Adsorption of polycyclic aromatic hydrocarbons at the air-water interface: molecular dynamics simulations and experimental atmospheric observations

Adsorption of benzene, naphthalene, anthracene, and phenanthrene at the aqueous surface is investigated by means of molecular dynamics simulations. Potentials of mean force, i.e., free energy profiles obtained when moving the studied molecules across an aqueous slab were evaluated. In all cases, deep surface free energy minima, corresponding to orders of magnitude of surface enhancement of the aromatic molecule, were located. This enhancement, which increases with the size of the solute, points to the importance of the aqueous surface for the chemistry of polycyclic aromatic hydrocarbons (PAHs). Supporting evidence in the atmospheric environment related to the heterogeneous chemistry of PAHs on water droplets and planar surfaces is summarized. There is good agreement between the hydration free energies computed from MD calculations and the experimentally determined values. Data pertaining to the importance of the air-water interface in the adsorption and transport of PAHs on micron sized water droplets are described. The relevant data on adsorption and reaction (ozonation and photochemical) at the air-water interface of planar surfaces and droplets are also summarized.     

Dual-scale modeling of benzene adsorption onto Ni(111) and Au(111) surfaces in explicit water.

We present a multiscale modeling approach for studying interactions of organic molecules with metal surfaces in explicit water. The approach is based on combining adsorption energies of isolated molecules on transition metal surfaces calculated by ab initio density functional methods and classical molecular dynamics simulations using atomistically detailed force fields. The interaction of benzene with Ni(111) and Au(111) surfaces was studied. It is shown that a strong affinity of water for the hydrophilic surfaces makes benzene adsorption on Au thermodynamically unfavorable, while on Ni there is no preference. The work presented here serves as a first step in modeling the interactions of larger organic molecules with metal surfaces.     

Atomic structures of benzene and pyridine on Si(5 5 12)-2 x 1

The adsorption structures of benzene and pyridine on Si(5 5 12)-2 x 1 were studied at 80 K by using a low-temperature scanning tunneling microscope and density functional theory calculations. These structures are different from those observed on low-index Si surfaces: benzene molecules exclusively bind to two adatoms, that is, with di-sigma bonds between carbon atoms and silicon adatoms, leading to the loss of benzene aromaticity; in contrast, pyridine molecules interact with adatom(s) through either Si-N dative bonding or di-sigma bonds. Dative bonding configurations with pyridine aromaticity are the dominant adsorption features and are more stable than di-sigma bonding configurations. Thus the dative bonding of nitrogen-containing heteroaromatic molecules provides a strategy for the controlled attachment of aromatic molecules to high-index surfaces.     

Impact of the Chirality and Curvature of Carbon Nanostructures on Their Interaction with Aromatics and Amino Acids

Understanding noncovalent interactions on the surfaces of carbon nanostructures (CNSs) is of fundamental importance and also has implications in nano‐ and biotechnology. The interactions of aromatic compounds such as benzene, naphthalene, and aromatic amino acids with CNSs of varying diameter, chirality, and curvature were systematically explored by using density functional theory. Planar graphene exhibits stronger binding affinity than curved carbon nanotubes (CNTs), whereas zigzag CNTs appear to show stronger binding affinity than armchair CNTs. For hydrocarbons, there exist two competing modes, namely, π–π stacking interactions and CH ??? π interactions, which bring the aromatic motifs into parallel and perpendicular dispositions with respect to the CNSs, respectively. Our results reveal that π–π stacking interactions override CH ??? π interactions in such cases. However, in the case of aromatic amino acids, π–π interactions can exist simultaneously along with a range of other interactions, including CH ??? π. The polarizability and HOMO energy of the CNSs were found to be the key factors that determine the binding energies. The HOMO–LUMO energy gaps of the CNSs were found to be undisturbed by the noncovalent functionalization of the aromatic molecules.     

