Molecular dynamics simulations of atmospheric oxidants at the air-water interface: solvation and accommodation of OH and O3

A comparative study of OH, O3, and H2O equilibrium aqueous solvation and gas-phase accommodation on liquid water at 300 K is performed using a combination of ab initio calculations and molecular dynamics simulations. Polarizable force fields are developed for the interaction potential of OH and O3 with water. The free energy profiles for transfer of OH and O3 from the gas phase to the bulk liquid exhibit a pronounced minimum at the surface, but no barrier to solvation in the bulk liquid. The calculated surface excess of each oxidant is comparable to calculated and experimental values for short chain, aliphatic alcohols. Driving forces for the surface activity are discussed in terms of the radial distribution functions and dipole orientation distributions for each molecule in the bulk liquid and at the surface. Simulations of OH, O3, and H2O impinging on liquid water with a thermal impact velocity are used to calculate thermal accommodation (S) and mass accommodation (alpha) coefficients. The values of S for OH, O3, and H2O are 0.95, 0.90, and 0.99, respectively. The approaching molecules are accelerated toward the liquid surface when they are approximately 5 angstroms above it. The molecules that reach thermal equilibrium with the surface do so within 2 ps of striking the surface, while those that do not scatter into the gas phase with excess translational kinetic energy in the direction perpendicular to the surface. The time constants for absorption and desorption range from approximately 35 to 140 ps, and the values of alpha for OH, O3, and H2O are 0.83, 0.047, and 0.99, respectively. The results are consistent with previous formulations of gas-phase accommodation from simulations, in which the process occurs by rapid thermal and structural equilibration followed by diffusion on the free energy profile. The implications of these results with respect to atmospheric chemistry are discussed.     

Adsorption preconcentration in the luminescence determination of polycyclic aromatic hydrocarbons

The adsorption of polycyclic aromatic hydrocarbons (PAHs)—pyrene, anthracene, phenanthrene, and fluorene—by a cellulose matrix of micellar media containing sodium dodecyl sulfate and a nonionogenic polymer, namely, polyethylene glycol is studied. Procedures are proposed for the luminescence determination of PAHs in the adsorbent phase.     

Residential wood combustion: A source of atmospheric polycyclic aromatic hydrocarbons

Samples were taken of the atmospheric particulate matter over Fairbanks Alaska in the winter of 1985, and from wood stoves burning the major wood types locally available. These samples were then analyzed for polycyclic aromatic hydrocarbon (PAH). A PAH emission profile was determined from the wood stove samples and applied to the atmospheric samples to determine the residential wood combustion contribution to the local atmospheric particulate burden. Emission profiles for coal burning and automobile emissions for PAH were also used to estimate their relative contributions.     

Adsorption of polycyclic aromatic hydrocarbons onto graphyne: Comparisons with graphene

Density functional theory calculations were implemented to expand the knowledge about graphyne and its interaction with polycyclic aromatic hydrocarbons (PAHs). Due to the porous character of graphyne, the adsorption strength of PAHs onto graphyne surfaces is expected to be lower with respect to graphene (a perfect π‐extended system). However, there are not quantitative evidences for this assumption. This work shows that the adsorption strength of adsorbed PAHs onto γ‐graphyne nanosheets (GY) is weakened in 12 ? 23% with respect to the adsorption onto graphene, with a decrease of 10 ? 20% in the dispersive interactions. The adsorption energies (in eV) of the GY–PAH systems can be straightforward obtained as E _ads/eV≈0.033N _H + 0.031N _C, where N _H and N _C is the number of H and C atoms in the aromatic molecule, respectively. This equation predicts the binding energy of graphene–graphyne bilayers with a value of ～31 meV/atom. Analysis of the electronic properties shows that PAHs behaves as n‐dopants for GY, introducing electrons in GY and also reducing its bandgap in up to ～0.5 eV. Strong acceptor or donor substituted PAHs decrease the bandgap of γ‐graphyne in up to ～0.8 eV, with changes in its valence or conduction band, depending on the chemical nature of the adsorbate. Finally, these data will serve for future studies related to the bandgap engineering of graphyne surfaces by nonaggressive molecular doping, and for the development of graphyne‐based materials with potential applications in the removal of persistent aromatic pollutants.     

