clear that there is no basis to assume that their AHH,values can be approximated by one value. However, if it is assumed that AC, = 0, subtraction of AHf from the reported AH,,values gives AH,values, which are listed in Table I. As is shown, these enthalpies of solution of super-cooled liquid solutes show less variation than the AH,, values for the six PCBs investigated by Dickhut et al. (1). In addition, the AHs values of two PCBs investigated by Opperhuizen et al. (3),as well as with those for penta- and hexachlorobenzene, are comparable. These data suggest that the AHs values of hydrophobic chemicals vary little with chemical structure. While AHf values of various solid hydrophobic chemicals vary with the melting point, AHBs values will be different for the different chemicals. As a consequence, variations of the aqueous solubility with temperature are not similar for different chemicals. Hence, it can be concluded that the dissolution of extremely hydrophobic chemicals is associated with a relatively constant endothermic enthalpy of solution (from the super-cooled liquid state) and an endothermic enthalpy of fusion that is proportional to the solute’s melting point. More important, however, is that the presented data show clearly that unfavorable entropy changes dominate dissolution processes of extremely hydrophobic chemicals. Registry No. Water, 7732-18-5.
Literature Cited (1) Dickhut, R. M.; Andren, A. A,; Armstrong, D. E. Environ. Sci. Technol. 1986, 20, 807-810. (2) Chothia, C.; Janin, J. Nature (London) 1975,256,705-708. (3) Opperhuizen, A.; Gobas, F. A. P. C.; Van der Steen, J. M. D.; Hutzinger, 0. Environ. Sci. Technol., in press. (4) Tomlinson, E.; Davis, S. S. J. Colloid Interface Sci. 1980, 76, 563-572. ( 5 ) Hildebrandt, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions;Van Nostrand Reinhold New York, 1970. (6) Hollenbeck, G. E. J . Pharm. Sci. 1980, 69, 1241-1242. (7) Riebesehl, W. Ph.D. Thesis, University of Amsterdam, The Netherlands, 1984. (8) Miller, M. M.; Ghodbane, S.; Wasik, S. P.; Tewari, Y. B.; Martire, D. E, J . Chem. Eng. Data 1984, 20, 184-190. (9) Benecke, J. I.; M.Sc. Thesis, University of Amsterdam, The Netherlands, 1986. (10) Hafkenscheid, T. L.; Tomlinson, E. Znt. J. Pharm. 1981, 8, 331-335. (11) Mackay, D.; Mascarenhas, R.; Shiu, W. Y.; Valvani, S. C.; Yalkowsky, S. H. Chemosphere 1980, 9, 257-264. (12) Yalkowsky, S. H. Ind. Eng. Chem. Fundam. 1979, 18, 108-1 11.
Antoon Opperhuizen,” Jeanet I. Benecke John R. Parsons
Laboratory of Environmental and Toxicological Chemistry University of Amsterdam 1018 WV Amsterdam, T h e Netherlands
SIR: In their correspondence Opperhuizen et al. ( I ) raise the following issues regarding our article on the effect of temperature on the aqueous solubility of six PCB congeners (2). First, they suggest that we should not have limited our discussion of the dissolution thermodynamics to the enthalpies of solution for the solid test compounds. Second, they objected to our assumption of a linear relationship for a plot of In x vs. 1/T resulting in a constant 926
Environ. Sci. Technol., Vol. 21, No. 9, 1987
-18 -18
-20
x
I
MCB
..
-22
’.
-24
‘’
-28
’’
TCB
C
I
.982
r2
-32
‘
2. 8
= ,958
.
I
3. 1
3.4
3. 7
T-’ X IO3(OK-l) Flgure 1. van’t Hoff plots for selected PCB congeners (B = biphenyl, MCB = 4-chlorobipheny1, PCB = 2,2’,4,5,5’-pentachlorobiphenyl, HCB = 2,2’,3,3‘,6,6’-hexachlorobiphenyl, OCB = 2,2’,3,3’,5,5‘,6,6’-octachlorobiphenyl, NCB = 2,2’,3,3’,4,4’,5,5‘,6-nonachlorobiphenyl, DCB = decachlorobiphenyl).
