to both promote the direct electron transfer with the recessed enzyme redox centers and impart electronic conduction to the composite, allowing it to function as an electrode for electrochemical detection.