The Role of the Specific Surface Area of an Adsorbent in the Optimization of Mixture Separation Conditions in Thin-Layer Chromatography

Abstract

The very important aspect of effect of the magnitude of specific surface area on R_M values obtained by using TLC method is presented. In experiments were performed on 4 adsorbents of different specific surface areas 50 – 500 m²/g and with mixed binary mobile phases.

It is shown that R_M values of chromatographed substances aromatic hydrocarbons are lineary dependente upon the specific surface area of adsorbents for each composition of mobile phase. This relationship can be described by a straight line, with the parameters a and b that can be tabularized. These lines can be used to calculate the R_M values of chromatographed substances for any adsorbent if is specific surface area is known. The illustrational comparison of experimentally obtained and theoretically predicted R_M values for different chromatographic system are presented.     

Theoretische Untersuchungen zur Molekülstruktur und Spektroskopie von Benzopyrylium-Verbindungen: Vollständig-optimierte S0- und S1-Molekülgeometrien des 2-Phenyl-benzopyrylium-Ions und Einfluß der Substitution auf sein elektronisches Spektralverhalten in Absorption und Fluoreszenz

Summary Completely-optimized S₀ and S₁ molecular geometries of 2-phenyl benzopyrylium ion are presented. Using these structural results the influence of different substituents on its spectral behaviour in absorption and fluorescence is studied theoretically and compared with the corresponding experimental data.

     

Theoretische Untersuchungen zur Molekülstruktur und Spektroskopie von Benzopyrylium-Verbindungen: Vollständig-optimierte S0- und S1-Molekülgeometrien des 5,6-Dihydrobenzo[c]xanthylium-Ions und Einfluß der Strukturfixierung sowie Substitution auf sein elektronisches Spektralverhalten in Absorption und Fluoreszenz

Summary Completely-optimized S₀ and S₁ molecular geometries of 5,6-dihydrobenzo[c]xanthylium ion are presented. Using these structural results the influence of structural fixation and of substitution on its spectral behaviour in absorption and fluorescence is studied theoretically and compared with experimental data.

     

Non-monotonic behaviour with concentration of the surface tension of certain binary liquid alloys

The surface tension σ(c) of most liquid binary alloys usually varies with concentration c in a monotonic way between the values σ₁ and σ₂ of the two pure metals, and this behaviour is well explained by current models. Some alloys show deviations from this ideal behaviour. One of those is Fe–B. The surface tension of this liquid alloy shows a minimum at 17 atomic % B, which corresponds well with the composition of the eutectic point in the phase diagram, followed by a maximum at a concentration of 24 atomic % B or higher. The usual models for the surface tension of liquid binary alloys do not explain those exceptional features, and we propose that a model involving the concentration fluctuations in the liquid alloy has the proper ingredients to account for the features in Fe–B and similar alloys.     

Thermodynamics of chemisorption: induced surface segregation in binary alloys

Numerical results are presented for the segregational behaviour in the hydrogen-adsorbed Ni_xCui_1-x alloy system in which the equilibria of adsorption and segregation are treated in a self-consistent manner. For the powdered systems the nickel atoms are found to segregate to the surface in the presence of H adatoms. In crystalline alloy systems the Cu atoms segregate to the surface, but the amount of Cu segregation in the presence of H adatoms is less than that for a clean alloy surface.     

Surface plasmon resonance as a tool for the estimation of adsorbed polymeric layer characteristics: Theoretical considerations and experiment

The potential application of Surface Plasmon Resonance (SPR) spectroscopy in evaluating the thickness and volume fraction of adsorbed macromolecular layers is discussed in this work. The sensitivity of SPR spectroscopy to different layer concentration and to the layer extension normal to the surface (thickness) is theoretically illustrated. A new approach for the interpretation of SPR data is presented, which is applicable whenever the functional form of the density profile is known. The use of the proposed procedure for the fitting of experimental results from PS‐PEO brush self‐assembly on alumina surface has allowed the determination of the layer parameters, which have been found to be in accordance with theoretical mean‐field and scaling predictions, being also in good agreement with previous results from neutron reflectivity experiments. Furthermore, it has been confirmed that the dependence of the brush layer thickness d on the molecular weight M_w obeys the scaling law d ～ M_w^0.63. Since surface plasmon measurements can be acquired quite fast, it is suggested that under the present analysis scheme, the technique may be implemented to probe the average conformational properties of adsorbed macromolecular layers during their formation or under external stimuli. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2060–2070, 2007     