AHs,. Last, they disagree with our suggestion of an alternative average value for the enthalpy of dissolution for PCBs. The intention of our study was to describe solubility variation with temperature for PCBs so that predictions of the effect of this variable on partitioning and transport processes (3, 4) could be validated. Hence, most of our data (2, 5 , 6) were measured in the range of 0-40 “C of environmental interest. We agree that an investigation of the full thermodynamics of dissolution for the PCBs will aid in understanding the nature of hydrophobic interactions, but reiterate that this was not the original purpose of our paper. To perform a rigorous analysis requires more information than was obtained in our study (Le,, vapor pressure and fusion data). The linearity of the van’t Hoff relationship for PCBs is evident from the data. Figure 1shows reciprocal plots for the PCB solubility-temperature data of ref 2 and 6-8. Correlation coefficients for linear fits to these data range from 0.958 to 1,000 (Figure 1). In these investigations and others (9, IO),where the temperature range is restricted to the environmental region, good linearity of In x vs. 1/T is observed. Presupposition that the relationship between T and In x is linear (the van’t Hoff equation) is tantamount to assuming that AH,, is constant with temperature within the region of interest, Given the linearity of the plots in Figure 1 and those of others (5, 9), we do not feel that the assumption is inappropriate when made for a limited temperature range. However, the preceding correspondence (1) cites that there is an “observed increasing deviation from linearity with increasing experimental temperature ranges” ( I 1). We do not dispute this. The PCB data do show increased curvature, although not greatly significant, when the experimental temperature range is large. This is evidenced by the biphenyl curve, temperature range 0.4-64.5 “C, and
Table I. Free Energy, Enthalpy, and Entropy Contributions to the Thermodynamic Dissolution Processes for Selected PCBs at 25 "C (All Values in kJ/mol)d solid
IUPAC no.
-
solid
aq soln
AG,,
L V I S
-ThSs,
3 77 101 136 202 206 209
34.8 39.1 59.6 51.7 55.0 64.0 68.5 77.8
33.0 28.5 50.7 31.9 45.6 50.7 49.8 66.6
IUPAC no.
AGsub
Wsub
0 3 77 101 136 202 206 209
28.5 32.9
84.1 90.8
-55.6 -57.9
26.2 30.0
54.5
101.7
-47.2
70.1
121.8
-51.7
0
solid
-
-
liquid
liquid
-
aq soln
AGius
mfus
-TASf",
AGd
W
10.6 8.9 19.8 9.4 13.3 18.7 11.2
2.28 2.8gb 8.50b 2.79 4.78 7.16 9.92b 13.9
17.5 19.2' 24.8b 18.8 21.1 22.8 26.3b 28.7
-15.2 -16.3 -16.3 -16.0 -16.3 -15.6 -16.4 -14.8
32.5 36.2 51.1 48.9 50.2 56.8 58.6 63.9
15.5 9.3 25.9 13.1 24.5 27.9 23.5 37.9
-TASsub
AGvap
-TAS,ap
AGsv
66.6 71.6
-40.4 -41.6
6.3 6.2
-51.1 -62.3
57.4 68.5
47.3
78.9
-31.6
9.5
-51.0
60.5
56.2
93.1
-36.9
7.7
-55.2
62.9
1.8
vapor
liquid
-
mvap
vapor
vapor
-TAS,,
l
-
17.0 26.9 25.2 35.8 25.7 28.9 35.1 26.0 aq soln -TAS,,
from hGss = -RT In x ; x from ref 2, 6, and 15. AHs8from ref 2. -TAS,, = AG,, - A",,.AGfus = -RT In x , = (AHiu8/TfuJ(Tfus - T). -TAS,l = AGsl - AH,]. AGsub = -RT In ref 15. -TASiUs= AGius - AHfu,; Tfu,from ref 12 and 15. l G s I = AGs3 - AGius. AH8]= A",,- Sfus. (paat/patm). Psatand S s u b from ref 14. -TLS,,b = LGSub- i?LHsUb. = AGsub - AGfUs. 4ffv,p = s s u b - flf,,. -TAS,,, = AG,,, - AHvap. AGsv = AC,, - AGsub. lHSv= AHss - i?LHssub.-TAS,, = AGsv - AHsv. bEstimated using ASfu, = 54.8 J/deg mol.
+ Aqueous
Solid
G55
Solution
Figure 2. Thermodynamic cycle for solid-to-aqueous solution process.
