Current Physical Chemistry - Volume 11, Issue 3, 2021

Aims: The aim is to search for newer and better antioxidants through kinetic spectroscopic studies in combination with product analysis and computation. Background: Antioxidant effect of caffeic acid, its derivative, and analogues have been well reported. The anti-oxidative efficiencies are related to their molecular structure, and two reaction pathways are well accepted, H-atom transfer (HAT) or single electron transfer. 1-hydroxy ethyl radical (1-HER) being ethanol-derived free radical might be causing the onset of liver injury detected after alcohol administration. 1-HER has also been reported to react with fatty acids and endogenous antioxidants such as glutathione, ascorbic acid and alpha-tocopherol. Objective: The present study is an attempt to understand the reaction mechanism of 1- HER with caffeic acid, its derivative and analogues in detail. Methods: Pulse radiolysis with kinetic absorption spectroscopy has been employed to follow the reaction pathway and identify the intermediates produced in the reaction. The reaction products have been detected using LCMS/MS. Based on these studies, a consolidated mechanism has been proposed. Cyclic voltammetry measurements and computational calculations have been used in support of the proposed mechanism. Results: In the reaction of 1-hydroxy ethyl radical (1-HER) with caffeic acid and its oligomers, reduction takes place below the pKa1, while oxidation occurs with the deprotonated phenolic moiety. The reduction of caffeic acid generates a carbon-centered radical at the double bond of the side chain with a bimolecular rate constant of 1.4x10¹⁰ dm³ mol^-1 s^-1. Notably, a low concentration of oxygen is able to regenerate a part of the caffeic acid molecules in the reduction process. At pH 10 a phenoxyl radical is formed due to oxidation with a much lower bimolecular rate constant of (4.2x10⁸ dm³ mol^-1 s^-1). In the case of di-hydrocaffeic acid, only phenoxyl radical is formed at pH 10 and, no reaction could be observed below pH 8. Conclusion: Change in reactive pattern from reduction to oxidation with change in pH within same set of reactants has been evidently established in the present study. The results point towards the importance of α−β unsaturation in the side chain of caffeic acid oligomers for their reaction with 1-HER at neutral pH. The effect of oxygen concentration on the antioxidative protection offered by this class of molecules might be intriguing for the quest of the effectiveness of antioxidants at low concentration. Other: It may be inferred that the effect of pH on the reactivity pattern as observed is not 1-HER, but substrate specific, in the present case phenolic acids. This study generates further scope for in-depth studies on other polyphenols where unsaturation exists in the side chain.

Background: The aim of this work is to propose an approach for estimating the Joule-Thomson coefficient as an important parameter necessary to the study of changes in fluid temperature at a given change in pressure at constant enthalpy. Objective: The analytical approach presented in this work is very appropriate for detailed studies of the Joule-Thomson inversion temperature at zero pressure for arbitrary temperature values. Methods: A new approach is suggested for the accurate determination of the Joule- Thomson inversion temperature at zero pressure using virial coefficient of the Lennard- Jones (12-6) potential. Results: The usefulness and efficiency of the method are tested by application to various gases Ar, He, Ne, H2, O2, CO2 CO, CH4, Xe, Kr, N2 and Air. The results obtained are in well agreement with other approximation and experimental data. Conclusion: The suggested formula enables correct and rapid calculation of the JT inversion temperature at zero pressure.

