Development of visible-light responsive and mechanically enhanced "smart" UCST interpenetrating network hydrogels.

An interpenetrating polymer network (IPN), chlorophyllin-incorporated environmentally responsive hydrogel was synthesized and exhibited the following features: enhanced mechanical properties, upper critical solution temperature (UCST) swelling behavior, and promising visible-light responsiveness. Poor mechanical properties are known challenges for hydrogel-based materials. By forming an interpenetrating network between polyacrylamide (PAAm) and poly(acrylic acid) (PAAc) polymer networks, the mechanical properties of the synthesized IPN hydrogels were significantly improved compared to hydrogels made of a single network of each polymer. The formation of the interpenetrating network was confirmed by Fourier Transform Infrared Spectroscopy (FTIR), the analysis of glass transition temperature, and a unique UCST responsive swelling behavior, which is in contrast to the more prevalent lower critical solution temperature (LCST) behaviour of environmentally responsive hydrogels. The visible-light responsiveness of the synthesized hydrogel also demonstrated a positive swelling behavior, and the effect of incorporating chlorophyllin as the chromophore unit was observed to reduce the average pore size and further enhance the mechanical properties of the hydrogel. This interpenetrating network system shows potential to serve as a new route in developing "smart" hydrogels using visible-light as a simple, inexpensive, and remotely controllable stimulus.

Multipetal-Structured and Dumbbell-Structured Gold-Polymer Composite Particles with Self-Modulated Catalytic Activity.

A simple synthesis route for gold-polymer composite particles with controlled structure (multipetal structure and dumbbell structure) is developed. It is intriguing to observe that by controlling the reaction time and size of gold nanoparticles (AuNPs), tetrapetal-, tripetal-, and dumbbell-structured gold-polystyrene composite are obtained via seeded polymerization. The average number of petals on a single AuNP increases with the AuNP diameter. These particles show potential applications as building blocks for advanced ordered and hierarchical supracolloidal materials. Further, with the incorporation of poly(N-isopropylacrylamide) (PNIPAm), "smart" thermoresponsive dumbbell-structured gold-PNIPAm/polystyrene composite particles are formed. Significant size variation is validated for particles with 83 and 91 wt % PNIPAm content around lower critical solution temperature (LCST), which results in self-modulated catalytic activity.

Core-shell and asymmetric polystyrene-gold composite particles via one-step Pickering emulsion polymerization.

Core-shell structured polystyrene-gold composite particles are synthesized from one-step Pickering emulsion polymerization. The surface coverage of the core-shell composite particles is improved with increasing gold nanoparticle (AuNP) hydrophobicity and concentration. At high surface coverage, the AuNPs exhibit an ordered hexagonal pattern, likely due to electrostatic repulsion during the emulsion polymerization process. In addition to core-shell structured polystyrene-gold composite particles, an intriguing observation is that at low AuNP concentrations, asymmetric polystyrene-gold nanocomposite particles are simultaneously formed, where a single gold nanoparticle is attached onto each polystyrene particle. It is found that these asymmetric particles are formed via a "seeded-growth" mechanism. The core-shell and asymmetric polystyrene-gold composite particles prove to be efficient catalysts as they successfully catalyze the Rhodamine B reduction reaction with stable performance and show high recyclability as catalysts.

Comparison of the capillary wave method and pressure tensor route for calculation of interfacial tension in molecular dynamics simulations.

We have studied the calculation of surface and interfacial tension for a variety of liquid-vapor and liquid-liquid interfaces using molecular dynamics (MD) simulations. Because of the inherently small scale of MD systems, large pressure fluctuations can cause imprecise calculations of surface tension using the pressure tensor route. The capillary wave method exhibited improved precision and stability throughout all of the simulated systems in this study. In order to implement this method, the interface was defined by fitting an error function to the density profile. However, full mapping of the interface from coordinate files produced enhanced accuracy. Upon increasing the system size, both methods exhibited higher precision, although the capillary wave method was still more reliable.

Adsorption and release of active species into and from multifunctional ionic microgel particles.

We synthesize monodisperse ionic microgel particles which undergo a large change in volume in response to environmental stimuli such as pH and temperature. In addition, the study elucidates the effective uptake and release of rheology modifiers from these microgel particles to alter the bulk viscosity of a surrounding fluid. Moreover, we found that the prepared ionic microgel particles can demonstrate abilities to adsorb and repel iron oxide nanoparticles (Fe3O4-NPs) upon pH variation. The extent of the loading of Fe3O4-NPs within the colloidal particles and morphology can be manipulated by tunable interactions between the Fe3O4-NPs and ionic microgel particles.

Spontaneous transport of microparticles across liquid-liquid interfaces.

