Showing papers on "Standard molar entropy published in 2021"

Understanding conformational entropy in small molecules

Abstract: The calculation of the entropy of flexible molecules can be challenging, since the number of possible conformers can grow exponentially with molecule size and many low-energy conformers may be thermally accessible. Different methods have been proposed to approximate the contribution of conformational entropy to the molecular standard entropy, including performing thermochemistry calculations with all possible stable conformations and developing empirical corrections from experimental data. We have performed conformer sampling on over 120,000 small molecules generating some 12 million conformers, to develop models to predict conformational entropy across a wide range of molecules. Using insight into the nature of conformational disorder, our cross-validated physically motivated statistical model gives a mean absolute error of ∼4.8 J/mol·K or under 0.4 kcal/mol at 300 K. Beyond predicting molecular entropies and free energies, the model implies a high degree of correlation between torsions in most molecules, often assumed to be independent. While individual dihedral rotations may have low energetic barriers, the shape and chemical functionality of most molecules necessarily correlate their torsional degrees of freedom and hence restrict the number of low-energy conformations immensely. Our simple models capture these correlations and advance our understanding of small molecule conformational entropy.

Calculation of the Vapour Pressure of Organic Molecules by Means of a Group-Additivity Method and Their Resultant Gibbs Free Energy and Entropy of Vaporization at 298.15 K.

Abstract: The calculation of the vapour pressure of organic molecules at 298.15 K is presented using a commonly applicable computer algorithm based on the group-additivity method. The basic principle of this method rests on the complete breakdown of the molecules into their constituting atoms, further characterized by their immediate neighbour atoms. The group contributions are calculated by means of a fast Gauss-Seidel fitting algorithm using the experimental data of 2036 molecules from literature. A ten-fold cross-validation procedure has been carried out to test the applicability of this method, which confirmed excellent quality for the prediction of the vapour pressure, expressed in log(pa), with a cross-validated correlation coefficient Q2 of 0.9938 and a standard deviation σ of 0.26. Based on these data, the molecules' standard Gibbs free energy ΔG°vap has been calculated. Furthermore, using their enthalpies of vaporization, predicted by an analogous group-additivity approach published earlier, the standard entropy of vaporization ΔS°vap has been determined and compared with experimental data of 1129 molecules, exhibiting excellent conformance with a correlation coefficient R2 of 0.9598, a standard error σ of 8.14 J/mol/K and a medium absolute deviation of 4.68%.

Removal of Nickel from Aqueous Solutions by Natural Bentonites from Slovakia

Abstract: In this study, the removal of nickel (Ni(II)) by adsorption from synthetically prepared solutions using natural bentonites (Lieskovec (L), Hlinik nad Hronom (S), Jelsový Potok (JP), and Stara Kremnicka (SK)) was investigated. All experiments were carried out under batch processing conditions, with the concentration of Ni(II), temperature, and time as the variables. The adsorption process was fast, approaching equilibrium within 30 min. The Langmuir maximum adsorption capacities of the four bentonite samples used were found to be 8.41, 12.24, 21.79, and 21.93 mg g-1, respectively. The results best fitted the pseudo-second-order kinetic model, with constant rates in a range of 0.0948-0.3153 g mg-1 min. The effect of temperature was investigated at temperatures of 20, 30, and 40 °C. Thermodynamic parameters, including standard enthalpy (ΔH0), Gibbs energy (ΔG0), and standard entropy (ΔS0), were calculated. The adsorption of Ni(II) by bentonite samples was an endothermic and spontaneous process. These results indicated that, of the bentonite samples used, the natural bentonites from JP and SK were most suitable for the removal of nickel from synthetically prepared solutions.

Kinetic and thermodynamic studies of the biosorption of Cr (VI) in aqueous solutions by Agaricus campestris.

Abstract: In the present study, biomass of Agaricus campestris was tested to evaluate its effectivity as a biosorbent for the removal of Cr (VI) ions from aqueous solutions. The influence of various process parameters such as pH, temperature, contact time, biosorbent dosage and desorption were studied. Pseudo-first order, pseudo-second order, Ritchies and intraparticle diffusion model were used to present the adsorption kinetics. Results obtained indicate that the adsorption process is fast and spontaneous within the first 60 min. The experimental data supports pseudo-second order model. The sorption data conformed well to the Langmuir isotherm model. The maximum adsorption capacity (qmax) onto A. campestris was 56.21 mg g-1 for Cr(VI) at 45°C when 0.1 g biomass was used. In addition, the mean values of thermodynamic parameters of standard free energy (ΔG0 = -1.635 kJ mol-1 at 45°C), standard enthalpy (ΔH0 = -9.582 kJ mol-1) and standard entropy (ΔS0 = -24.992 J mol-1K-1) of the adsorption mechanism were determined.

