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doi:10.1016/j.at
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(M.F. Costa G
Atmospheric Environment 42 (2008) 4724–4734
www.elsevier.com/locate/atmosenv
Atmosphere/water partition of halocyclohexanes from vapourpressure and solubility data
Sabine Sarrautea, Ilham Mokbelb, Margarida F. Costa Gomesa,�,Vladimir Majera, Jacques Joseb
aLaboratoire de Thermodynamique des Solutions et des Polymeres, UMR 6003, Universite Blaise Pascal, Clermont-Ferrand/CNRS,
63177 Aubiere, FrancebLaboratoire des Sciences Analytiques, UMR 5180, Universite Claude Bernard/CNRS, 69622 Villeurbanne, France
Received 20 November 2007; received in revised form 23 January 2008; accepted 24 January 2008
Abstract
Original values of the atmosphere/water partition coefficients of chlorocyclohexane and bromocyclohexane are reported
at temperatures between 0 and 40 1C. These values could be obtained from the Henry’s law constants calculated from
aqueous solubility and vapour pressure data for the two solutes. Both properties were measured in this work and are
reported in the temperature ranges 1 to 40 1C and �10 to 100 1C, respectively. Chlorocyclohexane is more volatile and
more soluble in water than bromocyclohexane. At 25 1C, chlorocyclohexane has a vapour pressure of 1077.7 kPa and a
mole fraction aqueous solubility of 3.22� 10�5. Bromocyclohexane has a vapour pressure of 409.5 Pa and a mole fraction
solubility of 0.781� 10�5 at the same temperature. These values lead to atmosphere/water partition coefficients of 24.4 and
38.2 for chlorocyclohexane and bromocyclohexane, respectively. A simple group contribution scheme is presented that
makes use of the present values and of literature data to allow the prediction of Henry’s law constants, with reasonable
accuracy, of halogenated-substituted organic compounds at 25 1C.
r 2008 Elsevier Ltd. All rights reserved.
Keywords: Chlorocyclohexane; Bromocyclohexane; Aqueous solubility; Vapour pressure; Henry’s law constant; Atmosphere/water
partition coefficient
1. Introduction
The knowledge of the partition of pollutantsbetween the atmospheric and aquatic systems, rainwater or aerosols is essential to study their transportand fate in natural ecosystems. Aqueous solubilitydata, when combined with the vapour pressure ofthe pure solute, allows the calculation of the
e front matter r 2008 Elsevier Ltd. All rights reserved
mosenv.2008.01.041
ing author. Fax: +33 473 407 185.
ess: [email protected]
omes).
Henry’s law constant and from there, direct accessto the air/water partition coefficient. Informationabout this partition coefficient is capital to under-stand the mechanisms that govern the fate ofpollutants in the atmosphere and to decide whetherthey are transferred and/or transported to othercompartments or transformed by photochemicalprocesses.
In addition to its interest for environmentalstudies, the knowledge of the Henry’s law constantis also extensively used in chemical engineering andgeochemistry for designing or describing processes
.
ARTICLE IN PRESSS. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–4734 4725
where dilute aqueous systems are involved. Henry’slaw constant is also related to the thermodynamicproperties that characterize the solubilization pro-cess and so contribute to the understanding of theinteractions and structure of aqueous solutionscontaining organic chemicals.
The use of the Henry’s law constant by differentscientific communities led to the establishment ofmultiple and alternative, sometimes confusing, defini-tions of this quantity. Henry’s law constant wasoriginally proposed more than 200 years ago as ameasure of gas solubility in a liquid, and expressed asa ratio of the partial pressure of a gaseous solute to itsequilibrium concentration in the liquid phase. Today,the perception and use of the Henry’s law constant ismuch broader as, from a physico-chemical point ofview, it is basically a coefficient relating the fugacity ofa dissolved non-electrolyte to its concentration in asolution. The thermodynamic essence behind theHenry’s law constant is often misunderstood ormisinterpreted. We have calculated, in the presentwork, the Henry’s law constants of the organiccompounds in water from the measured solubilitiesand vapour pressures of the pure solute using thecorrect thermodynamic definition and considering anumber of valid approximations (Sarraute et al.,2004). From the Henry’s law constants, otherimportant coefficients like the atmosphere/waterpartition coefficient are also calculated.
