A thermodynamic criterion for the identification of the functional group of aliphatic compounds by gas chromatography

Summary A thermodynamic criterion, Q, is suggested which permits to estimate the difference in the functional group energy of interactions for two different stationary phases. The linear dependence of Q on the homolog number m of any series R_m X can be used as the thermodynamic criterion for the identification of substances by GC analysis.Enlarged text of the paper presented at the Fourth Danube Symposium on Chromatography, Bratislava, Czechoslovakia, September 1983.     

Inorganic gas chromatography

Summary The possibilities of using gas-solid chromatography as a method for the separation of trace amounts of various elements such as volatile chlorides or oxychlorides are investigated. The use of quartz as the column material and graphite, quartz or quartz coated with different ionic chlorides of low volatility as the stationary phase allows the use of temperatures up to 1100 K and consequently the separation of many elements as chlorides. The theory of linear equilibrium chromatography is applied successfully to the investigation of the influence of temperature and the type of solid (stationary) phase, in order to optimize separations. The dependence of the peak width on the various parameters is more complex, but several general principles can be derived from the experimental results. From these data physicochemical properties of the adsorbate-adsorbent systems investigated can be derived, and the optimum conditions for isothermal separations can be estimated.     

Low-temperature heat capacity of β-glycine and a phase transition at 252 K

Low-temperature heat capacity of unstable -glycine was measured in a temperature range 5.5 to 295 K, and thermodynamic functions were calculated. At very low temperatures, heat capacity fits a sum of cubic (Debye) and linear terms: C_p=aT+bT ³. The linear contribution increases with temperature and disappears at the second-order phase transition near 252 K which was observed for the first time.     

Thermodynamics of linear polyurethanes on basis of 1,4-diisocyanatobutane with 1,4-butanediol and 1,6-hexanediol in the range from T → 0 to 490 K

The temperature dependence of heat capacity and characteristics of physical transformations of partially crystalline linear aliphatic polyurethanes based on 1,4-diisocyanatobutane with 1,4-butanediol and 1,6-hexanediol have been studied over the range 6.5-490 K by precision adiabatic vacuum and dynamic calorimetry. The calorimetric data were used to determine the thermodynamic quantities of devitrification and fusion and to calculate the standard thermodynamic functions , H⁰(T) − H⁰(0), S⁰(T) and G⁰(T) − H⁰(0) of linear polyurethanes in totally crystalline and amorphous states. The values of the fractal dimension D in the function of multifractal generalization of Debye's theory of the heat capacity of solids were estimated and the character of heterodynamics of their structures was detected. The energies of combustion of the substances were measured in a calorimeter with an isothermal shield and a static bomb. The enthalpies of combustion and the standard thermodynamic characteristics of formation of the polymers at T = 298.15 K were calculated too. The standard thermodynamic characteristics of polycondensation processes in bulk of 1,4-diisocyanatobutane with 1,4-butanediol and 1,6-hexanediol followed by the formation of linear polyurethanes were determined in the range from 0 to 350 K. A comparative analysis of the corresponding standard thermodynamic properties of the polymers under consideration and polyurethanes of isomeric structure was made and some dependences of their change on various conditions were found.     

Investigation of energetic heterogeneity of carbon black surfaces during adsorption of chromatographically low concentrations of n-pentane

Energetic heterogeneity of different types of industrial carbons was studied by gas-adsorption chromatography in regions of low surface coverages. The main thermodynamic characteristics of n-pentane adsorption on the carbon black surfaces were experimentally determined under the conditions close to the zero coverage of the surface (Henry region). These characteristics include the Henry constants, differential molar heats of adsorption, changes in the entropy of adsorption, and differences in molar heat capacities of the adsorbate in the adsorbed state and its vapors at a constant pressure. The specific features of the geometric structure and ratio of surface regions different in the adsorption energy make it possible to establish the presence of the Henry region in the adsorption isotherm. The gas-chromatographic criteria, which can be used to estimate the degree of energetic heterogeneity of the adsorbent surface, were considered.     

