Measurement of Vapor Pressures and Melting Properties of Five

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Measurement of Vapor Pressures and Melting Properties of Five Polybrominated Aromatic Flame Retardants Kazuko Yui,† Toshiyuki Motoki,† Humiaki Kato,† Hidetoshi Kuramochi,*,† Tomoya Tsuji,‡ Shin-ichi Sakai,§ and Frank Wania∥

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†

Center for Material Cycles and Waste Management Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-8506, Japan ‡ Shizen iKohza, Malaysia-Japan International Institute of Technology, Universiti Teknologi Malaysia, Off Jalan Sultan Yahya Petra, Kuala Lumpur, 54100, Malaysia § Environment Preservation Center, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan ∥ Department of Physical and Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, Ontario, M1C 1A4, Canada S Supporting Information *

ABSTRACT: While vapor pressure is among the most important properties used in the environmental fate assessment of organic contaminants, measured vapor pressures exist only for a few representatives of the group of novel brominated flame retardants (BFRs). To expand on this limited set of data, the vapor pressures of five BFRs1,2,4,5,tetrabromo-dimethylbenzene (TBX), 1,2,3,4,5-pentabromo-6methyl-benzene (PBT), 1,2,3,4,5-pentabromo-6-ethyl-benzene (PBEB), 2,3,4,5,6-pentabromo-phenol (PBP), and 1,3,5tribromo-2(2,3-dibromopropoxy)-benzene (TBP-DBPE) were measured in the temperature range 322.7−367.7 K with the gas saturation method. Enthalpies of sublimation or vaporization were determined from the slopes of semilogarithmic plots of measured vapor pressure against reciprocal temperature. The melting temperature and enthalpy of fusion were measured using differential scanning calorimetry. From these experimental data, the vapor pressures and subcooled liquid vapor pressures at 298.2 K, pi° and pi°,sl, respectively, were derived and compared with values estimated using group contribution methods, polyparameter linear free energy relationships, as well as EPISuite, SPARC, and COSMOtherm. Depending on the method and compound, deviations between measured and estimated log(pi°) or log(pi°,sl) values ranged from 8 to 77% of the measured values. The gas/particle partitioning behavior of the five BFRs was estimated using the measured pi°,sl values and the Junge−Pankow model. This estimation could account for the observed partitioning behavior of PBT and TBP-DBPE.

1. INTRODUCTION Brominated flame retardants (BFRs) are used in a variety of industrial products, household appliances, and transport vehicles. Although there are clearly benefits associated with the use of flame retardants, some BFRs are recognized as being ecotoxic and having characteristics of persistent organic pollutants (POPs), such as resistance to environmental degradation, long-range transport potential, and tendency to bioaccumulate.1−3 The use of certain kinds of BFRs such as some polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecanes (HBCDs) has been phased out as a result of the Stockholm Convention on POPs4 and the Restriction of Hazardous Substances (RoHS) Directive of the European Commission.5 As a result of these restrictions, novel BFRs are emerging to replace the banned substances. It is important to evaluate the fate of such novel BFRs in the environment and to identify those that have POP-like characteristics. © XXXX American Chemical Society

Estimating the environmental fate of organic pollutants requires knowledge of (subcooled liquid) vapor pressures, water solubilities, 1-octanol/water partition coefficients, and Henry’s law constants.6−8 Measured partitioning properties are rarely available for novel BFRs, and even some of the conventional BFRs, and therefore have to be frequently estimated. Currently, EPI Suite (U.S. Environmental Protection Agency)9 and SPARC (ARChem)10 are the two tools most commonly used to estimate the physical−chemical properties required for environmental fate assessment of organic pollutants. Vapor pressures and/or subcooled liquid vapor pressures can also be predicted using group-contribution Special Issue: Emerging Investigators Received: November 30, 2017 Accepted: June 6, 2018

A

DOI: 10.1021/acs.jced.7b01040 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Physical Properties and Purities of Chemicals Used in This Study chemical name (abbreviation)

CAS number

formula

molar mass (g·mol−1)

source

purity (%)

1,2,4,5,-tetrabromo-dimethyl-benzene (TBX) 1,2,3,4,5-pentabromo-6-methyl-benzene (PBT) 1,2,3,4,5-pentabromo-6-ethyl-benzene (PBEB) 2,3,4,5,6-pentabromo-phenol (PBP) 1,3,5-tribromo-2(2,3-dibromopropoxy)-benzene (TBP-DBPE)

