Molecular dynamics of zigzag single walled carbon nanotube immersion in water

The results of enthalpy of immersion in water for finite single-walled carbon nanotubes are reported. Using molecular dynamics simulation, we discuss the relation between the value of this enthalpy and tube diameters showing that the obtained plot can be divided into three regions. The structure of water inside tubes in all three regions is discussed and it is shown that the existence of the strong maximum of enthalpy observed for tube diameter ca. 1.17 nm is due to freezing of water under confinement. The calculations of hydrogen bond statistics and water density profiles inside tubes are additionally reported to confirm the obtained results. Next, we show the results of calculation for the same tubes but containing surface carbonyl oxygen groups at pore entrances. A remarkable rise in the value of enthalpy of immersion in comparison to the initial tubes is observed. We also discuss the influence of charge distribution between oxygen and carbon atom forming surface carbonyls on the structure of confined water. It is concluded for the first time that the presence of surface oxygen atoms at the pore entrances remarkably influences the structure and stability of ice created inside nanotubes, and surface carbonyls appear to be chaotropic (i.e. structure breaking) for confined water. This effect is explained by the pore blocking leading to a decrease (compared to initial structure) in the number of confined water molecules after introduction of surface oxygen groups at pore entrances.     

Phenol adsorption on different nano-sized carbon materials: first comparative study

Carbon nanofibres and multi-branched carbon nanotubes are synthesized by using a new, proposed in this study method. The acid–base, adsorption and geometric properties of nanomaterials are characterised. Next we present first comparative adsorption and calorimetric studies of phenol uptake from aqueous solutions on mentioned nano-sized carbon materials and on nanotubes having similar diameter. The comparison of carbon nanofibres, multi-branched carbon nanotubes, and multi walled carbon nanotubes shows that for non-porous carbon nanomaterials the concentration of basic surface functionalities determines the mechanism of phenol adsorption. In consequence, larger phenol adsorption is recorded for the most basic nanomaterials (nanofibres and multi walled nanotubes) and smaller for multi branched nanotubes having the smallest surface concentration of basic groups. Possible explanation of differences between enthalpy of phenol displacement on graphite and on studied carbon adsorbents is also given.     

Effect of the carbon surface layer chemistry on benzene adsorption from the vapor phase and from dilute aqueous solutions

We present a complex study of benzene adsorption on chemically modified commercial activated carbons. The porosity of studied carbons is almost the same, whereas the chemical composition and the acid-base properties of surface layers differ drastically from amphoteric (initial de-ashed carbon D43/1, Carbo-Tech, Essen, Germany) and acidic (carbon modified with concentrated HNO3 and fuming H2SO4) to strongly basic (carbon modified with gaseous NH3). Benzene adsorption isotherms measured from aqueous solution at three temperatures (298, 313, and 323 K) and at the neutral pH level are reported. They are supported by studies of water and benzene adsorption from the gaseous phase (volumetric and calorimetric data) and the data of benzene temperature-programmed desorption (TPD). Moreover, the data of the enthalpy of immersion in water and benzene are also presented. Obtained data of benzene adsorption from the gaseous phase are approximated by applying the method of Nguyen and Do (ND) and the Dubinin-Astakhov (DA) equation. The data of adsorption from solution are described by the hybrid DA-Freundlich (DA-F) model. We show that there are similarities in the mechanisms of benzene adsorption from the gaseous phase and from aqueous solutions and that the pore-blocking effect is the main stage of the adsorption mechanism. This effect strongly depends on the polarity of the carbon surface. The larger the ratio of the enthalpy of carbon immersion in water to the enthalpy of immersion in benzene, the larger the reduction in adsorption from solution, compared to that in the gaseous phase, that is observed.     

Relation between immersion enthalpies of activated carbons in different liquids,textural properties,and phenol adsorption

The immersion enthalpies in benzene, cyclohexane, water, and phenol aqueous solution with a concentration of 100 mg L^?1 are determined for eight activated carbons obtained from peach seeds (Prunus persica) by thermal activation with CO₂ at different temperatures and times of activation. The results obtained for the immersion enthalpy show values between ?4.0 and ?63.9 J g^?1 for benzene, ?3.0 and ?47.9 J g^?1 for cyclohexane, ?10.1 and ?43.6 J g^?1 for water, and ?11.1 and ?45.8 J g^?1 for phenol solution. From nitrogen adsorption isotherms, the surface area, micropore volume, and average pore diameter of the activated carbons were obtained. These parameters are related with the immersion enthalpies, and the obtained trends are directly proportional with two first parameters in the nonpolar solvents, which is a behavior of microporous activated carbons with hydrophobic character. Phenol adsorption from aqueous solution on activated carbons is proportional to their surface area and their immersion enthalpy in the solution.     

