Elizabeth Nofen

Particle self-assembly at ionic liquid-based interfaces

This review presents an overview of the nature of ionic liquid (IL)-based interfaces and self-assembled particle morphologies of IL-in-water, oil- and water-in-IL, and novel IL-in-IL Pickering emulsions with emphasis on their unique phenomena, by means of experimental and computational studies. In IL-in-water Pickering emulsions, particles formed monolayers at ionic liquid-water interfaces and were close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. Interestingly, other than equilibrating at the ionic liquid-water interfaces, microparticles with certain surface chemistries were extracted into the ionic liquid phase with a high efficiency. These experimental findings were supported by potential of mean force calculations, which showed large energy drops as hydrophobic particles crossed the interface into the IL phase. In the oil- and water-in-IL Pickering emulsions, microparticles with acidic surface chemistries formed monolayer bridges between the internal phase droplets rather than residing at the oil/water-ionic liquid interfaces, a significant deviation from traditional Pickering emulsion morphology. Molecular dynamics simulations revealed aspects of the mechanism behind this bridging phenomenon, including the role of the droplet phase, surface chemistry, and inter-particle film. Novel IL-in-IL Pickering emulsions exhibited an array of self-assembled morphologies including the previously observed particle absorption and bridging phenomena. The appearance of these morphologies depended on the particle surface chemistry as well as the ILs used. The incorporation of particle self-assembly with ionic liquid science allows for new applications at the intersection of these two fields, and have the potential to be numerous due to the tunability of the ionic liquids and particles incorporated, as well as the particle morphology by combining certain groups of particle surface chemistry, IL type (protic or aprotic), and whether oil or water is incorporated.

Understanding droplet bridging in ionic liquid-based Pickering emulsions

We have studied the unique bridging behavior of solid-stabilized oil-in-ionic liquid (IL) and water-in-ionic liquid emulsions with respect to particle concentration, particle size, and droplet phase using a confocal laser scanning microscope. The emulsions exhibited three morphology regimes: (1) single, sparingly covered droplets, (2) bridged clusters of droplets, and (3) fully covered droplets. The degree of bridging was directly proportional to the total potential bridging area which can be determined from the particle size and concentration. This type of emulsion diverges from much of the conventional wisdom of oil-water Pickering emulsions regarding the particle self-assembly onto droplet interfaces and liquid film stability. While the focus here is the bridging regime, we also report interesting observations, specifically, the deformed oil droplets and the transport of excess solid particles into the water droplets, in the fully covered droplet regime. The work identified new self-assembled particle structure and morphology in solid-stabilized emulsions.

A Combined Experimental and Molecular Dynamics Study of Iodide-Based Ionic Liquid and Water Mixtures

Iodide-based ionic liquids have been widely employed as iodide sources in electrolytes for applications utilizing the triiodide/iodide redox couple. While adding a low-viscosity solvent such as water to ionic liquids can greatly enhance their usefulness, mixtures of highly viscous iodide-containing ILs with water have never been studied. This paper investigates, for the first time, mixtures of water and the ionic liquid 1-butyl-3-methylimidazolium iodide ([BMIM][I]) through a combined experimental and molecular dynamics study. The density, melting point, viscosity, and conductivity of these mixtures were measured by experiment. The composition region below 50% water by mole was found to differ dramatically from the region above 50% water, with trends in density and melting point differing before and after that point. Water was found to have a profound effect on viscosity and conductivity of the IL, and the effect of hydrogen bonding was discussed. Molecular dynamics simulations representing the same mixture compositions were performed. Molecular ordering was observed, as were changes in this ordering corresponding to water content. Molecular ordering was related to the experimentally measured mixture properties, providing a possible explanation for the two distinct composition regions identified by experiment.

Dimeric anthracene-based mechanophore particles for damage precursor detection in reinforced epoxy matrix composites

The problem of catastrophic damage purveys in any material application, and minimizing its occurrence is paramount for general health and safety. We have successfully synthesized, characterized, and applied dimeric 9-anthracene carboxylic acid (Di-AC)-based mechanophore particles to form stress sensing epoxy matrix composites. As Di-AC had never been previously applied as a mechanophore and thermosets are rarely studied in mechanochemistry, this created an alternative avenue for study in the field. Under an applied stress, the cyclooctane-rings in the Di-AC particles reverted back to their fluorescent anthracene form, which linearly enhanced the overall fluorescence of the composite in response to the applied strain. The fluorescent signal further allowed for stress sensing in the elastic region of the stress–strain curve, which is considered to be a form of damage precursor detection. Overall, the incorporation of Di-AC to the epoxy matrix added much desired stress sensing and damage precursor detection capabilities with good retention of the material properties.