Halogenated compounds are an important class ofchemicals, mainly of anthropomorphic origin, whichare widely distributed in the different compartmentsof the environment (aqueous, atmospheric, bio-sphere, etc.). Several halogenated and polyhaloge-nated aliphatic and aromatic hydrocarbons areamong the principal types of known chemicalcarcinogens (Samiullah, 1990), and so it is importantto correctly predict their fate in the environment. Inthis work, we determine experimentally the aqueoussolubility and vapour pressure of two halogenatedcycloalkanes and from there we assess their atmo-sphere/water partition. We propose an empiricalgroup contribution scheme to rationalize and quan-tify the effect of halogen substitution on the atmo-sphere/water partition of alkanes and cycloalkanes.
2. Experimental
2.1. Materials
Chlorocyclohexane and bromocyclohexane wereobtained from Acros Organics, and the purity was,
respectively, at least 98% and 99%. Distilled waterand methanol for high-performance liquid chroma-tography, from Sigma-Aldrich, were also used assolvents.
2.2. Vapour pressure measurements
The apparatus for the vapour pressure determi-nation is a static device allowing reliable measure-ments within a large pressure interval ranging from1Pa up to 200 kPa. A description of the apparatusand the experimental procedure can be foundelsewhere (Kasehgari et al., 1993; Mokbel et al.,1995; Ruzicka et al., 1998), so only the mostpertinent information will be given here.
The measurement cell is constituted of two parts:the lower one is the sample cell immersed inside athermostatic bath while the upper part, including acondensation coil, is connected permanently to thesystem allowing degassing and pressure determina-tion (see the simplified scheme in Fig. 1). Whendegassing, the cell is heated and the coil is traversedby cold water so as to minimize losses of the sample.Air and volatile impurities, which are the principalsource of error in measurements of low vapourpressures, are eliminated during the degassing stage.
During this campaign of measurements, the staticapparatus (Sawaya et al., 2006) was equipped with adifferential manometer Baraton 616 A from MKS.The new pressure gauge was calibrated against a U-manometer filled with mercury or Apiezon oildepending on the pressure range. The reading ofthe levels was carried out using a cathetometer withan uncertainty of 71 mm, as claimed by themanufacturer. The calibration curve obtained islinear in agreement with the indications from thesupplier.
In addition, the calibration of the MKS pressuregauge was also verified by measuring the vapourpressures above the solid and liquid water in theranges between 13 and 280 Pa (233.7oT/Ko264.1)and 1250–70,000 Pa (283.4oT/Ko363.1), respec-tively. The mean relative deviations between ourexperimental data and the literature values are0.75% for ice (Fisher, 1988) and 0.50% for liquidwater (Haar et al., 1984). Similar measurementswere performed with dodecane in the pressure rangebetween 5 and 1157 Pa (283.3oT/Ko361.7), theyare in good agreement with the values calculatedfrom the Cox equation representing the recom-mended data presented by Ruzicka and Majer(1994), the mean relative deviation being 2.3%.
ARTICLE IN PRESS
SC
TP
SC
TP
P'
methanol
water
t
Fig. 2. Schematic diagram of the saturation column apparatus.
(a) Simplified version where P is the LLC pump delivering water;
SC, the saturation column; and T indicates the liquid thermostat.
(b) A second pump is used. P or P0 are LLC pump delivering
water or methanol, respectively; SC is the saturation column; T,
the liquid thermostat; and t, a fitting allowing the mixture of the
aqueous solution with methanol.
valve1
MKS differential pressure gauge
High vacuum
Vent
valve3
valve2
Knife-edge flanges with copper gasketCondensation Coil
Union nut
Sample reservoir (10cc) Thermocouple pocket Magnetic stirring
Fig. 1. Schematic diagram of the static apparatus used for the vapour pressure measurements.
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–47344726
Temperature was detected with a copper–con-stantan thermocouple calibrated against a 25Oplatinum resistance standard thermometer (accu-racy of 70.001K claimed by the manufacturer inthe IPTS 90 temperature scale) with a Leeds andNorthrup bridge (710�4O). As the uncertainty ofthe voltmeter for the thermocouple reading is0.1 mV, the precision of the temperature readings isabout 0.01 1C, the overall uncertainty on themeasured temperatures is estimated to be 70.02 1C.
