MOLECULAR PACKING IN TRIACYL-sn-GLYCEROLS: INFLUENCES OF ACYL CHAIN LENGTH AND UNSATURATION

The molecular packing in triacylglycerols having different acyl chains has been examined by differential scanning calorimetry, powder X-ray diffraction and vibrational spectroscopy (infrared and Raman) techniques. In the triacylglycerols examined, the acyl chain length, unsaturation or the position of substitution on the glycerol were changed systematically to observe their influence on the molecular packing in different polymorphic forms. Variation in the 3-acyl chain length of 1,2-dipalmitoyl-3-acyl-sn-glycerols (PPX) influenced the molecular packing along the long axis in the stable polymorphic forms. Three different modes of packing were observed. If X ≤ 4, the compounds packed in a bilayer structure similar to diacylglycerols, or if X ≥ 10 and ≤ 16 the compounds packed in a bilayer structure but similar to mono acid triacylglycerols. However for intermediate 3-acyl chain lengths, as in PP6 and PP8 the stable packing can occured only through chain segregation resulting in a trilayer structure. In the triacylglycerols containing unsaturated acyl chains, 1,2-dioleoyl-3-acyl-sn-glycerols (00X) and 1,3-dioleoyl-Z-acyl-sn-glycerols (0X0) the stable polymorphic forms packed in a trilayer structure where the odd acyl chains segregated and formed a middle layer. In a metastable hexagonal packing (α-phase) the long range ordering is minimal. Because of this lack of specific chain-chain interaction the 3-short acyl chain compounds of PPX packed in a unimolecular length structure (except PP2) whereas the 3-long acyl chain compounds packed in a bilayer structure. In orthorhombic perpendicular and triclinic parallel packing where the specific chain-chain interaction is increased, the end plane methyl packing and the glycerol conformation played important roles in the formation of bi-, tri- and hexalayer structures. The driving force in the formation of these different structures is to minimize the crystal defects created by the odd acyl chains and to enhance the specific chain-chain interactions. The presence of an odd acyl chain influenced the lateral chain packing as well, e.g., the stability of the orthorhombic perpendicular packing is enhanced by the presence of an odd acyl chain and even in some cases it is favored over the triclinic parallel packing. The odd acyl chain at the 1- or 3position of -sn-glycerol stabilized the orthorhombic perpendicular packing. This indicates the glycerol conformation is probably perpendicular to the layer plane and thus is different from the monoacid triacylglycerols.     

Synthesis of twist-twist pi-conjugated di-sec-alkylstilbenes and stilbene polymers

Twist-twist pi-conjugated (TTPC) pi systems promise unique properties with their 90 degrees twist angles. Di-sec-alkyl substituted stilbenes, 5, were prepared by low-valent titanium coupling of phenyl ketones, 4. Long alkyl chains stopped the coupling reaction. Stilbenes 5 were shown to be approximately 90% TTPC. Inserting TTPC units into poly(p-phenylene) polymers created highly fluorescent, soluble, TTPC pi systems with weak electronic segmentation for organic light emitting diode (OLED) studies.     

An improved detailed chemical kinetic model for C3-C4 linear and iso-alcohols and their blends with gasoline at engine-relevant conditions

Propanol and butanol isomers have received significant research attention as promising fuel additives or neat biofuels. Robust chemical kinetic models are needed that can provide accurate and efficient predictions of combustion performance across a wide range of engine relevant conditions. This study seeks to improve the understanding of ignition and combustion behavior of pure C3-C4 linear and iso-alcohols, and their blends with gasoline at engine-relevant conditions. In this work, a kinetic model with improved thermochemistry and reaction kinetics was developed based on recent theoretical calculations of H-atom abstraction and peroxy radical reaction rates. Kinetic model validations are reported, and the current model reproduces the ignition delay times of the C3 and C4 alcohols well. Variations in reactivity over a wide range of temperatures and other operating conditions are also well predicted by the current model. Recent ignition delay time measurements from a rapid compression machine of neat iso-propanol and iso-butanol [Cheng et al., Proc. Combust Inst. (2020)] and blends with a research grade gasoline [Goldsborough et al., Proc. Combust Inst. (2020)] at elevated pressure (20–40 bar) and intermediate temperatures (780–950 K) were used to demonstrate the accuracy of the current kinetic model at conditions relevant to boosted spark-ignition engines. The effects of alcohol blending with gasoline on the autoignition behavior are discussed. The current model captures the suppression of reactivity in the low-temperature and negative-temperature-coefficient (NTC) region when either isopropanol and isobutanol are added to a research grade gasoline. Sensitivity and reaction flux analysis were performed to provide insights into the relevant fuel chemistry of the C3-C4 alcohols.     