Learning Molecular Representations for Thermochemistry Prediction of Cyclic Hydrocarbons and Oxygenates.

Abstract: Accurate thermochemistry estimation of polycyclic molecules is crucial for kinetic modeling of chemical processes that use renewable and alternative feedstocks. In kinetic model generators, molecular properties are estimated rapidly with group additivity, but this method is known to have limitations for polycyclic structures. This issue has been resolved in our work by combining a geometry-based molecular representation with a deep neural network trained on ab initio data. Each molecule is transformed into a probabilistic vector from its interatomic distances, bond angles, and dihedral angles. The model is tested on a small experimental dataset (200 molecules) from the literature, a new medium-sized set (4000 molecules) with both open-shell and closed-shell species, calculated at the CBS-QB3 level with empirical corrections, and a large G4MP2-level QM9-based dataset (40 000 molecules). Heat capacities between 298.15 and 2500 K are calculated in the medium set with an average deviation of about 1.5 J mol−1 K−1 and the standard entropy at 298.15 K is predicted with an average error below 4 J mol−1 K−1 . The standard enthalpy of formation at 298.15 K has an average out-of-sample error below 4 kJ mol−1 on a QM9 training set size of around 15 000 molecules. By fitting NASA polynomials, the enthalpy of formation at higher temperatures can be calculated with the same accuracy as the standard enthalpy of formation. Uncertainty quantification by means of the ensemble standard deviation is included to indicate when molecules that are on the edge or outside of the application range of the model are evaluated

Solubility of Hydroxytyrosol in binary mixture of ethanol + water from (293.15 to 318.15) K: Measurement, correlation, dissolution thermodynamics and preferential solvation

Abstract: Hydroxytyrosol (HXT) (also known as 3,4-dihydroxyphenylethanol,) is a biophenol extracted from olive. HXT is known for its high antioxidant significance effect. In this work, we focused on the study of the behavior of the solubility of HXT in binary solvent mixtures (ethanol + water) as well as the thermodynamic proprieties. The solubility of HXT in water, ethanol and in binary solvent mixtures (ethanol + water) was measured at five different temperatures from (293.15 to 318.15) K. The enthalpy of fusion and the melting point of HXT were experimentally determined since they are essential for the study of the of solubility and crystallization process. Thermodynamic properties of dissolution of the HXT (Gibbs energy (ΔsolG°), molar enthalpy of dissolution (ΔsolH°), and molar entropy of dissolution (ΔsolS°)) are predicted using the van’t Hoff analysis, the Gibbs equation, and the measured solubilities data. The preferential solvation has been determined using the inverse Kirkwood–Buff integral (IKBI) theory.

Thermal expansion of ellinaite (β-CaCr 2 O 4 ): an in-situ high temperature X-ray diffraction study

Abstract: The thermal expansion of orthorhombic CaFe2O4-type β-CaCr2O4, ellinaite, was studied by high temperature in-situ synchrotron X-ray diffraction measurements in the temperature range of 301–973 K at atmospheric pressure. Based on the obtained data, the thermal expansion coefficients of β-CaCr2O4 were determined as 2.84(3) × 10–5 K−1, 1.08(1) × 10–5 K−1, 0.79(1) × 10–5 K−1, 0.99(1) × 10–5 K−1 for volume, a-, b- and c axis, respectively. An anisotropic behavior was observed because the axial expansivity for the b axis is smaller than those of the a- and c axis. Combined with available experimental results, the isobaric heat capacity (Cp) of β-CaCr2O4 was evaluated by using a Kieffer’s model with Raman spectroscopy data and compared with previous studies. The isochoric heat capacity (Cv), standard entropy (S0298) and Debye temperature (θD) of β-CaCr2O4 were also determined.

Thermodynamics of Helium-4 Dimerization and Trimerization.

Abstract: A new equation of state (EOS) for helium-4 is used to obtain the equilibrium thermochemical properties of helium-4 dimerization (24He ⇌ 4He2) and trimerization (34He ⇌ 4He3) between 3.0 and 10.0 K. It is shown that at sufficiently low temperatures there are appreciable populations of dimer and trimer. The calculations account only for monomer, dimer, and trimer. At 3.0 K, the respective KPo values for dimerization and trimerization are 0.4832 and 0.4876, respectively. The standard enthalpy changes at 3.0 K are -54.53 and -110.0 J/mol, and standard entropy changes are -24.22 and -42.97 J/mol-K. Statistical thermodynamic calculations provide results that are qualitatively consistent with those obtained from the EOS calculations.