Considering the technical specifications reportedby the supplier of the pressure gauge as well as theresults of the tests described above, it is estimatedthat the uncertainties in pressure determination are3% for the range 1pp/Pap1000 and 0.5% at higherpressures.
2.3. Solubility measurements
The experimental technique used in this work isbased on a dynamic method first described by Millerand Hawthorne (1998) for measurements of theaqueous solubility of organic compounds at rela-tively high temperatures. The present apparatus is asimplified version of the one previously described(Dohanyosova et al., 2004; Sarraute et al., 2004,2006). As the solubility of the compounds studied inthis work was expected to be higher than thosepreviously measured, the extraction column was nolonger necessary in the experimental arrangement.The new version of the apparatus used here isdepicted in Fig. 2a. The saturation cell (SC) is astainless-steel tube filled up with an inert stationaryphase (Gaz Chrom R 60/80 for Alltech) impreg-
nated with the organic solute and placed in thethermostatic bath (T). The liquid thermostat iscontrolled by a proportional-integral-derivativetemperature controller from Tronac Inc., modelPTC-40 which maintains the stability to within70.05K in the whole temperature range ofmeasurements. Water is pumped through a pre-heating coil into the saturation cell by means of ahigh-pressure pump model HPP 5001 from Labor-atorni Pristoje Praha (P) operating in a constant-flow mode. After allowing 30min for equilibration,
ARTICLE IN PRESS
Table 1
Experimental vapour pressures and deviations from the correla-
tion by Antoine equation
t (1C) psat (Pa) ðpexpsat � pcalcsat Þ (Pa)
Chlorocyclohexane
�9.55 109.7 +0.2
0.28 226.3 +0.8
10.20 436.3 �3.1
20.14 809.9 �2.6
30.11 1434 +0.0
40.09 2433 +6.4
50.08 3961 +8.2
60.06 6232 +11
70.04 9505 +11
80.01 14,101 +12
89.97 20,362 �22
94.94 24,245 �54
Bromocyclohexane
�9.49 34.94 �0.99
0.31 75.68 �0.41
10.21 158.7 +2.8
20.13 301.5 �0.3
30.08 553.5 �2.4
40.08 974.82 �5.5
50.07 1654.5 �3.5
60.05 2701.8 +0.6
70.02 4255.5 +0.1
80.01 6520.9 +8.6
89.95 9688.9 +7.8
94.92 11,707 +13
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–4734 4727
two or three samples are collected during 2min invials to which a methanolic solution of theappropriate internal standard was added.
The methanolic solutions were analysed with aPerkin-Elmer model Autosystem XL gas chromato-graphy equipped with FID and an on-columninjector. Each analysis was performed three times,and the calculation of the solubility is obtained fromthe average of these values. Chromatographicseparations in this case were accomplished with a30-m capillary column (Elite-5 from Perkin-Elmer0.32mm id, 0.25 mm film thickness). During theseparations, oven temperature was maintained at40 1C for 7min and then increased at 10 1Cmin�1 upto the final temperature of 120 1C that wasmaintained for at least 2min. For the analysis ofchlorocyclohexane, bromocyclohexane was used asthe internal standard and for the analysis ofbromocyclohexane, chlorocyclohexane was used asthe internal standard. A tree-point calibration curvewas generated in each case.
One of the drawbacks of the present technique isthe possibility of deposition of the solute in the tubebetween the saturation cell and the sample vial,which is placed outside the thermostatic bath, atroom temperature. This fact is particularly relevantat the highest temperatures studied as the aqueoussolubility of organic compounds normally increaseswith temperature. When the solution reaches thetube at room temperature the solute can phaseseparate, leading to an underestimation of theaqueous solubility. We have eliminated this sourceof error by modifying the experimental procedurepreviously described, adding methanol, a goodsolvent for the solutes tested, just outside of thesaturation cell (see Fig. 2b). As the first drop ofwater appears in the sample vial, a Gilson pumpmodel 306 (P0 in Fig. 2b) starts pumping methanolat constant-flow through fitting ‘‘t’’ in Fig. 2b. Thesolubility values obtained for bromocyclohexaneusing the two experimental procedures describedwere found to be similar to within the experimentalerror.