Effects of Cetylpyridinium Chloride on Phase and Rheological Behavior of the Diluted C(12)E(4)/Benzyl Alcohol/Water System

The influence of the pore size distribution of activated carbon on the adsorption of phenol from aqueous solutions was explored. Activated carbons with different porous structures were prepared by gasifying a bituminous coal char to different extents of burn-off. The results of adsorption experiments show that the phenol capacity of these carbons does not proportionally increase with their BET surface area. This reflects the heterogeneity of the carbon surface for adsorption. The pore size distributions of these carbons, determined according to the Dubinin-Stoeckli equation, were found to vary with the burn-off level. By incorporating the distribution with the Dubinin-Radushkevich equation using an inverse proportionality between the micropore size and the adsorption energy, the isotherms for the adsorption of phenol onto these carbons can be well predicted. The present study has demonstrated that the heterogeneity of carbon surface for the phenol adsorption can be attributed to the different energies required for adsorption in different-size micropores. Copyright 2000 Academic Press.     

Immersion enthalpy and the constants of Langmuir model in the 3-chloro phenol adsorption on activated carbon

The adsorption process of 3-chloro phenol from aqueous solution on a activated carbon prepared from African palm stone and which presents a specific surface area of 685 m² g⁻¹, a greater quantity of total acid groups and a pH_PZC of 6.8 is studied. The adsorption isotherms are determined at pH values of 3, 5, 7, 9 and 11. The adsorption isotherms are fitted to the Langmuir model and the values of the maximum quantity adsorbed that are between 96.2 and 46.4 mg g⁻¹ are obtained along with the constant K_L with values between 0.422 and 0.965 L mg⁻¹. The maximum quantity adsorbed diminishes with the pH and the maximum value for this is a pH of 5. The immersion enthalpies of the activated carbon in a 3-chloro phenol solution of constant concentration, of 100 mg L⁻¹, are determined for the different pH levels, with results between 37.6 and 21.2 J g⁻¹. Immersion enthalpies of the activated carbon in function of 3-chloro phenol solution concentration are determined to pH 5, of maximum adsorption, with values between 28.3 and 38.4 J g⁻¹, and by means of linearization, the maximum immersion enthalpy is calculated, with a value of 41.67 J g⁻¹. With the results of the immersion enthalpy, maximum quantity adsorbed and the constant K_L, establish relations that describe the adsorption process of 3-chloro phenol from aqueous solution on activated carbon.     

The Impact of Carbon Surface Composition on the Diffusion and Adsorption of Paracetamol at Different Temperatures and at Neutral pH

Commercial activated carbon D43/1 (Carbo-Tech, Essen, Germany) was deashed and modified chemically to increase surface acidity and basicity, as well as to introduce metal cations onto the surface and yet conserve the porosity. The five carbons obtained were applied as adsorbents. Based on the results of batch-reactor test kinetic measurements of the adsorption of 4-hydroxyacetanilide (paracetamol) from aqueous solution at the neutral pH and at three temperatures (300, 310, and 320 K), the values of the effective diffusion coefficient (D(e)) were calculated. It is shown that D(e) increases (up to the relative adsorption equal to approximately 0.6) with rise in the magnitude of the enthalpy of immersion of carbons in water. Based on the obtained results as well as those published previously, the role of surface composition in the mechanism of paracetamol adsorption and in the kinetics of this process is discussed. Copyright 2000 Academic Press.     