Prediction of thermodynamic data for radium suitable for thermodynamic database for radioactive waste management using an electrostatic model and correlation with ionic radii among alkaline earth metals

Abstract: Thermodynamic data for radium for radioactive waste management have been predicted using an electrostatic model and correlation with the ionic radii of the alkaline earth metals. Estimation of the standard Gibbs free energy of formation and standard molar entropy of aqueous radium species and compounds has been based on such approaches as extrapolation of the thermodynamic properties of strontium and barium, and use of a model of ion pair formation. The predicted thermodynamic data for radium have been compared with previously reported values.

Determination of chlorzoxazone crystal growth kinetics and size distribution under controlled supersaturation at 293.15 K

Abstract: Background: Chlorzoxazone (CHZ) is a water-insoluble drug having bioavailability problems. The absorption rate of such drugs can be improved by reducing their particle size. In this work, the crystal growth kinetics of CHZ–ethanol for different degrees of supersaturation (SS) has been studied. Method: The equilibrium solubility data of CHZ in ethanol is determined by the shake-flask method within the 283.15–313.15 K temperature range. The mole fraction solubility of CHZ is calculated and correlated with the modified Apelblat equation, λh equation, van’t Hoff equation, Wilson, and non-random two liquid (NRTL) equation. Batch crystallization experiments are performed on three different degrees of SS-1.16, 1.18, and 1.20 at 293.15 K as a function of time. Results: The maximum root mean square difference (RMSD) and relative average deviation (RAD) values of 169.24 x10-6 and 0.699 x10-2, respectively, are observed in the NRTL equation model. The dissolution properties such as standard enthalpy, standard entropy, and Gibbs free energy are predicted using van’t Hoff equation. Using a simple integral technique, the average crystal growth rate constant KG is calculated as 1.58 (μm/min) (mg/ml)-1 and the order n=1 for CHZ–ethanol at 293.15 K. Conclusion: The obtained result concludes that the crystals growth size is found to be varied at different SS ratio in batch crystallization. The particle size control in batch crystallization can be achieved by optimizing the operating conditions to get the desired size crystals.

Variation with Temperature of Phase Ratio in Reversed Phase HPLC for a Methanol/Water Mobile Phase

Abstract: The ratio between the volume of the stationary phase and the void volume of column in HPLC is known as the phase ratio (denoted by Φ). In almost all the thermodynamic studies based on van’t Hoff plots for estimating the values of standard enthalpy and standard entropy, the phase ratio of HPLC column is assumed to be temperature invariant. The validity of this assumption is examined in the present work by studying the variation of Φ with temperature (T) in the range of 20–50 °C for three commercially available C18 columns and two mobile phase compositions methanol/water. A procedure based on proportionality of retention factors k on octanol/water partition coefficients log Kow for several aromatic hydrocarbon homologues was used for the measurement of Φ. Variation of Φ with temperature was previously reported for RP-HPLC separations in an acetonitrile/water mobile phase. However, while for acetonitrile/water mobile phase the effective value of Φ is decreasing as the temperature increases, in the case of methanol/water mobile phase the variation is more complicated. In some cases Φ is decreasing similar to the case of acetonitrile/water mobile phase, but in other cases it decreases up to a point and then shows a slight increase. The study proves that the findings regarding variability of Φ with temperature for the case of acetonitrile/water mobile phase is not an unique effect. In addition, present study evaluates how much the calculation of standard enthalpy and entropy from van’t Hoff plots differ when the variation of Φ with temperature is taken into consideration as compared to the classic approach when Φ is considered a constant.

Low-temperature heat capacity of d -glucose and d -fructose

Abstract: The thermodynamics data of crystalline states of two representative components in blood sugar, d-glucose and d-fructose, are significant in researching artificial synthesis and composition transformation in vivo. The heat capacities of d-glucose and d-fructose over a temperature range of 1.9–300 K were measured and calculated by the heat capacity measurement module of physical property measurement system (PPMS). The heat capacities of two compounds increased steadily with temperature, showing a smooth curve without any thermal anomalies. The heat capacity of d-glucose is greater than that of d-fructose in the range of 0 K < T < 80 K, which is smaller than that of d-fructose between 80 and 300 K. Based on the lattice vibration mode, factors of generating the difference heat capacity data between the two isomers were investigated. Additionally, the heat capacity data were fitted by low-temperature heat capacity theoretical model. The thermodynamic data that molar entropy change and molar enthalpy change over the temperature range of 0–300 K were calculated. The standard molar entropy of d-glucose and d-fructose at 298.15 K was calculated by heat capacity fitting to be 214.64 and 217.56 J K−1 mol−1, all with an error of 0.21.