The densities and molar volumes of molten lead(II) dodecanoate/dodecanoic acid mixtures over their complete composition range: Evidence for non-ideal behaviour

Data are presented for the densities and molar volumes of the molten system lead(II) dodecanoate/dodecanoic acid over its complete composition range. For equimolar mixtures, plots of molar volume against temperature show curvature at high temperatures, suggesting deviations from ideal behaviour. Support for this comes from a plot of molar volume at constant temperature against acid mole fraction. Densities and molar volumes are reported for lead (II) carboxylate/carboxylic acid (0.5 mole fraction) and for pure carboxylic acid for the even chain acids C₁₀ to C₁₈. The molar volumes at constant temperature in these cases are linear functions of chain length, although the volume occupied per methylene group in equimolar mixtures is suggested to be slightly smaller than with pure soap or pure acid. An explanation for non-ideal behaviour becoming more marked at higher temperature is given in terms of acid monomer-dimer equilibria.     

A study on the adsorption voltammetry of cobalt(II)—dime thylglyoxime system

The behaviour of the cobalt complex with dimethylglyoxime (DMG), Co(II)A₂, at the mercury electrode has been investigated in details. The adsorption phenomena have been observed by both normal pulse polarography and voltammetry with linearly changing potential. Experimental results show that, under the condition of adsorption potentials ranging from ?0.60 to ?0.9 eV (vs. S.C.E.), Co(II)A₂ can be adsorbed on the surface of hanging mercury drop electrode (HMDE) very well. The superficial concentrations represents a Langmuir isotherm with both concentration of Co(II)A₂ and the preconcentration time. The superficial concentration equation for adsorption voltammetry, corresponding to the condition of the low coverage of the electrode surface, is deduced. The equation has been verified experimentally. The sensitivity of the proposed method, which has been analysed theoretically, is independent on the scan rate and the surface area of HMDE, but depends on the preconcentration time and the diffusion layer thickness. For the 120 sec accumulation, the lower limit of determination is 1.10^?9 M.     

Electrochemical induction of changes in the distribution of the hydrogen adsorption states on Pt (100) and Pt (111) surfaces in contact with sulphuric acid solution

Electrochemical adsorption-desorption of hydrogen has been shown to be a structure-sensitive reaction. The influence of the crystallographic surface structure on this process is typically represented by the voltammogram of such a surface in contact with 0.5 M H₂SO₄ solution.For a given crystallographic plane, the voltammetric profile shows several peaks which are ascribed to various H adsorption states. Defined polarization programmes, applied to such electrodes, changed in a controlled way the population of each state with the possibility of inducing new ones.These transformations in the shape of the Pt (100) voltammogram in the hydrogen adsorption region suggest that the surface undergoes reconstruction due to hydrogen adsorption above 0.3 of a monolayer. The combined effect of low oxygen coverage with high hydrogen coverage induces a new type of voltammogram. At least four types of voltammogram have been observed for this orientation, each related to a particular surface structure which remains unknown in the present state of knowledge. This behaviour is in agreement with the known behaviour of this surface in contact with the gas phase at low pressure whose surface structure depends on the presence or not of an adsorbate such as hydrogen and on thermal treatment. This work gives preliminary evidence for the existence of possibilities of passing reversibly, by an electrochemical route, from one surface structure to another.For the Pt (111) surface, only one ordered state and a disturbed state with a different degree of disorder are observed. There is no known electrochemical possibility of passing reversibly from one state to the other; the transformation always occurs from the ordered to the disordered state.Comparison of these various voltammograms with that of a polyoriented model surface represented by a sherical Pt single crystal suggest that for both orientations the most strongly bonded hydrogen states are directly connected with the presence of two-dimensional long-range ordered domains on these low index surfaces.     

