Calculating chemical equilibrium in the stepwise transesterification reactions of mixed triglycerides of fatty acids

Chemical equilibrium is studied in reactions that occur during stepwise transesterification reaction between mixed triglycerides, which comprise radicals of palmitic, oleic, and linoleic acids, and methanol. The results obtained in calculating the chemical equilibrium allow predicting the behavior of the transesterification reaction of mixed triglycerides in the supercritical state region.     

Development of heterogeneous catalysts for transesterification of triglycerides

This paper describes a preliminary work done towards the development of new metallic heterogeneous catalysts to be used in the transesterification reaction of triglycerides, which is of considerable interest in the production of biodiesel. Biodiesel is a mixture of mono-alkyl esters of fatty acids, and is currently manufactured by transesterification of triglycerides with methanol using NaOH or KOH as liquid base catalyst. Catalysts as such are corrosive to the equipment, and as these catalysts are present in the liquid phase, must be neutralized after the completion of the reaction, typically using HCl, thus producing salt streams. Moreover, due to the presence of free fatty acids, it reacts to form soaps as unwanted by-products, hence requiring more expensive separation processes. Therefore, there is a great need for the development of industrial processes for biodiesel production using solid acid catalysts. The key benefit of using solid acid catalysts is that no polluting by-products are formed, and the catalysts do not have to be removed since they do not mix with the biodiesel product.     

Chemical equilibrium of transesterification reactions between mixed triglycerides and methanol

The chemical equilibrium of the transesterification reactions between methanol and mixed triglycerides containing radicals of palmitic, oleic, and linoleic fatty acids are studied. Based on the results of chemical equilibrium calculation, the behavior of the transesterification reaction of mixed triglycerides in the state of chemical equilibrium is predicted, both in the ideal approximation and with allowance for nonideality, in particular, in the region of supercritical state of the reaction mixture.     

Molecular dynamics study of structural and thermodynamic characteristics of nanodroplets of simple fluid

Equilibrium configurations of Lennard-Jones nanodroplets composed of 10–15000 spherically symmetric molecules placed in the center of a spherical container are studied at constant temperature by the molecular dynamics method. The distribution of local density is found and size dependences of density in the center of droplet, first coordination number, and energy surface tension coinciding for equimolecular dividing surface with specific excess free energy of droplet are studied. Radial distribution function is also determined. It is established that the passage of structural characteristics to their macroscopic values is observed for droplets containing as little as about 300 molecules, while, for energy surface tension, analogous passage for energy surface tension occurs for droplets containing 700–6000 molecules.     

A simple synthesis of trisubstituted quinolines through transesterification

An efficient and simple method has been reported for the synthesis of 2,3,4‐trisubstituted quinolines through zwitterion intermediate under reflux condition in presence of sulfuric acid. The formed dicarboxylate subsequently undergoes transesterification in various alcohols with good yields. Most of the synthesized compounds are newly reported characterized by spectroscopic method. J. Heterocyclic Chem., (2011).     

Calculating the free energy of self-assembled structures by thermodynamic integration

We discuss a method for calculating free energy differences between disordered and ordered phases of self-assembling systems utilizing computer simulations. Applying an external, ordering field, we impose a predefined structure onto the fluid in the disordered phase. The structure in the presence of the external, ordering field closely mimics the structure of the ordered phase (in the absence of an ordering field). Self-consistent field theory or density functional theory provides an accurate estimate for choosing the strength of the ordering field. Subsequently, we gradually switch off the external, ordering field and, in turn, increase the control parameter that drives the self-assembly. The free energy difference along this reversible path connecting the disordered and the ordered state is obtained via thermodynamic integration or expanded ensemble simulation techniques. Utilizing Single-Chain-in-Mean-Field simulations of a symmetric diblock copolymer melt we illustrate the method and calculate the free energy difference between the disordered phase and the lamellar structure at an intermediate incompatibility chiN=20. Evidence for the first-order character of the order-disorder transition at fixed volume is presented. The transition is located at chi(ODT)N=13.65+/-0.10 for an invariant degree of polymerization of N=14 884. The magnitude of the shift of the transition from the mean field prediction qualitatively agrees with other simulations.     

Calculating the thermodynamic and structural properties of liquid noble gases

The structural and energy parameters in the liquid branch of the saturation line of noble gases, including ⁴He, are determined.     

Calculating the thermodynamic properties of aqueous solutions of alkali metal carboxylates

A modified Robinson-Stokes equation with terms that consider the formation of ionic hydrates and associates is used to describe thermodynamic properties of aqueous solutions of electrolytes. The model is used to describe data on the osmotic coefficients of aqueous solutions of alkali metal carboxylates, and to calculate the mean ionic activity coefficients of salts and excess Gibbs energies. The key contributions from ionic hydration and association to the nonideality of solutions is determined by analyzing the contributions of various factors. Relations that connect the hydration numbers of electrolytes with the parameters of the Pitzer-Mayorga equation and a modified Hückel equation are developed.     

Calculating bond ionicity in crystals from thermodynamic data

S. M. Kirov Polytechnical Institute of the Urals. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 2, pp. 34–39, March–April, 1990.     