Experimental and kinetic modeling study of tetralin: A naphtheno-aromatic fuel for gasoline,jet and diesel surrogates

Distillate fuels contain significant proportions of naphtheno-aromatic components and tetralin is a suitable surrogate component to represent this molecular moiety. The presence of aromatic and naphthyl rings makes kinetic modeling of tetralin very challenging. Primary radicals formed during the oxidation of tetralin can be aryl, benzylic or paraffinic in nature. Using available information on reaction paths and rate constants of naphthenes and alkyl-aromatics, a kinetic model of tetralin has been developed in the current study with emphasis on low-temperature chemistry and high-pressure conditions. Due to the lack of high-level quantum chemical calculations on reaction pathways of tetralin, analogous rates from ab-initio studies on benzylic and paraffinic radicals have been adopted here. Some modifications to the reaction rate rules are incorporated to account for the unique characteristics of tetralin's molecular structure. Important reaction channels have been identified using reaction path and brute force sensitivity analyses. In order to investigate the model performance at low temperatures, new experiments are carried out in a rapid compression machine on blends of tetralin and 3-methylpentane. Blending of low-reactivity tetralin with a high-reactivity alkane allowed the investigation of tetralin ignition at very low temperatures (665 – 856 K). The kinetic model developed in the current study is found to predict the current experiments and literature data adequately. The new model will aid in high-fidelity surrogate predictions at engine-relevant conditions.     

A comprehensive experimental and improved kinetic modeling study on the pyrolysis and oxidation of propyne

To improve our understanding of the combustion characteristics of propyne, new experimental data for ignition delay times (IDTs), pyrolysis speciation profiles and flame speed measurements are presented in this study. IDTs for propyne ignition were obtained at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10 and 30 bar, over a wide range of temperatures (690–1460 K) using a rapid compression machine and a high-pressure shock tube. Moreover, experiments were performed in a single-pulse shock tube to study propyne pyrolysis at 2 bar pressure and in the temperature range 1000–1600 K. In addition, laminar flame speeds of propyne were studied at an unburned gas temperature of 373 K and at 1 and 2 bar for a range of equivalence ratios. A detailed chemical kinetic model is provided to describe the pyrolytic and combustion characteristics of propyne across this wide-ranging set of experimental data. This new mechanism shows significant improvements in the predictions for the IDTs, fuel pyrolysis and flame speeds for propyne compared to AramcoMech3.0. The improvement in fuel reactivity predictions in the new mechanism is due to the inclusion of the propyne + H?₂ reaction system along with ?H radical addition to the triple bonds of propyne and subsequent reactions.     

An Ethenoadenine FAD Analog Accelerates UV Dimer Repair by DNA Photolyase

Reduced anionic flavin adenine dinucleotide (FADH^?) is the critical cofactor in DNA photolyase (PL) for the repair of cyclobutane pyrimidine dimers (CPD) in UV‐damaged DNA. The initial step involves photoinduced electron transfer from *FADH^? to the CPD. The adenine (Ade) moiety is nearly stacked with the flavin ring, an unusual conformation compared to other FAD‐dependent proteins. The role of this proximity has not been unequivocally elucidated. Some studies suggest that Ade is a radical intermediate, but others conclude that Ade modulates the electron transfer rate constant (k_ET) through superexchange. No study has succeeded in removing or modifying this Ade to test these hypotheses. Here, FAD analogs containing either an ethano‐ or etheno‐bridged Ade between the AN1 and AN6 atoms (e‐FAD and ε‐FAD, respectively) were used to reconstitute apo‐PL, giving e‐PL and ε‐PL respectively. The reconstitution yield of e‐PL was very poor, suggesting that the hydrophobicity of the ethano group prevented its uptake, while ε‐PL showed 50% reconstitution yield. The substrate binding constants for ε‐PL and rPL were identical. ε‐PL showed a 15% higher steady‐state repair yield compared to FAD‐reconstituted photolyase (rPL). The acceleration of repair in ε‐PL is discussed in terms of an ε‐Ade radical intermediate vs superexchange mechanism.     