Interaction of phenol and dopamine with commercial MWCNTs

We report the adsorption of phenol and dopamine probe molecules, from aqueous solution with NaCl, on commercial multiwall carbon nanotubes (MWCNT) and on their carboxylated derivative. The nanotubes were fully characterized by high resolution transmission electron microscopy (HRTEM), small angle X-ray scattering (SAXS), potentiometric titration, electrophoretic mobility, and nitrogen adsorption (77K) measurements. The experimental pollutant isotherms, evaluated using the Langmuir model, showed that only 8-12% and 21-32% of the BET surface area was available for phenol and dopamine, respectively, which is far below the performance of activated carbons. Influence of the pH was more pronounced for the oxidized MWCNT, particularly with dopamine. The strongest interaction and the highest adsorption capacity occurred at pH 3 with both model pollutants on both types of nanotubes. Although the surface area available for adsorption is far lower in MWCNTs than in activated carbons, it is nonetheless substantial. In particular, delayed release of toxic molecules that are either adsorbed on the surface or trapped in the inner bore of such systems could constitute an environmental hazard. The need for further adsorption studies with regard to their environmental aspects is therefore pressing, particularly for MWCNTs in their functionalized state.     

Investigation of Thermodynamic,Dynamic, and Structural Properties of H2 Adsorption on a Ag–Au Nanoalloy with a Carbon Nanotube Support

We have performed MD simulations to investigate H₂ adsorption on Ag–Au nanoclusters with the different Au mole fractions supported on the carbon nanotubes with the different diameters. Our thermodynamic results shown that the saturation value of coverage and the enthalpy of adsorption increases as the mole fraction of Au is increased. Our structural results showed that the presence of the H₂ gas exerts a significant effect on the nanocluster surface atoms and tends to stabilize the surface atoms on the nanocluster. Also, the structural changes are irreversible in such a way that by gradually decreasing the pressure to zero, the nanocluster geometry is not reversed to its initial structure in vacuum conditions. We have also shown that the nanoclusters have smaller values of the self‐diffusion coefficients in presence of H₂ molecules than those values in the initial state (vacuum), which is due to the increasing of the interface structure between the nanocluster and the nanotube.     

ZnCl2、KOH和HNO3改性MWCNT对苯酚的吸附行为研究

通过扫描电子显微镜、X射线衍射仪、N₂吸附分析仪及Boehm滴定法获得ZnCl₂、KOH和HNO₃化学处理对高纯多壁碳纳米管的结构和表面含氧官能团的影响,通过批处理实验考察吸附条件（吸附时间、初始浓度、温度）对处理前后的碳纳米管吸附苯酚行为的影响,并采用准一级、准二级、Evolich动力学模型和热力学方程拟合其吸附数据,分析其动力学行为、热力学行为和吸附机理。结果表明,虽然ZnCl₂、KOH和HNO₃化学处理法均未对碳纳米管BET比表面积产生显著影响,但会影响其表面化学性质（即,对于ZnCl₂和KOH化学处理降低表面羧基、内酯基含量和增大碱性官能团量,而对于HNO₃化学处理可以增大表面羧基、内酯基含量,而碱性官能团略有增加）;改性处理影响碳纳米管去除苯酚效率：由于ZnCl₂和KOH改性处理降低碳纳米管表面羧基量,故其提高了苯酚去除率,而HNO₃处理则略减小碳纳米管的苯酚去除率,可能是由于碳纳米管结构和表面化学性质共同影响所致;碳纳米管的苯酚去除率均随苯酚溶液初始浓度的增大而减小;高温不利于吸附;热力学研究发现碳纳米管吸附苯酚过程是自发的和放热的,属于物理吸附;动力学研究表明,吸附过程符合准二级动力学方程。通过ZnCl₂和KOH化学处理,可以显著提高碳纳米管对苯酚的吸附性能。     

Carbon surface chemical composition in para-nitrophenol adsorption determined under real oxic and anoxic conditions

A series of commercial unmodified and modified activated carbons was studied. The surface chemical composition was characterized using X-ray photoelectron spectroscopy and Boehm titration methods. Data on p-nitrophenol (pnp) adsorption isotherms determined under real oxic and anoxic conditions (at 310 K) are presented and described using bimodal Langmuir and lattice density functional theory models. The applicability of the pnp molecule for determination of surface area using adsorption from solution data is discussed. It is shown that under anoxic conditions adsorption and relative enthalpy of this process depend on the value of BET apparent surface area and DA micropore volumes. The differences between adsorption levels under both conditions increase with rise in solute equilibrium concentration. Moreover, the average difference between adsorption values under both conditions increases and next decreases with rise in the concentration of surface acidic groups. Applying quantum chemical calculations, we show that under anoxic conditions the influence of surface oxygen groups on pnp adsorption is small, whereas under oxic conditions the reverse situation is observed. Obtained theoretical results show very good correspondence to the experimental data and the origin of the relationships observed experimentally is explained and discussed.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

