Thermodynamics of adsorption of organic liquids on Ni-modified solid obtained from collodal silica

Summary In order to reduce the well known silica surface inhomogeneity and to get a material suitable for selective chromatographic separations, a modified variety of the material was prepared by a controlled sorption of Ni²⁺ ions at the solid surface; its adsorption properties were then investigated using the GSC method. Since the starting material was of very high purity, it was possible to follow the influence of Ni²⁺ ion on the adsorption of various organic liquids, including n-alkanes, cyclohexane, benzene and halomethanes. Adsorption experiments, spanning zero surface coverage and the finite concentration region, were performed in the temperature range from 333 to 443 K. Both the surface activity change resulting from the modification and the possible interactions between the organic substances and the prepared adsorbent were examined using the standard free energy change of adsorption in zero coverage conditions. The adsorption isotherms of all the adsorbates listed were obtained using the Elution by Characteristic Point (ECP) technique and the data were fitted to the BET equation. To get a better insight into the adsorption process in the finite concentration region, the thermodynamic parameters of adsorption were determined and are discussed in terms of surface coverage.     

A gas-solid chromatographic study of the adsorption of hydrocarbons and alcohols on graphite

Summary The Henry's law region of the adsorption isotherms for several hydrocarbons and alcohols on a graphite was studied by gas-solid chromatography. The thermodynamic functions at zero surface coverage were determined. The low heats of adsorption indicate a highly homogeneous surface. A constant increment of the standard free energy with the number of carbon atoms in the lineal adsorbates was found. Attemps were made to estimate the London contribution to the surface free energy of the adsorbents. A non-specific interaction between the graphite and the adsorbates was shown.     

Adsorption of organics on activated carbon cloth at zero surface coverage

Summary Adsorption properties of activated carbon cloth were investigated by gas-solid chromatography. Retention of several organic compounds was measured in the temperature range from 200 to 250°C. The gas/solid distribution coefficients and the related thermodynamic function of adsorption at zero surface coverage were determined. The obtained experimental data were used to explain the adsorbent-adsorbate interaction.     

Use of specific surface areas in inverse gas chromatography studies at zero surface coverage

Inverse gas chromatography (IGC) is frequently used to study adsorption processes at zero surface coverage on microporous activated carbons. This allows to determine the thermodynamic adsorption parameters as equilibrium constants, V(S), standard enthalpies of adsorption, Delta HA degrees, standard free energy of adsorption, Delta GA degrees, and so on. Nevertheless, the surface areas of the adsorbents (microporous carbons in this case) are needed for this purpose. The experimental determination of the surface areas of microporous solids is not univocal and the results depend on the adsorbate employed in the measurements, usually N2 or CO2. This means that the thermodynamic parameters obtained by IGC are subjected to a degree of uncertainty depending on whether N2 or CO2 is used to determine the surface area values. The aim of this paper is to discuss which of the two surface area values is more appropriate to be used in IGC measurements at zero surface coverage. Experimental and theoretical considerations are supplied in a thorough discussion which supports that CO2 surface area value is more appropriate. Thus, it is proposed that this should be used instead of the more generally extended nitrogen specific surface area obtained by the BET equation.     

Adsorption of n-alkanes on a graphitized carbon black and its activation products in dry air

Summary Adsorption of n-alkanes (n-heptane to n-decane) on a graphitized carbon black and its activation products in dry air was carried out by a gas chromatographic technique. Adsorption runs were performed at finite surface coverage, to obtain the surface area and the London component of the surface free energy of the solids from the adsorption isotherms. On the other hand, the adsorption of n-alkanes carried out at zero surface coverage gave the differential heats of adsorption. The results show that, after the activation in dry air of the graphitized carbon black used, the surface area has a linear relationship with the degree of activation, whereas the surface heterogeneity changes in a parabolic fashion.     

Adsorption of organic liquids on alkaline earth-metal modified silica

Summary Adsorption properties of alkaline earth-metal modified silica beads were investigated by the gas-solid chromatography (GSC) method. Silica beads, obtained from colloidal material, were characterized by their surface areas, crystallinity and amount of sorbed alkaline earth cations. Retentions of several aliphatic, alicyclic, chlorinated, and aromatic compounds were measured in the temperature range from 373 to 453 K.The gas/solid partition coefficients, K₅, and the related thermodynamic functions at zero surface coverage were determined. The results show that the specific surface areas of modified silica samples remain practically constant, whereas the modification leads to a decrease of silica surface heterogeneity.     

