Adsorption of argon from sub- to supercritical conditions on graphitized thermal carbon black and in graphitic slit pores: a grand canonical Monte Carlo simulation study

In this paper we consider the adsorption of argon on the surface of graphitized thermal carbon black and in slit pores at temperatures ranging from subcritical to supercritical conditions by the method of grand canonical Monte Carlo simulation. Attention is paid to the variation of the adsorbed density when the temperature crosses the critical point. The behavior of the adsorbed density versus pressure (bulk density) shows interesting behavior at temperatures in the vicinity of and those above the critical point and also at extremely high pressures. Isotherms at temperatures greater than the critical temperature exhibit a clear maximum, and near the critical temperature this maximum is a very sharp spike. Under the supercritical conditions and very high pressure the excess of adsorbed density decreases towards zero value for a graphite surface, while for slit pores negative excess density is possible at extremely high pressures. For imperfect pores (defined as pores that cannot accommodate an integral number of parallel layers under moderate conditions) the pressure at which the excess pore density becomes negative is less than that for perfect pores, and this is due to the packing effect in those imperfect pores. However, at extremely high pressure molecules can be packed in parallel layers once chemical potential is great enough to overcome the repulsions among adsorbed molecules.     

Molecular dynamics simulation studies of the conformation and lateral mobility of a charged adsorbate biomolecule: implications for estimating the critical value of the radius of a pore in porous media

The conformations, the values of the lateral transport coefficient of a charged biomolecule (desmopressin) in the adsorbed layer and in the liquid layers above the adsorbed layer, the potential energies of the interaction between the biomolecules located in different liquid layers with the charged solid surface and with the biomolecules in the adsorbed layer, the potential energies of the interaction between water molecules in the hydration layers surrounding the conformations of the biomolecules in different layers, as well as the structure and number of hydration layers between the different conformations of desmopressin, were determined by molecular dynamics simulation studies. The results show that the lateral mobility of the adsorbed desmopressin is approximately equal to zero and the value of the lateral transport coefficient of the biomolecule in the liquid layers located above the adsorbed layer increases as the distance of the liquid layer from the charged solid surface increases. But the values of the lateral transport coefficient of the biomolecule in the liquid layers above the adsorbed layer are lower in magnitude than the value of the transport coefficient of desmopressin along the direction normal to the charged solid surface in the liquid phase located above the vacant charged sites of the solid surface, and these differences in the values of the transport coefficients have important implications with respect to the replenishment of the biomolecules in the inner parts of a channel (pore), the overall rate of adsorption, and the form of the constitutive equations that would have to be used in macroscopic models to describe the mechanisms of mass transfer and adsorption in the pores of adsorbent media. Furthermore, a novel method is presented in this work that utilizes the information about the sizes of the conformations of the biomolecule in the adsorbed layer and in the liquid layers above the adsorbed layer along the direction that is normal to the charged solid surface, as well as the number and size of the hydration layers along the same direction, and could be used to estimate the value of the lower bound of the linear characteristic dimension of a pore (i.e., pore radius) in porous adsorbent media (e.g., porous adsorbent particles; skeletons of porous monoliths) in order to realize effective transport and overall adsorption rate.     

Adsorption from Ternary Liquid Mixtures on Partially Dehydroxylated Silica Gel

The formation of mixed adsorbed layers has been tested for ternary mixtures containing a specifically adsorbed component—acetone and binary solvent benzene +n-hepane. The specific excess adsorption isotherms from the liquid phase were measured on silica gel samples partially dehydroxylated. The competition of liquid components for silica surface is discussed on the basis of changes in the mixed solvent composition.     

On the physical adsorption of gases on carbon materials from molecular simulation

In this paper we present a series of work covering a range of aspects relating molecular simulation to experiment. The importance of surface mediation type effects to the adsorption of simple and complex gases is demonstrated. Coupled with the adsorption of simple gases is their projection area when used for surface area determination. The pressure dependence of a projection area is demonstrated for argon at 77 and 87.3 K. A simple model is used to account for the degree of graphitisation of a surface is demonstrated and used to account for the isosteric heat behaviour of non-graphitised carbon blacks. Turning from surfaces to porous solids, an alternative treatment of experiment data (either sub or super critical) is presented that avoids the ambiguity of excess amounts adsorbed. Using this method one is able to obtain pore size distributions and amounts adsorbed without relying on such things as helium expansion volumes. Since this type of method is usually applied to composite solids we also demonstrate the correct method for calculating the heat of adsorption using independent sets of simulations. The final topic covered in this paper is an example of the information that can be gained from the heat capacity of an adsorbed phase.     