23488-38-2 87-83-2 85-22-3 608-71-9 35109-60-5

C8H6Br4 C7H3Br5 C8H5Br5 C6HBr5O C9H7Br5O

421.7 486.6 500.6 488.6 530.7

Apollo Scientific Tokyo Chemical Industry Sigma-Aldrich Sigma-Aldrich AccuStandard

98.9b >99.5a 99.8b 100.0b 100.0a

a

Purities were provided by suppliers and were determined via GC/MS. bPurities were provided by suppliers and were determined via HPLC.

methods,11,12 polyparameter linear free energy relationships (ppLFER),13,14 and software such as COSMOtherm, which is based on quantum chemistry and statistical thermodynamics.15 However, without an adequate database of measured properties, it is not clear how accurate these prediction methods are for novel BFRs. Experimentally, subcooled liquid vapor pressures are typically obtained using a method that correlates gas chromatographic retention times (GC-RT) with gas−liquid partitioning properties. It requires experimental vapor pressure data of a reference compound in order to give reliable results. The Knudsen effusion and the gas saturation methods are both frequently used to obtain solid- or liquid-state vapor pressures. The two methods determine the vapor pressures directly, by measuring the amount of a substance in the gas phase (gas saturation method) or the rate of effusion through a hole of a cell that correlates with vapor pressure (Knudsen effusion method), respectively. Subcooled liquid vapor pressures can also be calculated from vapor pressure data and melting properties. So far, the GC-RT method has been used to determine subcooled liquid vapor pressures for PBDEs16−18 and hexabromobenzene (HBB),18 whereas the vapor pressures of PBDEs,19−21 HBCDs,20 HBB,21 tetrabromobisphenol A (TBBPA),20,22 and 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE)22 have been measured using either the Knudsen effusion19,20,22 or the gas saturation method.21 As for other brominated aromatic compounds, the vapor pressures of polybrominated dibenzo-p-dioxins (PBDDs),23 derivatives of BFRs,24 and brominated aromatic compounds with fewer Br substituents25−32 have also been measured using the Knudsen effusion method,23,26−30,32 gas saturation method,24,31 or other static methods using a manometer.25,32 Besides vapor pressures, water solubilities, 1-octanol/water partition coefficients, and Henry’s law constants of BFRs have also been measured or estimated, and we have previously reported experimentally determined properties other than vapor pressure for TBBPA,22 brominated phenols,33 brominated benzenes,34 and PBDEs.35 While experimentally measured properties are typically preferred in environmental fate assessments, it is not practical to measure the properties of all compounds of interest. We have previously evaluated the overall persistence (Pov) and long-range transport potential (LRTP) of 52 novel and conventional non-PBDE BFRs using estimated physical− chemical properties.36 A primary purpose of that study was to identify priority compounds whose properties should be measured first. Six BFRs had POP-like characteristics: 1,2,4,5tetrabromo-3,6-dimethylbenzene (TBX), 1,2,3,4,5-pentabromo-6-methylbenzene (PBT), 1,2,3,4,5,6-hexabromobenzene (HBB), 2,3,4,5-tetrabromo-6-chlorotoluene (TBCT), 2,3,4,5,6-pentabromobenzyl chloride (PBBC), and 2,3,4,5,6pentabromobenzyl bromide (PBBB). Other research groups similarly identified TBX,38,39 PBT,37−39 HBB,37−39 2,3,4,5,6-