the decachlorobiphenyl curve, range 25-80 "C, and by Bohon and Claussen (7). With respect to the use of an average value of AHs, for the PCBs, we agree with Opperhuizen et al. ( 1 ) that assumption of a single value for this quantity for 210 different compounds is unrealistic. As we stated in our earlier paper (2) "use of an average value for this quantity for all PCBs may lead to significant errors in estimating the temperature dependence of solubility". The suggestion to use an average AHssfor PCBs to determine the effect of temperature on solubility (12) was made in the absence of experimental data and without benefit of a forthcoming evaluation of the best method for making such estimations (13). At present, however, the use of an average AHs, and the van't Hoff equation is the best available means for estimating the temperature dependence of solubility for a PCB congener where experimental data are nonexistent. Finally, our colleagues request an examination of the full thermodynamics of dissolution for the PCBs. The available solubility (2,6-8), vapor pressure (14),and fusion (15) data are enough to allow a limited analysis. The examination can be made by using a thermodynamic cycle (Figure 2) and by partitioning free energy data into enthalpic and entropic components. The entire thermodynamic analysis is shown in Table I. Solid enthalpies of solution, AH,, values, show large variation among PCB congeners (Table I). This variation,
however, is not entirely explained by differences in melting point as Opperhuizen et al. ( 1 ) suggest. Table I lists enthalpies of solution for some PCBs for the solid (AH,,), liquid (AHsl),and vapor (AH,,) phases, where available. The variation in AH,, is 28.6% while the variation in AH,l, is 41.5%, significantly higher. This large variation in AHH,, is attributable to the heat required to vaporize the liquid (AHvap).AHsvvaries by less than 10% and thus is the only enthalpic quantity for PCBs to have little variation with chemical structure. Generally, the purpose for this differentiation of terms is to develop a mechanistic interpretation at the molecular level of the dissolution process. Hence, the pathway solid-to-liquid-to-vapor-to-aqueoussolution (Figure 2) is the best for interpretation since it breaks the solid-to-aqueous solution process into contributions due to solid crystalline interactions, nonbonded solute-solute interactions, and solute-solvent interactions. The solid-to-aqueous solution process is characterized by a positive free energy that increases exponentially with molecular surface area and is indicative of the hydrophobic nature of these solutes. The components of AG,, are a large positive enthalpy of solution and a much smaller negative entropy of solution. The decrease in entropy with this process can be attributed entirely to the solute-solvent interactions (Le., the structuring of water upon introduction of a nonpolar solute) since both the fusion and vaporization processes increase the overall entropy. The enthalpic contribution is mainly due to the heat necessary to break apart the noncrystalline solute (AHvap).This is largely compensated by the exothermic vapor dissolution process (AHsv),but when coupled with the heat of fusion, the enthalpy for the overall process remains significantly endothermic. Hence, it is the nonbonded solute-solute interactions and the solute-solvent interactions (structure formation) that control the dissolution process for the PCBs. It is clear that the dissolution process can be examined in much greater detail than was suggested by Opperhuizen et al. (I). If this is done, the dissolution process can be Environ. Sci. Technol., Vol. 21, No. 9, 1987
927
interpreted by means of contributing molecular interactions. Such a thermodynamic partitioning analysis requires an abundance of physical-chemical information but advances our knowledge of hydrophobicity. Registry No. Water, 7732-18-5.
Literature Cited Opperhuizen, A.; Benecke, J. I.; Parsons, J. R. Enuiron. Sci. Technol., preceding correspondence in this issue. Dickhut, R. M.; Andren, A. W.; Armstrong, D. E. Enuiron. Sci. Technol. 1986, 20, 807-810. Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. Enuiron. Sci. Technol. 1985, 19, 590-596. Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. Chemosphere 1985, 14, 1703-1716. Doucette, W. J. Ph.D. Dissertation, University of Wisconsin, Madison, 1985. Stolzenburg, T. R.; Andren, A. W. Anal. Chim. Acta 1983, 151, 271-274. Bohon, R. L.; Claussen, W. F. J . Am. Chem. SOC.1951, 73, 1571-1578.
928
Environ. Sci. Technol., Vol. 21, No. 9, 1967
(8) Wauchope, R.; Getzen, F. W. J. Chem. Eng. Data 1972,17, 38-41. (9) May, W. E.; Wasik, S. P.; Freeman, D. H. Anal. Chem. 1978, 50, 997-1000. (10) Schwarz, F. P. J . Chem. Eng. Data 1977, 22, 273-277. (11) Benecke, J. I. M.Sc. Thesis, University of Armsterdam, The Netherlands, 1986. (12) Mackay, D.; Mascarenhas, R.; Shiu, W. Y.; Valvani, S. C.; Yalkowsky, S. H. Chemosphere 1980, 9, 257-264. (13) Dickhut, R. M.; Armstrong, D. E.; Andren, A. W., submitted for publication in Enuiron. Sci. Technol. (14) Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. J. Chem. Eng. Data 1984, 29, 248-250. (15) Miller, M. M.; Ghodbane, S.; Wasik, S. P.; Tewari, Y. B.; Martire, D. E. J . Chem. Eng. Data 1984, 29, 184-190.
Rebecca M. Dickhut," Anders W. Andren David E. Armstrong Water Chemistry Program University of Wisconsin Madison, Wisconsin 53706
