Capillary complexation gas chromatography in analysis of cyclic and aromatic hydrocarbons

Summary Results of studies on stationary phases containing copper(II), nickel(II) and cobalt(II) chloride chemically bonded via cyano—or thiol groups are presented. The retention parameters—i.e. retention factor (k), retention index (I) and molecular retention index (ΔMe) and specific retention volume (Vg)-enabled the characteristics of specific interaction between aromatic and cyclic hydrocarbons and metal complexes chemically bonded to stationary phases to be determined. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

Evaluation of the Thermodynamic Parameters for the Adsorption of Some Hydrocarbons on Chemically Treated-Bentonites by Inverse Gas Chromatography

Inverse gas chromatography has been used to evaluate the adsorption parameters (ΔH_a, ΔH_st, ΔS_a and ΔG_a) of some probe molecules, each representing a class of organic (n-hexane, cyclohexane, benzene, n-octane, 1-octene and isooctane) on bentonite and chemically treated-bentonites. The adsorption parameters of the probes on the bentonite samples were determined in infinite dilution region. Adsorption of the organic species was investigated in the temperature range of 200–275^∘C, using a flame ionization detector, and nitrogen as a carrier gas. The net retention volumes (V_n) of the probes were determined by the help of the retention times (t_R) observed on gas chromatograms for each probe. Injection was made at least three times for each probe, obtaining reproducible results of ± 0.5%. It was found that benzene exhibits more negative ΔH than for n-hexane and cyclohexane on all of the adsorbents. In addition, it was found that 1-octene exhibits more negative ΔH than for n-octane and isooctane on the chemically treated-bentonites, whereas n-octane exhibits more negative ΔH than for 1-octene and isooctane on the natural bentonite. Also, interactions of benzene with the natural- and chemically treated-bentonites were found to be stronger than those of n-hexane and cyclohexane with the same carbon number. Again, interactions of the 1-octene with the chemically treated-bentonites were found to be stronger those of n-octane and isooctane with the same carbon number. On the contrary, interactions of n-octane with the untreated-bentonite were found to be stronger than those of 1-octene and isooctane.     

Study of catalase immobilized on a silicate matrix for non-aqueous biocatalysis

Catalytic activity of catalase (CAT) immobilized on a modified silicate matrix to mediate decomposition of meta-chloroperoxibenzoic acid (3-CPBA) in acetonitrile has been investigated by means of quantitative UV-spectrophotometry. Under the selected experimental conditions, the kinetic parameters: the apparent Michaelis constat (K _M ), the apparent maximum rate of enzymatic reaction (V _max ^app ), the first order specific rate constants (k _sp ), the energy of activation (E _a ) and the pre-exponential factor of the Arrhenius equation (Z₀) were calculated. Conclusions regarding the rate-limiting step of the overall catalytic process were drawn from the calculated values of the Gibbs energy of activation ΔG^*, the enthalpy of activation ΔH^*, and the entropy of activation ΔS^*.     

Surfactant assisted preparation and characterization of carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger

Carboxymethyl cellulose Sn(IV) phosphate composite nano-rod like cation exchanger with diameter in the range of 20–40 nm, length in the range of 100–150 μm and particle size in the range of 21–38 nm have been successfully prepared by surfactant assisted sol–gel method. Scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, fourier transform infra red spectroscopy and thermogravimetric analysis-differential thermal analysis studies were carried out to study the structure and morphology of this composite nano-rod like cation exchanger. Freundlich adsorption isotherm is well fitted for the adsorption of pyridine on the surface of this composite nano-rod like cation exchanger. The thermodynamic parameters such as Freundlich constant, thermodynamic equilibrium constant (K ₀), standard free energy changes (ΔG ⁰), standard enthalpy changes (ΔH ⁰) and standard entropy changes (ΔS ⁰) have been evaluated. These parameters indicated that the adsorption of pyridine on the surface of composite nano-rod like cation exchanger was feasible, spontaneous and exothermic in nature which suggests for the potential application of pyridine removal from water.     