反气相色谱法测定锌镁铝类水滑石的表面性能

采用水热合成法制备了锌镁铝类水滑石(ZnMgAl-HTLC),利用X射线衍射仪(XRD)对ZnMgAl-HTLC的晶体结构进行了表征,并以一系列非极性和极性分子为探针分子,采用反气相色谱法(IGC)研究了ZnMgAl-HTLC的表面性能.结果表明:XRD特征衍射峰窄、尖、高,水热合成法能够制得纯度较高的ZnMgAl-HTLC; ZnMgAl-HTLC表面吸附自由能小于零,表面色散自由能最大为6.02 mJ/m²,酸碱作用自由能最大为5.33 kJ/mol,吸附焓为43.6 kJ/mol,吸附熵为0.15 kJ/mol.本文的反气相色谱方法对研究锌镁铝类水滑石的表面性能具有重要的指导意义.     

Modification of silica surfaces by grafting of alkyl chains. I — Characterization of silica surfaces by inverse gas-solid chromatography at zero surface coverage

Summary The surface properties of fumed and precipitated silicas were modified by esterifying their surface silanol groups with methanol and hexadecanol. Characterization of the silicas by inverse gas-solid chromatography at zero surface coverage revealed that the surface properties of the initial solids varied with their method of preparation. Thermodynamic functions (ΔG, ΔH, ΔS) changed significantly with grafting. Grafting resulted in an significant decrease in the dispersive and specific components of the surface free energy of the solid. The changes were shown to be dependent on the method of preparation of the filler (the biggest effects being particularly observed with fumed silicas) and on the size of the grafted chain (hexadecyl grafts yielding the largest change).     

Time distribution of adsorption entropy of gases on heterogeneous surfaces by reversed-flow gas chromatography

The reversed-flow gas chromatography (RF-GC) technique has been applied to measure the adsorption entropy over time, when gaseous pentane is adsorbed on the surface of two solids (gamma-alumina and a silica supported rhodium catalyst) at 393.15 and 413.15K, respectively. Utilizing experimental chromatographic data, this novel methodology also permits the simultaneous measurement of the local adsorption energy, epsilon, local equilibrium adsorbed concentration, c(s)(*), and local adsorption isotherm, theta(p, T, epsilon) in a time resolved way. In contrast with other inverse gas chromatographic methods, which determine the standard entropy at zero surface coverage, the present method operates over a wide range of surface coverage taking into account not only the adsorbate-adsorbent interaction, but also the adsorbate-adsorbate interaction. One of the most interesting observations of the present work is the fact that the interaction of n-pentane is spontaneous on the Rh/SiO(2) catalyst for a very short time interval compared to that on gamma-Al(2)O(3). This can explain the different kinetic behavior of each particular gas-solid system, and it can be attributed to the fact that large amounts of n-C(5)H(12) are present on the active sites of the Rh/SiO(2) catalyst compared to those on gamma-Al(2)O(3), as the local equilibrium adsorbed concentration values, c(s)(*), indicate.     

反相气相色谱法表征1-烯丙基-3-甲基氯代咪唑离子液体的表面性质

采用反相气相色谱(IGC)技术研究了不同温度下1-烯丙基-3-甲基氯代咪唑(［AMIM］Cl)的表面性质。以正己烷、正庚烷、正辛烷和正壬烷作为非极性探针分子测定［AMIM］Cl在343.15、353.15、363.15和373.15 K温度下的表面色散自由能;以二氯甲烷、三氯甲烷、丙酮、乙酸乙酯、四氢呋喃作为极性探针分子测定离子液体Lewis酸碱性质,并测定了吸附自由能和吸附自由焓变等热动力学参数。实验结果表明,［AMIM］Cl的酸解离平衡常数Ka为0.34,碱解离平衡常数Kb为1.68,其表面呈Lewis两性偏碱性特点。在343.15、353.15、363.15和373.15 K温度下,［AMIM］Cl的表面色散自由能分别为52.26、50.82、46.08和42.05 mJ/m2。这一结果对研究离子液体的表面性质及应用有指导作用。     