pentabromoethylbenzene (PBEB),39 1,2,3,4,5-pentabromobenzene (PBBz),37 PBBB, and PBBC as having POP-like properties. In field-based studies, TBX, PBT, HBB, PBEB, PBBz, and 2,4,6-tribromophenyl 2,3-dibromopropyl ether (TBP-DBPE, also referred to as DPTE) as well as other novel and conventional BFRs have been detected in the environment,40−47 suggesting that they are actually being released into the environment. Accurate physical−chemical properties for the identified compounds are essential for more refined environmental fate assessments. In this study, the vapor pressures of five non-PBDE BFRs including TBX, PBT, PBEB, 2,3,4,5,6-pentabromophenol (PBP), and TBP-DBPE were measured in the temperature range 322.7−367.7 K using the gas saturation method. We selected these compounds because they have a single aromatic ring and were expected to have relatively high vapor pressures, which means they would be more likely to be emitted to the atmosphere than other novel BFRs. We employed the gas saturation method because it is suitable for measuring vapor pressures within the order of magnitude expected for these substances and also because we have experience generating reliable data with this method.21 The enthalpy of sublimation or vaporization and the vapor pressure at 298.2 K (standard temperature) were determined for each substance by fitting the Clausius−Clapeyron equation to the measured vapor pressures. The melting point and enthalpy of fusion for each compound were also measured using differential scanning calorimetry (DSC). This in turn allowed for the calculation of subcooled liquid vapor pressures. The liquid-state or subcooled liquid-state vapor pressure (pi°,sl) at 298.2 K (standard temperature) is used in some environmental fate estimations. The new vapor pressure data were used to evaluate the predictive performance of a number of existing methods for vapor pressure estimation for our compounds. Using the subcooled liquid vapor pressure data, we also calculated gas/ particle partitioning ratios of the BFRs and compared them with environmental observations in order to assess whether the measured property data can be used to reproduce the observed atmospheric behavior of BFRs.

2. MATERIALS AND METHODS 2.1. Chemicals. Table 1 lists details about the chemicals used in this study. Five BFRs of reagent grade or higher purity were purchased from commercial suppliers and used without further purifications. The abbreviations are based on the nomenclature advocated by Bergman et al.48 2.2. Vapor Pressure Measurements. Vapor pressures were measured using the gas saturation method.49 The experimental procedure was similar to that of an earlier study.21 The apparatus was comprised of a pure nitrogen gas supply (compressed gas cylinder), a gas preheating coil (2.2 mm [i.d.] × 3.0 m), a generator column (2.2 mm [i.d.] × 60 cm) filled with glass beads (Chromosorb-W/AW, 60/80 mesh, B



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Table 2. Vapor Pressures (pi) of Five Aromatic Brominated Flame Retardants at Various Temperatures (T)a TBX T (K) 322.8 327.7 332.7 337.5 342.5 347.5 352.4 357.5 362.4 367.4

PBT

pi (Pa)

T (K)

± ± ± ± ± ± ± ± ± ±

322.9 327.8 332.9 337.6 342.7 347.6 352.6 357.6 362.5 367.6

0.0024 0.0042 0.0072 0.0125 0.021 0.036 0.059 0.10 0.16 0.25

0.0003 0.0002 0.0004 0.0003 0.001 0.006 0.007 0.01 0.02 0.01

PBEB

pi (Pa)

T (K)

± ± ± ± ± ± ± ± ± ±

322.7 327.6 332.6 337.5 342.4 347.4 352.3 357.4 362.3 367.3

0.00027 0.00051 0.00091 0.00162 0.0029 0.0055 0.0090 0.015 0.027 0.042

0.00004 0.00005 0.00010 0.00029 0.0004 0.0005 0.0003 0.002 0.002 0.004

pi (Pa)

T (K)

± ± ± ± ± ± ± ± ± ±

322.9 327.8 332.8 337.7 342.6 347.6 352.6 357.6 362.6 367.4

0.0037 0.0061 0.0104 0.017 0.032 0.049 0.083 0.13 0.21 0.32

TBP-DBPE (liq)b

PBP 0.0005 0.0011 0.0006 0.004 0.005 0.009 0.013 0.01 0.01 0.04

pi (Pa)

T (K)

± ± ± ± ± ± ± ± ± ±

322.9 327.8 332.7 337.7 342.7 347.7 352.7 357.7 362.7 367.7

0.00019 0.00036 0.00067 0.0012 0.0023 0.0042 0.0078 0.014 0.027 0.045

0.00002 0.00003 0.00005 0.0001 0.0002 0.0003 0.0005 0.002 0.002 0.004

pi (Pa) 0.0019 0.0032 0.0055 0.0096 0.017 0.027 0.043 0.067 0.13 0.16

± ± ± ± ± ± ± ± ± ±

0.0002 0.0004 0.0004 0.0012 0.002 0.001 0.005 0.007 0.01 0.02

The standard uncertainties are u(T) = 0.04 K, and unbiased estimations of the standard deviation of vapor pressures are shown after each ± sign in the pi column. bTBP-DBPE was a liquid in the temperature range of the measurement. a

