Temperature dependence of the free energy of sorption ofn-alkanes and energy contribution of their methylene groups on columns with fullerene C60

The partial molar free energy, enthalpy, and entropy of sorption of C₁₁−C₂₃ n-alkanes were calculated on the basis of the GC data obtained on the glass capillary column coated with fullerene C₆₀ (Ful-60) as stationary phase. The thermodynamic parameters ofn-alkane sorption on a column with Ful-60 and a fused silica capillary column with polydimethylsiloxane OV-1 were determined and compared. The enthalpy-entropy compensation effect for the sorption ofn-alkanes on Ful-60 and OV-1 was found. A linear dependence of the partial molar free energy ofn-alkane sorption on the temperature of analysis and carbon chain length was found. The free energy contributions of the methylene groups were calculated, and their temperature dependences were studied. The differences in the temperature dependences of the energy contributions of methylene groups ofn-alkanes on Ful-60 and OV-1 were revealed. The entropy contribution is 68–82% of the enthalpy contribution which indicates a substantial role of the number of contacts with Ful-60 in retention ofn-alkanes. The ability of Ful-60 for dispersive interactions is similar to those of nonpolar liquid phases and substantially differs from that for carbon adsorbents. Fullerene columns were shown to be convenient for analysis of highly boiling organic substances in aqueous and organic solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1490–1495, August, 1999.     

Influence of intramolecular interaction on the enthalpy and entropy of methylene unit sorption for the homologous series of carbonyl-containing compounds analyzed by capillary GC

The enthalpy and entropy of sorption of methylene units in the homologous series of n-alkyl acetates; methyl, n-butyl, and phenyl n-alkyl ketones; and the methyl esters and chloroanhydrides of n-aliphatic carboxylic acids have been determined on an SE-54 capillary column. The enthalpy of sorption of the first methylene unit is anomalously high when the growing n-alkyl chain is connected directly to a carbonyl group. This effect is a result of intramolecular interaction between neighboring methyl and carbonyl groups. It has been shown by measurement of the enthalpy and entropy of sorption of the difluoromethylene unit in the series of n-hexyl esters of perfluorinated carboxylic acids that for these compounds the effect is absent. The intramolecular interaction was found to increase in the order methyl butyl ketone < methyl phenyl ketone < methyl acetate < acetyl chloride < acetone.     

Influence of structure and polarity of polysiloxanes on retention of N-TFA butyl esters of amino acids

In this work, we have investigated the influence of the polarities and structures of 13 polysiloxanes on the retention of some N-TFA N-butyl esters of amino acids. Gehrke [1] was the first to examine the possible relationship between polarity and retention data of amino acid derivatives for 9 stationary liquid phases. He concluded that the molecular structure of the stationary liquid phase is an important factor determining retention.     

Determination of thermodynamic functions of sorption for cyclohexyl-, piperidyl-, morpholyl-, and thiomorpholyl-n-alkanes by capillary gas-liquid chromatography

The values of partial molar free energy (G), enthalpy (H), and entropy (S) of sorption in the homologous series ofN-alkylpiperidines,N-alkylmorpholines,N-alkyl thiomorpholines, and alkylcyclohexanes were determined. It was found that the free energy of sorption is determined to a greater extent by the enthalpy term than by the entropy one. The free energy of sorption of the first homolog decreases when then-alkyl chain is attached directly to the carbon atom of the cycle and increases in the case ofN-alkylsubstituted heterocycles. The influence of the heteroatom nature on intermolecular interactions of homologs with the nonpolar stationary phase was quantitatively estimated on the basis of thermodynamic data.Dead time necessary for calculation of the retention factor was determined by the retention of methane injected into the column simultaneously with the sample.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2030–2032, August, 1996.     

Comparison of four cholesterol‐based stationary phases for the separation of steroid hormones

The chromatographic behavior of steroid hormones on four cholesterol‐bonded stationary phases with different structures in binary methanol/water mobile phases was studied. Of the stationary phases tested, the commercially available stationary phases Cogent UDC cholesterol? and COSMOSIL cholester? provided better separations of steroid hormones in comparison to homemade aminocholesterol and diaminocholesterol stationary phases. The results show that the temperature has a significant influence on the retention and selectivity for steroid hormones separation. The temperature increase may cause changes in the elution order. From the dependences of the retention (ln k) on temperature (1/T), the standard partial molar enthalpy and standard partial molar entropy were calculated and their enthalpic and entropic contributions to the retention were compared. The enthalpic effects principally control the retention mechanism.     

