Electroresponse of weak polyelectrolyte brushes.

End-tethered polyelectrolytes are widely used to modify substrate properties, particularly for lubrication or wetting. External stimuli, such as pH, salt concentration, or an electric field, can induce profound structural responses in weak polyelectrolyte brushes, which can be utilized to further tune substrate properties. We study the structure and electroresponsiveness of weak polyacid brushes using an inhomogeneous theory that incorporates both electrostatic and chain connectivity correlations at the Debye-Hückel level. Our calculation shows that a weak polyacid brush swells under the application of a negative applied potential, in agreement with recent experimental observation. We rationalize this behavior using a scaling argument that accounts for the effect of the surface charge. We also show that the swelling behavior has a direct influence on the differential capacitance, which can be modulated by the solvent quality, pH, and salt concentration.

Entropic Origin of Ionic Interactions in Polar Solvents.

Implicit solvent models that reduce solvent degrees of freedom into effective interaction potentials are widely used in the study of soft materials and biophysical systems. For electrolyte and polyelectrolyte solutions, coarse-graining the solvent degrees of freedom into an effective dielectric constant embeds entropic contributions into the temperature dependence of the dielectric constant. Properly accounting for this electrostatic entropy is essential to discern whether a free energy change is enthalpically or entropically driven. We address the entropic origin of electrostatic interactions in a dipolar solvent and provide a clarified physical picture of the solvent dielectric response. We calculate the potential of mean force (PMF) between two oppositely charged ions in a dipolar solvent using molecular dynamics and dipolar self-consistent field theory. We find with both techniques that the PMF is dominated by the entropy gain from the dipole release, owing to the diminished orientational polarization of the solvent. We also find that the relative contribution of the entropy to the free energy change is nonmonotonic with temperature. We expect that our conclusions are applicable to a broad range of problems involving ionic interactions in polar solvents.

Wetting behavior of polyelectrolyte complex coacervates on solid surfaces.

The wetting behavior of complex coacervates underpins their use in many emerging applications of surface science, particularly wet adhesives and coatings. Many factors dictate if a coacervate phase will condense on a solid surface, including solution conditions, the nature of the polymer-substrate interaction, and the underlying supernatant-coacervate bulk phase behavior. In this work, we use a simple inhomogeneous mean-field theory to study the wetting behavior of complex coacervates on solid surfaces both off-coexistence (wetting transitions) and on-coexistence (contact angles). We focus on the effects of salt concentration, the polycation/polyanion surface affinity, and the applied electrostatic potential on the wettability. We find that the coacervate generally wets the surface via a first order wetting transition with second order transitions possible above a surface critical point. Applying an electrostatic potential to a solid surface always improves the surface wettability when the polycation/polyanion-substrate interaction is symmetric. For asymmetric surface affinity, the wettability has a nonmonotonic dependence with the applied potential. We use simple scaling and thermodynamic arguments to explain our results.

Thermodynamics of electrolyte solutions near charged surfaces: Constant surface charge vs constant surface potential.

Electric double layers are ubiquitous in science and engineering and are of current interest, owing to their applications in the stabilization of colloidal suspensions and as supercapacitors. While the structure and properties of electric double layers in electrolyte solutions near a charged surface are well characterized, there are subtleties in calculating thermodynamic properties from the free energy of a system with charged surfaces. These subtleties arise from the difference in the free energy between systems with constant surface charge and constant surface potential. In this work, we present a systematic, pedagogical framework to properly account for the different specifications on charged bodies in electrolyte solutions. Our approach is fully variational-that is, all free energies, boundary conditions, relevant electrostatic equations, and thermodynamic quantities are systematically derived using variational principles of thermodynamics. We illustrate our approach by considering a simple electrolyte solution between two charged surfaces using the Poisson-Boltzmann theory. Our results highlight the importance of using the proper thermodynamic potential and provide a general framework for calculating thermodynamic properties of electrolyte solutions near charged surfaces. Specifically, we present the calculation of the pressure and the surface tension between two charged surfaces for different boundary conditions, including mixed boundary conditions.

The development of a comprehensive toolbox based on multi-level, high-throughput screening of MOFs for CO/N2 separations.

The separation of CO/N2 mixtures is a challenging problem in the petrochemical sector due to the very similar physical properties of these two molecules, such as size, molecular weight and boiling point. To solve this and other challenging gas separations, one requires a holistic approach. The complexity of a screening exercise for adsorption-based separations arises from the multitude of existing porous materials, including metal-organic frameworks. Besides, the multivariate nature of the performance criteria that needs to be considered when designing an optimal adsorbent and a separation process - i.e. an optimal material requires fulfillment of several criteria simultaneously - makes the screening challenging. To address this, we have developed a multi-scale approach combining high-throughput molecular simulation screening, data mining and advanced visualization, as well as process system modelling, backed up by experimental validation. We have applied our recent advances in the engineering of porous materials' morphology to develop advanced monolithic structures. These conformed, shaped monoliths can be used readily in industrial applications, bringing a valuable strategy for the development of advanced materials. This toolbox is flexible enough to be applied to multiple adsorption-based gas separation applications.

Nonelectrostatic Adsorption of Polyelectrolytes and Mediated Interactions between Solid Surfaces.