Adsorption of uranyl species onto the rutile (110) surface: a periodic DFT study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO₂ rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta‐ or hexa‐coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H₂O, OH^?, and CO₃^2? ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface–[UO₂(H₂O)₃]²⁺ system were well reproduced by our calculations. With respect to adsorbates, [UO₂(L1)_x(L2)_y(L3)_z]ⁿ (L1=H₂O, L2=OH^?, L3=CO₃^2?, x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H₂O to OH^? and CO₃^2? lengthens the U? O_surf and U? Ti distances. As a result, the uranyl–surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO₂ decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U?O distances that are longer than those of aquouranyl species by 0.1–0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U?O bond lengths, shorter uranyl–surface separations in most cases, and stronger sorption interactions.     

Surface energies of chain molecules from homogeneous nucleation

A model for the interpretation of homogeneous nucleation data for chain molecules is presented. The two surface energies σ_s and σ_e are related to interchain and intrachain bonding. Surface energies calculated from experimental data on n-alkanes from octane to dotriacontane and polyethylene agree with estimated values. The results are discussed in relation to surface energies measured from spherulite growth rates in polymers but these values are not known with sufficient reliability to provide a good basis for comparison.     

Study of the electronic and atomic structure of thermally treated SrTiO3(110) surfaces

The electronic structure of heated SrTiO₃(110) surfaces was investigated with metastable impact electron spectroscopy and ultraviolet photoelectron spectroscopy (He(I)). Scanning tunnelling microscopy and atomic force microscopy (AFM) were used to study the topology of the surface. The crystals were heated up to 1000°C under reducing conditions in ultrahigh vacuum or under oxidizing conditions in synthetic air for 1 h, respectively. Under both conditions microfacetting of the surface is observed. The experimental results are compared with ab initio Hartree–Fock calculations, also presented here, carried out for both ideal and reconstructed SrTiO₃(110) surfaces. The results give direct evidence for Ti termination of the faceted TiO₂ rows. Copyright © 2003 John Wiley & Sons, Ltd.     

Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures—A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study

We investigated the surface tension and surface composition of various mixtures of the two ionic liquids (ILs) 1-methyl-3-octyl-imidazolium hexafluorophosphate [C₈C₁Im][PF₆] and 1,3-bis(polyethylene glycol)imidazolium iodide [(mPEG₂)₂Im]I in the temperature range from 230 to 370 K under ultraclean vacuum conditions. The surface tension was measured using a newly developed apparatus, and the surface composition was determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). In the pure ILs, the alkyl chains of [C₈C₁Im][PF₆] and the PEG chains of [(mPEG₂)₂Im]I are enriched at the IL/vacuum interface. In the mixtures, a strong selective surface enrichment of the alkyl chains occurs, which is most pronounced at low [C₈C₁Im][PF₆] contents. For the surface tension, strong deviations from an ideal mixing behaviour take place. By applying a simple approach based on the surface composition of the mixtures as deduced from ARXPS, we are able to predict and reproduce the experimentally measured temperature-dependent surface tension values with astonishingly high accuracy.     

Effect of steam activation on the porosity and chemical nature of activated carbons from Eucalyptus globulus and peach stones

The effect of steam activation of chars prepared from Eucalyptus globulus (EU) and peach stones (PS) on both the porosity development and the amount of oxygen surface groups is presented. The N₂ (77 K) and CO₂ (273 K) adsorption isotherms and the mercury intrusion measurements show that, apart from the differences in macroporosity caused by the different texture of the original precursors, the development of porosity upon activation is small for the EU char and important for the PS char. A somewhat parallel behaviour is found for the oxygen surface groups, as determined by infrared spectroscopy and temperature programmed decomposition (TPD): the development of microporosity for PS chars is accompanied by an increase in the number of oxygen surface groups stable at the activation temperature, the number being low for activated carbons from EU chars. This behaviour indicates the more important role of diffusion control of the water molecule to the interior of the particle when activating the EU char in respect to the PS char.     

Surface tension and density measurements of the molten YCl3-KCl binary system

The surface tension and the density of the YCl₃-KCl system were determined as a function of temperature (1073–1248 K) by the maximum bubble pressure technique. These results are represented by linear empirical equations as functions of temperature. At a constant temperature, the surface tensions of YCl₃-KCl melts show a negative deviation from Guggenheim's equation for ideal solutions.     