3. Results
3.1. Vapour pressures
The vapour pressure measurements were per-formed at 12 temperatures in the range between �10and 95 1C. The experimental values as a function oftemperature are reported in Table 1 together with
their deviations from the fit using the Antoineequation
ln psat ¼ a� b=ðtþ cÞ (1)
where t is temperature in 1C. The parameters a, b, c
used for the calculation of the vapour pressure arelisted in Table 2. They were obtained from theweighted least-squares regression of the linear formof Eq. (1) (Majer et al., 1989). The minimizedobjective function s has the form
s ¼X
i
w2i ðti ln psat;i þ c ln psat;i � ati þ b� acÞ2
(2)
and the weighting factors wi of individual datapoints were estimated from the propagation ofexpected errors in pressure (3%) and temperature(0.02 1C) discussed above
w2i ¼ 1=ððti þ cÞ20:0009þ ðln psat;i � aÞ20:0004Þ (3)
An iterative procedure was used in which the firstapproximation of a and c parameters was obtained
ARTICLE IN PRESS
Table 2
Parameters for Antoine equation (Eq. (1) for pressure in Pa) used
to smooth the raw data from Table 1 along with the standard
deviation of the fit
a b c s
Chlorocyclohexane
21.2230 3556.36 224.737 0.11
Bromocyclohexane
21.3044 3807.09 223.999 0.23
Table 3
Experimental values of the solubilities in water of the two solutes
studied in this work
Chlorocyclohexane Bromocyclohexane
t
(1C)
Cs
(g cm�3)xsols
(10�5)
t
(1C)
Cs
(g cm�3)xsols
(10�6)
1.3 338.00 5.13 1.3 64.25 7.10
1.3 324.49 4.93 1.4 62.50 6.91
1.3 332.83 5.06 1.3 64.71 7.15
4.8 308.12 4.68 4.5 64.30 7.10
4.8 302.85 4.60 4.5 62.15 6.87
13.9 235.71 3.58 4.5 63.28 6.99
13.9 235.26 3.58 10.0 64.07 7.08
13.9 240.74 3.66 10.0 62.50 6.91
19.1 201.72 3.07 15.7 64.84 7.17
19.1 206.97 3.15 15.4 66.89 7.40
23.7 210.99 3.21 19.8 69.49 7.69
23.7 218.75 3.33 19.9 68.40 7.57
23.9 211.13 3.22 19.8 67.19 7.44
29.6 217.49 3.32 24.9 69.95 7.75
29.7 216.29 3.30 24.9 69.51 7.70
30.0 205.93 3.14 24.9 70.43 7.80
30.1 209.78 3.20 29.6 75.13 8.34
34.6 234.41 3.58 29.4 74.41 8.26
34.7 234.29 3.58 29.4 74.21 8.23
34.7 234.51 3.58 34.3 76.44 8.49
34.7 239.60 3.54 34.2 75.88 8.43
40.0 230.98 3.78 34.3 76.15 8.46
40.0 247.02 3.84 41.4 79.34 8.84
40.0 250.87 3.72 41.4 81.27 9.05
41.4 80.75 9.00
Table 4
Parameters of Eq. (4) used to smooth the raw experimental data
from Table 3 along with the standard deviation of the fit
A B C s
Chlorocyclohexane
�147.900 137.556 137.777 0.03
Bromocyclohexane
�31.7652 20.0051 22.2407 0.02
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–47344728
from the regression without weighting. The valuesof standard weighted deviation sw ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis=ðn� 3Þ
p,
where n ¼ 12, are equally presented in Table 2. Theyare well below unity, which indicates that thedeviations of the experimental vapour pressuresfrom the smoothed values are substantially lowerthan that of the estimated error in psat. Thepropagation of the uncertainty in temperature intopsat is minor at our conditions compared to the errorin the pressure determination (this error is mainlysystematic and is not reflected in the scatter ofexperimental data). The parameters a, b and c
calculated were used to generate the recommendedvalues listed in Table 5. It can be considered that theoverall uncertainty in the vapour pressure data liesbetween 3% and 5%.