一种亲水的酚羟基修饰聚苯乙烯树脂对酚类化合物的吸附热力学

在283-323K和研究的浓度范围内,苯酚、对甲苯酚、对氯苯酚和对硝基苯酚在亲水性的酚羟基修饰聚苯乙烯树脂（AM-1）与大孔吸附树脂（Amberlite XAD-4)上平衡吸附数据符合Freundlich吸附等温方程。酚类化合物在AM-1上的吸附容量比在Amberlite XAD-4上的吸附容量增加20%以上,这主要得益于AM-1表面的酚羟基入树脂的微孔结构。在较稀的溶液中AM-1对苯酚的吸附量比AmberliteXAD-4对苯酚的吸附量增加60%,表明AM-1对苯酚有特殊的选择性。Freundlich吸附等温线、相对吸附容量以及等量吸附焓表明,四种酚在两种树脂上的吸附是物理吸附过程。对酚类化合物被两种树脂吸附的吸附焓、自由能、吸附熵也作了测试,并对吸附行为作了合理的解释。     

Thermodynamic analysis of the immersion of a smectite substituted with Na or Ca: heat effect due to the cation

The enthalpy produced during immersion in water of a smectite, identified as a montmorillonite, substituted by Ca or Na is analyzed using three different approaches. The heat of immersion is measured using the classical calorimetric method. It is compared to an estimate using adsorption techniques. And it is calculated from a theoretical estimate of the surface energy of a crystal model of montmorillonite. The comparison of the different values allows the enthalpy linked to the swelling phenomenon to be estimated. The Ca form appears in every case to interact more strongly with water molecules than the Na form.     

Isotherm analysis of phenol adsorption on polymeric adsorbents from nonaqueous solution

Macroporous poly(methyl methacrylate-co-divinylbenzene) (PMMA), interpenetrating polymer adsorbent based on poly(styrene-co-divinylbenzene) (PS) and poly(methyl methacrylate-co-divinylbenzene) (PMMA/PS), and macroporous cross-linked poly(N-p-vinylbenzyl acetylamide) (PVBA) were prepared for the adsorption of phenol from cyclohexane. The sorption isotherms of phenol on the three polymeric adsorbents were measured and fitted to Langmuir and Freundlich isotherms. It is shown that the Langmuir isotherm, which is based on a homogeneous surface model, is unsuitable to describe the sorption of phenol on the adsorbents from nonaqueous solution and the Freundlich equation fits the tested three adsorption systems well. The isosteric enthalpy was quantitatively correlated with the fractional loading for the sorption of phenol onto the three polymeric adsorbents. The surface energetic heterogeneity patterns of the adsorbents were described with functions of isosteric enthalpy. The results showed that the tested three polymeric adsorbents exhibited different surface energetic heterogeneity patterns. The initial isosteric enthalpy of phenol sorption on polymeric adsorbent has to do with the surface chemical composition and is free from the pore structure of the polymeric adsorbent matrix. Forming hydrogen bonds between phenol molecules and adsorbent is the main driving force of phenol sorption onto PVBA and PMMA adsorbent from nonaqueous solution. When phenol is adsorbed on PMMA/PS, pi-pi interaction resulting from the stacking of the benzene rings of the adsorbed phenol molecules and the pendant benzene ring of adsorbent is involved.     

