Analysis of Formation Mechanisms of Sugar-Derived Dense Carbons via Hydrogel Carbonization Method

Four kinds of sugar (glucose, fructose, sucrose, and maltose) were selected as carbon precursors, and corresponding dense carbon products were prepared using a novel hydrogel carbonization method. The carbonization processes of sugar-polyacrylamide (sugar-PAM) hydrogels were studied in detail. The molecular structures in the raw materials were analyzed by proton nuclear magnetic resonance spectroscopy (¹H NMR). Samples prepared at different temperatures were characterized by thermogravimetry analysis (TGA) and Fourier-transform infrared (FTIR) spectroscopy. The morphology and microstructure of sugar-derived carbons were confirmed by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The results indicated that the sugar solution was surrounded by PAM with a three-dimensional network structure and formed hydrogels in the initial stage. The sugar solution was considered to be separated into nanocapsules. In each nanocapsule, sugar molecules could be limited within the hydrogel via walls formed by PAM chains. The hydroxyl group in the sugar molecules connected with PAM by the hydrogen bond and intermolecular force, which can strengthen the entire hydrogel system. The self-generated pressure of hydrogel constrains the foam of sugar during the heat treatment. Finally, dense carbon materials with low graphitization instead of porous structure were prepared at 1200 °C.

Coarse-grained molecular dynamics simulations of α-1,3-glucan

In this paper we present a computational study of aggregation in aqueous solutions of α-1,3-glucan captured using a coarse-grained (CG) model that can be extended to other polysaccharides. This CG model captures atomistic geometry (i.e., relative placement of the hydrogen bonding donors and acceptors within the monomer) of the α-1,3-glucan monomer, the directional interactions due to the donor-acceptor hydrogen bonds, and their effect on aggregation of multiple α-1,3-glucan chains without the extensive computational resources needed for simulations with atomistic models. Using this CG model, we conduct molecular dynamics simulations to assess the effect of varying α-1,3-glucan chain length and hydrogen bond interaction strengths on the aggregation of multiple chains at finite concentrations in implicit solvent. We quantify the hydrogen bonding strength needed for multiple chains to aggregate, the distribution of inter- and intra-chain hydrogen bonds within the aggregate and in some cases, the shapes of the aggregate. We also explore the effect of substitution/silencing of some randomly selected or specific hydrogen bonding sites in the chain on the aggregation and aggregate structure. In the unmodified α-1,3-glucan solution, the inter-chain hydrogen bonds cause the chains to aggregate into sheets. Random silencing of hydrogen bonding donor sites only increases the hydrogen bond strength needed for aggregation but retains the same aggregate structure as the unmodified chains. Specific silencing of the hydrogen-bonding site on the C6 carbon leads to the chains aggregating into planar sheets that then fold over to form hollow cylinders at intermediate hydrogen bond strength - 4.7 to 5.3 kcal mol-1. These cylindrical aggregates assemble end-to-end to form larger aggregates at higher hydrogen bond strengths.

Statistical thermodynamics for the unexpectedly large difference between disaccharide stereoisomers in terms of solubility in water

We unravel the physical origins of the large difference between cellobiose and maltose, which consist of two β-1,4 and α-1,4 linked d-glucose units, respectively, in terms of the solubility in water. We construct a thermodynamic theory where the chemical-potential difference between disaccharides in water and in vacuum is identified as the key free-energy function. Its energetic and entropic components are calculated for cellobiose and maltose by statistical-mechanical theories for solute hydration. The disaccharide structures are taken into account at the atomic level and a molecular model is adopted for water. Molecular dynamics simulations are used to account for the conformational fluctuation of a disaccharide molecule, which also enables us to estimate the conformational entropy. We show that the cellobiose/maltose solubility ratio calculated is in good agreement with the experimental value. The solubility becomes much lower for cellobiose due to conformational-entropy and water-entropy effects. The former effect is relevant to higher stability of the intramolecular hydrogen bond between oxygen atoms in the six-membered ring and in the neighboring hydroxyl group: the hydration alters the fluctuation of a molecular conformation to a larger or less regular one, but the degree of this alteration is smaller. The latter effect is attributed to more separation of two hydroxymethyl groups in a molecule, causing lower probability of the overlap of excluded volumes generated by the groups for water molecules. We suggest that physicochemical properties of disaccharides in water become variable depending on the stereoisomerism through hydration effects and the origins of the variety are entropic.

Molecular docking and molecular dynamics simulation analyses of urea with ammoniated and ammoxidized lignin

Ammoniated lignin, prepared through the Mannich reaction of lignin, has more advantages as a slow-release carrier of urea molecules than ammoxidized lignin and lignin. The advantages of the ammoniated lignin include its amine groups added and its high molecular mass kept as similar as that of lignin. Three organic molecules including guaiacyl, 2-hydroxybenzylamine and 5-carbamoylpentanoic acid are monomers respectively in lignin, ammoniated lignin and ammoxidized lignin. We studied the difference between the interactions of lignin, ammoniated lignin and ammoxidized lignin with respect to urea, based on radial distribution functions (RDFs) results from molecular dynamics (MD) simulations. Glass transition temperature (T_g) and solubility parameter (δ) of ammoniated and ammoxidized lignin have been calculated by MD simulations in the constant-temperature and constant-pressure ensemble (NPT). Molecular docking results showed the interaction sites of the urea onto the ammoniated and ammoxidized lignin and three different interaction modes were identified. Root mean square deviation (RMSD) values could indicate the mobilities of the urea molecule affected by the three different interaction modes. A series of MD simulations in the constant-temperature and constant-volume ensemble (NVT) helped us to calculate the diffusivity of urea which was affected by the content of urea in ammoniated and ammoxidized lignin.