Estimation of the energetic nonhomogeneity of the surface of oxide and carbon materials

The molecular statistical method for evaluating the distribution of active sites of various adsorbents relative to their energies has been improved. This method is used not only for the treatment of experimental data on the adsorption of hydrocarbons on various adsorbents, which is the usual procedure, but also data on the adsorption of polar water and methanol molecules on the active sites of adsorbent surfaces. Two types of active sites differing in energy have been shown to exist on the surface of graphitized carbon black, the complex shungite carbon/mineral adsorbent, and modified Silochrom. Chromatographic, calorimetric, and structural adsorption data were used to establish the relationship between the observed maxima of the energy distribution function of the adsorption sites with concrete adsorption sites or pores of the surface, on which the molecules are adsorbed. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 44, No. 5, pp. 315–320, September–October, 2008.     

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.     

Study on adsorption kinetic of aromatic hydrocarbons onto activated carbon in gaseous flow method

The adsorption behavior of benzene, toluene, o-xylene, m-xylene, and p-xylene onto activated carbon was investigated using the flow method. The removal efficiency of aromatic hydrocarbons in the gaseous phase was estimated based on the adsorption kinetic constants and the saturated amount of aromatic hydrocarbons adsorbed on the activated carbon. The saturated amount of benzene and toluene adsorbed was greater than that of xylene adsorbed because the molecular sizes of benzene and toluene are smaller than that of xylene. The adsorption kinetic constant increased in the order of xylene, toluene, and benzene. Those of the three xylene isomers were similar. These results indicated that the adsorption rate of benzene by the activated carbon was the fastest and the kinetic constant depended upon the different between the boiling point and the melting point and the molecular size of the aromatic hydrocarbons.     

Role of surface chemistry in adsorption of phenol on activated carbons

Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The structural properties and surface chemistry of the samples and their oxidized counterparts were characterized using sorption of nitrogen and Boehm titration, respectively. Phenol adsorption from solution (at trace concentrations) was studied at temperatures close to ambient without maintaining a specific pH of the solution. The results showed, as expected, that the phenol uptake is dependent on both the porosity and surface chemistry of the carbons. Furthermore, phenol adsorption showed a strong dependence on the number of carboxylic groups due to two factors: (1) phenol reacts with carboxylic groups on the carbon surface, forming an ester bond, and (2) carboxylic groups on the carbon surface remove the pi-electron from the activated carbon aromatic ring matrix, causing a decrease in the strength of interactions between the benzene ring of phenol and the carbon's basal planes, which decreases the uptake of phenol.     

Retention characteristics of alkylated polycyclic aromatic hydrocarbons in normal phase liquid chromatography

Summary Retention characteristics of series of polymethyl and mono-n-alkyl derivatives of benzene and pyrene, and also of parent polycyclic aromatic hydrocarbons (PAH), were studied using silica and aminopropyl- and cyanopropyl-modified silica. Differences in the selectivities for the studied compound groups were found between the three phases. Deviations from linear behaviour in plots of log (k′)vs. carbon number were observed for the methyl series. These are explained in terms of differences in π-electron delocalisation within the aromatic ring systems. Further, the effect of methyl substitution on selectivity decreased with an increasing number of aromatic rings. Results were obtained which indicated that the primary adsorption site in a cyano column used in normal phase mode, at least for PAH molecules, is the cyano group.     

Adsorption of volatile organic compounds onto carbon nanotubes, carbon nanofibers, and high-surface-area graphites

The adsorption of different alkanes (linear and cyclic), aromatics, and chlorohydrocarbons onto different nonmicroporous carbons--multiwalled carbon nanotubes (CNTs), carbon nanofibers (CNFs), and high-surface-area graphites (HSAGs)--is studied in this work by inverse gas chromatography (IGC). Capacity of adsorption was derived from the isotherms of adsorption, whereas thermodynamic properties (enthalpy of adsorption, surface free energy characteristics) have been determined from chromatographic retention data. HSAGs present the highest adsorption capacity, followed by CNTs and CNFs (although CNTs present an intermediate surface area between the two HSAG studied). Among the different adsorbates tested, benzene exhibits the highest adsorption capacity, and the same trend is observed in the enthalpy of adsorption. From surface free energy data, enthalpies of adsorption of polar compounds were divided into dispersive and specific contributions. The interactions of cyclic (benzene and cyclohexane) and chlorinated compounds (trichloroethylene, tetrachloroethylene, and chloroform) with the surfaces are mainly dispersive over all the carbons tested, CNTs being the material with the highest dispersive contribution, as was deduced also from the entropy parameter. Adsorption parameters were correlated with morphological and chemical properties of the materials.     