Calculating the thermodynamic parameters of the surface layer of the binary sodium-potassium system

The composition and number of surface monoatomic potassium layers in Na-K alloys are calculated on the basis of theory of the thermodynamic stability of surface layers of metal alloys with respect to their thickness. The surface layer of Na-K alloys is shown to be thermodynamically stable at surface layer thickness of two monoatomic potassium layers in the range of compositions with K in Na content of ∼13–15 at %. Good agreement is obtained using the surface activities of the components in binary metal melts as criteria.     

Correlation of the thermodynamic characteristics of polymorphic crystals

Analytical expressions for the correlation of volume and entropy, isobaric heat capacity, volume thermal expansion, and the compressibility of crystalline phases are obtained. The pair correlations of the above values and their correlation with the geometrical characteristics of the solid-phase equilibrium line (including the neighborhood of a triple point) are described using the obtained expressions. An analytical form for the correlation of the characteristics of polymorphic crystal polar modification is found.     

The thermodynamic characteristics of diethylzinc-diethylsulfur solutions

The p-T-x dependences for diethylsulfur and diethylzinc-diethylsulfur solutions were studied. The enthalpies and entropies of vaporization were determined, and the composition of the vapor phase calculated. It was shown that S(C₂H₅)₂ existed as monomers in the vapor phase. The component activity coefficients and excess Gibbs energies were obtained. The structures formed in solution were shown to be solvato complexes; specific intermolecular interactions in them involved the essentially lone 2s ²(c) electron pair of five-coordinate carbon. The energies of specific intermolecular interactions in the structures of solvato complexes decreased in the series \(D_{Zn \to S} (11.50) > D_{Zn - C_2 H_5 \to Zn} (9.70) > D_{S - C_2 H_5 \to S} (8.72) > D_{Zn - C_2 H_5 \to C_2 H_5 \to S} \)(4.5 kJ/mol).     

FABES and FAPES – transesterification of triglycerides with higher alcohols. Quantitation and mass spectrometric evaluation

In order to facilitate the gas chromatographic determination of butyric as well as other short chain fatty acids, triglycerides containing these fatty acids were transbutylated and transpentylated to give FABES (fatty acid butyl esters) and FAPES (fatty acid pentyl esters), respectively. This method allows the molecular weight of the compounds to be increased while their polarity is decreased. Short chain esters elute sufficiently separated from the solvent; due to the decreased polarity, elution of long chain fatty acids is only slightly retarded compared to the corresponding methyl esters. Quantitative evaluation over a large series of injections proved the linearity of correction factors in both cases, FABES and FAPES, with a slight deviation for 16:0 and 18:0, respectively. Structural characterization was performed by GC/MS. It was shown that there are characterstic fragmentations for FABES as well as for FAPES. The El-mass spectra give structural information about the alcohol and acid moieties, the ester molecule and the Cl-spectra indicate the molecular weight. The absence of abundant highly characteristic peaks in the spectra of these esters makes the detection of FABES or FAPES in complex mixtures more difficult than the detection of FAMES with their very characteristic ions at m/z 74 and 87.     

Thermodynamic characteristics of stepwise dissociation of glycyl-glycyl-glycine in aqueous solutions

The heats of interaction between glycyl-glycyl-glycine and solutions of nitric acid and potassium hydroxide are determined by calorimetry at 298.15 K and ionic strengths of 0.25, 0.50, and 0.75 in the presence of KNO₃. The standard thermodynamic characteristics (Δ_r H°, Δ_r G°, and Δ_r S°) of acid–base interaction in aqueous solutions of the peptide are calculated. The effect of background electrolyte concentration on the enthalpy of dissociation of glycyl-glycyl-glycine is discussed.     

Structural and thermodynamic characteristics of amide solvents

The structural and thermodynamic characteristics of amide solvents are calculated with different types of molecular self-assembly through hydrogen bonding. Under a model-based approach, the specific and nonspecific components of the total energy of intermolecular interactions are identified for primary, secondary, and tertiary amides of carboxylic acids. It is found that similarly to water, primary amides have a network of hydrogen bonds and belong to the class of liquids characterized by an increase in nonspecific interactions with temperature. In secondary amides with the chain self-assembly, the contribution of these interactions is practically independent of temperature, and in tertiary amides it decreases with an increase in temperature. The molar values of the specific and nonspecific components are used to analyze the intermolecular interactions and the structural properties of amides with different degrees of N-substitution.     

The thermodynamic characteristics of natural iron-lithium micas

A thermochemical study of natural lithium micas, iron-containing polylithionite and lepidolite, was performed on a high-temperature heat-flux Calvet microcalorimeter (Setaram, France). Melt solution calorimetry was used to measure the enthalpies of mineral formation from the elements Δ_f H°_el (298.15 K), ?5989.3 ± 9.6 and ?5981.3 ± 6.3 kJ/mol, respectively. The drop method was used to determine the enthalpy increments heating of the micas over the temperature interval 444–973 K. The equations for the temperature dependences of the heat capacities and enthalpies of Fe-polylithionite and Fe-lepidolite were obtained. The S° (298.15 K) and Δ_f G°_el (298.15 K) values were estimated. The thermodynamic functions of the micas were calculated over the temperature range 298.15–1000 K.