3.2. Aqueous solubility
Multiple data points for the aqueous solubilitiesof the two solutes were obtained in the temperatureinterval between 0 and 40 1C, in steps of approxi-mately 5 1C. All the direct experimental values forthe aqueous solubility expressed in mole fraction arepresented in Table 3. To obtain representativevalues of the solubility, the raw experimental datawere correlated as a function of temperature by theempirical equation
ln xsols ¼ Aþ B=tþ C ln t (4)
with t ¼ (t+273.15)/(t0+273.15) and t0 ¼ 25 1C.The adjustable parameters A, B and C are presentedin Table 4, together with the standard deviation ofthe fit, which reflects the scatter of the experimentaldata points (on average between 2% and 3% fromthe fitted values of xs
sol). These parameters were usedfor generating the recommended values for thesolubility listed in Table 5. They are considered tobe accurate within 710%, similar to our earlierdata (Sarraute et al., 2004, 2006; Dohanyosova
et al., 2004). Also listed in Table 5 are Henry’s lawconstants, KH and air/water partition coefficients,KAW, which were determined from the expressions
KH ¼ limxs!0
f s
xsffi
psats
xsols
(5)
KAW ¼ limxs!0
CAirs
CWs
ffiKHMW
RTrW(6)
ARTICLE IN PRESS
Table 5
Mole fraction aqueous solubility xssol, solute’s vapour pressure
pssat, Henry’s law constant KH and air–water partition coefficient
KAW as a function of temperature for the solutes studied in this
work
t (1C) 105 xsols
pssat (Pa) KH (MPa) 103 KAW
Chlorocyclohexane
0 5.44 221.1 4.06 32.3
5 4.45 312.0 7.01 54.6
10 3.83 433.8 11.3 86.6
15 3.46 595.0 17.2 129
20 3.27 805.7 24.6 182
25 3.22 1078 33.5 244
30 3.29 1425 43.3 311
35 3.49 1865 53.4 378
40 3.83 2415 63.0 440
Bromocyclohexane
0 0.695 74.32 10.69 84.9
5 0.703 107.7 15.33 119
10 0.715 153.7 21.49 165
15 0.732 216.0 29.49 222
20 0.754 299.3 39.69 294
25 0.781 409.5 52.43 382
30 0.813 553.2 68.06 489
35 0.850 738.9 86.94 615
40 0.893 976.1 109.4 763
Fig. 3. Mole fraction aqueous solubilities (upper plot) and
Henry’s law constant (lower plot) as a function of temperature
for aliphatic compounds: ’, octane; K, chlorooctane; J,
bromooctane; m, 1,8-dichlorooctane; W, 1,8-dibromooctane.
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–4734 4729
where fs and xs are the fugacity of the organic solutein the aqueous phase and its corresponding molefraction, respectively. MW is the molar mass ofwater and rW its density (taken from the equation ofWagner and Pruss (1993)). The two inequalities inEqs. (5) and (6) are considered as equalities fordilute solutions for which the molar volume of thesolution can be considered equal to that of the puresolute. Furthermore, the effect of water dissolved inthe organic phase must be negligible and the vapourof solute and air are expected to behave as idealgases.
4. Discussion
The analysis of the present data allows theevaluation of the influence of halogen substitutionin hydrocarbons in the air–water partition of thesechemical species. The influence of these substitu-tions in aliphatic hydrocarbon chains has beenstudied and quantified previously for haloalkanes(Sarraute et al., 2004) and dihaloalkanes (Sarrauteet al., 2006), the halogen being chloride or bromide.The results found are represented in Fig. 3 where themole fraction aqueous solubilities and the Henry’s
law coefficients for chlorooctane, bromooctane, 1,8-dichlorooctane and 1,8-dibromooctane are com-pared with the same values for n-octane. It isobserved that the aqueous solubility increases withthe halogen substitution and is slightly higher forthe chlorinated species (upper plot in Fig. 3).Henry’s law constants are always lower for 1,8-dibromooctane than 1,8-dichlorooctane in thetemperature range studied. The mono-substitutedspecies exhibit higher values for KH, and n-octanehas the highest Henry’s law constants. This meansthat for example at 25 1C, the atmosphere/waterpartition coefficient, the presence of halogen sub-stitution in an aliphatic hydrocarbon increases bytwo orders of magnitude the possibility of wetdeposition of the organic species when it is releasedin the environment.