Estimation of hydrogen bond energy in n-alkanols from the gas chromatography data

The retention factors of C1—C9 n-alkanols on a capillary column with the SE-30 nonpolar phase were determined for temperatures between 320 and 400 K. From the temperature dependence of the free energy change, variations in the enthalpy of sorption of n-alkanols on the SE-30 polymethylsiloxane phase were calculated. The difference between the enthalpies of condensation and sorption was used to estimate the contribution of hydrogen bonding to self-association of pure liquid alcohols. The calculated energies of hydrogen bonds in liquid n-alkanols are in good agreement with the published data.     

Thermodynamic characteristics of sorption of heterocyclic compounds m capillary gas chromatography

Dependences of sorption energies of sulfur-, oxygen-, and nitrogen-containing heterocyclic compounds on the nature of heteroatoms, structure of substituents and their position in the ring were studied. The contributions of heteroatoms, functional and alkyl groups to the energy of dispersive interaction of position isomers with nonpolar stationary phases were determined for imidazoles, piperidines, morpholines, thiomorpholines, dioxalanes, oxathiolanes, dithiolanes, thiophenes, and furans. The nonequivalence of contributions of the same substituents to the sorption energy of each of the heterocyclic compounds, depending on the nature of the heteroatom and position of the substituent, was shown. The obtained values of contributions of heteroatoms and substituents can be used for apriori calculation of retention indices of position isomers of five- and six-membered heterocycles containing one or two heteroatoms in the ring.Translated from lzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2033–2038, August, 1996.     

Peculiar features of sorption of positional isomers of formyl-, acetyl-, and aminopyridines in capillary gas-liquid chromatography

The GLC retention parameters of α-, β-, and γ-isomers of formyl-, acetyl-, and aminopyridines were determined on two columns with stationary phases of different polarities. The contributions of formyl, acetyl, methyl, and amino groups located in the α-, β-, and γ-positions of the ring to the retention indices were calculated. The contributions of these groups to the retention were found to depend on their positions with respect to the N atom of the ring and are the smallest for α-substituents. The difference between pyridines containing electron-withdrawing substituents and those containing electron-donating substituents is manifested as different orders of elution of α-, β-, and γ-derivatives. The distinctions between the GLC behaviors of pyridines, benzenes, and furans with the same substituents were identified. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 91–94, January, 1997.     

An equation for the calculation of retention indices in temperature-programmed gas chromatography with allowance for the nonlinear variation of the retention parameters ofn-alkanes

In view of the nonlinear variation of the temperature increments ofn-alkanes found previously, the accuracy of the calculations of the retention indices (I _pr) of substances in temperature-programmed capillary gas chromatography carried out in terms of six known equations was verified. A new four-parameter equation was proposed, and a general method for the calculation of its coefficients, suitable for all stationary phases, based on the adjusted retention times ofn-alkanes was suggested. The coefficients of the equation for 12 temperature variation programs were determined. Using the homologous series of methyl esters of fatty acids as an example, it was shown that the proposed equation ensures the minimum error of determination ofI _pr under various conditions. The equation also makes it possible to carry out interpolation and extrapolation calculations. The coefficients of the equation are found using the least-squares method based on data for any 4–5 referencen-alkanes. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 323–327, February, 1997.     

Retention characteristics in gas chromatography as function of molecular mass of the stationary phase

The thermodynamic characteristics of the selectivity of stationary phases in gas-liquid chromatography have been used to study the change in selectivity of a stationary phase with its increasing molecular mass within the limits of the same class of compounds. It is shown that the entropy of disssolution is the dominant thermodynamic factor of selectivity in the case of paraffinic stationary phases; a linear relatinship exists between the logarithm of relative retention of different sorbates and the reciprocal molecular mass of the stationary phase. In the case of solutions where hydrogen bonds can be formed, the enthalpy of dissolution serves as the dominant factor; in this case a linear relationship exists between the relative retention of alcohols and the reciprocal molecular mass of the stationary phase (polyethylene glycol). For the other classes of sorbates and stationary phases, the entropy as well as the enthalpy of dissolution contribute to the relative retention when the molecular phase varies within a single class of substances. The theoretical postulations have been confirmed by experimental data for paraffinic stationary phases, polyethylene glycols, and phthalic acid esters.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 81–90, January–February, 1988.     