Interpretation of interactions of halogenated hydrocarbons with modified silica adsorbent coated with 3-benzylketoimine group silane

The paper reports a new group of adsorbents obtained by modification of silica surface with silane containing 3-benzylketoimine groups with bonded Cu(II) and Ni(II) chlorides. The adsorbents obtained were subjected to a chromatographic study to establish the type of adsorbate-adsorbent interactions for aliphatic and aromatic halogenated hydrocarbons used as adsorbates. The following retention parameters were determined: retention factor (k), Kovats retention index (I), specific retention volume (V _g), and molecular retention index (ΔM _e). Values of the molecular retention index were subjected to complex statistical analysis providing qualitative relations between the adsorbates’ properties and spatial structure and their retention. It was observed that bonding of chlorides of Cu(II) and Ni(II) with the ketoimine groups resulted in an increase of the retention index and the molecular retention index indicating an increase of the adsorbate-adsorbent interaction in comparison with the reference column coated with silane including a 3-benzylketoimine group without transition metal chlorides. Along with the observed increase of the adsorbate-adsorbent interaction, larger difference between the retention indices was observed for both halogenated compounds. This indicates a positive influence of the introduced transition metals on the selectivity of the adsorbate-adsorbent interactions of the examined columns.     

Capillary complexation gas chromatography in analysis of halohydrocarbons

Summary Investigations were carried out on capillary columns coated with liquid stationary phases possessing cyano (3-cyanopropylmethylpolysiloxane) and thiol (3-mercaptopropylmethylpolysiloxane) groups. These phases were modified by bonding CuCl₂ and CoCl₂ to cyano groups or NiCl₂ and CoCl₂ to thiol groups. The modified phases were examined in order to assess their usefulness for the analysis of aliphatic and aromatic halohydrocarbons. A number of retention parameters (I, ΔMe, Vg), which allow characterisation of the specific interactions between the bonded metal and the halohydrocarbon molecule, were determined for the columns both with and without the metal. The separations performed show that the columns with bonded metals are applicable for the practical analysis of the halohydrocarbons.     

An indirect thermodynamic model developed for initial stage sintering of an alumina compacts by using porosity measurements

The specific micro- and mesopore volumes (V) of alumina compacts fired between 900 and 1250 °C for 2 h were determined from nitrogen adsorption/desorption data. The V value was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K _a) was estimated for each firing temperature by assuming K _a = (V _i − V)/V, where V _i is the largest value at 900 °C before sintering. Also, an arbitrary Gibbs energy (ΔG _a °) of sintering was calculated for each temperature using the K _a value. The graph of ln K _a versus 1/T and ΔG _a ° versus T were plotted, and the real enthalpy (ΔH°) and the real entropy (ΔS°) of sintering were calculated from the slopes of the obtained straight lines, respectively. On the contrary, real ΔG° and K values were calculated using the real ΔH° and ΔS° values in the ΔG° = −RT lnK = 165814 − 124.7T relation in SI units.     

Determination of the surface properties of untreated and chemically treated kaolinites by inverse gas chromatography

Inverse gas chromatography is used to study the surface properties of the untreated and chemically treated kaolinite samples. Changes in the enthalpy of adsorption for a variety of probes and in the surface energy of clays are measured and the effect of modification of the natural clay after chemical treatment with Na₂CO₃ is determined. The surface energy of the natural clay increased by the modification due to an increase in the surface area. It can be concluded that the dispersive component of surface free energy, γ _s ^d , decreases with temperature in the 200–275°C temperature range for both samples. Temperature coefficients of γ _s ^d for untreated and modified kaolinites are −0.1185 and −0.3966 mJ/(m² °C) with the correlation coefficients (R ²) of 0.8479 and 0.965, respectively. From the retention data for polar probes at infinite dilution, information on the accessibility of surface sites to the probes and on the acid-base character of the surface is obtained. The specific free energy of adsorption, the specific enthalpy of adsorption (ΔH ^sp), and the specific entropy of adsorption of polar probes on initial and modified kaolinites are determined. The ΔH ^sp values correlated with the donor numbers and modified acceptor numbers of the probes to quantify the acidity (K _A) and basicity (K _D) parameters of clay surfaces. The values of K _A and K _D for initial and modified kaolinites are determined to be 0.1202 and 0.2803; 0.0130 and 0.0408 with the correlation coefficients of −0.9805 and −0.9782, respectively. The unmodified clay sample indicated a more acidic character, while the modified clay sample conferred a largely basic character. Consequently, the predominant surface basicity of the modified kaolinite agrees with expectation, bearing in mind the treatment with Na₂CO₃, taking into consideration that such a modification contributes to a decrease in the hydrophilicity of the surface and also results in the surface showing only weak Lewis acidity. The text was submitted by the authors in English.     