Polymer-mediated interaction between two solid surfaces is directly connected to the properties of the adsorbed polymer layers. Nonelectrostatic interactions with a surface can significantly impact the adsorption of polyelectrolytes to charged surfaces. We use a classical density functional theory to study the effect of various polyelectrolyte solution properties on the adsorption and interaction between two like-charged surfaces. Our results show that nonelectrostatic interactions not only enhance polyelectrolyte adsorption but can also result in qualitatively different salt effects with respect to the adsorbed amount. In particular, we observe decreasing, increasing, and a previously unreported nonmonotonic behavior in the adsorbed amount of polymer with added salt under the conditions studied, although the nonmonotonic regime only occurs for a narrow range in the parameter space. With sufficient nonelectrostatic adsorption, the adsorbed polymer layers produce a long-range repulsive barrier that is strong enough to overcome dispersive interactions that cause surfaces to attract. Concurrently, a short-range bridging attraction is observed when the two polyelectrolyte layers span both the surfaces. Both the repulsive barrier and bridging attraction depend on the charge density of the polymer backbone and the bulk salt concentration but not on the chain length in the semidilute regime studied.

Wiz: A Web-Based Tool for Interactive Visualization of Big Data.

In an age of information, visualizing and discerning meaning from data is as important as its collection. Interactive data visualization addresses both fronts by allowing researchers to explore data beyond what static images can offer. Here, we present Wiz, a web-based application for handling and visualizing large amounts of data. Wiz does not require programming or downloadable software for its use and allows scientists and non-scientists to unravel the complexity of data by splitting their relationships through 5D visual analytics, performing multivariate data analysis, such as principal component and linear discriminant analyses, all in vivid, publication-ready figures. With the explosion of high-throughput practices for materials discovery, information streaming capabilities, and the emphasis on industrial digitalization and artificial intelligence, we expect Wiz to serve as an invaluable tool to have a broad impact in our world of big data.

Cucurbit[8]uril-Derived Graphene Hydrogels.

The scalable production of uniformly distributed graphene (GR)-based composite materials remains a sizable challenge. While GR-polymer nanocomposites can be manufactured at a large scale, processing limitations result in poor control over the homogeneity of hydrophobic GR sheets in the matrices. Such processes often result in difficulties controlling stability and avoiding aggregation, therefore eliminating benefits that might have otherwise arisen from the nanoscopic dimensions of GR. Here, we report an exfoliated and stabilized GR dispersion in water. Cucurbit[8]uril (CB[8])-mediated host-guest chemistry was used to obtain supramolecular hydrogels consisting of uniformly distributed GR and guest-functionalized macromolecules. The obtained GR hydrogels show superior bioelectrical properties over identical systems produced without CB[8]. Utilizing such supramolecular interactions with biologically derived macromolecules is a promising approach to stabilize graphene in water and avoid oxidative chemistry.

Modeling Nanoparticle Dispersion in Electrospun Nanofibers.

The quality of nanoparticle dispersion in a polymer matrix significantly influences the macroscopic properties of the composite material. Like general polymer-nanoparticle composites, electrospun nanofiber nanoparticle composites do not have an adopted quantitative model for dispersion throughout the polymer matrix, often relying on a qualitative assessment. Being such an influential property, quantifying dispersion is essential for the process of optimization and understanding the factors influencing dispersion. Here, a simulation model was developed to quantify the effects of nanoparticle volume loading (ϕ) and fiber-to-particle diameter ratios (D/d) on the dispersion in an electrospun nanofiber based on the interparticle distance. A dispersion factor is defined to quantify the dispersion along the polymer fiber. In the dilute regime (ϕ < 20%), three distinct regions of the dispersion factor were defined with the highest quality dispersion shown to occur when geometric constraints limit fiber volume accessibility. This model serves as a standard for comparison for future experimental studies and dispersion models through its comparability with microscopy techniques and as a way to quantify and predict dispersion in electrospinning polymer-nanoparticle systems with a single performance metric.

Influence of Particle Size and Loading on Particle Accessibility in Electrospun Poly(ethylene oxide) and ZIF-8 Composite Fibers: Experiments and Theory.

Developing electrospun nanofiber/nanoparticle composites (ENNCs) is an emerging strategy for immobilizing functional particles for a variety of applications. The radial location of the particle along the fiber, either at the surface or in the bulk, has implication into the resulting properties. To explore particle location along fibers, ZIF-8 impregnated poly(ethylene oxide) (PEO) nanofibers are formed by electrospinning particle suspensions. Fibers impregnated with two different ZIF-8 particle sizes (200 nm and 12.5 µm) were electrospun and shown by nitrogen porosimetry to be nearly completely wrapped by PEO in each case at loadings near 10 wt %. This was favorably compared to developed theory of polymeric membrane encapsulated particles and extended to other electrospun fiber/particle composites through a brief literature review. ENNCs with varying loadings of nanosized ZIF-8 particles were then fabricated and tested with nitrogen porosimetry to find that the particles became available for adsorption at the surface of the fibers starting from 25 wt % (28 vol %) loading. This suggests that the particles are kinetically trapped at this loading level since, if allowed to exhibit random close-packing, the ZIF-8 would be expected to fully imbedded inside the fibers up to 56 vol % loading.

Particle size studies to reveal crystallization mechanisms of the metal organic framework HKUST-1 during sonochemical synthesis.

Systematic studies of key operating parameters for the sonochemical synthesis of the metal organic framework (MOF) HKUST-1(also called CuBTC) were performed including reaction time, reactor volume, sonication amplitude, sonication tip size, solvent composition, and reactant concentrations analyzed through SEM particle size analysis. Trends in the particle size and size distributions show reproducible control of average particle sizes between 1 and 4µm. These results along with complementary studies in sonofragmentation and temperature control were conducted to compare these results to kinetic crystal growth models found in literature to develop a plausible hypothetical mechanism for ultrasound-assisted growth of metal-organic-frameworks composed of a competitive mechanism including constructive solid-on-solid (SOS) crystal growth and a deconstructive sonofragmentation.