Adsorption Behaviors of 4-Mercaptobenzoic Acid on Silver and Gold Films

The adsorption behaviors of 4-mercaptobenzoic acid on silver and gold nanoparticles were studied by surface-enhanced Raman scattering (SERS) and density functional theory. The silver and gold films by electrodeposition have the same excellent characteristics as SERS-active substrates. At the same, the SERS spectra indicate that 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of gold nanoparticles through the S atom, and that the carboxyl group is far away from surface of gold nanoparticles, and that there is a certain angle between the plane of benzene ring and gold film. However, 4-mercaptobenzoic acid molecules are adsorbed on the surfaces of silver nanoparticles through the carboxyl group, and the S atom is far away from surface of silver nanoparticles, and there is also a certain angle between the plane of benzene ring and the surface of silver nanoparticles. Here it is demonstrated the calculated Raman frequencies are in good agreement with experimental values, and the calculated Raman frequencies are also helpful to infer the adsorption behaviors of 4-mercaptobenzoic acid molecules.     

Adsorption of aromatic compounds from solutions in <Emphasis Type="Italic">n</Emphasis>-heptane by modified nanoporous silica

The adsorption of anisole, anthracene, benzoic acid, benzene, naphthalene, phenanthrene, phenol, and carbon tetrachloride from dilute solutions in n-heptane on the modified nanoporous silica sorbents, Silasorb C18 and Phenyl-Silasorb, at 298 K is investigated by developing liquid chromatography. The Henry law constants and Helmholtz energy of adsorption for these aromatic compounds are calculated from their retention values. The adsorption isotherms of the aromatic compounds are calculated from their chromatographic peaks with allowances made for longitudinal smearing. A correlation is made between the Henry law constants calculated from the retention values of the aromatic compounds and from their adsorption isotherms. The influences of the surface modification of the silica adsorbents and the molecular structure of the aromatic compounds on their elution from the chromatographic column with n-heptane as the eluent are discussed. Contributions of functional groups in the molecules of the aromatic compounds to the Helmholtz energy of adsorption on different surfaces are estimated.     

Adsorption of benzoic acid from organic solvents on calcite and dolomite: Influence of water

A study has been carried out of the adsorption of benzoic acid from cyclohexane solution onto the hydrophilic surface of calcite.

We determined initially the chemical and mineral composition of the solid, its specific surface area and its granulometry. This was followed by the determination of the enthalpies of immersion of calcite in different solvents. These thermodynamic properties gave information on the energetics of calcite—solvent interactions. In this way, we could construct a scale of affinities of the different organic molecules and water for the calcite surface. It was noted that the enthalpies were higher in unsaturated than in saturated organic solvents, and higher in water than in the organic solvents.

The adsorption isotherms and the differential molar enthalpies of displacement were determined in the presence and the absence of water. The role played by water in the adsorption of polar organic molecules from the oil phase has not been clearly explained previously. In this paper, we indicate how the presence of water can modify the adsorption of aromatic compounds on the surface of calcite. As regards the adsorption isotherms, the presence of water essentially increases the amount of adsorption. The results of the calorimetric studies were found to be surprising; we observed that the differential molar enthalpies of displacement were endothermic.

Similar experiments were carried out with dolomite and n-heptane solution and the results compared with those obtained with calcite and cyclohexane, leading to the formulation of a general model concerning the adsorption of small polar organic molecules from organic solvents onto the surfaces of the carbonates.     