The standard uncertainty of the oven temperature, u(T), was 0.08 K,21 and the actual fluctuations of the oven temperature in this study were at most 0.04 K. The thermometer had been calibrated by the supplier; and the systematic deviations were smaller than 0.01 K. The vapor pressure measurements were repeated three to six times for each condition. 2.3. Melting Properties Measurements. Melting properties were measured using differential scanning calorimetry (DSC). In each measurement, 2 mg of sample was placed in an aluminum pan with a lid. The lid was sealed, and the pan was introduced into a DSC analyzer (DSC200 F3Maia, NETZSCH, Japan). Scanning was performed from 273.2 K (for TBP-DBPE) or 303.2 K (for other compounds) to a temperature at least 10 K above the melting temperature of each compound at a heating rate of 1.0 K·min−1 (except for TBX) or 3.0 K·min−1 (in the case of TBX) in a stream of N2 gas (20 mL·min−1). The melting temperature (Tm) and heat of fusion (ΔfusH) were determined from the onset temperature and the area of the melting peak of each measurement, respectively. The temperature and heat flow were calibrated using standard materials, in accord with the methods used in a previous study.51 The measurements were performed five times for each substance. 2.4. Property Prediction. Vapor pressures (pi°) at the standard state were estimated using EPISuite (version 4.11),9 SPARC,10 and group contribution methods by Myrdal and Yalkowsky (MY).11 Subcooled liquid vapor pressures (pi°,sl) at the standard temperature (298.2 K) were calculated using EPISuite,9 SPARC,10 COSMOtherm (version C30_1401, COSMOlogic GmbH & Co. KG, Germany, 2014)15 with optimization of molecular geometry by TURBOMOLE at the BP_TZVP_C30_1401 level (version 6.6, 2014), two pp-LFER models by Quina et al. (QCC)13 and Bel’skii (BS),14 and two group contribution methods by Myrdal and Yalkowsky (MY)11 and Nannoolal et al. (Nan).12 In the calculations of subcooled liquid vapor pressures using SPARC, melting points were set as “not assumed”. Solute descriptors of the BFRs for the pp-LFER calculations were obtained using either Absolv (ACD/Labs)52 or an online estimation tool provided by the Helmholtz Centre for Environmental Research UFZ (Leipzig, Germany).53 Normal boiling points (Tb) required for the calculations of the group contribution methods were estimated via Joback− Reid’s method (JR)54 or Nannoolal’s method (Nan).55 The melting temperatures (Tfus) used in the calculations of the MY model were estimated using Joback−Reid’s method54 or the values measured in this study.

GL Sciences, Japan) coated with the target compound (BFR), a mini-column (4.0 mm [i.d.] × 10 cm), a sample extraction cartridge (Sep-Pak PS-Air cartridge, Waters Corporation, USA), an impinger, and an integrating flowmeter, all connected in series. The preheating coil and generator column were placed inside an oven (GC4000, GL Sciences, Japan), whose temperature was monitored with a quartz thermometer (DMT-624, Tokyo Denpa Co., Ltd. Japan). In a typical measurement, glass beads were soaked in the solution of a target compound in toluene (>99.5% purity, Kanto Chemical, Japan). The toluene was removed using a rotary evaporator and nitrogen gas purge, and approximately 0.7 g of the beads coated with the compound was packed into the generator column. The loaded generator column contained approximately 14 mg of a compound (2 mass % of the beads). Nitrogen gas was heated to the oven temperature in the preheating coil and then introduced into the generator column at a constant flow rate of 40 cm3 (STP)·min−1. The outlet gas from the generator column was introduced into a mini-column and an extraction cartridge, where it was cooled to room temperature. The amount of the target compound precipitated in the mini-column and trapped in the cartridge, Q (in units of g), was determined by solvent extraction followed by gas chromatography/mass spectrometry (GC/MS, 7890A/5975C, Agilent Technologies, USA). Toluene (>99.5% purity, Kanto Chemical, Japan) was used for extraction and GC/MS analysis. The concentration of the target compound in the extract was quantified according to a previously reported methodology.50 Finally, the vapor pressure (pi, in Pa) was determined using eq 1 pi =

