An Approach to the Estimation of Adsorption ... - ACS Publications

Article pubs.acs.org/JPCA

An Approach to the Estimation of Adsorption Enthalpies of Polycyclic Aromatic Hydrocarbons on Particle Surfaces Richard E. Cochran,† Alena Kubátová, and Evguenii I. Kozliak* Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202, United States S Supporting Information *

ABSTRACT: Current atmospheric models incorporate the values of vaporization enthalpies, ΔHvap, obtained for neat standards, thus disregarding the matrix effects on volatilization. To test the adequacy of this approximation, this study measured enthalpies of vaporization for five polycyclic aromatic hydrocarbons (PAHs) in the form of neat standards (ΔHvap) as well as adsorbed on the surface of silica, graphite, and graphene particles (ΔHvapeff), by using simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Measurement of the corresponding activation energy values, Eavap and Ea vapeff, by TGA using a derivative method was shown to be the most reliable and practical way to assess ΔHvap and ΔHvapeff. Enthalpies of adsorption (ΔHads) were then calculated from the differences between Eavap and Ea vapeff, thus paving a way to modeling the solid−gas phase partitioning in atmospheric particulate matter (PM). The PAH adsorption on silica particle surfaces (representing n−π* interactions) resulted in negative values of ΔHads, indicating significant interactions. For graphite particles, positive ΔHads values were obtained; i.e., PAHs did not interact with the particle surface as strongly as observed for PM. PAHs on the surface of graphene particles evaporated in two stages, with the bulk of the mass loss occurring at temperatures lower than those with the neat standard, just as on graphite. Yet, unlike graphite, a small PAH fraction did not evaporate until higher temperatures compared to case of the neat standards and other particle surfaces (37.4−145.7 K), signifying negative, more PM-relevant values of ΔHads, apparently reflecting π−π* interactions and ranging between −7.6 and +32.6 kJ mol−1, i.e., even larger than for silica, −3.3 to +8.3 kJ mol−1. Thus, current atmospheric models may underestimate the partitioning of organic species in the particle phase unless matrix adsorption is taken into account.

1. INTRODUCTION

ln

Polycyclic aromatic hydrocarbons (PAHs) constitute a large class of organic compounds commonly observed in the atmosphere.1−4 Emitted through combustion by various anthropogenic and natural sources, PAHs have been found to be highly toxic, affecting human health.5,6 Therefore, much interest has been given to various transformation pathways that may occur during their lifetime in the atmosphere.7−9 In the environment, PAHs occur in both the gas and condensed (on the surface of suspended particulate matter, PM) phases.1,10,11 The phase where the PAH molecule resides dictates which transformation pathways it may undergo.12−14 Accurate modeling of the distribution/partitioning between the two phases is of significant importance in determining their impacts on the climate. Currently, the most common way to evaluate the partitioning of an organic species in the atmosphere is through its partitioning coefficient (Kp), which is inversely proportional to the corresponding vapor pressure (p).15−17 The vapor pressure is a function of enthalpy of vaporization (ΔHvap) as defined by the Clausius−Clapeyron equation (eq 1).16,18 © 2016 American Chemical Society

ΔH vap p =− +c po RT

(1)

where po is a constant standard pressure and c is a constant entropic term, ΔSvap/R (ΔSvap is the entropy of vaporization and R is the ideal gas constant). PM is either emitted by primary sources (e.g., combustion engines) or formed from the condensation of gas-phase compounds of low volatility.3,4,19 During their lifetime, the surface of aerosols represents a dynamic mixture of condensed chemicals that are continuously changing, making them even more complex and further deviating from an ideal surface.14,20−25 These nonideal surfaces may exhibit enhanced interactions with PAHs that shift the partitioning more toward the condensed phase than would be expected on the basis of theoretical partitioning in an ideal system (i.e., decrease in p from an increase in ΔHvap). Consequently, an effective enthalpy of vaporization (ΔHvapeff) is larger for the substances adsorbed on interacting particles than the values tabulated for neat Received: April 8, 2016 Revised: June 14, 2016 Published: July 11, 2016 6029

DOI: 10.1021/acs.jpca.6b03611 J. Phys. Chem. A 2016, 120, 6029−6038

Article

The Journal of Physical Chemistry A Table 1. List of PAH Standards Used in This Study along with Their Physical Parametersa compd naphthalene phenanthrene anthracene pyrene fluoranthene

MW (g mol ) 128.17 178.22 178.22 202.25 202.25

ref 491.2 ± 0.1 611.6 ± 0.9 614.5 ± 0.4 667 ± 6 653 ± 5

ΔHvap0 (kJ mol−1)

pio (Pa)b

Tboil (K) −1

41 41 41 41 41

ref 10.6 2.51 2.49 6.03 6.72

× × × ×

10−2 10−3 10−4 10−4

42 42 42 42 42

Ea0 (kJ mol−1)

ref 57.6 ± 6.8 77.8 ± 6.8 79 ± 10 89.7 ± 6.1 87.3 ± 6.7 c

43 43 43 43 43

8.7 10.7 11.2 4.3 7.6

± 6.6 ± 7.6 ± 10.1 ± 7.0 ± 7.3

d

manufacturer

purity (%)

Sigma-Aldrich Sigma-Aldrich Fluka Fluka Sigma-Aldrich

98 98 99 99 99

MW = molecular weight; Tboil = boiling temperature; pi0 = standard vapor pressure; Tfus = fusion temperature; ΔHvap0 = standard vaporization enthalpy. bVapor pressure values at 298.13 K cAverage values of ΔHvap0 along with pooled variances were calculated using all reported values reported in ref 43, which were obtained by a range of analytical techniques. dValues for Ea0 were calculated using values of Eavap determined for neat standards used in this study using TGA (integrated method) and those calculated using the values reported in ref 43. Variances for Ea0 are the pooled variances between those calculated for Eavap in this study and those reported for ΔHvap0 in ref 43. a