The data originally reported in this work, permitsthe study of the influence of the halogen substitution
ARTICLE IN PRESSS. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–47344730
on the atmosphere–water partition of cycloalkanes.In Fig. 4 are represented the aqueous solubility data(upper plot) and the Henry’s law constants (lowerplot) for aqueous solutions of cyclohexane, chlor-ocyclohexane and bromocyclohexane. As can beobserved, the aqueous solubility of chlorocyclohex-ane is higher than that of cyclohexane, butbromocyclohexane dissolves less in water than thenon-substituted cycloalkane in the temperaturerange covered in this work. The association of thevapour pressure of the pure solutes to the aqueoussolubility measured allows the calculation of theHenry’s law constant (or the atmosphere/waterpartition coefficient) and the analysis of the atmo-spheric implications associated with the presence ofhalogen substitution in a hydrocarbon cycle. As canbe observed in Fig. 4, the presence of halogensubstitution increases, by slightly more than oneorder of magnitude, the possibility of wet depositionof the organic compounds. Furthermore, the pre-
Fig. 4. Mole fraction aqueous solubilities (upper plot) and
Henry’s law constants (lower plot) as a function of temperature
for cyclic compounds: ’, cyclohexane; ~, chlorocyclohexane;
}, bromocyclohexane.
sence of chlorine accentuates this phenomenonmore than the presence of bromine. The atmo-sphere/water partition coefficient for chlorocyclo-hexane has been reported by other authors(Bakierowska and Trzeszczynski, 2003) and isconfirmed by the results of the present work to bewithin the mutual experimental uncertainties (at10 1C the previously published value for KAW is 82compared with the value of 86.6 reported here).
The data obtained here can be associated toliterature values in order to rationalize, at least at abasic level, the atmospheric impact of releasingorganic compounds containing the same moleculargroups in to the environment. We have decided tomake use of a simple group contribution schemesimilar to the one presented in previous publications(Dohanyosova et al., 2004; Sarraute et al., 2006), tocalculate the Henry’s law constants. The groupcontribution scheme presented here uses a small setof reliable data on the Henry’s law constants for aselected group of substances with the objective ofproviding an appropriate way for predicting the KH
for a series of medium molecular mass alkanes,cycloalkanes and halogen-substituted alkanes. Weexpect that this prediction method is sufficientlysimple and reliable to be used rapidly by a largecommunity of scientists. From a more fundamentalpoint of view, it allows the quantification of theeffect on the atmospheric/water partition of thehalogen substitution in a linear of cyclic hydro-carbon.
Rather than developing a comprehensive groupcontribution scheme to predict the Henry’s lawconstant using a large training data set, we haveopted for an inverse strategy. Our aim is todemonstrate that the effects of the molecularstructure on the atmospheric/water partitioningof organic compounds can be quantitatively cap-tured by only a small set of accurate data on theHenry’s law constants of selected substances. Thegroup contributions determined on the basis ofthis limited experimental information are thenable to provide a realistic prediction for a varietyof organic substances including saturated linearand cyclic hydrocarbons and halogen-substitutedhydrocarbons. We applied a simple first-ordergroup contribution scheme, which, in general,ignores nearest-neighbour and steric-hindranceeffects:
ln KH ¼X
i
ni ln KHð Þi (7)
ARTICLE IN PRESSS. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–4734 4731
where (lnKH)i stands for the contribution of group i