A Proactive Crew Recovery Decision Support Tool for Commercial Airlines During Irregular Operations

In this paper, a decision support tool that automates crew recovery during irregular operations for large-scale commercial airlines is presented. The tool is designed for airlines that adopt the hub-spoke network stru cture. The advance of this tool over the existing ones is that it recovers projected crew problems that arise due to current system disruptions. In other words, it proactively recovers crew problems ahead of time before their occurrence. In addition, it gives a wide flexibility to react to different operation scenarios. Also, it solves for the most efficient crew recovery plan with the least deviation from the originally planned schedule. The tool adopts a rolling approach in which a sequence of optimization assignment problems is solved such that it recovers flights in chronological order of their departure times. In each assignment problem, the objective is to recover as many flights as possible while minimizing total system cost resulting from resource reassignments and flight delays. The output of this tool is in the form of new crew trippairs that cover flights in the considered horizon. A test case is presented to illustrate the model capabilities to solve a real-life problem for one of the major commercial airlines in the U.S.     

Contact angles of drops on curved superhydrophobic surfaces

Superhydrophobic surfaces have contact angles that exceed 150 degrees and are known to reduce surface fouling, protect surfaces, and improve liquid-liquid separations. Electrospun sub-micron fiber mats can perform as superhydrophobic surfaces. Superhydrophobic behavior is typically measured on planar surfaces, whereas applications may require curved surfaces. This paper discuses the measurement of water contact angles of fiber mats formed on cylindrical surfaces to create superhydrophobic behavior on curved surfaces. Equations are derived that relate the radius of curvature of spherical and cylindrical surfaces and drop size to the observed contact angle on the curved surfaces. Calculations from the equations agree well with experimental observations on spherical surfaces reported in literature and on cylindrical surfaces created in our lab.     

Ion-implantation-induced structural modifications in Y1Ba2Cu3O7−δ superconductor

Ion-implantation-induced structural modifications in Y₁Ba₂Cu₃O_7−δ superconductor are examined by a grazing angle X- ray diffraction technique. By employing a range of grazing angles from 0.3° to 10° it is shown that 100 KeV Ar⁺ inplantation of the superconductor leads to amorphization as well as modification of grain size and orientation at dose values lower than 10¹⁶ ions/ cm². At the dose of 5 × 10¹⁶ ions/ cm² the X-ray diffraction intensity is a factor of 6 less as compared to the original pellet, though the lines themselves are sharp. This shows coexistence of perovskite grains and amorphous matrix.     

Room temperature LPG resistive sensor based on the use of a few-layer graphene/SnO<Subscript>2</Subscript> nanocomposite

A nanocomposite consisting of a few layers of graphene (FLG) and tin dioxide (SnO₂) was prepared by ultrasound-assisted synthesis. The uniform SnO₂ nanoparticles (NPs) on the FLG were characterized by X-ray diffraction in terms of lattice and phase structure. The functional groups present in the composite were analyzed by FTIR. Electron microscopy (HR-TEM and FE-SEM) was used to study the morphology. The effect of the fraction of FLG present in the nanocomposite was investigated. Sensitivity, selectivity and reproducibility towards resistive sensing of liquid propane gas (LPG) was characterized by the I-V method. The sensor with 1% of FLG on SnO₂ operated at a typical voltage of 1 V performs best in giving a rapid and sensitive response even at 27 °C. This proves that the operating temperature of such sensors can be drastically decreased which is in contrast to conventional metal oxide LPG sensors.

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Graphical abstract Schematic of a room temperature gas sensor for liquefied petroleum gas (LPG). It is based on the use of a few-layered graphene (1 wt%)/SnO₂ nanocomposite that was deposited on an interdigitated electrode (IDEs). A sensing mechanism for LPG detection has been established.