The intra- and intermolecular interactions of chitin in NaOH/urea aqueous system were studied by a combination of NMR measurements (including 13C NMR, 23Na NMR, and 15N NMR) and differential scanning calorimetry. The results revealed that the NaOH and chitin formed a hydrogen-bonded complex that was surrounded by the urea hydrates to form a sheath-like structure, leading to the good dissolution. The optimal concentration range, in which chitin was molecularly dispersed in NaOH/urea aqueous, was found to investigate the chain conformation in the dilute solution with a combination of static and dynamic light scattering. The weight-average molecular weight (Mw), radii of gyration (⟨Rg⟩z), and hydrodynamic radii (⟨Rh⟩z) values of chitin were determined, and the structure-sensitive parameter (ρ) and persistent length (Lp) were calculated to be >2.0 and ∼30 nm, respectively, suggesting an extended wormlike chain conformation. The visualized images from TEM, cryo-TEM, and AFM indicated that, chitin nanofibers were fabricated from the parallel aggregation of chitin chains in NaOH/urea system. This work would provide a theoretical guidance for constructing novel chitin-based nanomaterials via "bottom-up" method at the molecular level.
The dissolution of cellulose in NaOH/urea aqueous solution at low temperature is a key finding in cellulose science and technology. In this paper, 15N and 23Na NMR experiments were carried out to clarify the intermolecular interactions in cellulose/NaOH/urea aqueous solution. It was found that there are direct interactions between OH– anions and amino groups of urea through hydrogen bonds and no direct interaction between urea and cellulose. Moreover, Na+ ions can interact with both cellulose and urea in an aqueous system. These interactions lead to the formation of cellulose–NaOH–urea–H2O inclusion complexes (ICs). 23Na relaxation results confirmed that the formation of urea–OH– clusters can effectively enhance the stability of Na+ ions that attracted to cellulose chains. Low temperature can enhance the hydrogen bonding interaction between OH– ions and urea and improve the binding ability of the NaOH/urea/H2O clusters that attached to cellulose chains. Cryo-TEM observation confirmed the formation of cellulose–NaOH–urea–H2O ICs, which is in extended conformation with mean diameter of about 3.6 nm and mean length of about 300 nm. Possible 3D structure of the ICs was proposed by the M06-2X/6-31+G(d) theoretical calculation, revealing the O3H···O5 intramolecular hydrogen bonds could remain in the ICs. This work clarified the interactions in cellulose/NaOH/urea aqueous solution and the 3D structure of the cellulose chain in dilute cellulose/NaOH/urea aqueous solution.
According to the case that mass concrete in solid sections of lower pylon column of Jiashao Bridge was easy to crack in construction because of the bigger section size, larger content of cementitious material and lower water binder ratio, the temperature and thermal stresses distribution of mass concrete was simulated and temperature control scheme was adjusted constantly based on the results of field temperature monitoring. Through taking some temperature control measures such as applying circulating cooling water and prolonging the time appropriately, thermal insulation and moisture retention curing, extending the form removal time and controlling the quality of concrete, harmful cracks did not appear in solid sections of lower pylon column of Jiashao Bridge and anticipated temperature control requirements were achieved.
Abstract Low‐field nuclear magnetic resonance technique combined with chemometrics was employed to establish the quantitative models for rapidly monitoring the changes of indicators (hydroxyl value (OHV), epoxy value (EV), polymeric hydroxyl value (POHV, reflecting the degree of polyetherification)) during the ring opening of epoxidized soybean oils. Comprehensively considering the variations of peak retention times and peak areas of LF‐NMR spectra, OHVs of vegetable oils‐based polyols were divided into low OHV (LOHV, 1.7–91.9 mg KOH/g) group and high OHV (HOHV, 94.1–229.4 mg KOH/g) group. In both LOHV and HOHV groups, OHV presented good linearity with T 2w , S 23 , and S Total of relaxation property with correlation coefficient of >0.90. EV and POHV also presented good linearity with relaxation property in both LOHV and HOHV groups. In LOHV group, data segment 1–1000 ms was taken with Deresolve, no preprocessing, and Deresolve as preprocessing methods for OHV, EV, and POHV, respectively, and partial least square (PLS) as the modeling method for all the three indicators, where RMSEPs of OHV, EV, and POHV models were 4.282, 0.114, and 4.061, respectively. In HOHV group, data segment 1–1000 ms was taken with no preprocessing as the preprocessing method and PLS as the modeling method for all the three indicators, where RMSEPs of OHV, EV, and POHV models were 3.117, 0.088, and 4.964, respectively. The established method will benefit polyol industry to online adjust the production parameters according to the monitoring results and achieve high‐quality of vegetable oils‐based polyols.
Quantum chemistry calculations were used to study the structure and energy of strontium (Sr) ion hydrated clusters [Sr(H2O)1−25]2+. The saturated hydration number of the first hydration layer of Sr2+ was 8, and the hydration distance was 2.58 Å. The second hydration layer had 1–9 hydration numbers, and the hydration distance was in the range of 4.4–4.6 Å. This work also developed the relationship between the thermodynamic data (average water binding energy En and successive water binding energy ΔEn,n−1, etc.) of the aforementioned low-energy structure and the hydration structures. The first hydration layer was formed by the strong electrostatic interaction between Sr2+ and water molecules, and the decrease in ΔEn,n−1 was relatively large. Hydrogen bonds were formed between water molecules of the second hydration layer and water molecules of the inner layer, and the decrease in ΔEn,n−1 was relatively small. When one water molecule was added beyond the second hydration layer, ΔEn,n−1 was close to the hydrogen bond energy 8.88 kcal/mol (37.1 kJ/mol) of dimer water molecule, indicating that there was very weak interaction between Sr2+ and the water molecules beyond the second hydration layer.
Four kinds of π-complexation adsorbents are synthesized via ion exchange method or incipient wetness impregnation method with Amberlyst 35, SBA-15, TUD-1, and KIT-6 as supports, and AgNO3 as active ingredient. The samples are characterized by N2 adsorption/desorption. Fourier transformed infrared spectroscopies, transmission electron microscopy, X-ray diffraction spectrum analysis, and coupled plasma optical emission spectrometry are also used as adsorbents for ethylene/ethane adsorptive separation. The results show that the specific surface area and the dispersion of silver ions affect the separation performance of the adsorbent. Ag–Amberlyst 35 has the highest ethylene/ethane selectivity among these adorbents while the adsorbed amounts of ethylene of the three mesoporous silica complexation adsorbents are higher. Adsorption thermodynamics analysis suggests that the interaction of ethylene with adsorbent is a mild chemical adsorption. An adsorption kinetics study indicates that the adsorption of ethylene on silver-supported mesoporous materials is not a simple diffusion-control process. The adsorption behavior of ethylene on the π-complexation adsorbent has an energy barrier in the range of 24–33 kJ/mol. Among the adsorbents in this work, the KIT-6-based adsorbent has the best mass kinetic performance due to its three-dimensional regular interconnected mesoporous structure.
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