Q RT V Mw

(1)

where V is the total volume of nitrogen gas that passed through the generator column (m3), Mw is the molar mass of the target compound (g·mol−1), R is the gas constant (8.314 J·mol−1· K−1), and T is the oven temperature (K). The sampling time, tsampling, was set to 100−1300 min, depending on the value of the vapor pressure. The volume V was calculated using eq 2 V = ν·tsampling(T /TSTP)

(2)

where v is the volumetric flow rate of the gas and TSTP is the temperature of the flowmeter (K), which was maintained at 298 K. In the actual measurements, the total volume of the gas, v·tsampling (m3), was measured using an integrating flowmeter. C



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Table 3. Physicochemical Properties of Five Aromatic Brominated Flame Retardantsa TBX

PBT

PBEB

Ttr (K)

PBP

TBP-DBPE

Tfus (K)

527.4

559.0

408.9

ΔfusH (kJ·mol−1)

23.9 ± 3.0

29.3 ± 1.1

13.4 ± 0.3

ΔsubH (kJ·mol−1) ΔvapH (kJ·mol−1) pi° (Pa) pi°,sl (Pa) ϕJP (−)

103.5 ± 0.5

112.4 ± 0.7

100.0 ± 0.8

416.2 441.5b 11.8 ± 0.3 11.29b 496.3 502b 469.8c 18.5 ± 0.5 19.14 ± 0.5b 27.6c 121.8 ± 1.2

(9.6 ± 2.6) × 10−5 (6.3 ± 1.7) × 10−3 0.0041 ± 0.0011

(8.2 ± 4.4) × 10−6 (2.1 ± 1.1) × 10−3 0.012 ± 0.0065

(1.6 ± 0.9) × 10−4 (7.1 ± 4.1) × 10−4 0.035 ± 0.019

(4.1 ± 3.7) × 10−6 (3.1 ± 2.8) × 10−4 [0.077 ± 0.065]

ΔtrH (kJ·mol−1)

307.9 17.1 ± 1.1 313.7

2.56 ± 0.56

99.7 ± 1.4 (6.7 ± 9.5) × 10−5 (8.7 ± 9.0) × 10−5 0.23 ± 0.18

a ΔfusH, enthalpy of fusion; Tfus, temperature of fusion (=melting point); ΔtrH, enthalpy of solid-state phase change; Ttr, temperature of solid-state phase change; pi°, vapor pressure at 298.2 K; pi°,sl, subcooled liquid vapor pressure at 298.2 K; ΔsubH, enthalpy of sublimation; ΔvapH, enthalpy of vaporization; ϕJP, Junge−Pankow gas-particle partitioning ratio. The melting properties and solid-state phase-change properties were measured at atmospheric pressure (p) and standard uncertainty of the pressure, u(p) = 5 kPa. The standard uncertainties for melting and phase-change temperatures are u(Tfus) = 0.3 K and u(Ttr) = 0.2 K. The standard uncertainties in ΔfusH and ΔtrH are shown after the ± sign. The standard uncertainties of ΔsubH or ΔvapH were obtained from the standard deviations of the vapor pressure data. The standard uncertainties of ϕJP were calculated from the uncertainties of the pi°,sl values, and the standard uncertainties for pi°,sl were calculated from the uncertainties of the vapor pressure and temperature using the Gauss error propagation law. ϕJP for PBP was indicated in a square bracket because the JP model may not be applicable to the substances that can ionize in aqueous solution. bReference 59. cReference 33.