Effect of Alkyl Chain Length on the Interfacial Dilational Properties of Hydroxy-Substituted Alkyl Benzenesulfonates

Dilational rheological behaviors of adsorption layers of three surfactants, sodium 2-hydroxy-3,5-dioctyl benzene sulfonate (C₈C₈), sodium 2-hydroxy-3-octyl-5-decyl benzene sulfonate (C₈C₁₀), and sodium 2-hydroxy-3-octyl-5-dodecylbenzenesulfonate (C₈C₁₂) formed at air–water and decane–water interfaces, have been investigated as a function of concentration and frequency (0.002–0.1 Hz) by the oscillating bubble/drop method. The experimental results show that the dilational moduli of hydroxy-substituted alkyl benzenesulfonates are obviously higher than those of the common surfactants, because the interfacial interactions between alkyl chains are improved drastically by the unique arrangement of C₈C₈ molecules at the interface. However, the moduli at the decane–water interface are much lower than those at the surfaces because decane molecules will insert into the surfactant molecules adsorbed at the interface and destroy the interactions between alkyl chains. With an increase in the number of carbon atom of 5-alkyl, the surface dilational modulus decreases because the orientation of the surfactant molecules at the surface varies from parallel to tilt. On the other hand, the diffusion-exchange process dominates the interfacial behavior and the interfacial modulus improves with the increase in the length of the alkyl chain.     

Adsorption and structural characteristics of the surface of modified nanodiamond powders

The adsorption of benzene and water vapors on the surface of detonation nanodiamond powders that differed in particle size, degree of their strong aggregation, and chemical state of their surface was studied. Specific features of the processes of adsorption of polar and nonpolar molecules on chemically modified surfaces of nanodiamonds are analyzed.     

Adsorption of aromatic hydrocarbons and ozone at environmental aqueous surfaces

Adsorption of environmentally important aromatic molecules on a water surface is studied by means of classical and ab initio molecular dynamics simulations and by reflection-absorption infrared spectroscopy. Both techniques show strong activity and orientational preference of these molecules at the surface. Benzene and naphthalene, which bind weakly to water surface with a significant contribution of dispersion interactions, prefer to lie flat on water but retain a large degree of orientational flexibility. Pyridine is more rigid at the surface. It is tilted with the nitrogen end having strong hydrogen bonding interactions with water molecules. The degree of adsorption and orientation of aromatic molecules on aqueous droplets has atmospheric implications for heterogeneous ozonolysis, for which the Langmuir-Hinshelwood kinetics mechanism is discussed. At higher coverages of aromatic molecules the incoming ozone almost does not come into contact with the underlying aqueous phase. This may rationalize the experimental insensitivity of the ozonolysis on the chemical nature of the substrate on which the aromatic molecules adsorb.     

Predicting whether aromatic molecules would prefer to enter a carbon nanotube: A density functional theory study

The interaction of a carbon nanotube (CNT) with various aromatic molecules, such as aniline, benzophenone, and diphenylamine, was studied using density functional theory able to compute intermolecular weak interactions (B3LYP-D3). CNTs of varying lengths were used, such as 4-CNT, 6-CNT, and 8-CNT (the numbers denoting relative lengths), with the lengths being chosen appropriately to save computation times. All aromatic molecules were found to exhibit strong intermolecular binding energies with the inner surface of the CNT, rather than the outer surface. Hydrogen bonding between two aromatic molecules that include N and O atoms is shown to further stabilize the intermolecular adsorption process. Therefore, when benzophenone and diphenylamine were simultaneously allowed to interact with a CNT, the aromatic molecules were expected to preferably enter the CNT. Furthermore, additional calculations of the intermolecular adsorption energy for aniline adsorbed on a graphene surface showed that the concavity of graphene-like carbon sheet is in proportion to the intermolecular binding energy between the graphene-like carbon sheet and the aromatic molecule.     