Structure of the nonionic surfactant triethoxy monooctylether C8E3 adsorbed at the free water surface, as seen from surface tension measurements and Monte Carlo simulations

The adsorption layer of the nonionic surfactant triethoxy monooctylether C8E3 has been investigated at the free water surface by means of both experimental and computer simulation methods. The surface tension of the aqueous solution of C8E3 has been measured by pendant drop shape analysis in the entire concentration range in which C8E3 is soluble in water. The data obtained from these measurements are used to derive the adsorption isotherm. The critical micellar concentration and the surface excess concentration of the saturated adsorption layer are found to be 7.48 mM and 4.03 micromol/m2, respectively, the latter value corresponding to the average area per molecule of 41 A2. In order to analyze the molecular level structure of the unsaturated adsorption layer, Monte Carlo simulations have been performed at four different surface concentration values, i.e., 0.68, 1.36, 2.04, and 2.72 micromol/m2, respectively. It has been found that the water surface is already almost fully covered at the lowest surface density value investigated, and the adsorbed molecules show a strong preference for lying parallel with the interface in elongated conformations. No sign of the penetration of the hydrophilic triethoxy headgroups into the aqueous phase to any extent has been observed. With increasing surface densities the preferential orientation of the apolar octyl tails gradually turns from lying parallel with the interface to pointing toward the vapor phase by their CH3 end, whereas the conformation of the adsorbed molecules becomes gradually less elongated. Both of these changes lead to the increase of the number of C8E3 molecules being in a direct contact (i.e., forming hydrogen bonds) with water. However, the increasing number of the C8E3 molecules hydrogen bonded to water is found to be accompanied by the weakening of this binding, i.e., the decrease of both the number of hydrogen bonds a bound C8E3 molecule forms with water and the magnitude of the average binding energy of the adsorbed C8E3 molecules.     

Controlled-rate thermal analysis

Controlled-rate thermal analysis (CRTA) and differential scanning calorimetry (DSC) were used to investigate the adsorbed water layers and the surface properties of different commercial activated carbons. A simple method is proposed to obtain information on the properties of the adsorbed water film and the surface heterogeneity of the materials studied. This method utilizes TG mass loss and the first derivative of the DTG mass loss curves with respect to temperature and time, obtained during programmed liquid thermodesorption. The obtained TG mass loss curves, which reflect the energetic heterogeneity, consisted of steps and inflections which were associated with the mechanism of wetting of the solid surfaces. The heights of these steps and inflections depend on the adsorption capacity, the adsorption potential and the nature and number of the active centers of the carbon samples studied. The values of the total porosity and the surface phase capacity obtained by this method are in good agreement with those estimated on the basis of independent methods. The behaviour of water/carbon samples was studied by means of DSC at subambient and elevated temperatures. The experimental results provided novel data on the structural heterogeneity, the thermal stability of the water/carbon interface and its phase and structural transitions.Support from the Research Council of Kent State University (Ohio, USA) is acknowledged. The author is pleased to thank Drs M. Jaroniec, R. K. Gilpin, J. Choma and R. Dobrowolski for fruitful discussions and the active carbon samples.     

Spreading of proteins and its effect on adsorption and desorption kinetics

The kinetics of adsorption of lysozyme and alpha-lactalbumin from aqueous solution on silica and hydrophobized silica has been studied. The initial rate of adsorption of lysozyme at the hydrophilic surface is comparable with the limiting flux. For lysozyme at the hydrophobic surface and alpha-lactalbumin on both surfaces, the rate of adsorption is lower than the limiting flux, but the adsorption proceeds cooperatively, as manifested by an increase in the adsorption rate after the first protein molecules are adsorbed. At the hydrophilic surface, adsorption saturation (reflected in a steady-state value of the adsorbed amount) of both proteins strongly depends on the rate of adsorption, but for the hydrophobic surface no such dependency is observed. It points to structural relaxation ("spreading") of the adsorbed protein molecules, which occurs at the hydrophobic surface faster than at the hydrophilic one. For lysozyme, desorption has been studied as well. It is found that the desorbable fraction decreases after longer residence time of the protein at the interface.     