standards.26,27 Unfortunately, current models only take into account the standard values of ΔHvap, thus not accounting for any possible added interactions between the compound of interest and the surface of the aerosol.17,18,26−30 To incorporate these interactions into the models, the values of ΔHvapeff must first be determined for individual species across a range of different particle matrix types. The values of ΔHvap can be estimated experimentally using thermal techniques such as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Different approaches have been developed to accurately calculate both Eavap (activation energy for vaporization, assuming that Eavap ≈ ΔHvap) and ΔHvap values of pure homogeneous substances using TGA-DSC with a nonisothermal oven program.31−35 Using the TGA data, Eavap values are calculated by treating the sample cup as a Knudsen cell and monitoring the mass loss with respect to temperature. Calculations relate Eavap to the rate of mass loss through different linearization methods of the firstorder Arrhenius equation (as detailed below). Alternatively, the DSC data set can be used to calculate ΔHvap, again using the Knudsen cell approach.31,33 Though these methods have been established for various classes of pure homogeneous organic compounds, their suitability in estimating ΔHvap for a wide range of PAHs on particle surfaces has yet to be evaluated.36−40 The main focus of this work was to use TGA-DSC to investigate possible interactions of PAHs with different particle surfaces. The enthalpy of adsorption (ΔHads) was determined through the differences between the vaporization enthalpy values for neat standards and those in the presence of particles. TGA was first optimized to accurately determine values of Eavap for different PAHs, by using two previously reported methods for calculating Eavap for neat standards and by comparing these values to previously reported ΔHvap values. Experiments were then performed using single PAH species present on the surface of different types of model particles to gain an understanding of the types of interactions that occur. Three types of surrogate particles were chosen as to mimic a range of surface types and polarities occurring in various real-world PM matrices. Silica particles were used to mimic polar surfaces (e.g., highly oxidized secondary organic aerosol with a high oxygen-tocarbon ratio) whereas graphite (carbon allotrope with a relatively disordered lattice structure) and graphene (highly ordered two-dimensional sheet) particles represented nonpolar aerosol surfaces (e.g., soot particles with relatively high levels of elemental or black carbon compared to levels of organic carbon).

2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. The standards as well as model particles used in this study are listed in Tables 1 and 2, Table 2. Model Particles Used in This Study and Their Physical Parameters surrogate particle

manufacturer

purity (%)

Dpmean (μm)a

surface area (m2 g−1)

silica graphite graphene

Sigma-Adrich Sigma-Aldrich USRNb

99 ≥99.9 ≥99.5

75−100 ≤20 4−12

480 50

a

Dpmean = mean particle diameter. bUSRN stands for U.S. Research Nanomaterials, Inc. (Houston, TX, USA).

respectively, along with their pertinent physical parameters.41−43 All stock solutions and particle suspensions were prepared in dichloromethane (DCM, high-purity GC grade, Fisher Scientific, Pittsburgh, PA, USA). 2.2. Preparation of Standards and Model Particles. All particle surfaces were cleaned prior to use by calcination in an oven at 550 K for 12 h and then stored in a sealed glass jar until used. To ensure the homogeneous coating of individual organic standards onto particle surfaces, stock solutions of individual PAHs (15−30 mg mL−1) were added to the particles (∼50 mg) and the suspensions were brought to ca. 1 mL with DCM in a 2 mL autosampler vial. The DCM was then evaporated under a gentle stream of nitrogen with the vial on a shaker plate at 250 rpm (Labnet, Edison, NJ, USA) until the particles moved freely at the bottom of the vial. The final fraction of the individual PAH relative to the mass of particles was ∼150 μg mg−1. Select experiments showed that changes in the mass loadings between 80−210 μg mg−1 resulted in statistically similar ΔHvap and Eavap values (as calculated using the methods described in sections 2.3 and 2.4), with RSD values being between 1−8%. 2.3. Simultaneous Thermogravimetry-Differential Scanning Calorimetry. All TGA-DSC analyses were performed using a SDT-Q600 system (TA Instruments, New Castle, DE, USA). Analyses were conducted with a dynamic oven program using linear heating rates (5−100 K min−1) with nitrogen as a purge gas (20−100 mL min−1). Preliminary experiments showed that an oven heating rate of 20 K min−1 and purge gas flow rate of 20 mL min−1 were optimal in providing quality calorimetry and differential thermogram peak shapes. Additional optimization of the TGA-DSC method was performed by comparing two different sample cup types: alumina and aluminum pans with and without lids. Previous work showed that a lid containing a small pierced hole provided 6030


Article

The Journal of Physical Chemistry A

Figure 1. (a) Example TGA curve (A) with overlaid DSC (B) and DTG (C) curves obtained during the TGA-DSC analysis of an anthracene standard (solid crystal) and (b) TGA mass loss curve from panel A showing characteristic temperatures: Tlow, Tavg, and Thigh.

more accurate estimations of ΔHvap for semivolatile organics.33 The use of loose lids resulted in early onset of vaporization whereas crimped nonpierced lids led to overpressurization of the sample cup (Supporting Information, Figure S1a). A comparison of different sample cup configurations showed that more accurate estimates of vaporization enthalpies were obtained using the aluminum sample cups with crimped and pierced lids. On the basis of these findings, a crimped lid with a pierced hole was used for all subsequent work. The diameter of the hole was determined with scanning electron microscopy (Figure S1b) and was consistently between 150−200 μm. Each standard was analyzed three times (n = 3). ΔHvap and Eavap values were determined from the DSC and TGA data, respectively. 2.4. Calculations of Vaporization Enthalpies and Activation Energies. Vaporization enthalpies were estimated through two different techniques. The first of them utilized the heat flow signal measured by differential scanning calorimetry during the TGA-DSC runs (see Figure 1a for an example heat flow curve), using calculation schemes reported by Rojas and Orozco.33 The heat flow curve was integrated within the temperature range by which vaporization occurred with respect to a baseline (determined through the analysis of blank sample cups). This area under the heat flow curve, in the units of watts (W) per second (s), was then divided by the mass lost during vaporization to give the specific heat vaporization per gram, Q (J g−1) (eq 2). Qi =

A (W · s ) m initial − mfinal

ization.34−36,40 The second method called the “integral method” relates the activation energy to the mass differential functions in the following manner:34,45 ⎛ m ⎜ −ln m0 ln⎜ 2 ⎜ T ⎝

( ) ⎞⎟⎟ = ln⎡⎢

(2)

(3)

The mass loss curve (TGA curve; an example is shown in Figure 1) was used to calculate the values for the activation energy of vaporization Eavap, using two separate calculation methods based on the linearization of the Arrhenius equation.34,44 The first, referred to as the “derivative method”, uses the Friedman equation in the following form:34 ⎛ ⎡ dm ⎤ m 0 ⎞ E vap ⎟ = ln(A) − a ln⎜ −⎢ ⎥ ⎝ ⎣ dT ⎦ m ⎠ RT

(5)

Plotting the left-hand term against 1/T, similarly to the derivative method, allows for the determination of Eavap from the slope of the near-linear curve (in the range of sublimation/ vaporization).34,35 The regions used for estimating ΔHvap for each compound using both methods are shown in Figure S2. ΔHvap should be either equal or slightly smaller than Eavap, as with any highly endothermic reaction. In addition to those previously reported, a fourth method was developed in this work utilizing a characteristic temperature along the thermogram: Tavg. Tavg was the temperature where the maximum value of dm/dT was observed. Other characteristic temperatures included Tlow (at which the tangent of dm/ dT at Tavg intersected a straight line at m/m0 = 1) and Thigh (at the point the tangent line intersected a line at m/m0 = 0). An example of how these temperature values were determined is shown in Figure 1b. Using PAHs as neat chemicals, the correlation was established between the calculated ΔHvap values (using the integral method) and Tavg using linear regression analysis. Then, this correlation was used as a calibration curve to estimate the unknown ΔHvapeff values for the given surface on the basis of the experimentally observed Tavg. Although the other above-mentioned characteristic temperatures were not utilized in this study, except for Tavg, they also may be used to estimate thermodynamic values using similar calibrations. 2.5. Data Processing. Integrations of heat flow curves and the two-tailed Student’s t test were performed using Origin 9.1 software (OriginLab, Northampton, MA, USA). All other processing of TGA-DSC data was done using Microsoft Excel.