Adsorption of DNA to octadecylamine monolayers at the air-water interface

In this work, surface properties of octadecylamine (ODA) monolayers in the presence of different concentrations of calf thymus DNA in the aqueous subphase covering a range of 2-8μM have been investigated. The increase of DNA concentration is accompanied by a marked increment in the expansion of the corresponding isotherms. In addition, there is a change in the profile of the isotherms ranging from an abrupt liquid-solid transition for the lipid monolayer on pure water to a slow condensation of the monolayer in a liquid state when DNA is added to the subphase, demonstrating the effective adsorption of the polynucleotide to the long chain amine monolayer. Additional phase transitions appear in the isotherms upon addition of sufficient amount of DNA, revealing the existence of specific processes such as folding or squeezing out of the DNA. This system is, however, highly reversible during compression-expansion cycles due to the strong interaction between the two components. These results are also supported by Brewster Angle Microscopy (BAM) images showing significant changes in the morphology of the film. Integral reflectivity of the BAM microscope has been used to study both isotherms themselves and the kinetic process of DNA inclusion into the lipid-like ODA monolayer. This parameter has been proven to be very effective for quantification of the monolayer processes showing high consistency with the compressibility and kinetics results.     

Adsorption of ionic surfactants at an expanding air-water interface

A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surface is presented for surfactant concentrations below and above the critical micelle concentration (cmc). For surfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in the adsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfaces under steady-state conditions. Experimental measurements of the surface excess for solutions of cationic surfactants CH3(CH2)n-1N+(CH3)3 Br- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactant sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreement with the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below the cmc. For surfactant concentrations above cmc, the diffusion ofmicelles and monomers are handled separately under the assumption of fast micellar breakdown. This simplified model gives excellent agreement for the system C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB + 0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longer fast on the time scale of the OFC (ca. 0.1 s).     

Organometallic chemistry at the edge of polycyclic aromatic hydrocarbons

Zirconocene and bisphosphine nickel chemistry developed in our labs and directed towards the derivatization and synthesis of polycyclic aromatic carbon compounds is reviewed. Complexes with the formula Cp₂ZrMe(η¹-PAC) (PAC=anionic polycyclic aromatic carbon ligand) eliminate methane to produce zirconacycles and yne complexes. Treatment of the zirconacycles with L₂NiX₂ (L=phosphine, X=Cl, Br) in the presence of alkynes results in metallacycle transfer to nickel and cycloaddition of the alkyne. The resulting polycyclic aromatic carbon compounds contain an additional ring. The nickelacycles may also be accessed by oxidative addition of Ni(0) to polycyclic aromatic dihalides followed by reduction. The application of this chemistry to the step-growth synthesis of single-walled carbon nanotubes is proposed.     

Microwave assisted extraction of polycyclic aromatic hydrocarbons from atmospheric particulate samples

For several years, microwave assisted extraction (MAE) was applied to extract organic compounds such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, etc., from soils, sediments and standard reference materials. Very few authors applied this methodology for the extraction of PAHs from atmospheric particulate matter. In the present study, MAE of polycyclic aromatic hydrocarbons with hexane/acetone (1:1) from real atmospheric particulate samples was investigated and the effect of microwave energy and irradiation time studied. The yields of extracted compounds obtained by microwave irradiation were compared with those obtained using traditional Soxhlet extraction. MAE was evaluated using spiked real atmospheric particulate samples and two standard reference materials. Analytical determinations of PAHs were carried out by high performance liquid chromatography (HPLC) with ultraviolet and fluorescence detection. The best recoveries were achieved with a microwave energy of 400 W and an irradiation time of 20 min.     

Self-assembly of a catechol-based macrocycle at the liquid-solid interface: experiments and molecular dynamics simulations

This combined experimental (STM, XPS) and molecular dynamics simulation study highlights the complex and subtle interplay of solvent effects and surface interactions on the 2-D self-assembly pattern of a Schiff-base macrocycle containing catechol moieties at the liquid-solid interface. STM imaging reveals a hexagonal ordering of the macrocycles at the n-tetradecane/Au(111) interface, compatible with a desorption of the lateral chains of the macrocycle. Interestingly, all the triangular-shaped macrocycles are oriented in the same direction, avoiding a close-packed structure. XPS experiments indicate the presence of a strong macrocycle-surface interaction. Also, MD simulations reveal substantial solvent effects. In particular, we find that co-adsorption of solvent molecules with the macrocycles induces desorption of lateral chains, and the solvent molecules act as spacers stabilizing the open self-assembly pattern.     