Adsorption from ternary liquid mixtures on silica gel and aluminium oxide

Formation of mixed adsorbed layers was tested for ternary liquid mixtures containing methanol or acetone and the binary solvent benzene+n — heptane. The specific excess adsorption isotherms from the liquid phase were measured on silica gel, silanized silica gel and aluminium oxide. The experimental adsorption data are discussed on the basis of the changes in the mixed solvent composition.     

Charged molecular films on brownian particles: structure, interactions, and relation to stability

The interfacial film of physically adsorbed ionic amphiphilic molecules on submicron particles dispersed in water was studied by a combination of surface tension measurements, laser light scattering (LLS) and high-shear experiments in a microchannel. General features in the structure and morphology of the molecular film are identified and understood in the framework of the two-step Langmuir adsorption model deduced from the adsorption isotherm. On the basis of this approach, the phase transitions and structural ordering of the film at the solid-liquid interface are analyzed in detail. A novel methodology based on high-shear aggregation experiments subsequently analyzed by means of LLS is proposed and turns out to be able to provide significant information on the phase transitions and structural arrangements of the adsorbed molecules (in substantial agreement with the adsorption isotherm model) as well as on the resulting interactions. Particularly important for applications is the result that, with no added salt, the films on two particles can adhere/fuse, leading to aggregation as long as an uncovered (hydrophobic) patch is present (unsaturated molecular layers). In the opposite case of fully developed layers, by analyzing the mechanism of shear aggregation of charged particles in the low-salt limit, we show that, when the hydrophobic attraction is absent, short-range hydration repulsive forces dominate over Derjaguin-Landau-Verwey-Overbeek (DLVO) forces and adhesion can never be achieved even upon application of very high collision energies. Consistently, a lower limiting boundary for the hydration interaction is calculated and found to be in agreement with data in the literature.     

Electrochemical behaviour of mono- and oligonucleotides: IV. The adsorption of adenine oligonucleotides at the mercury-solution interface

The adsorption behaviour of some adenine oligonucleotides was studied at the hanging mercury drop electrode by phase sensitive a.c.-voltammetry and sweep voltammetry. These substances are adsorbed in the whole potential range from most positive potentials adjustable up to the potential of their reduction. The adsorbed layer is transformed at a fresh drop rather rapidly to a condensed film at all concentrations due to significant stacking forces acting between adjacent adenine base rings. The time dependence of the voltammetric reduction response under conditions where only the contribution due to the adsorbed species is measured shows that diffusion controlled adsorption takes place. Differences in adsorption behaviour for adsorption at a fresh surface and at a surface partially covered by slowly desorbing blocking reduction products have been elucidated. From the limiting value of the surface concentration an average surface area per adsozbed mononucleotide unit of about 40 Ų was found which is comparable to the value resulting under the same conditions for poly-A. It was concluded that in both cases the adsorption is occurring via the adenine base residues of the nucleotide chain. As the distance between adjacent bases in the chain is fixed by its backbone formed by sugar and phosphate residues the average adsorption area is essentially the same in both cases. The possible orientation of the bases attached to the electrode is discussed.     

DNA and cationic surfactant complexes at hydrophilic surfaces. An ellipsometry and surface force study

The adsorption and formation of DNA and cationic surfactant complexes at the silica-aqueous interface have been studied by ellipsometry. The interaction between the DNA-surfactant complexes at the mica-aqueous interface has been determined by the interferometric surface force apparatus. Adsorption was as expected not observed on negatively charged hydrophilic surfaces for DNA and when DNA-cationic surfactant complexes were negatively charged. However, adsorption was observed when there is an excess of cationic surfactant, just below the point of phase separation. The adsorption process requires hours to reach steady state. The adsorbed layer thickness is large at low surface coverage but becomes more compact and thinner at high coverage. A long-range repulsive force was observed between adsorbed layers of DNA-cationic surfactant complexes, which was suggested to be of both electrostatic and steric origin. The forces were found to be dependent on the equilibration time and the experimental pathway.     