Background: The non-covalent molecular interactions of 1-Ethyl-3- methylimidazolium chloride-aluminum chloride and pure alcohols are attracting attention in the industry, academic and research. Chemists, engineers, designers, and some researchers are interested in accessing trustworthy databases. Objective: 1-Ethyl-3-methylimidazolium chloride-aluminum chloride interacts with pure alcohols with non-covalent interactions. Physicochemical properties with their convincing data interpret the interactions occurring there. Methods: For that limiting apparent molar volume, molar refraction, and limiting apparent molar isentropic compressibility of the binary systems viz., {([EMIm]Cl/AlCl3) +methanol}, {([EMIm]Cl/AlCl3) +ethanol}, {([EMIm]Cl/AlCl3) +1-propanol}, and {([EMIm]Cl/AlCl3)+ 1-butanol} have been calculated using physicochemical properties i.e.,. density, refractive index, and speed of sound, respectively within the temperature range T=293.15K-318.15K (with the interval of 5K). Results: The ionic liquid strongly interacts with 1-butanol (10⁶φ^ov =874.52 m³ mol^-1, 10⁶·RM = 211.13 m³ mol^-1, and 10^-11·φ^ok= -0.10 m3 mol^-1 Pa^-1, 10⁸· (∂φ^oE/∂T)P = 1.52 m³ mol^-1 K^-2) than other chosen primary alcohol at a higher temperature (318.15K). Among the individual ions, the 10⁶·φ^ov(ion) is higher for ALCI4-(522.96 m³ mol^-1) than [EMIm]+ (351.56 m³ mol^-1) at high temperature (318.15K) in 1-butanol. Conclusion: The molecular interactions occurring between the ionic liquid and solvent molecules are due to the structure-making capacity caused by intermolecular forces and non-covalent interactions. Where the 1-butanol strongly interacts with ionic liquids. In between the ions, the anaion interaction is greater than cation to solvents.

Introduction: Flory’s statistical theory (FST), for the first time, has been applied successfully to two pure ionic liquids, [C3mim][NTf2] and [C5mim][NTf2], in an extended range of pressure (0.10 – 59.9 MPa) and at different temperatures (298.15 – 333.15 K). Methods: Density and sound speed data have been employed to compute a number of useful and important properties of these ionic liquids in the light of FST. Using Flory parameters (P*, T*, V*, P, T, V), the expression for the surface tension (σ) has been deduced in the form σ = σ* σ (V), with σ* and σ (V) being the characteristic and reduced values of surface tension. Since the experimental σ of liquids is not known, the validity of FST has been tested by calculating “u” using four different u-ρ- σ correlations, namely Auerbach (1948), Altenberg (1950), Singh et al. (1997), and Modified Auerbach (2016). Results: Several useful and important properties of ionic liquids, under varying physical conditions, have been deduced and compared to the observed ones with a quite satisfactory agreement. Such properties include Pint, van der Waals constants (a & b), parachor [P], Eötvas constant (kB), energy (ΔEV) and heat of vaporization (ΔHV), cohesive energy density (ced), polarity index (n), and solubility parameter (δ). Conclusion: In this study, the validity of FST to two ionic liquids under the present study has been confirmed.

Aims: The aim of this study is to show the anti-corrosive properties of 1,3- dimethyl-7H-purine-2,6-dione on aluminium corrosion in 1 M hydrochloric acid and to study the synergy effect between iodide ions and this molecule. Background: There is a need for a research on eco-friendly, low toxic, and biodegradable corrosion inhibitors capable of protecting metals in order to support industrialists who spend large sums of money on replacing their corroded equipment. Objective: The main objective is to study the anti-corrosive properties of theophylline on aluminium corrosion in 1 M HCl. Methods: The anti-corrosive properties of theophylline on aluminium corrosion in 1 M HCl were evaluated using mass loss, Density Functional Theory at B3LYP/6-31G (d), and Quantitative Structure-Property Relationship methods. Results: The results obtained show that theophylline inhibition efficiency increases with concentration but decreases with increasing temperature with a maximum value of 88% for 5.10^-3M at T = 298 K. The result from absorption isotherms reveals that theophylline adsorbs to the aluminium surface according to the modified Langmuir isotherm. Adejo Ekwenshi's isotherm has shown that the molecule adsorption on aluminium is essentially of a physical nature. Thermodynamic adsorption and activation parameters were calculated and analyzed. A synergistic effect between the studied molecule and the iodide ions was found. Furthermore, global and local reactivity were analyzed through density functional theory calculations. The quantitative structure-property relationship model has been used to correlate experimental and theoretical inhibition efficiencies. Conclusion: Theophylline is an excellent aluminum corrosion inhibitor in the current studied solution. Theoretical results were in agreement with the experimental data. Other: Finally, the best set of parameters to model the inhibition efficiency of analogous molecules were found.