Solvent-Free Formation of Cyclodextrin-Based Pseudopolyrotaxanes of Polyethylene Glycol: Kinetic and Structural Aspects

Pseudopolyrotaxanes (PPRs) are supramolecular structures consisting of macrocycles able to thread on a linear polymer chain in a reversible, non-covalent way, often referred to in the literature as “molecular necklaces”. While the synthesis and reaction mechanisms of these structures in solution have been widely described, their solvent-free production has received little attention, despite the advantages that this route may offer. We propose in this work a kinetic mechanism that describes the PPR formation in the solid phase as a process occurring in two consecutive stages. This mechanism has been used to investigate the spontaneous formation of a PPR that occurs when grinding α-Cyclodextrin (α-CD) with polyethylene glycol (PEG). In the threading stage, the inclusion of the polymer and subsequent release of the water molecules lodged in the cavity of the macrocycle cause vibrational changes that are reflected in the time-dependence of the FTIR-ATR spectra, while the further assembly of PPRs to form crystals produces characteristic reflections in the XRD patterns, due to the channel-like arrangement of CDs, that can be used to track the formation of the adduct in crystalline form. The effects that working variables have on the kinetics of the reaction, such as temperature, feed ratio, molar mass of the polymer and the introduction of an amorphous block in the polymer structure, have been investigated. The rate constants of the threading step increase with the temperature and the activation energy of the process increases at lower proportions of CD to PEG. This is attributed to the lower degree of covering of the polymer chain with CDs that reduces the hydrogen-bonding driven stabilization between adjacent macrocycles. The formation of crystalline PPR, which takes place slowly at room temperature, is markedly promoted at higher temperatures, with lower proportions of CD favoring both the formation and the growth of the crystals. The molar mass of the polymer does not modify the typical channel-like arrangement of packed PPRs but the conversion into crystalline PPR diminishes when using PEG1000 instead of PEG400. At a microscopic level, the crystals arrange into lamellar structures, in the order of hundreds of nm, embedded in an amorphous-like matrix. The introduction of a polypropylene oxide block in the structure of the polymer (Pluronic L62) renders poorer yields and a considerable loss of crystallinity of the product of the reaction. The methodology here proposed can be applied to the general case of inclusion complexes of CDs with drugs in the solid phase, or to multicomponent systems that contain polymers as excipients in pharmaceutical formulations along with CDs.

Effect of Pre-Injection on Combustion and Emission Characteristics of a Diesel Engine Fueled with Diesel/Methanol/n-Butanol Blended Fuel

In this study, the combustion and emission characteristics of a diesel/methanol/n-butanol blended fuel engine with different pre-injection timings and pre-injection mass ratios were investigated by a computational fluid dynamics (CFD) model. The CFD model was verified by the measured results and coupled with a simplified chemical kinetics mechanism. Firstly, the corresponding three-dimensional CFD model was established by CONVERGE software and the CHEKMIN program, and a chemical kinetic mechanism containing 359 reactions and 77 species was developed. Secondly, the combustion and emission characteristics of the diesel engine with different diesel/methanol/n-butanol blended fuels were analyzed and discussed. The results showed that increases in the pre-injection timing and the pre-injection mass ratio could increase cylinder pressure and cylinder temperature and decrease soot, HC, and CO emissions. At 100% load, the maximum cylinder pressures at the start of pre-injection timing from −15 °CA to −45 °CA, were 7.71, 9.46, 9.85, 9.912, and 9.95 MPa, respectively. The maximum cylinder pressures at pre-injection fuel mass ratios from 0.1 to 0.9 were 7.98, 9.10, 9.96, 10.52, and 11.16 MPa, respectively. At 50% load, with increases of the pre-injection timing and pre-injection fuel mass ratio, the soot emission decreased by 7.30%, 9.45%, 27.70%, 66.80%, 81.80% and 11.30%, 20.03%, 71.32%, 83.80%, 93.76%, respectively, and CO emissions were reduced by 5.77%, 12.31%, 22.73%, 53.59%, 63.22% and 8.29%, 43.97%, 53.59%, 58.86%, 61.18%, respectively. However, with increases of the pre-injection timing and pre-injection mass ratio, NOx emission increased. In addition, it was found that the optimal pre-injection timing and optimal pre-injection mass ratio should be −30 °CA and 0.5, respectively. Therefore, through this study we can better understand the potential interaction of relevant parameters and propose pre-injection solutions to improve combustion and emission characteristics.