Transporting micrometer-sized particles through the liquid-liquid interface generally requires high shear force and sometimes surfactant functionalization. Without these aids, particles adhere to the interface due to strong capillary forces (can be on the order of 10(6) kT). Thus, spontaneous transport of microparticles through the liquid-liquid interface has not yet been reported. However, we present a new phenomenon here: some ionic liquids (ILs) possess powerful extraction capabilities and can cause microparticles to migrate across the interface without the aid of any shear forces. Both single particles and clusters of particles were observed to adsorb to, then "jump" across the interface and finally detach. In the absence of external mixing, particles as large as 4 µm (in diameter) could completely penetrate the IL/water interface, despite the significant adhesive forces. We have presented a hypothesis that these forces were overcome by ions dissolved in the non-IL phase, which helped by covering the particle surfaces, allowing for more favorable interactions with the IL.

Nonconvective mixing of miscible ionic liquids.

Ionic liquids (ILs) are ionic compounds that are liquid at room temperature. We studied the spontaneous mixing behavior between two ILs, ethylammonium nitrate (EAN) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), and observed notable phenomena. Experimental studies showed that the interface between the two ILs was unusually long-lived, despite the ILs being miscible with one another. Molecular dynamics (MD) simulations supported these findings and provided insight into the micromixing behavior of the ILs. We found that not only did the ions experience slow diffusion as they mix but also exhibited significant ordering into distinct regions. We suspect that this ordering disrupted concentration gradients in the direction normal to the interface, thus hindering diffusion in this direction and allowing the macroscopic interface to remain for long periods of time. Intermolecular interactions responsible for this behavior included the O-NH interaction between the EAN ions and the carbon chain-carbon chain interactions between the [BMIM](+) cations, which associate more strongly in the mixed state than in the pure IL state.

Controlled morphology of thin film silicon integrated with environmentally responsive hydrogels.

Environmentally responsive hydrogels hold multiple important applications. However, the functionality of these materials alone is often limited in comparison to other materials like silicon; thus, there is a need to integrate soft and hard materials for the advancement of environmentally sensitive materials. Here we demonstrate the capability of integrating a thermoresponsive hydrogel, poly(N-isopropylacrylamide), with thin film silicon ribbons, enabling the stiff silicon ribbons to become adaptive and drivable by the soft environmentally sensitive substrate. This integration provides a means of mechanical buckling of the thin silicon film due to changes in environmental stimuli (e.g., temperature, pH). We also investigate how advanced lithographic techniques can be used to generate patterned deformation on the aforementioned integrated structures. Furthermore, we explore multilayer hybrid hydrogel structures formed by the integration of different types of hydrogels that have tunable curvatures under the influence of different stimuli. Silicon thin film integration on such tunable curvature substrates reveal characteristic reversible buckling of the thin film in the presence of multiple stimuli. These results open new opportunities for developing stretchable and intelligent devices for multiple applications.

Molecular dynamics studies on the adaptability of an ionic liquid in the extraction of solid nanoparticles.

Recently, a number of publications have suggested that ionic liquids (ILs) can absorb solid particles. This development may have implications in fields like oil sand processing, oil spill beach cleanup, and water treatment. In this Article, we provide a computational investigation of this phenomenon via molecular dynamics simulations. Two particle surface chemistries were investigated: (1) hydrocarbon-saturated and (2) silanol-saturated, representing hydrophobic and hydrophilic particles, respectively. Employing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) as a model IL, these nanoparticles were allowed to equilibrate at the IL/water and IL/hexane interfaces to observe the interfacial self-assembled structures. At the IL/water interface, the hydrocarbon-based nanoparticles were nearly completely absorbed by the IL, while the silica nanoparticles maintained equal volume in both phases. At the IL/hexane interface, the hydrocarbon nanoparticles maintained minimal interactions with the IL, whereas the silica nanoparticles were nearly completely absorbed by it. Studies of these two types of nanoparticles immersed in the bulk IL indicate that the surface chemistry has a great effect on the corresponding IL liquid structure. These effects include layering of the ions, hydrogen bonding, and irreversible absorption of some ions to the silica nanoparticle surface. We quantify these effects with respect to each nanoparticle. The results suggest that ILs likely exhibit this absorption capability because they can form solvation layers with reduced dynamics around the nanoparticles.

Molecular dynamics simulations of charged nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces.

Nanoparticle self-assembly at liquid-liquid interfaces can be significantly affected by the individual nanoparticle charges. This is particularly true at ionic liquid (IL) based interfaces, where Coulombic forces play a major role. Employing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF(6)]) as a model IL, we have studied the self-assembly of hydrophobic nanoparticles with different surface charges at the IL/water and IL/oil (hexane) interfaces using molecular dynamics simulations. In the IL/water system, the nanoparticles were initially dispersed in the water phase but quickly equilibrated at the interface, somewhat in favor of the IL phase. This preference was lessened with increased nanoparticle charge. In the IL/hexane system, all charged nanoparticles interacted with the IL to some extent, whereas the uncharged nanoparticles remained primarily in the hexane phase. Potential of mean force calculations supported the observations from the equilibrium studies and provided new insights into the interactions of the nanoparticles and ionic liquid based interfaces.