Molecular orbital analysis of the XPS spectrum of a fluorine containing polyimide: PMDA–BDAF

The results of XPS measurements and molecular orbital calculations performed on the fluorine containing polyimide, PMDA–BDAF, are presented. The calculated carbon 1s (C1s) core energy level positions are compared with the level positions inferred from the XPS measurements. Within Koopman's approximation, the observed shape of the main XPS peak is consistent with the calculated distribution of C1s levels under this peak. Comparison of the magnitude of the carbonyl XPS peak intensity with the main peak intensity indicates a carbonyl C1s signal deficiency compared with that expected for “ideal bulk stoichiometry” i.e., for a polymer with no crosslinks or chain terminations. Comparison of data obtained from a grazing emission (surface sensitive) geometry with that obtained from a normal emission geometry, which probes more deeply into the bulk, indicates a signal enhancement of the C1s levels associated with carbon atoms of the CF₃ groups as one nears the polymer surface. Such enhancement might be due to either actual differences in chemical composition, or to preferential structural ordering near the polymer surface.     

Dependence of the effective coefficient of separating a mixture of NH3-O2 and NH3-N2 on the rate of distillation

The effective coefficient for separating a mixture of NH₃-O₂ and NH₃-N₂ at different rates of distillation is determined experimentally and theoretically. The effect of the temperature of a surface layer of liquid ammonia on the purification process from permanent gases (O₂ and N₂) is studied. The results indicate that upon an increase in the rate of distillation, the efficiency of separating a mixture of ammonia and impurities rises as heat transfer in the surface layer becomes difficult.     

Surface structure and roughness of Nickel-Titanium wires

Nickel-Titanium wires not only show the well known shape memory effect, but also pseudoelastic behaviour. This means, as the material is bent in a wide range of tension, the stress in the material stays constant. This special behaviour allows the improvement of orthodontic devices. The investigation of wires for this application reveals great differences in the surface roughness of the wires from various suppliers. The roughness causes a loss of spring energy for the orthodontic wire by frictional effects. The investigation of the mechanical behaviour of these materials leeds to differences compared with the ideal behaviour expected for the stress/strain curve. Obviously these differences are caused by surface effects of the wires. To evaluate these questions intensive roughness measurements with profilometry, laser spectroscopy and scanning force microscopy have been performed. Further more defined bending and tensile experiments have been carried out and corresponding roughness and structure analysis with the scanning force microscope has been taken into account. By etching the wires significant surface effects could be found.     

Surface structure and roughness of Nickel-Titanium wires

Nickel-Titanium wires not only show the well known shape memory effect, but also pseudoelastic behaviour. This means, as the material is bent in a wide range of tension, the stress in the material stays constant. This special behaviour allows the improvement of orthodontic devices. The investigation of wires for this application reveals great differences in the surface roughness of the wires from various suppliers. The roughness causes a loss of spring energy for the orthodontic wire by frictional effects. The investigation of the mechanical behaviour of these materials leeds to differences compared with the ideal behaviour expected for the stress/strain curve. Obviously these differences are caused by surface effects of the wires. To evaluate these questions intensive roughness measurements with profilometry, laser spectroscopy and scanning force microscopy have been performed. Further more defined bending and tensile experiments have been carried out and corresponding roughness and structure analysis with the scanning force microscope has been taken into account. By etching the wires significant surface effects could be found.     

On the Constant Pressure Specific Heat CP of a Simple Fluid

Abstract

Calculation of C_P from a model soft-core equation of state reveals a line in the phase diagram on which C_P is equal to its zero pressure value C _{P 0} . This line commences on the temperature axis where the second virial coefficient has a point of inflexion. At higher temperatures (and pressures) C_P falls below C _{P 0} . The detailed behaviour of C_P is presented via contour maps, illustrating the effects of changing the exponent N(= 3/n, where n is the repulsive potential exponent) which parameterizes the model. For soft-core fluids at high temperatures C_P deviates only slightly from the ideal gas value over a wide range of temperature and density, in marked contrast to the behaviour of hard-core models.