Adsorption of polycyclic aromatic hydrocarbons and inversion barriers of curved conjugated systems inside the molecular cage ExCage6+

We investigated the hosting of planar and curved π systems by ExCage⁶⁺. The M06‐2X/6‐311G* method and including basis set superposition error (BSSE) corrections showed good agreement with the experimental free energy changes in solution. The mean absolute deviation (MAD) with respect to experiment was 1.3 kcal/mol. The M06‐2X/6‐31G* method exhibited a MAD of 2.1 kcal/mol, so it may be useful to investigate large systems. The good agreement between the M06‐2X/6‐311G*+BSSE results and the M06‐2X/6‐31G* ones was due to a fortuitous error cancelation between basis set incompleteness and BSSE. The interaction energies decreased linearly with the number of CC double bonds present in the PAH, but the shape of the PAH is an important factor in determining the binding strength. Finally, we studied how effective is ExCage⁶⁺ in reducing the inversion barriers of buckybowls. For corannulene, sumanene and dibenzo[a,g]corannulene they are reduced by 2.0, 2.7, and 2.0 kcal/mol, respectively. Although these values indicate an induced fit catalysis by ExCage⁶⁺, they are far from those values calculated inside bilayer graphene. Therefore, much work is necessary to replicate the reduction of inversion barriers observed for graphene.     

Adsorption of nonionic mixtures at the air-water interface: effects of temperature and electrolyte

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.     

Adsorption and solvation of ethanol at the water liquid-vapor interface: a molecular dynamics study

The free energy profiles of methanol and ethanol at the water liquid-vapor interface at 310K were calculated using molecular dynamics computer simulations. Both alcohols exhibit a pronounced free energy minimum at the interface and, therefore, have positive adsorption at this interface. The surface excess was computed from the Gibbs adsorption isotherm and was found to be in good agreement with experimental results. Neither compound exhibits a free energy barrier between the bulk and the surface adsorbed state. Scattering calculations of ethanol molecules from a gas phase thermal distribution indicate that the mass accommodation coefficient is 0.98, and the molecules become thermalized within 10 ps of striking the interface. It was determined that the formation of the solvation structure around the ethanol molecule at the interface is not the rate-determining step in its uptake into water droplets. The motion of an ethanol molecule in a water lamella was followed for 30 ns. The time evolution of the probability distribution of finding an ethanol molecule that was initially located at the interface is very well described by the diffusion equation on the free energy surface.     

Nonspecific interaction forces at water-membrane interface by forced molecular dynamics simulations

Nonspecific interactions are the main driving forces for the behavior of molecules with great affinity for biologic membranes. To investigate not only the molecular details of these interactions but to estimate their magnitude as well, the theoretical method of Forced Molecular Dynamics Simulations, based on the Atomic Force Spectroscopy experimental technique, was applied. In this approach, an additional one-dimensional elastic force, representing the cantilever probe, was incorporated to the force field of a Molecular Dynamics computational program. This force represents a spring fixed on one end to a selected atom of the molecule; the other end of the spring is displaced at constant velocity to pull the molecule out of the membrane. The force experimented by the molecule due to the spring, is proportional to the spring elongation relative to its equilibrium position. This value is registered during the entire simulation, and its maximum value will determine the molecule-membrane interaction force. Nonexplicit medium simulations were carried out. Polar and apolar media were considered according to their polarizability degree and a specific dielectric constant value was assigned. In this approach, the membrane was considered as the apolar region limited by two flat surfaces with a polar aqueous medium. The potential energy discontinuity at the interfaces was smoothed by considering the polarization-induced effects using the image method. The results of this methodology are presented using a small system, a single Alanine amino acid model, which enables extended simulations in a microsecond time scale. The confinement of this amino acid at the interface reduces its degrees of freedom and forces it to adopt one of the six defined conformations. A correlation between these stable structures at the water-membrane interface and the interaction force value was determined.     

Determination of polycyclic aromatic hydrocarbons in atmospheric particulate matter of Valencia city

Summary Polycyclic aromatic hydrocarbons (PAHs) were determined in atmospheric particulate matter in 11 sites of the Valencia area and at several times during the year. Sample analysis was carried out by ultrasonic acetonitrile extraction followed by reverse phase HPLC separation and fluorescence detection. The maximum concentration of total PAH developed in winter and spring. Mean values per sampling site varied from 0.193 to 1.668 g/m³ of filtered air. Environmental noise and temperature were determined at those same 11 sites and correlated with PAH levels.     