Hydration and sorption characteristics of a polyfunctional weak-base anion exchanger after the sorption of vanillin and ethylvanillin

Features of the sorption of substituted aromatic aldehydes by a weak-base anion exchanger under equilibrium conditions are investigated using vanillin and ethylvanillin as examples. Analysis of the sorption isotherms of carbonyl compounds at different temperatures allows us to calculate the equilibrium characteristics of their sorption and assess the entropy and enthalpy contributions to the energy of the process. Hydration characteristics of the macroporous weak-base anion exchanger before and after the sorption of aromatic aldehydes are compared.     

Effect of ester molecular structure and column temperature on the retention of eight esters in gas chromatography

Summary An experimental design has been used to study the effect of column temperature on the gas chromatographic retention of eightpara-hydroxy benzoic esters. A rapid procedure has been developed to reduce the number of experiments compared with traditional methods. Capacity factors were determined and the enthalpy and entropy of transfer from the mobile phase to the stationary phase, H^o and S^o, respectively, were calculated using the linear Van't Hoff equation (dependence of ln k on 1/T). A retention prediction system (RPS) for these compounds in GC was investigated. The molecular connectivity index was used to describe the quantitative structure relationships. Enthalpy — entropy compensation revealed that the mechanism was similar for all the compounds studied.     

Polycyclic aromatic hydrocarbons as test probes to investigate the retention behavior of 1,3‐alternate calix[4]arene silica‐bonded stationary phases

A series of polycyclic aromatic hydrocarbons (PAHs) of different size and shape has been used to characterize the chromatographic behavior of five calix[4]arene stationary phases in 1,3‐alternate conformation synthesized in our laboratory. The selection of linear, four‐ring nonlinear, and five‐ring PAHs gave data on selectivity changes across range of the calix[4]arene columns. Retention of the 12 aromatic solutes has been evaluated at various methanol contents in the mobile phase (70–100% v/v) and column temperatures (20–45°C). The thermodynamic parameters underlying the retention mechanisms revealed that each of the five calix[4]arene columns exhibited variation in selectivity and retention of PAHs caused by enthalpy and entropy effects. The calixarene stationary phases substituted with electron‐withdrawing groups exhibit enhanced selectivity toward PAHs in comparison to the rest of the investigated columns. The observed divergences are due to differences in solute–stationary phase interactions and originate in π–π and π‐electron transfer specific to the analytes and the type of calix[4]arene functionalization at the upper rim, as well as steric and sorption phenomena.     

Gas chromatography study of silica modified with polyfluoroalkyl groups

Silicas with attached polyfluoroalkyl groups were studied by gas chromatography (GC) at zero surface coverage. Organic molecules (n-alkanes, ethers, esters, ketones, etc.) were used as test compounds. The contributions of molecule groups to the Gibbs energy of adsorption deltaGi, and the contributions of specific energy interactions deltaG(sp) as well as Kováts retention indices were calculated. The strong and weak points of the two approaches (deltaGi, deltaG(sp)) were discussed. It was shown on the basis of both approaches that adsorption process on polyfluoroalkyl silicas is to be described by the low contribution of hydrocarbon groups (dispersion interactions) and sufficiently high contribution of groups reinforcing specific interactions. The significant role of entropy in thermodynamic of retention was shown by example of adamantane adsorption.     

Identification and confirmation of traces of chlorinated fatty acids in fish downstream of bleached kraft pulp mills by gas chromatography with halogen-specific detection

Methyl esters of threo-9,10-dichlorooctadecanoic, threo-7,8-dichlorohexadecanoic, and threo-5,6-dichlorotetradecanoic acids, present in transesterified extracts of filets, gonad, intestinal fat and carcass of white sucker (Catostomus commersoni) sampled in receiving waters of bleached kraft pulp mill effluents, were identified by gas chromatography with halogen-specific detection (XSD). Identification was based on (1) a comparison of the retention times of a sample peak with a prospective reference standard on two stationary phases of very different polarities by spiking, and (2) elution behavior of configurational and positional isomers of dichloro fatty acid methyl esters.     