Particle self-assembly in ionic liquid-in-water Pickering emulsions.

We report the self-assembly of a single species or a binary mixture of microparticles in ionic liquid-in-water Pickering emulsions, with emphases on the interfacial self-assembled particle structure and the partitioning preference of free particles in the dispersed and continuous phases. The particles form monolayers at ionic liquid-water interfaces and are close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. In contrast to those at oil-water interfaces, no long-range-ordered colloidal lattices are observed. Interestingly, other than equilibrating at the ionic liquid-water interfaces, the microparticles also exhibit a partitioning preference in the dispersed and continuous phases: the sulfate-treated polystyrene (S-PS) and aldehyde-sulfate-treated polystyrene (AS-PS) microparticles are extracted to the ionic liquid phase with a high extraction efficiency, whereas the amine-treated polystyrene (A-PS) microparticles remain in the water phase.

Formation and dynamics of capillary-flow-induced colloidal rings.

We have directly observed the ring formation of colloidal particles of 1 µm diameter at the contact lines of air, water, and oil using a laser scanning confocal microscope. Colloidal rings form and grow through the transport of particles induced by capillary flow due to water evaporation. In addition, we observe the sudden "jump in" of particles into the ring and the "depletion" of particles in the ring. Particle-tracking experiment shows that the particles within the ring exhibit 1D-like motion along the circular ring geometry, and the pair correlation function of the ring configuration suggests an equilibrium interparticle distance of approximately 2.8 µm. It is also found that the structure and formation speed of the colloidal rings can be controlled by accelerating water evaporation by the addition of methanol as a cosolvent.

Molecular dynamics simulations of nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces.

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][PF(6)])/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 but 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. The equilibrium positions of the nanoparticles were also supported and explained by potential of mean force (PMF) calculations. 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 were immersed more in the hexane phase and packed efficiently to reduce steric hindrance. The ordering likely contributed to a heightened IL density and a 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.

Two-particle interfacial microrheology at polymer-polymer interfaces.

We continue to develop two-particle interfacial microrheology and have applied the technique to study the interfacial viscoelastic properties of immiscible poly(dimethylsiloxane) (PDMS)-poly(ethylene glycol) (PEG) interfaces. The interfacial storage and loss moduli are measured over a wide frequency range: at low frequencies, the interfaces are dominated by viscous responses whereas elasticity dominates at high frequencies. The zero-shear interfacial viscosity, estimated following the Cox-Merz rule and Cross model, falls between the bulk viscosities of the two individual polymers. Surprisingly, the interfacial relaxation time, calculated from the crossover of the storage and loss moduli, is observed to be an order of magnitude larger than that of the PDMS bulk polymers. The effects of tracer particle surface chemistry and size have also been investigated and show minimum influences on two-particle interfacial microrheology.

Understanding nanoparticle diffusion and exploring interfacial nanorheology using molecular dynamics simulations.

We have studied the dynamics of nanoparticles at polydimethylsiloxane (PDMS) oil-water interfaces using molecular dynamics (MD) simulations. The diffusion of nanoparticles in pure water and low-viscosity PDMS oil is found to be reasonably consistent with the prediction by the Stokes-Einstein equation. In addition, we have calculated the shear moduli and viscosities of bulk oil and water, as well as oil-water interfaces from single nanoparticle tracking and demonstrated the potential of probing nanorheology from an MD simulation approach. Surprisingly, we found that the lateral diffusion of nanoparticles as well as apparent interfacial nanorheology at the PDMS oil (low viscosity)-water interface are independent of the position of the nanoparticle at the interface.

Effects of methanol on nanoparticle self-assembly at liquid-liquid interfaces: a molecular dynamics approach.

Molecular dynamics simulations have been performed to study the influence of methanol on the self-assembly of nanoparticles at liquid-liquid interfaces. The simulation shows that the methanol molecules, at low volume fraction, stay in the aqueous phase but with a preference of the water-trichloroethylene (TCE) interfaces. However, at higher methanol volume fraction, methanol dominates the aqueous phase and no preferable location of methanol molecules is observed. The simulations also suggest that the interfacial tension decreases and the interfacial thickness increases with increasing methanol concentration. The presence of the nanoparticles at water-TCE interfaces has minor effect on the interfacial properties compared to those of methanol, and the presence of methanol drives the noncharged nanoparticle clusters closer to the interfaces. Although the methanol molecules do not affect the monolayer distribution of the negatively charged nanoparticles at the water-TCE interfaces, they increase the three-phase contact angles of these nanoparticles.