Thermodynamic investigation of effect of salt concentrations on denatured α-Amylase adsorbed onto a moderately hydrophobic surface

The displacement adsorption enthalpies (ΔH) of denatured α-Amylase (by 1.8 mol L⁻¹ GuHCl) adsorbed onto a moderately hydrophobic surface (PEG-600, the end-group of polyethylene glycol) from solutions (x mol L⁻¹ (NH₄)₂SO₄, 0.05 mol L⁻¹ KH₂PO₄, pH 7.0) at 298 K are determined by microcalorimeter. Further, entropies (ΔS), Gibbs free energies (ΔG) and the fractions of ΔH, ΔS, and ΔG for net adsorption of protein and net desorption of water are calculated in combination with adsorption isotherms of α-Amylase based on the stoichiometric displacement theory for adsorption (SDT-A) and its thermodynamics. It is found that the displacement adsorptions of denatured α-Amylase onto PEG-600 surface are exothermic and enthalpy driven processes, and the processes of protein adsorption are accompanied with the hydration by which hydrogen bond form between the adsorbed protein molecules favor formation of β-sheet and β-turn structures. The Fourier transformation infrared spectroscopy (FTIR) analysis shows that the contents of ordered secondary structures of adsorbed α-Amylase increase with surface coverages and salt concentrations increment.     

Physicochemical constants as a factor determining the need for the derivatization of organic substances in analysis by gas chromatography

A criterion was proposed to estimate the necessity of the derivatization of organic substances for their determination on conventional nonpolar phases, based on such characteristic of analytes as molecular weight (M _r), normal boiling point (T _bp), and molar refraction (MR _D). All these constants can be presented as indices relative to nonpolar n-alkanes (similarly to chromatographic retention indices), I(M), I(T), and I(MR _D), which can be compared to each other as differences Δ_{T − M} = I(T) − I(M) and Δ_{T − M} R _D = I(T) − I(MR _D). Substances do not require derivatization if Δ_{T − M} < 400 and Δ_{T − M} R _D < 600, while at Δ_{T − M} > 600 and Δ_{T − MRD} > 800, derivatization is necessary.     

Isocratic separation of ginsenosides by high-performance liquid chromatography on a diol column at subambient temperatures

An improved high-performance liquid chromatographic method for separation of a number of ginsenosides has been developed. The influence of temperature (from 0 to 25°C) on the retention and separation of the ginsenosides was studied by applying a binary mobile phase (acetonitrile/water, 82:18 v/v) and a diol column (LiChrospher 100 Diol). The column temperature is one of the more important parameters for the retention and separation of the components investigated. Selected thermodynamic parameters, including changes of enthalpy (ΔH°) and entropy (ΔS°), were estimated from linear van’t Hoff plots, and possible retention mechanisms were discussed. Moreover, the best separation conditions were selected based on optimization criteria including maximum retention time (t _{R max}), minimum resolution (R _{s min}), and relative resolution product (r). Temperature regions close to 14°C offered the highest selectivity and almost equal distribution of the ginsenosides peaks across the chromatogram. Under such isocratic conditions, excellent separation of chromatographic standards and selected ginseng samples was achieved in less than 16 min.     

Micellar parameters of diblock copolymers and their interactions with ionic surfactants

The interactions of non-ionic amphiphilic diblock copolymer poly（oxyethylene/oxybutylene）(E₃₉B₁₈) with anionic surfactant sodium dodecyl sulphate（SDS） and cationic surfactant hexadecyltrimethylammonium bromide（CTAB） were studied by using various techniques such as surface tension,conductivity,steady-state fluorescence and dynamic light scattering.Surface tension measurements were used to determine the critical micelle concentration（CMC） and thereby the free energy of micellization(△G_mic),free energy of adsorption(△G_ads),surface excess concentration（Γ） and minimum area per molecule（A）.Conductivity measurements were used to determine the critical micelle concentration（CMC）,critical aggregation concentration（CAC）,polymer saturation point（PSP）,degree of ionization（α） and counter ion binding（β）. Dynamic light scattering experiments were performed to check the changes in physiochemical properties of the block copolymer micelles taken place due to the interactions of diblock copolymers with ionic surfactants.The ratio of the first and third vibronic peaks（I₁/I₃） indicated the polarity of the pyrene micro environment and was used for the detection of micelle as well as polymer-surfactant interactions.Aggregation number（N）,number of binding sites（n） and free energy of binding （△G_b） for pure surfactants as well as for polymer-surfactant mixed micellar systems were determined by the fluorescence quenching method.     