The calculated value of Q was then multiplied by the molecular weight of the compounds being vaporized to obtain the molar vaporization enthalpy, ΔHvap (J mol−1) (eq 3). ΔH vap, i = Q i·MW

⎟ ⎠

Ea vap 2RT ⎞⎤ AR ⎛ ⎥ vap ⎜1 − vap ⎟ − ⎢⎣ CEa ⎝ Ea ⎠⎥⎦ RT

3. RESULTS AND DISCUSSION 3.1. Comparing Calculation Methods for Eavap and ΔHvap of PAHs. The applicability of the TGA-DSC methods for five representative PAHs (those containing two to four aromatic rings) was assessed on the basis of ΔHvap values calculated using the Rojas method (employing DSC) and comparing them to both the Eavap values obtained by TGA and previously reported ΔHvap values. The PAHs in this study were specifically chosen as they are typically the most abundant PAH compounds in PM.46 Both five- and six-ring PAHs were

(4)

By plotting ln(m0/m[dm/dT]) against 1/T, we can calculate the activation energy from the slope of the resulting curve, which is close to a straight line for sublimation/vapor6031


Article


Table 3. Calculated ΔHvap Values for Two- to Four-Ring PAHs Using Different Methods, Which Utilize Either Heat Flow (Rojas Method) or Mass Loss Data (Integral and Derivative Methods)a Tboil (K) compd naphthalene phenanthrene anthracene pyrene fluoranthene

ref 491 613 613 667 653

± ± ± ± ±

5 4 2 2 3

41 41 41 41 41

Tlow (K) 494.5 603.1 606.3 641.0 627.3

± ± ± ± ±

1.1 2.6 4.2 2.4 3.4

Thigh (K) 500.7 612.1 614.6 664.5 653.7

± ± ± ± ±

0.6 0.5 0.8 0.7 1.0

Tavg (K) 497.8 609.7 612.6 659.8 651.2

± ± ± ± ±

0.8 0.7 0.5 1.8 1.9

ΔHvap Rojas method (DSC) (kJ mol−1) 71.6 95.0 92.7 103.6 102.9

± ± ± ± ±

3.1 2.4 2.0 1.9 1.7

Eavap integral method (TGA) (kJ mol−1) 63.0 87.9 90.4 99.8 97.4

± ± ± ± ±

4.0 3.6 1.1 2.0 1.3

Eavap derivative method (TGA) (kJ mol−1) 66.3 88.4 89.8 94.0 94.8

± ± ± ± ±

1.6 2.4 1.1 2.4 2.7

a

Tboil = boiling temperature; Tlow, Thigh, and Tavg are characteristic temperatures of the TGA mass loss curve (see section 2.4). Each average and standard deviation was calculated from the analysis of three separate samples (n = 3).

considered as well; however, due to their high boiling points the process of pyrolysis competed with vaporization, prohibiting the estimation of ΔHvap. The high boiling points of these PAHs also reduce the vapor pressures under ambient atmospheric conditions to rather low values, even without the PM matrix assistance, thus rendering them virtually nonvolatile.26,27 The use of the Rojas derivative or integral methods have not been previously reported for PAHs with nonisothermal TGA and DSC analysis.33 These methods allow for the calculation of ΔHvap from a single nonisothermal run, thereby decreasing analysis time compared to traditional isothermal methods that require multiple runs at different heating rates to accurately calculate the values. Comparing ΔHvap values calculated with the Rojas method to the activation energies determined by TGA using two methods, derivative and integral (Table 3), resulted in a linear correlation observed for all of the studied PAHs, with the Rojas method consistently yielding higher values (Figure 2a). A comparison using the Student’s t test (two-tailed; df = 4) showed that the values obtained using the Rojas method were statistically similar to results from both of the TGA methods. The obtained values of ΔHvap were also similar to those previously reported using other experimental methods (shown in Figure 2b for the integral method). All of the analyses were performed with the same heating rate (0.33 K s−1) because it has been reported that both the derivative and integral methods are sensitive to changing temperature ramps from run to run.34 During each run, mass losses can occur through either sublimation or vaporization; however, sublimation was not observed (i.e., mass loss began after melting occurred, or Tlow ≫ Tfus). Therefore, all enthalpies and activation energies resulted from vaporization alone (see Figure S3 for the TGA, DSC, and differential thermal gravimetry (DTG) curves for each neat standard). In the linearization methods, limitations can arise from the somewhat arbitrary determination of which points to include in the linear region of the resulting curve. To avoid potential errors arising from this constraint, both methods were performed using the same points within the same temperature region. Although the Rojas method provides accurate determinations of ΔHvap from the DSC data for sufficient sample amounts, it is dependent on obtaining enough heat flow signal per unit of time that is distinguishable from the baseline. In evaluating the desorption of a small PAH concentration from a particle surface (e.g., at concentrations of 0−200 μg analyte/mg particle), heat flow signals are difficult to distinguish from the baseline. Therefore, the methods utilizing the TGA thermograms are expected to be more accurate and applicable, just as observed. From these, in addition to yielding Eavap values that were similar

Figure 2. (a) Comparison of ΔHvap values determined using the TGA methods (derivative and integral methods) to the values obtained by DSC (Rojas method) and (b) comparison of ΔHvap values determined in this work using the derivative method to those reported in the literature using other experimental methods (gas chromatography, GC, determining vapor pressures and correlation gas chromatography, CGC, measuring chromatographic retention times).43,58

to ΔHvap values reported in literature, the integral method consistently provided greater linearity (Figure S2) and therefore was used for the remainder of this study to accurately determine the strength of PAH adsorption on particle surfaces. 6032