Stress-sensing thermoset polymer networks via grafted cinnamoyl/cyclobutane mechanophore units in epoxy

The incorporation of mechanophores into networked thermoset polymers, such as epoxy, is notably missing from the mechanochemistry literature, which focuses more on traditional thermoplastic and elastomeric polymers. In this work, we develop novel approaches for direct covalent grafting of photoactive mechanophore units into an epoxy matrix (a two-part network polymer), to create a self-sensing thermoset network nanocomposite, linked by both epoxide and mechanophore bonds. Two routes of grafting mechanophore units into an epoxy system to form a self-sensing nanocomposite were explored, including grafting of the mechanophore precursor molecule cinnamamide to the epoxy resin, with subsequent hardener addition and ultraviolet curing to form the mechanically sensitive cyclobutane rings, and the separate grafting of the solution-made mechanophore di-cinnamamide to the epoxy resin to allow for maximum cyclobutane concentration in the formed nanocomposites. Under a compressive force, the cyclobutane rings in the mechanophore units break, increasing the overall fluorescence, which can then be correlated with the applied stress. The goals of this work included detecting early damage by fluorescence spectroscopy, environmental robustness, and retention of the mechanical and thermal properties of the composite. Overall, there was successful formation of self-sensing nanocomposites and achievement of the early damage detection functionality. This systematic work additionally aims to provide further fundamental understanding of mechanochemistry as a whole.

Pyrrole-based poly(ionic liquids) as efficient stabilizers for formation of hollow multi-walled carbon nanotubes particles

Poly(ionic liquid) (PIL) derivatives with pyrrole intrinsically conducting polymer (ICP) backbones were synthesized and utilized as novel dispersants of multi-walled carbon nanotubes (MWCNTs) in various aqueous and non-aqueous systems, including polar and nonpolar solvents. This is due to the highly tunable nature of the PIL, in which the PILs of varying polarity with the same pyrrole-based polycation can be synthesized. The dispersions are exceedingly stable over many months, and with the addition of hexane, Pickering (solid-stabilized) emulsions with the PIL-stabilized MWCNTs at the droplet interfaces were formed. Depending on the hydrophobicity of the PIL, hexane-in-water and hexane-in-acetonitrile emulsions were formed, the latter marking the first non-aqueous CNT-stabilized emulsions, further advancing the processability of CNTs. The PIL-stabilized CNT Pickering emulsion droplets generated hollow conductive particles by subsequent drying of the emulsions. With emulsion templating, the hollow shells could be used as a payload carrier, depending on the solubility of the payload in the droplet phase of the emulsion. This was demonstrated with silicon nanoparticles, which have limited dispersibility in aqueous environments, but great scientific interest due to their potential electrochemical applications. Overall, this work explored a new class of efficient PIL-ICP hybrid stabilizers with tunable hydrophobicity, with hollow particle formation capability.

MEMS accelerometer based on Molecular Electronic Transducers using Ionic Liquid

This paper demonstrates a novel MEMS accelerometer based on Molecular Electronic Transducers (MET) using Ionic Liquids (ILs) as sensing body to enable operation in a wide temperature range with better sensitivity. The Sensitivity has been improved for 8 times and the low operation temperature limit expands to at least -90 °C.

Multiscale Modeling and Characterization of Stress-sensitive Mechanophore-embedded Nanocomposites

This paper presents the characterization and multiscale modeling of novel stresssensitive mechanophore-embedded nanocomposites. Stress-sensitive mechanophores that emit fluorescence under mechanical loading have been developed recently. In this study, a cyclobutane-based mechanophore is incorporated into thermoset polymer matrix for early damage detection. Tris-Cinnamoyloxymethyl-Ethane monomer is used to make the cyclobutane-based mechanophore using ultra violet (UV) light by a process called UV-dimerization. Test results indicate that the cyclobutane-based mechanophore embedded thermoset polymer matrix is capable of capturing crack nucleation by exhibiting a color change under mechanical loading. A multiscale modeling framework connecting sub-nanoscale phenomena to a bulk polymer system is also developed by implementing a hybrid MD simulation approach. The multiscale modeling framework simulates cyclobutane-based mechanophore synthesis and mechanophore activation successfully. Local force analysis implemented by the multiscale modeling framework enables quantitative analysis of the mechanophore activation process and shows good correlation with experimental results. doi: 10.12783/SHM2015/277

Ionic Liquids in Multiphase Systems

Ionic liquids (ILs) can be used to replace one or more phases in conventional oil/water emulsions including Pickering emulsions—surfactant-free emulsions which utilize nanoor micron-sized particles to stabilize the immiscible liquid-liquid interface. Due to the extreme tunability of both the ILs and particles used, the study of IL-based Pickering emulsions yields novel emulsion morphologies and insights into the ionic liquid-liquid-particle interactions present. This work discusses extensive experimental work on IL-based Pickering emulsions and IL/liquid interfaces, emphasizing unique phenomena—such as “bridging” between emulsion droplets and spontaneous particle transport across the interface—never observed in more conventional Pickering emulsions. Molecular dynamics (MD) simulations of particles at the IL/liquid interface are also discussed, and fundamental insights from these simulations are used to enhance understanding of the unique interface behavior revealed by experiment.

Colloidal Lattices of Environmentally Responsive Microgel Particles at Ionic Liquid–Water Interfaces

This work reports new evidence of the versatility of soft and environmentally responsive micron-sized colloidal gel particles as stabilizers at ionic liquid-water droplet interfaces. These particles display a duality with properties ascribed typically to both polymeric and colloidal systems. The utilization of fluorescently labeled composite microgel particles allows in-situ and facile visualization without the necessity of invasive sample preparation. When the prepared particles form monolayers equilibrated at the ionic liquid-water interface on fully covered droplets, the colloidal lattice re-orders itself depending on the surface charge of these particles. Finally, we demonstrate that the spontaneously formed and densely packed layer of microgel particles can be employed for extraction applications, as the interface remains permeable to small active species.