Thermodynamics of interactions between lead(II) and cadmium(II) ions and azulene-based complexing polymer films

In order to understand the essential processes/interactions between the metal ions and modified electrodes which are based on complexing polymeric films, access to thermodynamic characteristics is compulsory. The paper enlarges the information concerning the sorption of metal ions within complexing polymer films, particularly based on azulene, which can be involved in metal detection sensors. Interactions between lead(II) or cadmium(II) ions and complexing polymer films have been studied using chemical preconcentration–anodic stripping method. The films have been obtained by controlled potential electrolysis in millimolar solutions of 4-azulen-1-yl-2,6-bis(2-thienyl)pyridine (L) in acetonitrile. PolyL films affinities towards these metal ions have been quantified at different temperatures by means of sorption isotherms. Parameters for sorption of lead(II) and cadmium(II) ions within polyL films have been calculated for Freundlich, Langmuir and Redlich–Peterson isotherms. The best fit was obtained when using Langmuir isotherm. The results evidence that lead ions are better sorbed than cadmium within polyL film. Thermodynamic parameters for the chemical sorption of lead(II) and cadmium(II) ions within polyL films have been calculated.     

Gas chromatography of halogenated adamantanes

The main thermodynamic characteristics of the adsorption of haloadamantanes on graphitized thermal carbon black were determined by experimental measurements and by calculations in terms of molecular-statistic theory of adsorption. Using experimental data, the Kovac retention indices were calculated and the optimum conditions for gas-chromatographic separation of haloadamantanes on stationary phases with different polarity were elucidated. The influence of the cage effect in the adamantane unit on the chromatographic properties of haloadamantanes was established.     

Thermodynamics of the extraction of europium(III) radiotracer by benzyl-phosphonic acid monoester

The extraction of Eu³⁺ from perchloric acid by ethyl hydrogen benzyl phosphonate (HEBP) dissolved in a series of organic diluents, has been studied at different temperatures. From the variation of the distribution ratio with temperature, the thermodynamic functions H, S and G have been determined. The meaning of the experimentally obtained thermodynamic quantities is discussed.     

Calculation of retention indices and peak widths in temperature programmed gas-liquid chromatography

Summary Isothermal chromatographic measurements lead directly to H _v ^o and A (entropy term) of solutes, and three constants of an empirical relationship between peak width and column temperature. From the thermodynamic parameters H _v ^o and A retention temperatures have been computed by means of a theoretical model including temperature dependence of carrier gas viscosity, and subsequently retention times; programmed retention indices have been determined by linear and polynomial interpolations. By substituting the value of the calculated retention temperature in the above-mentioned relationship, peak width at half-height for a linear temperature may be estimated. Predicted retentions correlate with observed data, with a P-value 0.01; simulation accuracy is generally 6–10% for peak widths.Retention indices of some organochlorine species, separated on an OV-101 capillary column, may differ by as much as 26 units depending on the method of calculation. Polynomial-calculated indices are more consistent with the retention index scheme, and have smaller standard deviations and better constancy at different heating rates.     

Base strength of amines at high temperatures. Ionization of cyclohexylamine and morpholine

A potentiometric technique using a hydrogen electrode concentration cell with flowing solutions was employed to measure the ionization equilibria of cyclohexylamine and morpholine over the temperature range 50–295°C in dilute KCl media. Also, the isothermal pressure coefficients were determined to 250°C in the same media. Smoothing functions are presented which fit these and other selected data, and thermodynamic quantities for the ionization process were derived. Nearly linear variation of H vsT S was observed for this process as well as other similar reactions, and the significance is discussed. Differences in basicities of amines are found to decrease at elevated temperatures, and reversals in relative base strengths are seen.     

Fulfillment of the Conditions of Thermodynamic Equilibrium in an Ice/Electrolyte Solution System with Constrained Geometry

For small volumes of a NaCl solution (10^–6 cm³) with concentrations of 0.1 and 1 M, temperature dependences of the length lof solution columns frozen in thin quartz capillaries (5–10 m in radius) are obtained. At the temperatures t above –4 and –8°C (for 0.1 and 1 M solutions, respectively), the l(t) dependences are reversible, independent of the direction of changes in temperature, and, hence, correspond to the equilibrium conditions of ice/solution system. From the constant mass condition of the solute, an expression for l(t) is derived that includes only one thermodynamic characteristic, namely, the temperature dependence of the solution concentration in equilibrium with ice. Deviations from the calculated l(t) dependences are observed at a temperatures below –2 and –5°C (for 0.1 and 1 M solutions, respectively), which can be explained by the adhesion of frozen solution to the capillary walls. The arising internal stresses lead to the deviations from the thermodynamic equilibrium conditions known for the bulk systems. On approaching the melting zone, the adhesion is failed because of the formation of thin nonfreezing water interlayers on the quartz surface.     