不同覆盖度下Li原子在Si(001)表面上的吸附构型和电子结构

采用基于赝势平面波基组的密度泛函理论, 对不同Li原子覆盖度下Li/Si(001)体系的吸附构型、电子结构以及吸附Li原子对表面性质的影响进行了系统研究. 计算结果表明, 在所考察的覆盖度范围内, Li原子倾向于吸附在相邻两个Si-Si二聚体之间各种对称性较高的空穴位, 其中覆盖度为0.75 ML(monolayer)时具有最小的平均吸附能. 由能带结构分析结果可知, 随着覆盖度的增大, Si(001)表面存在由半导体→导体→半导体的变化过程. 在覆盖度为1.00 ML时, 由于表层二聚体均受到显著破坏, 使得体系带隙明显增大. 吸附后, 有较多电子从Li原子转移到底物, 导致Si(001)表面功函显著下降, 并随着覆盖度的增加表面功函呈现振荡变化. 此外, 从热力学稳定性角度上看, 覆盖度为0.75 ML的Li/Si(001)表面较难形成.     

Single‐Molecule Measurement of Adsorption Free Energy at the Solid–Liquid Interface

Adsorption plays a critical role in surface and interface processes. Fractional surface coverage and adsorption free energy are two essential parameters of molecular adsorption. However, although adsorption at the solid–gas interface has been well‐studied, and some adsorption models were proposed more than a century ago, challenges remain for the experimental investigation of molecular adsorption at the solid–liquid interface. Herein, we report the statistical and quantitative single‐molecule measurement of adsorption at the solid–liquid interface by using the single‐molecule break junction technique. The fractional surface coverage was extracted from the analysis of junction formation probability so that the adsorption free energy could be calculated by referring to the Langmuir isotherm. In the case of three prototypical molecules with terminal methylthio, pyridyl, and amino groups, the adsorption free energies were found to be 32.5, 33.9, and 28.3 kJ mol^?1, respectively, which are consistent with DFT calculations.     

多溴吩噻嗪系列化合物的稳定性和热力学性质的密度泛函理论研究

用Gaussian 03程序, 在B3LYP/6-31G*水平上全优化计算了吩噻嗪和135个多溴吩噻嗪系列化合物(PBPTHs)在298.15 K和101.3 kPa状态时的热力学参数. 设计等键反应, 计算了PBPTHs系列化合物的标准生成热( )和标准生成自由能( ). 同时研究了这些参数与溴原子的取代位置及取代数目(NPBS)之间的关系. 结果表明: 多溴吩噻嗪的热能校正值(Eth)、恒容热容( )、标准熵( )、标准焓( )以及标准自由能( )与NPBS之间有很强的相关性(r2≥0.998). 在相关方程中, 溴原子的取代个数对多溴代吩噻嗪热力学数值的大小有很大影响. 根据 的相对大小, 从理论上求得异构体的相对稳定性顺序.     

Surface properties of a calcium carbonate filler treated with stearic acid

The fixation of stearic acid on a CaCO₃ filler and the induced changes in surface properties have been studied. The chemisorption isotherm of stearic acid was established using a tracer (tritium) method. Nitrogen adsorption, contact angle measurements and inverse gas chromatography at infinite dilution revealed a drastic decrease in surface energy, as a function of the degree of surface coverage by stearic acid. The totally covered CaCO₃ exhibits a surface energy close to that of pure stearic acid, with no surface polarity.     

Effect of moisture on the surface free energy and acid-base properties of mineral oxides

Surface energetic properties of mineral oxides are important in many applications. Since oxide surfaces in practice have generally come in contact with water molecules, it is important to know how water coverage affects the surface properties. In this work, five oxide samples, namely MgO, Al2O3, TiO2, SnO2 and SiO2 are heat-treated to various extents, to produce different degrees of hydration, and characterized thereafter by inverse gas chromatography. Water contents of the treated samples are determined independently by Karl Fischer titration, and specific surface areas are measured by the BET method. The results show that in general as water coverage decreases, the Lifshitz-van der Waals component of the specific surface free energy (sigma(S)LW) increases, but the acid-base interaction potential (-deltaG(AB)) decreases. These attributes are more sensitive to changes in water coverage at lower coverages, where the surface is presumed to consist of patches of molecular water and unhydrated hydroxyl groups.     

Two-photon photoemission study of the coverage-dependent electronic structure of chemisorbed alkali atoms on a Ag(111) surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].     