Article


determine ΔHads from the differences between ΔHvap for neat compounds and their effective vaporization enthalpies (ΔHvapeff) while adsorbed to particle surfaces. Due to the extreme rigidity of PAH molecules, the impact of changes in the vibrational heat capacity with temperature should be minimal within the considered temperature range and thus ΔHvap and ΔHvapeff will have minimal dependence on temperature; also, these changes are expected to be offset when ΔHvapeff is subtracted from ΔHvap and to yield ΔHads. Therefore, the values of ΔHads will still accurately reflect the added energy of adsorption due to the interaction between the PAH molecule and the particle surface, regardless of the temperature. 3.2.1. PAHs Spiked to Silica Particles: The Influence of n−π* Interactions on ΔHvap. Values of ΔHvapeff calculated for two- to four-ring PAHs spiked to silica were compared to ΔHvap of the corresponding neat standards (Figure 4; detailed

The observed slight positive bias in Eavap compared to ΔHvap can be explained by the occurrence of a nonzero intrinsic activation barrier according to the Polanyi equation: Ea vap = g ΔH vap + Ea 0

(6)

where Ea0 is assumed to be small and γ is equal to 1 for very endothermic processes.47 The values of Ea0 calculated as the difference between Ea0 and the published values of ΔHvap summarized in ref 43 (see also Table 1) corroborate this assumption. On the basis of statistical comparisons between the averages and variances of Eavap in this work and literature ΔHvap values, Ea0 is either insignificant or very small compared to the magnitude of ΔHvap. Furthermore, this value of Ea0 will be offset when ΔHads is calculated as the difference between Eavap and Ea vapeff in the next section. On the basis of these observations, and for clarity of presentation, Eavap and Ea vapeff will be referred to as ΔHvap and ΔHvapeff, respectively, for the remainder of this paper. The determined Thigh, Tlow, and Tavg values for the neat standards of each PAH all showed a linear relationship to the calculated Eavap values (comparisons to Tavg are presented in Figure 3). Therefore, Tavg of a vaporization process may be used to estimate values of Eavap.

Figure 4. Comparison of ΔHvapeff calculated for PAHs on surrogate particle surfaces to ΔHvap calculated for neat standards. αΔHvapeff values are shown for graphene.

data are shown in Table 4). Naphthalene exhibited the strongest adsorption behavior, with ΔHvapeff being greater than ΔHvap by 8.3 ± 0.2 kJ mol−1. Phenanthrene and anthracene (both three-ring PAHs) yielded slightly higher ΔHvapeff values relative to ΔHvap (yielding ΔHvap − ΔHvapeff = ΔHads values of −3.3 ± 0.6 and −3.8 ± 3.5 kJ mol−1, respectively). However, by contrast, the four-ring PAHs, pyrene and fluoranthene, showed more negative ΔHads values, at −7.4 ± 2.2 and −6.6 ± 1.2 kJ mol−1, respectively. It therefore seems that there are two overlapping trends; i.e., the adsorption is stronger for both smaller two-ring PAH and larger four-ring PAH structures than for the three-ring PAHs. These observations may reflect steric factors involved in the interaction of the PAHs with the nonhydrated surface of silica, i.e., a mixture of silanol groups and oxo bridges, both of which offer a nonbonding pair of electrons and will be considered when ΔHads is correlated with the PAH electronic parameters. 3.2.2. PAHs Spiked to Graphite and Graphene Particles: The Influence of π−π* Interactions on ΔHvap. When PAHs were spiked to graphite particles, electron donor−acceptor (π−π*; EDA) interactions of PAHs with sp2 elemental carbon, i.e., graphite, were expected to result in even more negative values of ΔHads than those observed when spiked to silica.48,49

Figure 3. Correlation of calculated ΔHvap values (using the integral method) to Tavg of mass loss curves for PAH standards.

3.2. ΔHvapeff and ΔHads for PAHs on Surrogate Particle Surfaces. The Rojas method could not be applied to the experiments with adsorbents, as we had to use small PAH amounts to ensure their specific interaction with surfaces. As a result, the mass of the PAH vaporized during the TGA-DSC runs was too small for the DSC to register a significant response that could be accurately distinguished from the baseline. Thus, the integral TGA method was used to evaluate the contribution of surrogate particle surfaces to the adsorption of two- to four-ring PAHs. The surfaces considered in this work (silica, graphite, and graphene) were selected to mimic the different types of surfaces observed in real-world PM as well as addressing the common types of molecular interactions between polyaromatic hydrocarbons and a particle surface (i.e., n−π* and π−π * interactions). In these experiments, the major focus was to 6033


Article

The Journal of Physical Chemistry A Table 4. Calculated Values of ΔHvapeff and ΔHads for Two- to Four-Ring PAHs on Surfaces of Surrogate Particlesa adsorbed analyte naphthalene

phenanthrene

anthracene

pyrene

fluoranthene

substrate surface

analyte level

neat silica graphite graphene

bulk bulk bulk bulk trace bulk bulk bulk bulk trace bulk bulk bulk bulk trace bulk bulk bulk bulk trace bulk bulk bulk bulk trace

neat silica graphite graphene neat silica graphite graphene neat silica graphite graphene neat silica graphite graphene

Tlow (K) 494.5 488.0 407.6 405.2 538.5 603.1 587.1 500.6 479.5 578.6 606.3 552.2 498.7 501.2 613.9 641.0 618.9 543.8 523.4 629.2 627.3 609.6 542.5 522.8 628.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.1 2.6 2.0 1.2 1.5 2.6 3.2 2.4 1.5 1.4 4.2 2.4 2.8 2.0 2.5 2.4 2.8 2 1.7 2.0 3.4 4.8 3.1 2.4 2.0

Thigh (K) 500.7 555.6 449.9 478.4 683.5 612.1 650.0 560.5 558.4 715.7 614.6 639.6 567.5 596.4 720.1 664.5 691.2 614.8 615.4 783.2 653.7 692.5 611.8 614.6 780.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Tavg (K)

0.6 2.9 1.5 1.2 1.5 0.5 3.8 6.1 2.0 1.5 0.8 3.9 5.5 1.7 1.3 0.7 2.6 1.2 1.7 2.6 1.0 2.8 0.7 1.6 2.5

497.8 524.8 430.7 445.4 643.5 609.7 625.4 523.5 526.3 659.3 612.6 610.0 522.6 551.2 662.6 659.8 661.7 578.5 573.7 697.2 651.2 658.0 574.7 572.4 690.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.8 5.0 2.1 0.8 1.6 0.7 4.3 4.3 0.8 7.5 0.5 5.7 2.5 4.2 0.6 1.8 3.4 1.3 0.8 1.0 1.9 5.4 0.7 1.8 2.0

calcdb,c ΔHvapeff (kJ mol−1) 63.1 71.3 66.8 60.8 95.7 87.9 91.2 83.6 66.1 99.2 90.4 91.8 87.2 64.9 99.9 99.9 107.3 97.2 67.1 107.7 97.4 104.0 95.7 67.1 106.2