BFRs, together with the literature values for PBP.33,59 The DSC curves of each substance were shown in the Supporting Information. The standard uncertainties in this study were u(Tfus) = 0.3 K and u(ΔfusH) = 0.6 kJ·mol−1, respectively. Relative deviations between measured and literature values for PBP were within 5% (27 K) for Tfus but up to 50% for ΔfusH. The large difference in ΔfusH can probably be attributed to the difference in the state of the PBP used in the two studies. The purity of PBP used here was almost 100%, whereas that of the PBP used in our earlier study33 was only 96%, because a higher purity sample was not available at that time. In another earlier study, we had confirmed that the DSC system (which is exactly the same as the one used here) provides ΔfusH values within at most 10% deviations from other literature data.51 The order of the melting temperatures was PBT > TBX > PBP > PBEB > TBP-DBPE. Among the five BFRs, PBP and TBP-DBPE had an additional peak, respectively, and we supposed that these peaks were due to a solid-state phase transition. However, for TBP-DBPE, the two phase-change temperatures were so close that they might be the melting of two different polymorphs of that compound. For further understanding, some other studies such as temperaturecontrolled powder X-ray diffractions will be useful. Anyway, because the melting temperature of TBP-DBPE was lower than the temperature range of the vapor pressure measurement, we measured the vapor pressure of liquid TBP-DBPE. And because the temperature of the lower phase-change peak of PBP was higher than the temperature range of the vapor pressure measurements, the vapor pressures of PBP were all obtained against the same solid state, and that solid state was also stable at room temperature. The other BFRs were solids at the temperatures of the vapor pressure measurements as well as at room temperature. 3.3. Solid and Subcooled Liquid Vapor Pressures at 298.2 K. Figure 1 shows the temperature dependence of the vapor pressures measured in this study. In all plots, the

2.5. Gas/Particle Partitioning Ratios. Particle-to-gas partitioning ratios (ϕJP) were estimated using the Junge− Pankow model, which only requires subcooled liquid vapor pressure as a compound-specific parameter:56−58 ϕJP = cθ /(pi° ,sl + cθ )

(3)

The parameter c characterizes sorption and has been proposed to equal 17.2 Pa·cm for urban air,56 and the particle surface area per unit volume of air (θ) is 1.5 × 10−6 cm2aerosol/cm3-air for average background air.58 The subcooled liquid vapor pressures were calculated from the measured vapor pressure data and the melting data as described below. This model does not account for the possibility of ionic dissociation in aqueous aerosols, so for a compound that can ionize in aqueous solution, this model may not be applicable.

3. RESULTS AND DISCUSSION 3.1. Vapor Pressures. Table 2 lists the measured vapor pressures of the five BFRs. The standard uncertainties in Table 2 are the unbiased estimates of the standard deviations of the vapor pressures and were about 1−10% of the corresponding vapor pressures. These uncertainties were very similar to those determined in our previous study (2−6% of the measured values).21 The order of the vapor pressures was PBEB ≈ TBX ≈ TBPDBPE > PBT ≈ PBP. Together with our previously reported value for hexabromobenzene (HBB),21 the sequence of the vapor pressures of the substituted benzenes was TBX (4 Br) > PBT (5 Br) ≈ PBP (5 Br) > HBB (6 Br). TBX had the highest vapor pressure, and the vapor pressure decreased by about 1 order of magnitude if OH or CH3 was replaced with a Br atom. This is similar to what has been observed for other BFRs.17,18,20,26,28 3.2. Melting Properties. Table 3 summarizes the measured melting and phase-change properties of the five D



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subcooled liquid vapor pressure at standard temperature (pi°,sl). For the other four compounds, subcooled liquid vapor pressures were calculated from the solid-state vapor pressures at standard temperature (pi°) as follows: ln F =

ΔfusH ij T y jj1 − fus zzz RTfus k T° {

(5)

F = pi° /pi° ,sl

(6)

Here, F is the fugacity ratio of the solid to the hypothetical subcooled liquid at 298.2 K. If there is more than one solidstate phase change between the highest temperature of the vapor pressure measurement and the melting point, as is the case for PBP, eq 5 should be replaced by eq 7

Figure 1. Vapor pressures (pi) of five aromatic polybrominated flame retardants against the reciprocal temperatures (1/T) in a semilogarithmic plot. ○, TBX; △, PBT; ▽, PBEB; □, PBP; ◇, TBPDBPE.

ln F =

logarithm of vapor pressure relates linearly with the reciprocal of temperature. From the slope of a plot, the enthalpy of sublimation (ΔsubH) or enthalpy of vaporization (ΔvapH) was determined using the Clausius−Clapeyron equation (eq 4)