Surface characteristics of layered silicates: Influence on the properties of clay/polymer nanocomposites

A sodium montmorillonite and six organophilic montmorillonites coated with different surfactants were characterized in various ways in the study. Particle and surface characteristics were determined by nitrogen adsorption and inverse gas chromatography, respectively. The gallery structure of organophilic montmorillonite, the orientation of surfactants in the galleries, and surface coverage were estimated by X-ray diffraction measurements and model calculations. The effect of organophilization on the properties of polypropylene/clay composites was determined by the measurement of tensile properties. The results showed that the surface energy of uncoated layered silicates is large; thus, the forces keeping the layers together are very strong. The long chain surfactants used for the coating of montmorillonite orientate more or less parallel to the surface and usually cover the platelets in a single layer in commercial silicates. Surplus surfactant is not located in the galleries, but among the particles, and might influence the properties of composites negatively. Organophilization leads to the drastic decrease of surface free energy. Surface tension of all coated clays is practically the same, irrespective of the type of the surfactant used for treatment. Low surface energy leads to weaker forces between the layers, which might facilitate exfoliation. This effect can be further enhanced by the use of surfactants with two long aliphatic chains, one of which orientates vertically to the surface, leading to larger gallery distance. Polymer/silicate interaction also decreases as an effect of decreasing surface tension proved by the decrease of tensile yield stress of polypropylene/montmorillonite composites. Besides surface tension, the exfoliation of layered silicates is influenced by several other factors as well, like gallery distance, mutual solubility of the components, competitive adsorption, or possible chemical reactions.     

Experimental evidence of the reversibility of the first stage of protein adsorption at a hydrophobic quartz surface near the isoelectric point

The reversibility of the first stage of adsorption of zwitterionic cytochrome c on a hydrophobic quartz surface was investigated using time‐resolved slab optical waveguide (SOWG) absorption spectroscopy. Using a novel prism‐free broadband coupling approach, absorbance data were collected successfully at a 50 ms time interval during the first few seconds after solution–surface contact. Near the isoelectric point where the cytochrome c molecules possess a net charge of zero and hence cannot be influenced by an electric field, the speed at which adsorption proceeded was found to be dependent on cytochrome c concentration as well as on surface hydrophobicity. It was also observed that the degree of protein adsorption increased as the surface hydrophobicity was increased. Within 6 s the adsorption process appeared to be reversible, as revealed by extremely low chi‐squared values when the absorbance data were fitted into the reversible Langmuir‐type kinetic model. The standard Gibbs free energy of adsorption was also calculated from the absorbance data. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermodynamics of Adsorption ofn-Alkanes on Maleated Wood Fibers by Inverse Gas Chromatography

The adsorption ofn-heptane,n-octane,n-nonane, andn-decane on untreated wood fiber and wood fiber treated with maleated polypropylene was studied by inverse gas chromatography (IGC) at infinite dilution or zero surface coverage. The specific retention volume increased with increasing probe chain length, decreased with increasing column temperature, and increased with increasing maleated polypropylene concentration. The enthalpy of adsorption increased with increasing chain length of the probe vapors. The enthalpy of adsorption remained constant after the treatment of wood fiber. The London dispersive component of the surface free energy decreased with the column temperature and showed no dependency with either the type of wood fiber or the maleated polypropylene concentration.     

Two-dimensional condensation in camphor-10-sulphonic acid films at the mercury/electrolyte interface

The electrosorption properties of camphor-10-sulphonic acid (CS) in different electrolytes (Na₂SO₄, LiClO₄, KNO₃, KCl, KBr, KJ) were investigated experimentally by ac polarographic measurements at different temperatures. Depending on the type of electrolyte and on the temperature, film condensation was observed.The Frumkin interaction coefficient, determined from the critical degree of coverage, does not depend linearly on the reciprocal temperature as would be the case in correspondence to the temperature dependence of the pit width. This contradiction does not occur if the interaction coefficient is determined from the critical degree of coverage using the lattice gas model. The temperature dependence of the pit width of the differential capacity-potential curves, found in the experiment, can be theoretically described by both these models. The interaction coefficient in the models is inversely proportional to the temperature. Furthermore, the parameters of the standard free energy of adsorption, the interaction energy, the interaction coefficient of the Frumkin isotherm, the adsorption coefficient, and the maximal surface concentration of CS in the film were estimated and compared.