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4.1 1.0 0.8 0.7 0.4 3.6 0.6 0.7 0.3 1.7 0.2 3.5 1.1 2.6 0.1 2.1 2.2 0.4 0.1 0.2 1.4 1.2 0.9 0.3 0.5

calcdb,c ΔHads (kJ mol−1) −8.3 ± 0.2 −3.7 ± 1.8 ND −32.7 ± 0.9 −3.3 ± 0.6 +4.3 ± 0.7 ND −10.3 ± 2.1 −3.8 ± 3.5 +0.8 ± 1.1 ND −11.8 ± 1.5 −7.4 ± 2.2 +2.7 ± 0.4 ND −7.6 ± 2.1 −6.6 ± 1.2 +1.7 ± 0.9 ND −8.8 ± 2.1

Each individual PAH was applied at ∼150 μg mg−1 relative to the particle mass. ΔHvapeff values were calculated using the integral method with the TGA data. Tlow, Thigh, and Tavg are characteristic temperatures of the TGA mass loss curve (see section 2.4). bValues of ΔHvapeff for all bulk levels of analyte were calculated using the Integral method from the TGA curve data. Values are shown as averages (n = 3) with one standard deviation. c Values of ΔHvapeff for all trace levels of analyte were calculated using our method based on Tavg. Values are shown as averages (n = 3) with one standard deviation. a

Figure 5. (a) TGA thermograms of anthracene as a neat standard and spiked to the surfaces of model particles. (b) Thermogram of anthracene on graphene showing the two phases of the vaporization/desorption process.

To evaluate other sp2 carbon surfaces, PAHs were also spiked to graphene nanoparticles. Similarly to graphite, the bulk of all tested PAHs evaporated from the particle surface at lower temperatures than the neat standards. Presumably, the first step of this two-process mechanism involves the vaporization of a bulk of the PAH from the surface (i.e., βΔHvapeff; shown in Figure 5b for anthracene). However, in contrast to those observed with graphite, the thermograms of PAHs evaporating from the graphene surface (Figure 5) showed that the remaining small fraction of residual PAH appears to access “high-energy” active sites on the particle surface. Enthalpies in

Unexpectedly, when added to the graphite-like surfaces, all of the two- to four-ring PAHs evaporated at significantly lower temperatures compared to that observed with the neat standard (Figure 4, Table 4). This, in turn, resulted in lower ΔHvapeff values relative to ΔHvap (i.e., with positive values of ΔHads), indicating poor interaction. Corroborating this assumption, scanning electron microscopy (SEM) images showed that, when spiked to the surface, PAHs recrystallized and did not appear to be associated with the surface (see Figure S4 for an example with anthracene). 6034


Article


Figure 6. Correlation of negative ΔHads values calculated in this work for naphthalene and pyrene adsorbed on silica and graphene to (a) their theoretical HOMO−LUMO gap energies in the gas phase (HOMO−LUMOgas) and (b) their theoretically estimated transition energies to the first excited singlet state (S0−S1). All values for HOMO−LUMOgas and S0−S1 were obtained from ref 55.

of model particles) using the TGA methods and the Tavg method are shown in Figure 3. For all PAHs considered, the calculated values of αΔHvapeff were significantly higher for graphene than for silica or graphite particles (Figure 4). This observation is consistent with strong π−π* interactions postulated by Beránek et al., who observed greater thermal desorption temperatures for very small amounts of PAHs evolving from both graphite carbon and PM surfaces.51 Similarly to silica, the values of ΔHads showed a correlation to the PAH size, with naphthalene exhibiting the strongest interactions to the graphene surface, as expected for the trends in π−π* energy gaps.48,50,52 3.4. Correlation of ΔHads to PAH Electronic Parameters. The increased interaction of PAHs with graphene can thus be explained in terms of the energy gaps between the highest occupied orbital and the lowest unoccupied orbital of each individual PAH (i.e., the electronic HOMO−LUMO gap). Figure 6a compares the ΔHads values calculated in this work to the theoretical energy gaps reported previously.53,54 As the LUMO appears to be a triplet, Figure 6b plots ΔHads vs the theoretical energy gaps55 between the ground (S0) and first excited singlet states (S1), yielding a similar correlation. The graphs show a general trend of increasing ΔHads with the PAH HOMO−LUMO gap, although this correlation is significantly skewed, apparently due to the above-mentioned steric effects of thermodynamic nature including the difference in binding of linear and nonlinear PAHs. The resulting energy of the system being lower than that of the homomolecular interactions of both components can be explained as follows. All PAHs considered in this work have a greater HOMO−LUMO energy gap in the gas phase53,56 than the HOMO−LUMO gap for graphene, which is less than 190 kJ mol−1.57 Interaction of the HOMO of the individual PAH molecule with the LUMO for graphene occurs during adsorption, replacing similar homomolecular interactions (i.e., the graphene and PAH π-stacking, the latter of which is directly measured by the corresponding HOMO−LUMOgas). For naphthalene, the propensity to form heteromolecular π−π* interactions is so high (apparently, due to both its smallest size reducing steric effects and the greatest energy gain effected among the PAHs used) that its ΔHads is negative not only for graphene but also for graphite, for which adsorption was unfavorable for all other PAHs (Table 4).

this range would then refer to the trace amount only (αΔHvapeff). This observation corroborates the report of Lazar et al., who showed that for graphene only 0.24 ± 0.03% of the total surface area includes such high-energy binding sites.48 It may be assumed that the interaction of PAHs with these stronger adsorption sites reflects the true adsorption process characteristic for interactions of small amounts of PAHs embedded in organic matrices of PM. Such an adsorption process would essentially require, at least within the temperature range used, ΔHvapeff values greater than ΔHvap and the mass loss to continue at temperatures beyond the Tavg value observed for the neat standard (i.e., 339.5 ± 0.5 K). The latter requirement was met by the TGA curve for PAHs on graphene but not on graphite. Thus, structural differences of graphite particles seem to not favor interactions of PAH molecules with high-energy sites that may represent only a small fraction of the surface. The physical nature of these stronger adsorption sites on graphene rather than graphite was explained by ChakarovaKäck et al.,50 who showed the occurrence of two binding modes in PAH π-stacking. The strongest binding sites with the shortest bond require a perfect lineup of the interacting molecules whereas an imperfect overlap yields only weaker and longer intermolecular interactions. When PAHs with a size greater than that of naphthalene are forced to interact with such a surface due to its high abundance, at the expense of more energy favored homomolecular interactions, a positive value of ΔHads = ΔHvap − ΔHvapeff is effected. 3.3. Estimating ΔHvapeff from Tavg for PAHs Adsorbed on the Surface of Graphene. For graphene, the mass loss rate (dm/dT) observed in the temperature range above Tavg was linear but lower than that observed for the bulk anthracene region, resulting in calculated αΔHvapeff values being even lower than βΔHvapeff. This would yield positive values of adsorption enthalpy (eq 1), which would be inconsistent with the PAH evaporation from the surface at temperatures above its boiling point. Apparently, the evaporating masses are too small so that the slope is artificially decreased for kinetic reasons. Therefore, αΔHvapeff, i.e., the value characterizing the strong PAH adsorption on graphene, was estimated using our alternately proposed method based on the observed Tavg values (described in section 2.4). The calibration plots comparing ΔHvap values for PAHs (as neat standards and on the surfaces 6035