THERMODYNAMIC STUDY OF ADSORPTION OF PHENOLIC COMPOUNDS FROM AQUEOUS SOLUTION BY A WATER-COMPATIBLE HYPERCROSSLINKED POLYMERIC ADSORBENT

Equilibrium data for the adsorption of phenolic compounds, i.e., phenol, p-cresol, p-chlorophenol and p-nitrophenol from aqueous solutions by a water-compatible hypercrosslinked polymeric adsorbent (NJ-8) within temperature range of 283-323 K were obtained and correlated with a Freundlich-type of isotherm equation, so that equilibrium constants KF and n were obtained. The capacities of equilibrium adsorption for all the four phenolic compounds on the NJ-8 from aqueous solutions are around 2 times as high as those of Amberlite XAD-4, which may be attributed to the unusual micropore structure and the partial polarity on the network. The values of the enthalpy (always negative) are indicative of an exothermic process, which manifests the adsorption of all the four phenolic compounds on the two polymeric adsorbents to be a process of physical adsorption. The negative values of free energy change show that the solute is more concentrated on the adsorbent than in the bulk solution. The absolute free energy values of adsorption for NJ-8 are always higher than those for Amberlite XAD-4, which indicates that phenolic compounds are preferentially adsorbed on NJ-8. The negative values of the adsorption entropy are consistent with the restricted mobilities of adsorbed molecules of phenolic compounds as compared with the molecules in solution. The adsorption entropy values of phenolic compounds for NJ-8 are lower than those for Amberlite XAD-4, which means the micropores of NJ-8 require more orderly arranged adsorbate.     

Immersion Calorimetry: Molecular Packing Effects in Micropores

Repeated and controlled immersion calorimetry experiments were performed to determine the specific surface area and pore‐size distribution (PSD) of a well‐characterized, microporous poly(furfuryl alcohol)‐based activated carbon. The PSD derived from nitrogen gas adsorption indicated a narrow distribution centered at 0.57±0.05 nm. Immersion into liquids of increasing molecular sizes ranging from 0.33 nm (dichloromethane) to 0.70 nm (α‐pinene) showed a decreasing enthalpy of immersion at a critical probe size (0.43–0.48 nm), followed by an increase at 0.48–0.56 nm, and a second decrease at 0.56–0.60 nm. This maximum has not been reported previously. After consideration of possible reasons for this new observation, it is concluded that the effect arises from molecular packing inside the micropores, interpreted in terms of 2D packing. The immersion enthalpy PSD was consistent with that from quenched solid density functional theory (QSDFT) analysis of the nitrogen adsorption isotherm.     

Theoretical Study of Phenol Adsorption on Pristine, Ga-Doped, and Pd-Decorated (6,0) Zigzag Single-Walled Boron Phosphide Nanotubes

Phenol adsorption on the external surface of H-capped pristine, Ga-doped, and Pd-decorated (6,0) zigzag boron phosphide nanotubes (BPNTs) was studied by using density functional theory (DFT) calculations. The results indicate that the hydroxyl group of phenol prefers to attach to the Ga and Pd sites and thus the Ga-doped and Pd-decorated (6,0) can be used for removing phenol. The calculated adsorption energy of phenol on the Ga-doped and Pd-decorated (6,0) BPNTs are ?0.724 and ?420 eV, respectively and about 0.28 and 0.27 electrons are transferred from phenol to the nanotubes. In addition, the value for the fractional number of electrons transferred is negative, indicating that phenol act as an electron donor. Frontier molecular orbital theory (FMO) and structural analyses show that the high polar surface bonds and large bond lengths of the Ga-doped and Pd-decorated (6,0) BPNT surfaces increase the adsorption of phenol on the nanotube models. This study can be useful in removing phenol and development of many catalytic processes for formation of a variety of useful compounds.     

Interaction of isoniazid drug with the pristine and Ni-doped of (4, 4) armchair GaNNTs: a first principle study

In this research, the interaction of isoniazid drug (INH) with the pristine and Ni-doped Gallium nitride nanotubes (GaNNTs) is investigated by using density function theory. The adsorption energy, deformation energy, natural bond orbital (NBO), quantum parameters, molecular electrostatic potential (MEP) and thermodynamic parameters of all adsorption models are calculated from optimized structures. The values of adsorption energy, enthalpy and Gibbs free energy of all adsorption models are negative and all adsorption process are favorable in view of thermodynamic points. It is notable that Ni-doped decrease adsorption strength and it is not suitable for INH adsorption on the GaNNTs surface. The MEP, NBO and maximum amount of electronic charge ΔN results demonstrate that the negative potential are localized around adsorption position and the positive potential are localized around INH molecule. The calculated results indicate that the GaNNTs is a good candidate to making absorber and sensor for detecting INH drug.