氢在多壁碳纳米管上吸附行为研究

根据热力学平衡原理推导了通用吸附等温方程.通过比较氢在碳纳米管和炭狭缝孔上的高阶维里吸附系数，分析了77～297 K温度区间，温度、管径(孔宽)对碳纳米管、炭狭缝孔吸附空间储氢容量的影响，并由氢在石墨平面上的最大吸附容量计算了本次试验多壁碳纳米管(MWCNTs)在各平衡温度时的最大氢吸附容量.运用确定参数后的吸附等温方程，线性回归分析了氢在本次试验MWCNTs上的吸附数据.结果表明，在160～180 K温度区间，管内被吸附氢分子之间由于吸附受压产生的排斥能出现极大值；随着温度升高，氢分子之间以吸引力为主，提高氢气压力后才发生明显吸附.     

Negative azeotropy in mixed adsorption of hexanol and dodecylammonium chloride at water/air interface

Miscibility of hexanol and dodecylammonium chloride (DAC) in adsorbed films and micelles was investigated by evaluating the compositions of the adsorbed films and micelles from surface tension measurements. Judging from the phase diagram of adsorption, negative azeotropy of adsorption was observed for the mixed adsorption of hexanol and DAC at water/air interfaces. The nonideal mixing in the adsorbed film was clarified using excess functions of adsorption. Interaction between hexanol and DAC in the adsorbed film was compared with that between other alkanols and surfactants. It was found that the range of azeotropes is narrower for the hexanol-DAC mixture than for the heptanol-octylsulfinylethanol mixture, because interaction between different species in an adsorbed film is weaker in the former than in the latter.     

Interfacial behavior of Cyanex 302 and kinetics of lanthanum extraction

In this paper, interfacial tension of Cyanex 302 is measured by a Sigma-701 tensiometer and the adsorption parameters are calculated according to the Gibbs and Szyszkowski adsorption isotherms. The interfacial adsorbed behavior of Cyanex 302 is investigated. The results demonstrate that the dimer is the predominant species in the bulk organic phase; however, the monomer is adsorbed at the interface and more interfacially active. The effects of aqueous pH, ion strength, and temperature on the interfacial activity of Cyanex 302 in heptane are discussed and explained in detail. The lower interfacial activity of Cyanex 302 in aromatic hydrocarbon than in aliphatic hydrocarbon has also been determined. The values of interfacial excess at the saturated interface increase in the order n-heptane>cyclohexane>toluene>benzene, which is consistent with the order of extractability of lanthanum by Cyanex 302 in these diluents. The interfacial activity data are used to discuss the kinetic mechanism of lanthanum(III) extraction. It is shown that an interfacial mechanism is very probable, and the extraction limiting step is the reaction between the Cyanex 302 molecules in the organic phase sublayer and the adsorbed intermediate complex.     

Adsorption Equilibrium of Binary Mixtures in Zeolites and State of Adsorbed Phase

The criterion of ideal behavior of a mixture of a few molecules within a separate zeolite cavity is formulated on the basis of the statistical thermodynamics. The criterion determines the dependence of the Helmholtz free energy, internal energy, and entropy of a molecular aggregate on the ratio of the number of molecules of components 1 and 2. The similarity between this criterion and the criterion of ideal behavior for bulk solutions is shown. Expressions of excess thermodynamic functions of the molecular mixture in a cavity are obtained. The negative magnitude of these excess functions is proposed to be due to rearrangement of molecules under influence of energetic heterogeneity. The calculation procedure of the excess functions has been demonstrated for the system CO₂-C₂ H₆-zeolite NaX, the information of both isotherms and isosteric adsorption heats being used simultaneously. The approach offered allows the state of adsorbed mixture in a separate cavity to be analyzed from pure-component and multicomponent experimental data.