Tailoring intermolecular interactions to develop a low-temperature electrolyte system consisting of 1-butyl-3-methylimidazolium iodide and organic solvents.

Ionic liquids (ILs) exhibit remarkable properties and great tunability, which make them an attractive class of electrolyte materials for a variety of electrochemical applications. However, despite the promising progress for operating conditions at high temperatures, the development of their low-temperature viability as electrolytes is still limited due to the constrains from thermal and ion transport issues with a drastic decrease in temperature. In this study, we present a liquid electrolyte system based on a mixture of 1-butyl-3-methylimidazolium iodide ([BMIM][I]), γ-butyrolactone (GBL), propylene carbonate (PC), and lithium iodide (LiI) and utilize its molecular interactions to tailor its properties for extremely low-temperature sensing applications. In particular, the carbonyl group on both PC and GBL can form hydrogen bonds with the imidazolium cation, as indicated by Fourier transform infrared spectroscopy (FTIR), and the extent of these interactions between ions and molecules was also characterized and quantified via proton nuclear magnetic resonance (1H NMR) spectroscopy. More importantly, at the optimal ratio, the organic solvents do not have excess content to form aggregates, which may cause undesired crystallization before the glass transition. The microscopic evolutions of the systems are correlated with their bulk behaviors, leading to improvements in their thermal and transport properties. The optimized formulation of [BMIM][I]/PC/GBL/LiI showed a low glass transition temperature (T g) of -120 °C and an effectively reduced viscosity of 0.31 Pa s at -75 °C. The electrochemical stability of the electrolyte was also validated to support the targeted iodide/triiodide redox reactions without interference.

The structure and dynamics of microparticles at pickering emulsion interfaces.

We have employed a laser scanning confocal microscope (LSCM) to study the structure and dynamics of microparticles at Pickering emulsion interfaces. The microparticles can have rich morphology at the emulsion interfaces, ranging from an aggregated structure to colloidal lattices, with a possibility of involving heterogeneous particles. With a specific interest in colloidal lattices, we find that although the enhanced electrostatic repulsion explains the formation of colloidal lattices by sulfate-treated polystyrene (S-PS) particles, it fails to interpret the unsuccessfulness of assembling lattices containing single-species carboxylate-treated polystyrene (C-PS) particles. A small percentage of C-PS particles in the colloidal mixture does not disturb the formation of lattices made of S-PS particles. The LSCM also provides a meaningful way to probe dynamic information. The diffusion of single particles at the emulsion interfaces depends strongly on the oil phase viscosity, particle size, and particle wettability. A highly curved emulsion interface slows the motion of microparticles at oil-water interfaces but the interface curvature effect decreases with increasing oil phase viscosity. Although the confocal microscope was originally used as an imaging tool, we find that the thermodynamic equilibrium of colloidal lattices can be disturbed and even destroyed when increasing the output laser intensity.

A Dual Ionic Liquid-Based Low-Temperature Electrolyte System.

Ionic liquids (ILs) show a promising future as electrolytes in electrochemical devices. In particular, IL-based electrolytes bring operations at extreme temperatures to realization that conventional electrolytes fail to accomplish. Although IL electrolytes demonstrate considerable progress in high-temperature applications, their breakthroughs in devices operating at low temperatures are still very limited due to undesirable phase transitions and unsatisfying transport properties. In this study, we present an approach where, by tuning molecular interactions in the system, the designed electrolyte of an IL-based mixture can reach a lower operating temperature with improved transport properties. We have discovered that the incorporation of the IL, ethylammonium nitrate ([EA][N]), can contribute to reforming the molecular interactions within the system, which effectively resolve the crystallization accompanied with the excess of water and retain a low glass transition temperature. The reported liquid electrolyte systems based on a mixture of 1-butyl-3-methylimidazolium iodide ([BMIM][I]), [EA][N], water, and lithium iodide exhibit a glass transition temperature below -105 °C. Furthermore, the optimized electrolyte system shows significant viscosity reduction and ionic conductivity enhancement from 25 to -75 °C. The influence is also noticeable on the increased ionicity, which made the developed electrolyte comparable with other good ILs under the Walden rule. The electrochemical stability of the electrolyte system is revealed by a steady and reproducible profile of iodide/triiodide redox reactions at room temperature over a proper potential window via cyclic voltammetry. The results from this work not only provide a potential solution to applications of the iodide/triiodide redox couple-based electrochemical devices at low temperatures but also show a practical approach to obtain tailored properties of a mixture system via modifying molecular interactions.

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.