Deviations of dynamic parameters characterizing enthalpic and dielectric relaxations in glass forming alkyl phosphates.

In our recent study [T. Wu et al., J. Chem. Phys. 147, 134501 (2017)], an alkyl phosphate glass former was studied and it suggested that the enthalpy relaxation involving the motions of all parts of the molecule is global, while the dielectric relaxation detects the local rotation of the polar core. In this work, we study a series of trialkyl phosphates using calorimetric and dielectric measurements over a wide temperature range. The results indicate a departure of the dielectric fragility indexes from the enthalpic ones as the length of the branch chain increases in the trialkyl phosphates. The Kirkwood correlation factor (g k ) is found to coincide at ∼0.6 at glass transition temperature (T g ) from triethyl phosphate to tributyl phosphate, indicating a similar structural alignment. The enthalpic relaxation serving as the more fundamental relaxation relevant to the structural relaxation is confirmed. Strikingly, we observed the relation of T g to the chain length in alkyl phosphates, revealing a minimum T g behavior, and its explanation assists in the understanding of the glass transition in relation to the structure of the glass-formers.

Interplay of intermolecular interactions and flexibility to mediate glass forming ability and fragility: A study of chemical analogs.

We have investigated the enthalpic and dielectric relaxations of four groups of quinoline analogs having similar structural properties (i.e., rigidity, stiffness, and bulkiness) but a different steric character and the nature of intermolecular interactions and flexibility. The dielectric fragility index (md) and the enthalpic one (mH), determined by the Tool-Narayanaswamy-Moynihan-Hodge formalism, are comparable. Generally, for the four sets of molecules of similar structures, both the interactions and flexibility are found to be critical in making the large span of fragility (i.e., from 59 to 131) and glass forming ability. By contrast, individual impacts of the interaction and flexibility can only explain fragility partly among each group of isomers. We found that the molecules with high fragility are of relatively low liquid density, reflecting the joint impact of the interactions and flexibility. An interesting result is observed among the isomers that the molecules which are fragile have enhanced glass forming ability. The results are unveiling the joint impacts of molecular structure (flexibility) and intermolecular interaction on the molecular dynamics.

Presence of global and local α-relaxations in an alkyl phosphate glass former.

The dynamics of a molecular glass former, tributyl phosphate (TBP), with an alkyl phosphate structure (three alkyl branches emanating from a polar core of PO4) is studied in the supercooled regime by dielectric and thermal (or enthalpic) relaxations. The dielectric fragility index md and the stretching exponent ßd of the Kohlrausch-Williams-Watts correlation function are determined. Analyses of the enthalpic relaxation data by the Tool-Narayanaswamy-Moynihan-Hodge formalism yield the enthalpic fragility index mH and stretching exponent ßH. The large difference between the dielectric md and the enthalpic mH, as well as between ßd and ßH, is a remarkable finding. The differences are interpreted by the formation of molecular self-assemblies. The interpretation is supported by the quite comparable fragility determined by viscosity and the enthalpic relaxation. The Kirkwood factor calculated at low temperatures is also consistent with the interpretation. The results suggest that the enthalpic relaxation involving the motions of all parts of TBP is global, while the dielectric relaxation detects the local rotation, which might originate from the rotation of the dipole moment of the core. The presence of two structural α-relaxations, one global and one local, with a large difference in dynamics is revealed for the first time in a molecular glass former.

Assessment of PAH toxicity and mutagenicity in emissions from coal and biofuel combustion.

Emissions from combustion of coal, wood and cowdung cakes in domestic cookstoves were sampled through a Stack Monitor on Glass Fibre thimbles. 16 PAH compounds were quantified in the samples extracted in dichloromethane by Gas Chromatography using FID detector. The toxic potencies of the quantified PAHs were determined by Toxicity Equivalence (TEF) Approach and their mutagenecities were tested by using Ames Plate Incorporation Method. All the 16 PAHs were determined in the emission of wood, including genotoxic compounds: carcinogens (BaA, Chy, BbF, BkF, BaP, DbA and IP) and co-carcinogens (Fla, Pyr, BghiP). In coal smoke, only 12 compounds were detected, while in cowdung cake 15 PAHs were detected except Phenanthrene. Emission factors on a fuel weight basis are highest for cowdung cake (120.23 mg/kg), followed by wood (48.97 mg/kg) and coal (28.85 mg/kg). Most of the contribution to the total PAH concentrations was from the high molecular weight compounds. Considering the genotoxic PAHs, the emission factor ranking order was from cow dung cake (115.85 mg/kg) to wood (43.03 mg/kg) and lowest for coal fuel (25.97 mg/kg). The emission factor for BaP was highest for cowdung cake (78.83 mg/kg) followed by coal (5.53 mg/kg) and wood fuel (4.47 mg/kg). Calculation of toxic potencies reveals that the carcinogenic contribution from low molecular weight PAHs is relatively much lower than high molecular weight PAHs for each tested fuel. Cow dung cake extracts did not show mutagenic activity in the Ames Salmonella test probably due to lower concentration of the direct-mutagens like Pyr, Chy. In contrast, the extracts of coal and wood had higher concentrations of direct-mutagens like Pyr, Chy, so positive results were obtained.