Effect of carbon nanotubes’ crystal structure on adsorption kinetics of small molecules An experimental study utilizing ultra-high vacuum thermal analysis techniques

The effect of carbon nanotubes’ (CNT) crystal structure on chemical reactivity has been studied in much detail in the liquid phase using CNT suspension. This type of information is pertinent for developing CNT separation strategies. However, few experimental studies are available providing data for gas–CNT interactions utilizing ultra-high vacuum (UHV) surface science techniques. Structure–activity relationships (SAR) for gas–surface interactions are important for sensor designs and heterogeneous catalysis exploring, for example, CNT’s potential as a support for fuel cell catalysts. We report on UHV kinetics experiments with single-wall metallic, semiconducting, and mixed CNTs in order to provide the experimental basis to correlate CNT’s crystal structure and chemical activity. Thermal desorption spectroscopy (TDS), a simple temperature ramping technique, has been used to determine the binding energies of a number of probe molecules including alkanes, alcohols, thiophene, benzene, and water on CNTs at UHV conditions. TDS allows for the identification of adsorption sites of probe molecules in CNT bundles, using gold foil or silica as a support for the drop-and-dry technique. A weak and probe molecule dependent SAR is present for adsorption inside the CNTs but not for the population of external sites by the probe molecules. The experimental data are in part consistent with current theoretical predictions by other groups. In addition, the effect of different solvents (methanol, SDS, and NMP) and cleaning procedures will briefly be discussed using results of spectroscopic (Auger electron spectroscopy) and kinetic techniques. Furthermore, molecular beam scattering techniques were utilized to characterize the adsorption dynamics, i.e., the gas-to-surface energy transfer processes of alkanes on CNTs. For example, opening the CNT tube ends by high temperature annealing, increases the so-called initial adsorption probability, that is, the probability for adsorption in the limit of zero surface concentration (coverage). This result directly illustrates the effect of large surface areas of CNTs, using internal and external surfaces, for gas adsorption.     

Eluotropic strength in non-aqueous liquid chromatography with porous graphitic carbon

Protein adsorption can be either endothermic or exothermic depending upon the protein, the sorbent and process conditions. In the case of protein adsorption onto ion-exchange surfaces exothermic adsorption heats are usually characterized as representing the electrostatic interaction between two oppositely charged surfaces. Endothermic adsorption heats are typically characterized as representing protein reconfiguration and/or repulsive interactions between adsorbed molecules. In certain segments of the literature surface dehydration and solution non-idealities have been suggested as possible sources of endothermic heats of adsorption. Each of these phenomena was investigated during studies concerning the adsorption of bovine serum albumin and ovalbumin onto an anion-exchange sorbent. The results demonstrated that electrostatic repulsive interactions between adsorbed molecules appears to be a larger contributor to endothermic heats of adsorption than surface dehydration or solution non-idealities. The presence of mobile phase cations can reduce the magnitude of endothermic adsorption heats by screening repulsive interactions between adsorbed molecules. Although water release was not found to be a major contributor to endothermic adsorption heats, it is likely to be a contributor to the entropic driving force associated with the adsorption of bovine serum albumin.     

Evaluation of molecular mechanics calculated binding energies for isolated and monolayer organic molecules on graphite

The calculated molecule-surface binding energy, E(cal)( *), for physical adsorption was determined using molecular mechanics MM2 parameters for a model graphite surface and various organic molecules. The results for E(cal)( *) were compared to published experimental binding energy values, E( *), from gas chromatography (GC) or thermal desorption (TD). The binding energies from GC were for isolated molecules in the Henry's law region of adsorption, and the binding energies from TD were for molecules in monolayer coverage on a highly oriented pyrolytic graphite (HOPG). A simple desorption model was used to allow the calculation of monolayer coverage to include both molecule-surface and molecule-molecule interactions and then the results were compared to experimental values. For the 14 TD organic adsorbates (polyaromatic hydrocarbons, alcohols, benzene, substituted benzenes, methane, chloroalkanes, N,N-dimethylformamide, and C(60) Buckyball), the experimental versus calculated binding energies were E( *)=1.1193E(cal)( *) and r(2)=0.967. The GC E( *) values were also well correlated by calculated E(cal)( *) values for a set of 11 benzene and methyl substituted benzenes and for another set of 10 alkanes and haloalkanes. The TD E(cal)( *) mechanics computation provides a useful comparison to the one for GC data since adsorbate-adsorbate interactions as well as adsorbate-surface must be considered.