Article

The Journal of Physical Chemistry A By contrast, the PAH ΔHads on silica show virtually no correlation to the corresponding PAH HOMO−LUMOgas or the S0−S1 transition energies, indicating the high impact of steric hindrance. Smaller two-ring PAHs can achieve a closer contact with these electron donor groups, whereas larger PAHs cannot. On the contrary, although the compact four-ring PAHs used in this study are not much larger in size compared to their three-ring analogs, they possess more aromatic rings and therefore potentially enable multiple interactions with the surface. A combination of these two conflicting trends may explain the observed stronger adsorption of both two-ring and four-ring PAHs on silica. The large ΔHvap − αΔHvapeff values reported in this study for the strong PAH adsorption on graphene (Figures 3 and 6) may significantly affect the PAH evaporation from the PM surface, as this value would be pertinent to a greater PAH fraction due to a lower abundance of PM-adsorbed PAH compared to the case in this study. However, even for silica the deviations in PAH vapor pressure may be significant despite the observed low ΔHvap − αΔHvapeff difference, as this value pertains to the whole adsorbed PAH fraction, as explained in section 3.2.1. Estimates based on the Clausius−Clapeyron equation (eq 1) show that current atmospheric models may underestimate the partitioning of organic species in the particle phase unless their matrix adsorption is taken into account. This can lead to increased particle phase PAH concentrations than expected on the basis of standard vaporization enthalpies.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: 701-777-2145. Fax: 701-777-2331. E-mail: evguenii. [email protected]. Address reprint requests to: 151 Cornell St., Stop 9024, Grand Forks, ND 58202, USA. Present Address †

University of California San Diego, Department of Chemistry and Biochemistry, La Jolla, CA 92093-0314. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by the National Science Foundation through the CAREER grant of Dr. Kubátová (Grant No. ATM0747349) as well as the North Dakota Experimental Program to Stimulate Competitive Research program (Grant No. EPS0184442). We thank Laura Elsbernd for some experimental contributions.

■

REFERENCES

(1) Ravindra, K.; Sokhi, R.; Van Grieken, R. Atmospheric Polycyclic Aromatic Hydrocarbons: Source Attribution, Emission Factors and Regulation. Atmos. Environ. 2008, 42, 2895−2921. (2) Wang, W.; Jariyasopit, N.; Schrlau, J.; Jia, Y.; Tao, S.; Yu, T.-W.; Dashwood, R. H.; Zhang, W.; Wang, X.; Massey Simonich, S. L. Concentration and Photochemistry of PAHs, NPAHs, and OPAHs and Toxicity of PM 2.5 during the Beijing Olympic Games. Environ. Sci. Technol. 2011, 45 (16), 6887−6895. (3) Pandis, S. N.; Seinfeld, J. H. Atmospheric Chemistry and Physics, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2006. (4) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Upper and Lower Atmosphere, 1st ed.; Academic Press: San Diego, CA, 2000. (5) Ravindra, K.; Mittal, A. K.; Van Grieken, R. Health Risk Assessment of Urban Suspended Particulate Matter With Special Reference to Polycyclic Aromatic Hydrocarbons: A Review. Rev. Environ. Health 2001, 16 (3), 169−189. (6) U.S. Environmental Protection Agency. Clean Air Act Amendment; 1990. (7) Perraudin, E.; Budzinski, H.; Villenave, E. Kinetic Study of the Reactions of Ozone with Polycyclic Aromatic Hydrocarbons Adsorbed on Atmospheric Model Particles. J. Atmos. Chem. 2007, 56 (1), 57−82. (8) Miet, K.; Le Menach, K.; Flaud, P. M.; Budzinski, H.; Villenave, E. Heterogeneous Reactions of Ozone With Pyrene, 1-Hydroxypyrene and 1-Nitropyrene Adsorbed on Particles. Atmos. Environ. 2009, 43, 3699−3707. (9) Perraudin, E.; Budzinski, H.; Villenave, E. Kinetic Study of the Reactions of NO2 with Polycyclic Aromatic Hydrocarbons Adsorbed on Silica Particles. Atmos. Environ. 2005, 39, 6557−6567. (10) Allen, J. O.; Dookeran, N. M.; Taghizadeh, K.; Lafleur, A. L.; Smith, K. A.; Sarofim, A. F. Measurement of Oxygenated Polycyclic Aromatic Hydrocarbons Associated with a Size-Segregated Urban Aerosol. Environ. Sci. Technol. 1997, 31 (7), 2064−2070. (11) Bamford, H. A.; Bezabeh, D. Z.; Schantz, M. M.; Wise, S. A.; Baker, J. E. Determination and Comparison of Nitrated-Polycyclic Aromatic Hydrocarbons Measured in Air and Diesel Particulate Reference Materials. Chemosphere 2003, 50 (5), 575−587. (12) Ringuet, J.; Albinet, A.; Leoz-Garziandia, E.; Budzinski, H.; Villenave, E. Reactivity of Polycyclic Aromatic Compounds (PAHs, NPAHs and OPAHs) Adsorbed on Natural Aerosol Particles Exposed to Atmospheric Oxidants. Atmos. Environ. 2012, 61, 15−22. (13) Asa-Awuku, A.; Miracolo, M. A.; Kroll, J. H.; Robinson, A. L.; Donahue, N. M. Mixing and Phase Partitioning of Primary and

4. CONCLUSIONS Adsorption of PAHs to the surface of silica particles (modeling n−π* interactions) showed a significant dependence on the PAH molecule size, with naphthalene yielding the most negative ΔHads values. Unexpectedly, PAHs evaporated from the surface of graphite particles (π−π* interactions) at temperatures much lower than those observed for the neat standards. Experiments with PAHs adsorbed on graphene yielded a specific temperature profile featuring two separate modes apparently reflecting the multisite evaporation. The majority of the PAH mass loss associated with vaporization occurred at temperatures lower than for the neat standards; however, a significant amount did not evaporate until temperatures higher than the boiling point (Tavg; temperature with maximum mass loss rate). In this temperature range, the methods for calculating ΔHads successfully used for silica did not provide accurate results. Yet, using the developed calibration method that relates Tavg to ΔHvap, more realistic values of ΔHvapeff for PAHs on graphene were estimated. Combined with the most accurately determined values of ΔHvap for the corresponding neat compounds, the values of ΔHads determined in this study can be used to estimate ΔHvapeff for PAHs on atmospheric particles. The significant difference obtained between the values of ΔHvap and ΔHvapeff shows that ignoring ΔHads may result in an underestimation of the partitioning of organic species in the particle phase.