Dependence of the energy of surface-active substances/metal interaction on their ionization potentials. Evaluation of hydrophilicity of metal

The function Δ(ΔG _A ⁰), which is the difference of Gibbs energies characterizing surface-active substance (surfactant, SAS) adsorption at metal/solution and air/solution surfaces, has been introduced. The equation connecting the function Δ(ΔG _A ⁰) with SAS ionization potential has been obtained using the elementary theory of donor-acceptor interactions. Published experimental data on SAS adsorption at mercury, bismuth and gold have been used for Δ(ΔG _A ⁰) calculation. The dependence of Δ(ΔG _A ⁰) on ionization potentials can be described by an equation derived in this work. It has been demonstrated that the value of the hydrophilicity of gold is much higher than the values for mercury and bismuth. The lifetime of SAS molecules at a metal surface has been estimated. The question of the possibility of theoretica l estimation of standard energies ΔG _A ⁰ characterizing SAS adsorption at a metal/solution surface has been discussed. Received: 9 December 1996 / Accepted: 13 January 1997     

Dissolution properties of hexanitrohexaazaisowurtzitane (CL-20) in ethyl acetate and acetone

The enthalpies of dissolution in ethyl acetate and acetone of hexanitrohexaazaisowurtzitane (CL-20) were measured by means of a RD496-2000 Calvet microcalorimeter at 298.15 K, respectively. Empirical formulae for the calculation of the enthalpy of dissolution (Δ_diss H), relative partial molar enthalpy (Δ_diss H _partial), relative apparent molar enthalpy (Δ_diss H _apparent), and the enthalpy of dilution (Δ_dil H _1,2) of each process were obtained from the experimental data of the enthalpy of dissolution of CL-20. The corresponding kinetic equations describing the two dissolution processes were \frac\textda\textdt = 1.60 ×10 ^{- 2} (1 - a)^0.84 {\frac{{{\text{d}}\alpha }}{{{\text{d}}t}}} = 1.60 \times 10^{ - 2} (1 - \alpha )^{0.84} for dissolution process of CL-20 in ethyl acetate, and \frac\textda\textdt = 2.15 ×10 ^{- 2} (1 - a)^0.89 {\frac{{{\text{d}}\alpha }}{{{\text{d}}t}}} = 2.15 \times 10^{ - 2} (1 - \alpha )^{0.89} for dissolution process of CL-20 in acetone.     

Curing of a phenol–formaldehyde–tannin adhesive in the presence of wood

The curing of a phenol–formaldehyde–tannin (PFT) adhesive in the presence of pine or eucalyptus wood has been studied using differential scanning calorimetry. The influence of the adhesive/wood ratio on the activation energy (E_a), the temperature of the maximum of the exothermic peak (T_p) and the enthalpy of the curing process (ΔH) was analysed. E_a, T_p and ΔH of the curing reaction decreased when wood was added in the curing system. The adhesive/wood interaction did not depend significantly on wood species.     

Inverse gas chromatographic characterization of Porapak Q as an extractant of pollutants from aqueous media

The surface of Porapak Q was characterized by inverse gas chromatography (IGC) carried out by the determination of dispersive and specific adsorption components of Porapak Q surface, calculating γ_s^d and -ΔH _a^s parameters, respectively. An interpretation was given based on the chemical structure of Porapak Q. The obtained results were confirmed through the surface Lewis acidity and basicity constants, K _a and K _b, respectively, correlating well with the determined thermodynamic parameters.     

Quantum-chemical study on the relation between thermodynamic parameters in the iodination of benzene with N-I and O-I reagents and p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of the corresponding NH and oh acids

Thermodynamic parameters of direct iodination of benzene with several iodinating agents were calculated using semiempirical (PM3), ab initio (3–21G**), and DFT (B3LYP/LanL2DZ) methods, as well as in terms of the polarization continuum model (PCM or Tomasi model). A close to linear correlation was found between the calculated thermodynamic parameters (ΔH ^≠, ΔG ^≠) and pK _T and experimental pK _a values of acids whose anions are incorporated into iodine-containing intermediates.     

