Strength, number, and kinetics of hydrogen bonds for water confined inside boron nitride nanotubes.

Water has shown a myriad of highly interesting properties and behaviors, such as very low friction, phase transition under unexpected conditions, massive property alterations, etc. inside strong nanoconfinements of few-nanometer to sub-nanometer diameters. Water-water hydrogen bonding is one of the most important factors dictating such water behavior and properties inside such strong nanoconfinements. In this paper, we employ Reactive Force Field (ReaxFF) molecular dynamics (MD) simulations for studying multiple facets of such water-water hydrogen bonds (HBs) inside boron-nitride nanotubes (BNNTs) having diameters ranging from a few nanometers to sub-nanometers. First, the strength of the water-water HB interactions, as a function of the HB configuration, is quantified by studying the corresponding PMF (potential of mean force). For water present in extreme confinements (BNNTs with sub-nanometric diameters), we see completely isolated HB basins. On the other hand, for bulk water the HB basin is connected (via a saddle point) to a nearby second PMF well. Therefore, our analysis successfully distinguishes the HB characteristics between the cases of water in extreme confinement and bulk water. Second, we study the kinetics of such water-water HBs: HBs formed by a given pair of water molecules in extreme confinements show a much larger probability of remaining intact once formed or re-forming after they have been broken. Both these results, which shed new light on water-water hydrogen bonding inside strong nanoconfinements, can be explained by the single-file structure formed by the water molecules in extreme BNNT confinements.

A cellulose-derived supramolecule for fast ion transport.

Supramolecular frameworks have been widely synthesized for ion transport applications. However, conventional approaches of constructing ion transport pathways in supramolecular frameworks typically require complex processes and display poor scalability, high cost, and limited sustainability. Here, we report the scalable and cost-effective synthesis of an ion-conducting (e.g., Na+) cellulose-derived supramolecule (Na-CS) that features a three-dimensional, hierarchical, and crystalline structure composed of massively aligned, one-dimensional, and ångström-scale open channels. Using wood-based Na-CS as a model material, we achieve high ionic conductivities (e.g., 0.23 S/cm in 20 wt% NaOH at 25 °C) even with a highly dense microstructure, in stark contrast to conventional membranes that typically rely on large pores (e.g., submicrometers to a few micrometers) to obtain comparable ionic conductivities. This synthesis approach can be universally applied to a variety of cellulose materials beyond wood, including cotton textiles, fibers, paper, and ink, which suggests excellent potential for a number of applications such as ion-conductive membranes, ionic cables, and ionotronic devices.

Atomistic explorations of mechanisms dictating the shear thinning behavior and 3D printability of graphene flake infused epoxy inks.

Nanofiller-based epoxy inks have found extensive use in fabricating 3D printed nanocomposites for applications in aerospace, automobile, and marine systems. In this paper, we employ an all-atom molecular dynamic (MD) simulation to atomistically explore the mechanisms dictating the shear-thinning behavior of the graphene flake-infused (GFI) epoxy inks. We compare our findings with those for pure epoxy inks: non-equilibrium MD (NEMD) simulations reveal that both the GFI epoxy ink and pure epoxy ink demonstrate shear thinning behavior, i.e., their viscosities decrease with an increase in the shear rate. However, interestingly, the viscosity of the GFI epoxy ink is larger than that of pure epoxy for smaller shear rates, while for higher shear rates, the viscosities of these two materials are similar. This indicates a much more favorable viscosity profile for the GFI epoxy inks in the context of 3D printing. From the context of exploring the nanoscale mechanism, we identify the tendency of the bisphenol F molecules (the key constituent of the epoxy inks) and the graphene flakes (for the case of GFI epoxy inks) to align along the shear planes (in the presence of a shear flow) allowing the dissipation of viscous force among them ensuring shear-thinning behavior for both pure epoxy and GFI epoxy inks. In this context, we also identify that the bisphenol F chains prefer to localize along a given shear plane to reduce the effect of tension forces: such an alignment ensures that the radius of gyration for the bisphenol F molecules (for both pure epoxy and GFI epoxy inks) is larger for the case of finite shear and has a non-monotonic variation with the shear rate. Finally, the equilibrium MD (EMD) simulations establish that the presence of the graphene flakes significantly slows down the rotational dynamics of the bisphenol F molecules that are adsorbed to these graphene flakes and, as a result, causes the zero-shear viscosity of the GFI epoxy to be three orders of magnitude larger than that of the pure epoxy. This difference provides a qualitative justification of the viscosity of the GFI epoxy being larger than that of pure epoxy at smaller shear rate values.

All-atom molecular dynamics simulations of weak polyionic brushes: influence of charge density on the properties of polyelectrolyte chains, brush-supported counterions, and water molecules.

All atom molecular dynamics (MD) simulations of planar Na+-counterion-neutralized polyacrylic acid (PAA) brushes are performed for varying degrees of ionization (and thereby varying charge density) and varying grafting density. Variation in the PE charge density (or degree of ionization) and grafting density leads to massive changes of the properties of the PE molecules (quantified by the changes in the height and the mobility of the PE brushes) as well as the local arrangement and distribution of the brush-supported counterions and water molecules within the brushes. The effect on the counterions is manifested by the corresponding variation of the counterion mobility, counterion concentration, extent of counterion binding to the charged site of the PE brushes, water-in-salt-like structure formation, and counterion-water-oxygen radial distribution function within the PE brushes. On the other hand, the effect on water molecules is manifested by the corresponding variation of water-oxygen-water-oxygen RDF, local water density, water-water and water-PE functional group hydrogen bond networks, static dielectric constant of water molecules, orientational tetrahedral order parameter, and water mobility. Enforcing such varying degree of ionization of weak polyelectrolytes is possible by changing the pH of the surrounding medium. Thus, our results provide insights into the changes in microstructure (at the atomistic level) of weak polyionic brushes at varying pH. We anticipate that this knowledge will prove to be vital for the efficient design of several nano-scale systems employing PE brushes such as nanomechanical gates, current rectifiers, etc.