■

linearization results, DTG differential mass loss rate curves (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b03611. DSC heat flow curves for phenanthrene, SEM images of phenanthrene and anthracene, TGA thermograms and 6036


Article

The Journal of Physical Chemistry A Secondary Organic Aerosols. Geophys. Res. Lett. 2009, 36 (15), L15827. (14) Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; et al. The Formation, Properties and Impact of Secondary Organic Aerosol: Current and Emerging Issues. Atmos. Chem. Phys. 2009, 9 (14), 5155−5236. (15) Kroll, J. H.; Chan, A. W. H.; Ng, N. L.; Flagan, R. C.; Seinfeld, J. H. Reactions of Semivolatile Organics and Their Effects on Secondary Organic Aerosol Formation. Environ. Sci. Technol. 2007, 41 (10), 3545−3450. (16) Kroll, J. H.; Seinfeld, J. H. Chemistry of Secondary Organic Aerosol: Formation and Evolution of Low-Volatility Organics in the Atmosphere. Atmos. Environ. 2008, 42, 3593−3624. (17) Donahue, N. M.; Robinson, A. L.; Stanier, C. O.; Pandis, S. N. Coupled Partitioning, Dilution, and Chemical Aging of Semivolatile Organics. Environ. Sci. Technol. 2006, 40 (8), 2635−2643. (18) Epstein, S. A.; Riipinen, I.; Donahue, N. M. A Semiempirical Correlation Between Enthalpy of Vaporization and Saturation Concentration for Organic Aerosol. Environ. Sci. Technol. 2010, 44 (2), 743−748. (19) Donahue, N. M.; Robinson, A. L.; Trump, E. R.; Riipinen, I.; Kroll, J. H. Volatility and Aging of Atmospheric Organic Aerosol. Top. Curr. Chem. 2012, 339, 97−144. (20) Couvidat, F.; Kim, Y.; Sartelet, K.; Seigneur, C.; Marchand, N.; Sciare, J. Modeling Secondary Organic Aerosol in an Urban Area: Application to Paris, France. Atmos. Chem. Phys. 2013, 13 (2), 983− 996. (21) Calvert, J. G.; Atkinson, R.; Becker, K. H.; Kamens, R. M.; Seinfeld, J. H.; Wallington, T. J.; Yarwood, G. The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons, 1st ed.; Oxford Press: New York, NY, 2002. (22) Riccobono, F.; Schobesberger, S.; Scott, C. E.; Dommen, J.; Ortega, I. K.; Rondo, L.; Almeida, J.; Amorim, A.; Bianchi, F.; Breitenlechner, M.; et al. Oxidation Products of Biogenic Emissions Contribute to Nucleation of Atmospheric Particles. Science (Washington, DC, U. S.) 2014, 344 (6185), 717−721. (23) Shen, X.; Zhao, Y.; Chen, Z.; Huang, D. Heterogeneous Reactions of Volatile Organic Compounds in the Atmosphere. Atmos. Environ. 2013, 68, 297−314. (24) Shiraiwa, M.; Seinfeld, J. H. Equilibration Timescale of Atmospheric Secondary Organic Aerosol Partitioning. Geophys. Res. Lett. 2012, 39 (24), n/a. (25) Virtanen, A.; Joutsensaari, J.; Koop, T.; Kannosto, J.; Yli-Pirilä, P.; Leskinen, J.; Mäkelä, J. M.; Holopainen, J. K.; Pöschl, U.; Kulmala, M.; et al. An Amorphous Solid State of Biogenic Secondary Organic Aerosol Particles. Nature 2010, 467 (7317), 824−827. (26) Donahue, N. M.; Epstein, S. A.; Pandis, S. N.; Robinson, A. L. A Two-Dimensional Volatility Basis Set: 1. Organic-Aerosol Mixing Thermodynamics. Atmos. Chem. Phys. 2011, 11 (7), 3303−3318. (27) Donahue, N. M.; Kroll, J. H.; Pandis, S. N.; Robinson, A. L. A Two-Dimensional Volatility Basis Set − Part 2: Diagnostics of Organic-Aerosol Evolution. Atmos. Chem. Phys. 2012, 12 (2), 615− 634. (28) May, A. A.; Saleh, R.; Hennigan, C. J.; Donahue, N. M.; Robinson, A. L. Volatility of Organic Molecular Markers Used for Source Apportionment Analysis: Measurements and Implications for Atmospheric Lifetime. Environ. Sci. Technol. 2012, 46 (22), 12435− 12444. (29) Saleh, R.; Donahue, N. M.; Robinson, A. L. Time Scales for GasParticle Partitioning Equilibration of Secondary Organic Aerosol Formed from Alpha-Pinene Ozonolysis. Environ. Sci. Technol. 2013, 47 (11), 5588−5594. (30) Jathar, S. H.; Donahue, N. M.; Adams, P. J.; Robinson, A. L. Testing Secondary Organic Aerosol Models Using Smog Chamber Data for Complex Precursor Mixtures: Influence of Precursor Volatility and Molecular Structure. Atmos. Chem. Phys. 2014, 14 (11), 5771−5780.