Bentonite blended with bagasse ash as an adsorbent for reactive red 198 dyes

Adsorption offers efficient, cost-effective, and eco-friendly method for the treatment of dye-laden wastewater. This work presents, reactive red 198 (RR198) removal by adsorption using bentonite clay (BC) blended with sugar cane bagasse ash (SCBA). The adsorbent's surface morphologies, crystalline phase structures, functional groups, and specific surface before and after adsorption were examined using SEM, XRD, FTIR, and BET respectively. Central composite design (CCD) under response surface methodology (RSM) was applied to optimize independent and dependent variable values. The optimal parameters for RR198 removal using the blended adsorbent were 107 minutes contact time, 0.934 g/L adsorbent dose, and 15 mg/L initial dye concentration, and 85.2% RR198 removal efficiency was achieved. The sorption isotherms and kinetics were evaluated using various existing models. The Freundlich isotherm model (R2 = 0.95) and the pseudo-second-order equation best described the adsorption parameters and the RR198 adsorption kinetic mechanism, respectively. Desorption and reusability experiments in batch study confirmed that BC blended with SCBA can be used multiple times for dye removal from wastewater.

Analysis of the Influence of Droplet Breakup Time using Kelvin-Helmholtz Model, on the Diesel Spray Formation, Evaporation and Combustion

A numerical simulation of the intake and compression stroke and fuel spray and combustion in a direct injection compression ignition engine was performed using the Converge CFD software. For this purpose, the Reynolds Average Navier-Stokes (RANS) k-e RNG model and an n-dodecane kinetic mechanism were used in order to obtain the flow fields in the cylinder and to perform the breakup time analysis of the Kelvin-Helmholtz model in the variables related to the spray and combustion. The turbulent flow inside the cylinder was analysed, obtaining consistent results with experimental pressure data and other research authors. The droplet breakup time is evaluated as a function of the breakup time constant (B1), the initial droplet radius (ro), the wavelength and the maximum growth rate ( . The results indicate that the numerical method and the models used in this work are adequate to perform subsequent representative combustion analyses with values of B1=7. It was possible to show that the formation of the species OH is greater for low values of B1. Also, higher values of the breakup time, variables such as temperature, pressure, fuel evaporation, ignition delay, and species formation are affected.

Identification and Modelling of Chlorine Decay Mechanisms in Reclaimed Water Containing Ammonia

Keeping an effective disinfectant residual concentration in reclaimed water is still a challenge, due to its high levels of ammonia and organic matter when compared with those in drinking water. This research proposes the integration of the reaction schemes of monochloramine auto-decomposition with an empirical kinetic mechanism accounting for reactive chlorine species decay in the presence of organic matter, for which three mechanisms were hypothesized and tested. A parallel second order mechanism, where monochloramine reacts both with fast and slow organic matter reactive fractions, was identified as the most suitable. The model, comprising two rate constants and two fictive concentrations of organic matter as parameters, was further successfully calibrated with real reclaimed waters with two initial free chlorine doses of 8.01 × 10−5 M (≈5 mg/L) and 2.67 × 10−4 M (≈20 mg/L). The proposed model is believed to support future studies aiming to predict and manage chlorine decay in reclaimed water distribution systems.

Pre-Steady-State Kinetics of the SARS-CoV-2 Main Protease as a Powerful Tool for Antiviral Drug Discovery

The design of effective target-specific drugs for COVID-19 treatment has become an intriguing challenge for modern science. The SARS-CoV-2 main protease, Mpro, responsible for the processing of SARS-CoV-2 polyproteins and production of individual components of viral replication machinery, is an attractive candidate target for drug discovery. Specific Mpro inhibitors have turned out to be promising anticoronaviral agents. Thus, an effective platform for quantitative screening of Mpro-targeting molecules is urgently needed. Here, we propose a pre–steady-state kinetic analysis of the interaction of Mpro with inhibitors as a basis for such a platform. We examined the kinetic mechanism of peptide substrate binding and cleavage by wild-type Mpro and by its catalytically inactive mutant C145A. The enzyme induces conformational changes of the peptide during the reaction. The inhibition of Mpro by boceprevir, telaprevir, GC-376, PF-00835231, or thimerosal was investigated. Detailed pre–steady-state kinetics of the interaction of the wild-type enzyme with the most potent inhibitor, PF-00835231, revealed a two-step binding mechanism, followed by covalent complex formation. The C145A Mpro mutant interacts with PF-00835231 approximately 100-fold less effectively. Nevertheless, the binding constant of PF-00835231 toward C145A Mpro is still good enough to inhibit the enzyme. Therefore, our results suggest that even noncovalent inhibitor binding due to a fine conformational fit into the active site is sufficient for efficient inhibition. A structure-based virtual screening and a subsequent detailed assessment of inhibition efficacy allowed us to select two compounds as promising noncovalent inhibitor leads of SARS-CoV-2 Mpro.