Adsorption of uranium ions by crosslinked polyester resin functionalized with acrylic acid from aqueous solutions

In this paper, the crosslinked polyester resin containing acrylic acid functional groups was used for the adsorption of uranium ions from aqueous solutions. For this purpose, the crosslinked polyester resin of unsaturated polyester in styrene monomer (Polipol 353, Poliya) and acrylic acid as weight percentage at 80 and 20%, respectively was synthesized by using methyl ethyl ketone peroxide (MEK_p, Butanox M60, Azo Nobel)-cobalt octoate initiator system. The adsorption of uranium ions on the sample (0.05 g copolymer and 5 mL of U(VI) solution were mixed) of the crosslinked polyester resin functionalized with acrylic acid was carried out in a batch reactor. The effects of adsorption parameters of the contact time, temperature, pH of solution and initial uranium(VI) concentration for U(VI) adsorption on the crosslinked polyester resin functionalized with acrylic acid were investigated. The adsorption data obtained from experimental results depending on the initial U(VI) concentration were analyzed by the Freundlich, Langmuir and Dubinin–Radushkevich (D–R) adsorption isotherms. The adsorption capacity and free energy change were determined by using D–R isotherm. The obtained experimental adsorption data depending on temperature were evaluated to calculate the thermodynamic parameters of enthalpy (ΔH°), entropy (ΔS°) and free energy change (ΔG°) for the U(VI) adsorption on the crosslinked polyester resin functionalized with acrylic acid from aqueous solutions. The obtained adsorption data depending on contact time were analyzed by using adsorption models such as the modified Freundlich, Elovich, pseudo-first order and pseudo-second-order kinetic models.     

The strength and length of donor-acceptor bonds in molecular complexes

Numerous published data on the structure and thermodynamics of formation of molecular complexes are analyzed. The enthalpies of complexation (−ΔH) are related to the characteristic parameter Δr = [r _DA−a ₁(r _D+r _A)], where r _DA is the donor-acceptor bond length determined by microwave spectroscopy and X-ray analysis, r _D and r _A are the tabulated values of the homopolar covalent radii of the heteroatoms that form the donor-acceptor bond, and a ₁ is an empirical coefficient equal to 0.901±0.007. The relation between −ΔH and Δr values has the form −ΔH = a ₂/Δr (a ₂ = 21.6±1.6 kJ Å mol⁻¹), with a mean relative error of approximation of about 15% and a correlation coefficient of 0.97. As the strength of the complex increases, the donor-acceptor bond length approaches the sum of the heteropolar covalent radii of the atoms involved in the bond (Δr tends to zero). At Δr ≫ 1, the strength of complexes is determined by weak van der Waals interactions between the complex components and the −ΔH values tend to zero. Dedicated to the memory of E. N. Guryanova (1911–2004), a prominent scientist who formulated the basic principles elaborated by her disciples in this review. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1869–1878, October, 2007.     

Marked differences between acetonitrile/water and methanol/water mobile phase systems on the thermodynamic behavior of benzodiazepines in reversed phase liquid chromatography

Summary Two different methods were used to determine the separation factor α at different temperatures and the Gibbs-Helmholtz parameters (Δ(ΔH), Δ(ΔS)) of two adjacent benzodiazepines on a chromatogram were obtained from plots of lnα versus 1/T. We first studied each factor (fraction of water ϕ in the ACN/water mixture and column temperatureT), which controls the retention mechanism, and then we examined the simultaneous variation of all these factors. The changes in Δ(ΔH) and Δ(ΔS) in relation to a volume fraction of water ϕ in an ACN/water mixture were examined. In the ACN/water system, Δ(ΔH) was fairly constant in the acetonitrile region of ϕ≤0.52 and appears to be a roughly linear function of ϕ for ϕ≥0.52. In this system Δ(ΔS) is approximately a parabolic function of ϕ with an optimum at ϕ≊0.52. The retention mechanism of ten benzodiazepines was found to be significantly different in the methanol/water and ACN/water mixtures. The separation optimization of these ten benzodiazepines was then considered. A fraction of water of 0.43 in the ACN/water mixture and a column temperature of 44 °C gave the most efficient separation conditions in the ACN/water mixture.