(31) Cedeño, F. O.; Prieto, M. M.; Espina, A.; García, J. R. Fast Method for the Experimental Determination of Vaporization Enthalpies by Differential Scanning Calorimetry. J. Therm. Anal. Calorim. 2003, 73 (3), 775−781. (32) Everson, R. C.; Neomagus, H. W. J. P.; Njapha, D. Kinetic Analysis of Non-Isothermal Thermogravimetric Analyser Results Using a New Method for the Evaluation of the Temperature Integral and Multi-Heating Rates. Fuel 2006, 85 (3), 418−422. (33) Rojas, A.; Orozco, E. Measurement of the Enthalpies of Vaporization and Sublimation of Solids Aromatic Hydrocarbons by Differential Scanning Calorimetry. Thermochim. Acta 2003, 405 (1), 93−107. (34) Hillier, J.; Bezzant, T.; Fletcher, T. H. Improved Method for the Determination of Kinetic Parameters from Non-Isothermal Thermogravimetric Analysis (TGA) Data. Energy Fuels 2010, 24 (5), 2841− 2847. (35) Popescu, C.; Segal, E. Critical Considerations on the Methods for Evaluating Kinetic Parameters from Nonisothermal Experiments. Int. J. Chem. Kinet. 1998, 30 (5), 313−327. (36) Rodante, F.; Vecchio, S.; Catalani, G.; Guidotti, M. Thermal Analysis and Non-Isothermal Kinetic Study of Some Pesticides Part II. Chlorinate Derivatives. J. Therm. Anal. Calorim. 2000, 60 (2), 605− 622. (37) Vecchio, S.; Brunetti, B. Vapor Pressures, Standard Molar Enthalpies, Entropies Gibbs Energies of Sublimation and Heat Capacities of 2,5- and 3,5-Dibromobenzoic Acids. Fluid Phase Equilib. 2013, 338, 148−154. (38) Vecchio, S.; Brunetti, B. Vapor Pressures and Standard Molar Enthalpies, Entropies, and Gibbs Free Energies of Sublimation of 2,4and 3,4-Dinitrobenzoic Acids. J. Chem. Thermodyn. 2009, 41 (7), 880− 887. (39) Zhao, H.; Unhannanant, P.; Hanshaw, W.; Chickos, J. S. Enthalpies of Vaporization and Vapor Pressures of Some Deuterated Hydrocarbons. Liquid - Vapor Pressure Isotope Effects. J. Chem. Eng. Data 2008, 53 (3), 1545−1556. (40) Pena, R.; Ribet, J. P.; Maurel, J. L.; Valat, L.; Lacoulonche, F.; Chauvet, A. Sublimation and Vaporisation Processes of S(−) Efaroxan Hydrochloride. Thermochim. Acta 2003, 408 (1−2), 85−96. (41) Haynes, W. M. Handbook of Chemistry and Physics, 94th ed.; CRC Press: Boca Raton, FL, 2013. (42) Site, A. D. The Vapor Pressure of Environmentally Significant Organic Chemicals: A Review of Methods and Data at Ambient Temperature. J. Phys. Chem. Ref. Data 1997, 26 (1), 157−193. (43) Roux, M. V.; Temprado, M.; Chickos, J. S.; Nagano, Y. Critically Evaluated Thermochemical Properties of Polycyclic Aromatic Hydrocarbons. J. Phys. Chem. Ref. Data 2008, 37 (4), 1855−1996. (44) Keuleers, R. R.; Janssens, J. F.; Desseyn, H. O. Comparison of Some Methods for Activation Energy Determination of Thermal Decomposition Reactions by Thermogravimetry. Thermochim. Acta 2002, 385 (1−2), 127−142. (45) Cai, J.; Bi, L. Precision of the Coats and Redfern Method for the Determination of the Activation Energy without Neglecting the LowTemperature End. Energy Fuels 2008, 22, 2172−2174. (46) Cochran, R. E.; Dongari, N.; Jeong, H.; Beránek, J.; Haddadi, S.; Shipp, J.; Kubátová, A. Determination of Polycyclic Aromatic Hydrocarbons and Their Oxy-, Nitro-, and Hydroxy-Oxidation Products. Anal. Chim. Acta 2012, 740, 93−103. (47) Masel, R. I. Chemical Kinetics and Catalysis, 1st ed.; John Wiley & Sons, Inc.: New York, 2001. (48) Lazar, P.; Otyepková, E.; Banás,̌ P.; Fargašová, A.; Šafárǒ vá, K.; Lapčík, L.; Pechoušek, J.; Zbořil, R.; Otyepka, M. The Nature of High Surface Energy Sites in Graphene and Graphite. Carbon 2014, 73, 448−453. (49) Zhu, D.; Pignatello, J. J. Characterization of Aromatic Compound Sorptive Interactions With Black Carbon (Charcoal) Assisted by Graphite as a Model. Environ. Sci. Technol. 2005, 39 (7), 2033−2041. (50) Chakarova-Käck, S. D.; Vojvodic, A.; Kleis, J.; Hyldgaard, P.; Schröder, E. Binding of Polycyclic Aromatic Hydrocarbons and 6037


Article

The Journal of Physical Chemistry A Graphene Dimers in Density Functional Theory. New J. Phys. 2010, 12 (1), 013017. (51) Beránek, J.; Kozliak, E.; Kubátová, A. Evaluation of Sequential Solvent and Thermal Extraction Followed by Analytical Pyrolysis For Chemical Characterization of Carbonaceous Particulate Matter. J. Chromatogr. A 2013, 1279, 27−35. (52) Tran-Duc, T.; Thamwattana, N.; Cox, B. J.; Hill, J. M. Adsorption of Polycyclic Aromatic Hydrocarbons on Graphite Surfaces. Comput. Mater. Sci. 2010, 49 (4), S307−S312. (53) Kubicki, J. D.; Blake, G. A.; Apitz, S. E. Molecular Models of Benzene and Selected Polycyclic Aromatic Hydrocarbons in the Aqueous and Adsorbed States. Environ. Toxicol. Chem. 1999, 18 (8), 1656−1662. (54) Malloci, G.; Cappellini, G.; Mulas, G.; Mattoni, A. Electronic and Optical Properties of Families of Polycyclic Aromatic Hydrocarbons: A Systematic (Time-Dependent) Density Functional Theory Study. Chem. Phys. 2011, 384 (1−3), 19−27. (55) Wang, S.; Sheng, Y.; Feng, M.; Leszczynski, J.; Wang, L.; Tachikawa, H.; Yu, H. Light-Induced Cytotoxicity of 16 Polycyclic Aromatic Hydrocarbons on the US EPA Priority Pollutant List in Human Skin HaCaT Keratinocytes: Relationship between Phototoxicity and Excited State Properties. Environ. Toxicol. 2007, 22 (3), 318−327. (56) Fan, X.-L.; Wang, X.-Q.; Wang, J.-T.; Li, H.-D. Structural and Electronic Properties of Linear and Angular Polycyclic Aromatic Hydrocarbons. Phys. Lett. A 2014, 378 (20), 1379−1382. (57) Kuc, A.; Heine, T.; Seifert, G. Structural and Electronic Properties of Graphene Nanoflakes. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81 (8), 085430. (58) Chickos, J. S.; Hosseini, S.; Hesse, D. G. Determination of Vaporization Enthalpies of Simple Organic Molecules by Correlations of Changes in Gas Chromatographic Net Retention Times. Thermochim. Acta 1995, 249 (94), 41−62.

6038


An Approach to the Estimation of Adsorption ... - ACS Publications

Recommend Documents