Studies of adsorption of polyacrylonitrile and polyacrylates at solid-liquid interface

The adsorption of poly(acrylonitrile), poly(methyl acrylate), poly(ethyl acrylate) and poly(butyl acrylate) was studied at different temperatures fromN,N-dimethyl formamide solution on various solid surfaces, i.e. silica gel, alumina and calcium carbonate. The thermodynamic quantities for adsorption processes were computed by using a) infinite dilution and b) surface coverage approaches. Both procedures tend to give similar conclusions. These reveal the importance of polymer-adsorbent interactions besides many other properties including solvent power.     

"分光光度法测定碘化铅溶度积常数"实验存在的问题及改进

针对"分光光度法测定碘化铅溶度积常数"实验测定方法中存在的问题,提出了改进方法。用改进后的方法测定碘化铅溶度积常数,减少了实验药品和污染,简化了实验环节和数据处理过程,使实验测定的数据更准确。并且根据热力学公式推导、计算和作图,给出了lnK_(sp)~θ与1/T关系的直线方程,可以判断不同温度时测试结果的准确程度。     

Etching and the morphology of branched and cross-linked polyethylene

A permanganic acid etching technique was used to characterize the genuine morphologies of bulk crystallized samples of linear, branched, and crosslinked polyethylene. Four different kinds of artifacts as large as 15–20 m were detected. It was also noted that artifact density could be drastically reduced by proper selection of etching conditions. The morphologies of artifacts were independent of specimen crystallinity and the orientation of lamellar crystals; however, their rate of production was susceptible to the crystallinity of samples. The formation of a fresh surface involved the preferential etching of edge-on and secondary lamellae. As the growth features of cross-linked polyethylene were less than 6 m and were mainly sheaf-like structures, growth features could be readily distinguished from the artifacts by transmission electron microscopy. Another type of artifact (globules morphology) produced at angstrom scale was found to depend on the crystallinity and the surface structure of lamellar crystals. It is demonstrated that the fine lamellar details of branched and cross-linked polyethylene can be observed using transmission electron microscopy. Increasing the number of cross-links in polyethylene, modifies the size and nature of supermolecular structures, the periodic twisting of lamellar crystals, and the nature of the nucleation process. The study of a blend of branched and cross-linked polyethylene showed that the structurally dissimilar molecules co-crystallized at low crystallization temperatures, while there is a tendency for molecular fractionation at higher crystallization temperatures.     

Complete Resolution and Thermodynamic Parameters of the Ionization Equilibria of Pyridoxal in Water—Dioxane Mixtures

A potentiometric method has been used to determine the thermodynamicequilibrium constants for the macroscopic ionization processes of pyridoxal inwater—1,4-dioxane mixtures (0–70% weight fraction dioxane)at temperatures ranging from 10 to 50°C. These data, combined with the equilibrium constants for the tautomericand hemiacetalization processes, allow complete resolution of the microconstantsystem and calculation of the microscopic ionization equilibrium constants underall our experimental conditions. The standard thermodynamic function changesfor the macroscopic and microscopic ionization processes were obtained in variouswater—1,4-dioxane mixtures at 25°C. The values of any given microscopic pKfor the different solvents and temperatures fit very well to a single equation. Thefree energy, enthalpy, and entropy obtained for the different ionization processesin water—dioxane mixtures correlate with Kamlet and Taft's solvatochromicparameters ^* and , which are a measure of the dipolarity/polarizability andhydrogen-bonding capacity of the solvent, respectively. These correlations explainmore fully the mutual compensation between the contributions of enthalpy andentropy and the origin of the solvent effect on the pK.     