ΔfusH ij T y jj1 − fus zzz + RTfus k T° {

i Δtr H i y jj1 − Ttr yzzzzz j zzz T ° {{ k RTtr k

∑ jjjjj tr

(7)

where Ttr and ΔtrH are the temperature and enthalpy, respectively, of the phase change. In the case of TBP-DBPE, we converted the value for the subcooled liquid pi°,sl (obtained as described above) into the solid-state vapor pressure at the standard temperature using eqs 6 and 7, and we denoted that value as pi°. Table 3 lists the pi°,sl and pi° values for the five BFRs. The pi° values ranged from 4.1 × 10−6 to 1.6 × 10−4 Pa. These are similar in magnitude to the vapor pressures of PBDEs with four to six bromines.21 The pi°,sl values are also of a similar order of magnitudes as those for the PBDEs with four to six bromines.21

ij p yz Δ Hi1 1 yz zz lnjjjj i zzzz = − sub jjj − j p° z R kT T° { (4) k i { where T° and pi° are the temperature at any reference state and the vapor pressure at T°, respectively. In this study, we set T° to the standard temperature (298.2 K). Table 3 lists the values of ΔsubH and pi° determined from eq 4 for TBX, PBT, PBEB, and PBP. For TBP-DBPE, because the vapor pressures were measured for the liquid, eq 4 provides values of ΔvapH and the

Table 4. Comparison of Measured and Estimated Vapor Pressures at 298.2 Ka methods (log(pi) − Tb(−Tfus))

TBX

measured EPISuitec SPARC-EPISuite (Tfus)d SPARC-measured (Tfus) MY-JR (Tb) - JR (Tfus) MY-JR (Tb) - measured (Tfus) MY-Nan (Tb) - JR (Tfus) MY-Nan (Tb) - measured (Tfus)

−4.02 −2.26 −3.63 −4.09 −5.02 −5.17 −3.38 −3.53

measured EPISuite SPARC COSMOtherm MY-JR (Tb) MY-Nan (Tb) Nan-Nan (Tb) QCC (Absolv)e QCC (UFZ)f BS (Absolv)e BS (UFZ)f

−2.20 −1.33 −3.22 −3.23 −3.36 −1.72 −1.47 −2.23 −1.79 −1.99 −1.47

PBT

PBEB

log10(pi°/Pa) −5.08 −3.78 −4.71 −3.21 −4.41 −4.72 −4.92 −4.71 −6.72 −7.93 −6.74 −6.22 −4.78 −5.92 −4.80 −4.21 log10(pi°,sl/Pa) −2.69 −3.15 −1.79 −2.07 −3.95 −4.22 −3.93 −2.21 −4.42 −5.03 −2.48 −3.03 −2.07 −2.51 −3.03 −3.52 −2.93 −3.30 −2.76 −3.30 −2.60 −2.99

PBP

TBP-DBPE

AARD%b

−5.39 −5.17 −4.68 −4.92 −9.07 −7.74 −6.60 −5.28

−4.18 −4.08 −5.67 −5.18 −10.3 −6.49 −7.56 −3.70

15 19 12 77 45 36 9

−3.51 −2.97 −4.12 −2.61 −5.98 −3.51 −3.59 −2.88 −3.49 −2.60 −3.25

−4.06 −2.80 −5.10 −4.59 −6.24 −3.46 −3.49 −4.83 −4.25 −4.72 −4.08

31 34 32 60 10 19 13 8 12 10

a Abbreviations and references for the models: EPISuite (ref 9); SPARC (ref 10); MY, Myrdal and Yalkowsky (ref 11); Nan, Nannoolal et al. (ref 12); QCC, Quina et al. (ref 13); BS, Bel’skii (ref 14); COSMOtherm (ref 15); JR(Tb) and JR(Tfus), Joback−Reid (ref 54); Nan(Tb), Nannoolal et

al. (ref 55). bAARD% =

1 5

∑

|log(Pmeasured / Pa) − log(Pestimated / Pa)| log(Pmeasured / Pa) e

× 100 cEstimates taken from our previous study (ref 36). dTfus were estimated by

EPISuite (weighted mean value). Solute descriptors were estimated using Absolv (ACD/Labs). fSolute descriptors were estimated using the UFZLSER database. E