Thermodynamics of the sorption of 1,3,4-oxadiazole and 1,2,4,5-tetrazine derivatives from solutions on hypercrosslinked polystyrene

The sorption of a series of aromatic heterocycles on neutral hypercrosslinked polystyrene from maximally dilute acetonitrile and water.acetonitrile solutions of them is investigated by high-performance liquid chromatography in a range of column temperatures from 308 to 348 K at 5 K intervals. It is found that for all heterocycles, the logarithm of the retention factor versus the reciprocal of the temperature dependences can be approximated by linear functions with r ² ≥ 0.999. It is established that the true compensation effect (which does not result from the correlation between enthalpy and entropy determination errors) occurs during the sorption of heterocycles on hypercrosslinked polystyrene from both acetonitrile and water-acetonitrile solutions. It is shown that in the compensation diagram, the points corresponding to sorbates are grouped together according to the type of sorbate-sorbent interactions. It is found that changes in the Gibbs energy, enthalpy, and entropy of sorption from acetonitrile solutions and for sorption from water-acetonitrile solutions are related to one another by linear dependences.     

Temperature changes of sorption parameters of di-n-alkylketones and methylcyclohexanones in capillary gas chromatography

The gas chromatographic behavior of di-n-alkylketones and isomeric methylcyclohexanones at variable temperatures in the isothermal runs has been studied using three capillary columns coated with SE-30, OV-225, and PEG-40M/KF stationary phases. The retention indices and their temperature increments were determined, and the partial molar free energies of the compounds under study were calculated. A linear dependence of the values of the partial molar free energy of sorption on the homologue number in di-n-alkylketones is not observed for the first member on the nonpolar SE-30 phase and for the third member on the polar OV-225 and PEG-40M/KF phases. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 328–332, February, 1997.     

Ionization of aqueous toluic acids: Conductance and thermodynamics

Conductivities of aqueous solutions ofortho-, meta-, andpara-toluic acids have been measured for the concentration range 0.1–2 millimolar and at 5° intervals from 5 to 100°C. At each temperature pK _a(m) andA ₀ have been calculated using the paired ion model recently described by Fuoss. Thermodynamic parameters have been calculated for the ionization of each acid, and Walden products for the anions. Results are discussed in terms of contributions to acidity by enthalpy and entropy changes as well as by hydration of the various solute species.     

Chromatographic Behavior of C60 and C70 Fullerenes at Subambient Temperature with n-Alkanes Mobile Phases

The influence of temperature on the retention and separation of C60 and C70 fullerenes was studied under HPLC conditions. Particularly, chromatographic experiments were conducted using moderate carbon loaded octadecylsilica stationary phase and homologous series of n-alkanes including n-pentane, n-hexane and n-heptane as the mobile phases. All studies were performed across wide range of subambient temperature from −80 to +20 °C. From practical point of view the best chromatographic conditions for baseline separation of the components of interest were selected. The retention of analytes was strongly affected by temperature and below minus 30 °C strong deviation from van't Hoff behavior was observed. To explore this phenomenon selected thermodynamic parameters including changes of enthalpy (ΔH^o) and changes of entropy (ΔS^o) were estimated. Positive values of the ΔH^o and ΔS^o at low temperature region may indicate the lack of the interaction with the stationary phase ligands. A possible retention mechanism at different temperatures for C60 and C70 molecules has been discussed.     

Some thermodynamic quantities of n-alkanes as a function of chain length

Summary Results from high pressure dilatometry onn-alkanes and linear polyethylene and literature data yield a linear relation between specific volume, entropy and enthalpy of fusion, the reciprocal melting temperature and 1/n, wheren denotes the number of C-atoms per molecule. Extrapolating towards infiniten one always obtains polyethylene data.The differences in the properties ofn-alkanes and polyethylene can be ascribed to the influence of the chain ends. Assuming entropy and enthalpy contributions from these chain ends as made probable by a molecular model one can quantitatively explain then-dependence of the above mentioned quantities including their pressure dependence.With 10 figures and 2 tables