Gas chromatographic and IR spectroscopic characteristics of tri- and tetramethylcyclohexenyl butenyl ketones

Infrared spectroscopic characteristics and gas chromatographic retention indices of tri- and tetramethylcyclohexenyl butenyl ketones were determined at different temperatures of analysis. Thermodynamic characteristics of sorption on the apolar stationary phase were calculated. The sorption characteristics of tri- and tetramethylcyclohexenyl butenyl ketones show that under conditions of capillary gas chromatography these compounds are retained due to the dispersive energy. It was shown by FTIR spectroscopy that the formation of conjugation systems of double bonds is responsible for the higher retention of the -isomers compared with that of the -isomers. The shortening of the distance between the carbonyl and trimethylcyclohexenyl groups was found to be accompanied by a decrease in the retention of the isomeric compounds.     

Conductance study of some transition and heavy metal complexes with 1,10-diaza-18-crown-6 in binary acetonitrile-dimethylsulfoxide mixtures

A conductance study of the interaction between cobalt, nickel, copper, zinc, cadmium, and lead ions with 1,10-diaza-18-crown-6 in different acetonitrile-dimethylsulfoxide mixtures has been carried out at various temperatures. The formation constants of the resulting 11 complexes were determined from the molar conductance-mole ratio data and found to vary in the order Zn²⁺2+2+2+2+2+. The enthalpy and entropy of complexation reactions were determined from the temperature dependence of the formation constants. A linear relationship is observed between the log K_f of different complexes and mole fraction of acetonitrile in the solvent mixtures. The TS vs. H plot of all thermodynamic data obtained shows a fairly good linear correlation indicating the existence of an enthalpy-entropy compensation in the complexation reactions.     

Surface and bulk interactions of organic molecules with calixarene layers

Modified calixarenes can be used as model compounds to study molecular recognition since their molecular cavities reversibly incorporate small organic molecules. This effect has been used in chemical sensors with bulk and surface acoustic wave devices, coated with thin calixarene layers. These devices sensitively convert the mass changes during molecule/calixarene interactions into electronic signals. Thin films of modified calixarenes were prepared with various side groups and various sizes by Knudsen sublimation under well-defined, ultra-high-vacuum conditions. The interaction with perchloroethylene, chloroform, benzene, and toluene at constant temperatures, T, and partial pressures, p_i, was studied systematically for different film thicknesses, d, by means of mass changes m=f(T,p _i,d). From the thickness dependence of values, m_eq, obtained under thermodynamic equilibrium conditions, surface and bulk effects during molecule calixarene interactions were separated. Pronounced excesses of surface concentrations of organic molecules have been found. Activation energies for molecular desorption from surface sites and for diffusion to subsurface sites were determined from mass spectroscopic results of thermal desorption behaviour. Experimental data of adsorption and desorption energies were confirmed by theoretical force field calculations.     

Novel enhanced applications of QSPR models: Temperature dependence of aqueous solubility

A model developed to predict aqueous solubility at different temperatures has been proposed based on quantitative structure–property relationships (QSPR) methodology. The prediction consists of two steps. The first one predicts the value of k parameter in the linear equation , where S_w is the value of solubility and T is the value of temperature. The second step uses Random Forest technique to create high‐efficiency QSPR model. The performance of the model is assessed using cross‐validation and external test set prediction. Predictive capacity of developed model is compared with COSMO‐RS approximation, which has quantum chemical and thermodynamic foundations. The comparison shows slightly better prediction ability for the QSPR model presented in this publication. © 2016 Wiley Periodicals, Inc.     

Nanoparticles synthesis by electron beam radiolysis

Electron beam (EB) irradiation is a useful method to generate stable silver nanoparticles without the interference of inherent impurities generated from chemical reactions. Our experiments were carried out using linear electron beam accelerators with two different EB absorbed dose rates: 2 kGy min^?1 and 7–8 kGy s^?1, and with different absorbed dose levels. The optimum conditions for silver nanoparticles (AgNPs) generation by radiolysis, or by radiolysis combined with chemical reduction, were established. In order to obtain a good yield for AgNPs synthesized by radiolysis, a high dose rate is required, resulting in a rapid production process. At low absorbed dose rates, the utilization of a stabilization agent is advisable. By modifying the experimental conditions, the ratio between the chemical and radiolytic reduction process can be adjusted, thus it is possible to obtain nanoparticles with tailored characteristics, depending on the desired application.

Open image in new window