and ni is the number of its occurrences in the solutemolecule, with the summation being over all groups.Besides the contributions of the groups alone,
Table 7
Comparison of the experimental and calculated solubility, at 25 1C for
nC Compound Group construction
Alkanes
5 Pentane 2CH3+3CH2
5 2-Methylbutane 2CH3+CH2+CH Me
5 2,2-Dimethylpropane 2CH3+C 2Me
6 Hexane 2CH3+4CH2
6 2-Methylpentane 2CH3+2CH2+CH Me
6 3-Methylpentane 2CH3+2CH2+CH Me
6 2,2-Dimethylbutane 2CH3+CH2+C 2Me
6 2,3-Dimethylbutane 2CH3+2CH Me
7 Heptane 2CH3+5CH2
7 2-Methylhexane 2CH3+3CH2+CH Me
7 3-Methylhexane 2CH3+3CH2+CH Me
7 3,3-Dimethylpentane 2CH3+2CH2+C 2Me
7 2,2-Dimethylpentane 2CH3+2CH2+C 2Me
7 2,3-Dimethylpentane 2CH3+CH2+2CH Me
7 2,4-Dimethylpentane 2CH3+CH2+2CH Me
8 n-Octane 2CH3+6CH2
8 3-Methylheptane 2CH3+4CH2+CH Me
8 2,2-Dimethylhexane 2CH3+3CH2+C 2Me
8 2,5-Dimethylhexane 2CH3+2CH2+2CH Me
8 2,2,4-Trimethylpentane 2CH3+CH2+CH Me+C
9 Nonane 2CH3+7CH2
10 Decane 2CH3+8CH2
11 Undecane 2CH3+9CH2
12 Dodecane 2CH3+10CH2
Table 6
Numerical values of the group contributions to lnKH, at 25 1C
together with the indication of the route used in their evaluation
Group/
structure
Contribution
to lnKH
Based on data from
CH3 10.850 (C5, C6, C7, C8)
CH2 0.284 (C5, C6, C7, C8)
CH-Me 0.788 2,5-Dimethylhexane
Me-C-Me 1.287 2,2-Dimethylhexane
CH2-Cl or
CH2-Br
6.986 (ClC5, ClC6, ClC7, ClC8) (BrC5,
BrC6, BrC7, BrC8)
Cycle 17.553 (cyC7, cyC8)
cyCH2 0.369 (cyC7, cyC8)
CyCH-Me 1.327 cis and trans-1,2
Dimethylcyclohexane
CyCH-Et 2.323 Ethylcyclohexane
CyCH-Cl or
CyCH-Br
�1.849 Chlorocyclohexane and
bromocyclohexane
contributions of some major structural features(e.g. ring) were also considered explicitly. Thecontributions are listed in Table 6 together withthe indication of the route of their evaluation.
The Henry’s law constants calculated using thevalues of Table 6 are compared in Table 7 with theexperimental values (the group construction foreach substance, together with the literature sourceof experimental values are always indicated). It canbe observed from the values of Table 7 that ourgroup contribution scheme reproduces the Henry’slaw constants 45 linear, branched, cyclic andhalogen-substituted aliphatic hydrocarbons with aroot-mean standard deviation (RMSD) of 0.39 inlnKH (i.e. about 48% in KH). This level of errorfavourably compares with the performance ofreliable contribution schemes by other authors(Plyasunov and Shock, 2000). A closer analysis ofthe data in Table 7 shows that the prediction of theHenry’s law constants for the cycloalkanes with thelowest number of carbon atoms in the ring has alarge error associated. This large deviation can alsobe seen in Fig. 5. This is probably due to the fact
several hydrocarbons, using a simple group contribution scheme
lnKH (exp) lnKH (calc) DlnKH
22.551b 22.554 0.003
22.656c 22.773 0.118
22.965d 22.988 0.023
22.808b 22.838 0.030
22.986b 23.058 0.071
22.950b 23.058 0.107
23.001b 23.273 0.271
22.702b 23.277 0.575
23.158b 23.122 �0.035
23.675b 23.342 �0.333
23.232b 23.342 0.109
23.061b 23.557 0.496
23.586b 23.557 �0.029
22.985b 23.561 0.576
23.519b 23.561 0.043
23.404e 23.407 0.003
23.758d 23.626 �0.132
23.841f 23.841 0.000
23.846f 23.846 0.000
2Me 23.639b 24.061 0.422
24.025b 23.691 �0.335
24.170g 23.975 �0.194
24.745g 24.259 �0.485
24.555b 24.544 �0.011
RMSD 0.268
ARTICLE IN PRESS
Table 7 (continued )
nC Compound Group construction lnKH (exp) lnKH (calc) DlnKH
Cycloalkanes
5 Cyclopentane 5cyCH2+cycle 20.721d 19.400 �1.321
6 Cyclohexane 6cyCH2+cycle 20.764b 19.770 �0.994
6 Methylcyclopentane Me+cyCH+4cyCH2+cycle 21.409b 20.358 �1.052
7 Methylcyclohexane Me+cyCH+5cyCH2+cycle 21.568b 20.727 �0.841
7 Cycloheptane 7cyCH2+cycle 20.139h 20.139 0.000
8 Cyclooctane cy+8cyCH2 20.509f 20.509 0.000
8 Ethylcyclohexane cy+et+5cyCH2+cyCH 21.724f 21.724 0.000