Characterisation of polycyclic aromatic hydrocarbons in atmospheric aerosols by gas chromatography-mass spectrometry

An accurate and reliable method for determining polycyclic aromatic hydrocarbons (PAHs) in atmospheric aerosols is described. This optimised gas chromatography-mass spectrometry (GC-MS) method permits a wide range of concentrations to be analysed without the influence of interferences.Pre-treatment comparison of four kinds of aerosol collector filters determined that quartz and glass fibre filters were the most suitable. Solvents like cyclohexane, toluene, acetonitrile and dichloromethane were evaluated for their PAH-extraction capacity. Ultrasonic extraction using CH₂Cl₂ was selected because it is rapid and easy; moreover, this solvent increases the sample-throughput capacity.PAH compounds were quantitatively collected and ultrasonically extracted twice using 15 mL of CH₂Cl₂ for 15 min for each replicate. Rotavapor concentration, fractionation and dissolution were also optimised.A certified standard mixture (16 EPA PAHs), a deuterated compound and precision recovery assays were used for validating the proposed methodology. Adequate analytical parameters were obtained. Detection limits were (1.6-26.3) × 10⁻⁵ ng and quantification limits were (5.2-87.6) × 10⁻⁵ ng.Analysis of the environmental samples detected 4-10 EPA list PAH compounds. In addition, 2-11 tentative compounds were found, and their molecular structures were described for the first time.Our study of both Youden method and the standard addition method has shown that the proposed determination of PAHs in environmental samples is free of systematic errors.In conclusion, this unbiased methodology improves the identification and quantification of PAH compounds. High sensitivity as well as acceptable detection and quantification limits were obtained for the environmental applications.     

Determination of polychlorobiphenyls and polycyclic aromatic hydrocarbons in the atmospheric aerosol of the Venice Lagoon

An analytical method for simultaneous determination of particle-associated and gaseous-phase concentrations of polychlorinated biphenyls (PCB) and polycyclic aromatic hydrocarbons (PAH) in atmospheric aerosol samples obtained by high-volume samplers using polyurethane foam adsorbent (PUF) and quartz fibber filters (QFF) has been investigated. Quality control of the analytical procedure was carried out by blank control and by evaluating limits of detection, recoveries, accuracy, and repeatability. The proposed method was subsequently used to determine PAH and PCB in the gaseous and particulate phases of the aerosols that enter the Venice Lagoon atmosphere. The highest concentrations of PCB and PAH were predominantly in the gaseous phase. In both particulate and gaseous phases the penta-CB congeners dominated total PCB concentrations whereas phenanthrene, fluoranthene, and pyrene dominated the PAH concentrations. Total (gaseous plus particulate) PCB and PAH concentrations were higher at the site directly influenced by the industrial plants but the concentrations in marine aerosol samples were lower by a factor four only and must be taken into consideration when studying the chemical contamination of the Venice Lagoon.     

Unimolecular and collision induced fragmentations of molecular ions of polycyclic aromatic hydrocarbons

The unimolecular and collision induced fragmentation reactions of 19 polycyclic aromatic hydrocarbons have been investigated, using B/E linked scans, accelerating voltage scans, and measurements of appearance energies. From the point of view of simple analytical fingerprinting within isomeric series, B/E linked scans without collision gas offered the best compromise. Attempts to correlate trends observed in fragmentation intensities with various molecular parameters were generally unsuccessful, although some tentative conclusions have been drawn. The evidence concerning fragmentation mechanisms and ion structures is also discussed.     

Ozone interaction with polycyclic aromatic hydrocarbons and soot in atmospheric processes: theoretical density functional study by molecular and periodic methodologies

The ozonization mechanism for polycyclic aromatic hydrocarbons (PAHs) and soot is investigated by quantum mechanical calculations carried out on molecular and periodic systems. PAHs, interesting per se, serve also to model the local features of the graphenic soot platelets, for which another model is provided by a periodic representation of one graphenic layer. A concerted addition leads to a primary ozonide, while a nonconcerted attack produces a trioxyl diradical (in which one of the two unpaired electrons is pi-delocalized). Easy loss of (i) (1)O(2) or (ii) (3)O(2) from either intermediate, with spin conservation, would yield stable (i) singlet or (ii) triplet pi-delocalized species which carry an epoxide group. The trioxyl diradical pathway is estimated to be preferred, in these systems. An intersystem crossing, taking place in the trioxyl diradicals, can be invoked to allow the even easier loss of a ground-state oxygen molecule with the formation of a ground-state epoxide in a more exoergic and less demanding step. We propose that soot ozonization can take place by such a process, with ultimate functionalization of the graphenic platelets by epoxide groups.