Article

3.5. Gas/Particle Partitioning. The particle-to-gas partitioning ratio indicates the fraction of the total compound in air that is adsorbed on atmospheric particles. The ϕJP values calculated from eq 3 are given in the last row of Table 3. Among the five BFRs, TBX, PBT, and PBEB had ϕJP values smaller than 0.1 and are predicted to be present mostly as vapors in air, whereas a notable fraction of TBP-DBPE can be expected to sorb to atmospheric particles. In the case of PBP, the ϕJP value is indicated in square brackets in Table 3 because the Junge−Pankow model may not be applicable to substances that ionize in aqueous solutions. While the calculated ϕJP value of PBP was smaller than 0.1, it may not be present as vapor as much as expected from this value: With a pKa of 4.62,60 PBP would be mostly ionic in aqueous aerosol (except under extremely acidic conditions). In order to evaluate whether our pi°,sl values can be used to reproduce the observed environmental partitioning behavior of the studied BFRs, we compared ϕJP estimated with the Junge− Pankow model with measured distributions reported in the literature. The predicted distribution patterns seemed consistent with the reported behavior: the dust/air partitioning coefficient of PBT calculated from the measured concentrations in dust and air is equal to or smaller than that of TBPDBPE.61

4. CONCLUSIONS The vapor pressures and melting properties for five aromatic BFRs were determined experimentally. From the experimental data, the vapor pressures and subcooled liquid vapor pressures at a standard temperature of 298.2 K were calculated. The vapor pressures of the five BFRs at 298.2 K were similar in magnitude to those of the PBDEs with four to six bromines, which are classified as POPs. The newly obatined vapor pressure data were compared with estimated values. The estimates by EPISuite and SPARC were close to the measured solid vapor pressures at 298.2 K, whereas ppLFERs and group contribution methods provided good estimates of the subcooled liquid vapor pressures at 298.2 K. For a comprehensive evaluation of the performance of these prediction methods, measured and estimated values for a broader range of compounds will need to be compared, which is beyond the scope of the present study. The gas/particle partitioning ratios of the BFRs were estimated using the Junge−Pankow model, and TBX, PBT, and PBEB were estimated to be largely gas phase compounds. This is consistent with the observed indoor behavior of those compounds as reported in the literature and suggests that the environmental fate of semivolatile compounds can be estimated using experimentally determined vapor pressure data.

Figure 2. Comparison between the measured and estimated vapor pressures (a) and subcooled liquid vapor pressures (b) at 298.2 K. In part a: ■, EPISuite; ●, SPARC with Tfus by EPISuite; ○, SPARC with Tfus from this study; ◇, MY-JR-JR; △, MY-JR-Nan; ▽, MY-Nan-JR; +, MY-Nan-Tfus from this study. In part b: ■, EPISuite; ●, SPARC; ◇, COSMOtherm; △, MY-Nan; ▽, Nan-Nan; +, QCC-UFZ; ×, BSUFZ. The lines correspond to perfect estimation.

3.4. Evaluation of Prediction Methods for Solid and Subcooled Liquid Vapor Pressures at 298.2 K. In Table 4, we compared the logarithmic values of the measured and estimated vapor pressures and subcooled liquid vapor pressures at 298.2 K. In this table, AARD refers to the average absolute relative deviation. Figure 2 plots the estimated vapor pressures and subcooled liquid vapor pressures at 298.2 K against the measured values. As far as the five BFRs are concerned, EPI-Suite (AARD < 15%) and SPARC (AARD < 19%) gave the best estimates of pi° values among the estimation methods that did not rely on experimental melting temperatures. By using the measured melting temperatures, the predictive performance of the MY models improved greatly. Among the methods estimating the subcooled liquid vapor pressure, the group contribution methods using Nannoolal’s Tb (AARD < 19%) and the two ppLFER models (13%) performed better than EPISuite (31%) and SPARC (34%). For our five BFRs, EPI-Suite tended to overestimate the measured subcooled liquid vapor pressures, whereas SPARC underestimated them, and this is the same tendency as noted previously for selected PBDEs and other non-BFR aromatic compounds.37

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b01040.

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DSC thermograms of TBX, PBT, PBEB, PBP, and TBPDBPE (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F



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ORCID

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Kazuko Yui: 0000-0001-7588-403X Hidetoshi Kuramochi: 0000-0003-2992-8358 Frank Wania: 0000-0003-3836-0901 Funding

This work was supported by the Environment Research and Technology Development Fund (3K163005) of the Environmental Restoration and Conservation Agency. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Mr. Tiange Yuan (University of Toronto Scarborough) for supporting the calculation of the vapor pressures using COSMOtherm.

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Measurement of Vapor Pressures and Melting Properties of Five

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