8 cis cy+2Me+4cyCH2+2cyCH 21.421f 21.684 0.263
8 trans cy+2Me+4cyCH2+2cyCH 21.947f 21.684 �0.263
RMSD 0.721
RMSD 0.166a
Halogenated alkanes
5 Clpentane CH3+CH2Cl+3CH2 18.611i 18.689 0.078
5 Brpentane CH3+CH2Br+3CH2 18.528i 18.689 0.161
6 Clhexane CH3+CH2Cl+4CH2 18.712i 18.974 0.262
6 Brhexane CH3+CH2Br+4CH2 19.032i 18.974 �0.058
7 Clheptane CH3+CH2Cl+5CH2 19.217i 19.258 0.041
7 Brheptane CH3+CH2Br+5CH2 19.303i 19.258 �0.045
8 Cloctane CH3+CH2Cl+6CH2 19.276e 19.542 0.266
8 Broctane CH3+CH2Br+6CH2 19.516e 19.542 0.026
8 Dicloctane 2CH2Cl+6CH2 15.863j 15.678 �0.186
8 Dibroctane 2CH2Br+6CH2 15.248j 15.678 0.429
RMSD 0.199
Halogenated cycloalkanes
6 Bromocyclohexane cy+5cyCH2+1 cyCHBr 17.775k 17.551 �0.224
6 Cholorocyclohexane cy+5cyCH2+1 cyCHCl 17.327k 17.551 0.224
RMSD 0.224
Total RMSD 0.392
Total RMSD 0.240a
aCalculated without cyclopentane, cyclohexane, methylcyclopentane and methylcyclohexane.bSolubilities data from Plyasunov and Shock (2000) and vapour pressures from Boublik et al. (1984).cSolubilities data from Plyasunov and Shock (2000) and vapour pressures from Wilhoit and Zwolinski (1971).dSolubilities data from Plyasunov and Shock (2000) and vapour pressures from Dykyj and Repas (1979).eData from our previous work (Sarraute et al., 2004).fData from our previous work (Dohanyosova et al., 2004).gData solubilities from Tolls et al. (2002) and vapour pressures from Boublik et al. (1984).hSolubilities data from Plyasunov and Shock (2000) and vapour pressures from Anand et al. (1975).iSolubilities dat from Horvath and Getzen (1999) and vapour pressures from Li and Rossini (1961).jData from our previous work (Sarraute et al., 2006).kData from this work.
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–47344732
that our group contribution scheme does not takeinto account second-order effects like the tension inthe C–C bonds in hydrocarbon cycles of less thansix carbon atoms. If these cases are not consideredfor the calculation of the total RMSD, the valueobtained is as low as 0.24 in lnKH (i.e. about 27% inKH). This comparison with the experimental valuesshows that our group contribution analysis iscapable of portraying the effect of the halogensubstitution on the aqueous solubility of alkanesand cycloalkanes.
5. Conclusions
In this work it is proven that the association ofaqueous solubility data and vapour pressures ofwell-chosen organic solutes can serve as a basis forbuilding empirical schemes for rationalizing theeffect of halogen substitution on the atmosphere/water partition of organic compounds and so ofthe relative importance of the wet depositionof halogen-substituted organic pollutants in theenvironment.
ARTICLE IN PRESS
Fig. 5. Aqueous solubility of hydrocarbons and halogen-
substituted hydrocarbons. Experimental values versus those
predicted from the group contribution scheme: ’, alkanes; K,
cycloalkanes; &, halogenated alkanes; J, halogenated cycloalk-
anes.
S. Sarraute et al. / Atmospheric Environment 42 (2008) 4724–4734 4733
A valid group contribution scheme was built inorder to predict the Henry’s law constant of severalalkanes, cycloalkanes, halogenated alkanes andhalogenated cycloalkanes. This simple empiricalapproach deserves to be extended, as it seems tocorrectly take into account the effect of simplestructural modifications of organic molecules ontheir partition between the atmospheric and theaqueous environmental compartments. The simpli-city of the model allows its use by a largecommunity of environmental scientists. Further-more, the approach described in the present papercan perhaps inspire other scientists, that measurevaluable experimental data, to rationalize them andto render them easily available to a wider scientificcommunity.
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