The solubility of toluene in aqueous salt solutions

Talanta 48 (1999) 633 – 641

The solubility of toluene in aqueous salt solutions Simon R. Poulson *, Rebecca R. Harrington, James I. Drever Department of Geology and Geophysics, Uni6ersity of Wyoming, Laramie, WY 82071 -3006, USA Received 8 June 1998; accepted 24 August 1998

Abstract The solubility of toluene has been measured in distilled water, and in various inorganic salt solutions. Values of the Setschenow constant, KS, which quantify toluene solubility versus salt concentration, have been determined for each salt. Values of KS are compared to the activity of water for the salt solutions. Data from this study, consistent with earlier data, suggests that the effects of salts upon toluene solubility are non-additive. This contrasts the additive behavior of inorganic salts upon the solubility of nonpolar organic compounds, such as benzene and naphthalene, reported in the literature. Specific interaction between slightly polar toluene and ions in solution is suggested as a possible explanation for the non-additive effect of salts on the solubility of toluene. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Setschenow constant; Toluene solubility; Salt concentration; Non-additive

1. Introduction The solubility of aromatic hydrocarbons, such as toluene, in water, and the decrease in solubility in aqueous salt solutions is an important factor controlling the behavior of hydrocarbons in a variety of environments. Waterflood recovery of mobile, separate-phase hydrocarbons in the subsurface is a well established technique, using a well injection-and-recovery methodology. It was initially developed as a secondary recovery process to increase petroleum production efficiency [1], but has also been applied as a remedial technology at sites contaminated with a nonaqueous

* Corresponding author. Tel.: +1-702-784-6050; fax: + 1702-784-1833; e-mail: [email protected].

phase liquid [2,3]. Brines of various compositions may be used during waterflood recovery, as salinity influences the wettability of an organic liquid phase [4], and hence affects the efficiency of organic phase recovery [5,6]. Salinity can also affect the adsorption behavior of organic compounds onto natural organic matter [7], and hence will affect the mobility of organic compounds in the subsurface. For example, sorption coefficients of pyrene onto organic matter are 15% higher in a 0.34 molal NaCl solution compared to distilled water [8], i.e. pyrene is more strongly adsorbed onto organic matter in the presence of NaCl. Lastly, the solubility of hydrocarbons in salt solutions also has implications for petroleum migration [9]. Toluene is an important compound to consider as it is a significant, and very soluble,

0039-9140/99/$ - see front matter © 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 9 8 ) 0 0 2 9 2 - 6

634

S.R. Poulson et al. / Talanta 48 (1999) 633–641

component of both gasoline and petroleum, and is also an important industrial chemical in common use worldwide. The solubility of neutral, nonpolar organic compounds in salt solutions has long been known to decrease in salt solutions of most inorganic ionic species, an effect commonly known as ‘salting-out’, although salts of large, organic cations can increase solubilities (known as ‘salting-in’). An empirical relationship between the solubility of an organic compound and the salt concentration has long been recognized [10], where: log(C0/Csalt) =SKS (1) where C0 is the solubility of the organic compound in distilled water, Csalt is the solubility of the organic compound in the salt solution, S is the salt concentration, and KS is the salting, or Setschenow, constant. The units of KS are discussed in a later section. The major effect of inorganic salts in solution upon organic compound solubility is the formation of hydration shells around the ionic species, which effectively reduces the availability of free water to dissolve the inorganic compound. As the size and strength of ionic hydration shells are dependent upon the salt composition, values of KS are also dependent upon the salt composition. Hence, it might be expected that there would be a correlation between the activity of water (i.e. the availability of free water to dissolve an organic compound) and the value of KS. However, values of KS are also dependent upon the molar volume of the organic compound [11], and upon the magnitude of any possible interaction between the organic compound and the dissolved salts. In the case of nonpolar organic compounds, such as benzene [12] and naphthalene [13,14], where there is no specific interaction between the organic compound and the dissolved salts, the effect of a mixture of salts is a simple summation of the effects of the contributions from the individual salts present: KS = %K iSSi

(2)

where K iS is the Setschenow constant for salt i, and Si is the concentration of salt i in the salt mixture.

This study has measured the solubility of toluene in various aqueous salt solutions, by direct equilibration of liquid toluene with an aqueous solution, followed by direct analysis of the toluene concentration in solution by UV spectrophotometry. Calculation of the activity of water in salt solutions has also been performed, in order to investigate a possible correlation between the value of KS and the activity of water.

2. Experimental methods Each salt under investigation was prepared at three different concentrations, using distilled, deionized water and ACS grade reagents. Ten gram aliquots of salt solution (or distilled deionized water) were transferred into glass centrifuge tubes with PTFE-lined screw caps. All experiments were performed in triplicate. Approximately 0.4 g of pure ACS grade toluene was then added to each centrifuge tube, and the samples were mechanically agitated for 3 days. Samples were then centrifuged at approximately 2000 rpm for 8 min, to separate liquid toluene (r=0.87 g/cm3) from the aqueous solution. The underlying salt solution was sampled by passing a first, outer pipette through the lens of toluene, and then sampling the salt solution from the bottom of the centrifuge tube with a second, inner pipette. Toluene concentrations in the salt solutions were measured by UV-visible spectrophotometry at a wavelength of 261.3 nm, using a Shimadzu UV160U dual beam UV-VIS scanning spectrophotometer (Shimadzu, Columbia, MD). Solutions were measured in a quartz cuvette with a PTFE stopper, with minimal headspace. Analytical standards were prepared daily from a stock solution of toluene dissolved in methanol. Triplicate analyses of each experiment indicated analytical reproducibilities with a typical standard deviation of 1.5%, with a maximum standard deviation of 3.4%. Linear regression of experimental data to determine values of KS had values of R 2 \ 0.99 for all salts except CsBr, which had a value of R 2 = 0.97. All experiments were performed at 23°C. The majority of previous studies have been conducted at 25°C, but other studies

S.R. Poulson et al. / Talanta 48 (1999) 633–641

[15,16] indicate that toluene solubility has little dependence upon temperature over the range of 20–25°C. Additional salts were investigated, but experimental problems precluded quantification of the effect of these salts. These salts included Ca(NO3)2, KI, and CsI, which all interfered at the wavelength used to measure toluene concentrations, and Na2SO4 and CsCl, which resulted in emulsification of toluene, and prevented accurate measurement of toluene concentrations. Activities of water in 1 molal salt solutions were calculated using the program PHRQPITZ [17]. Calculation of water activity for a CsBr solution was not possible as the appropriate Pitzer constants are not available for Cs + , so the value for CsBr is taken from the literature [18].

3. Units of KS The units of KS are dependent upon concentration in two ways, and need to be addressed in order to compare solubilities determined by different experimental studies. Firstly, inspection of Eq. (1) indicates that the KS has units of 1/(salt concentration), where salt concentration may be molal (mol/kg water) or molar (mol/l solution). This distinction is not always clearly delineated, and has a small but significant effect upon the numerical value of KS, depending upon solution density. For example, a 1 molal solution of BaCl2 (17.2 wt.% BaCl2), has a density of 1.169 g/cm3 [18], and therefore is equivalent to a 0.967 molar solution of BaCl2. Hence, KS (l/mol) =0.034KS (kg/mol). Secondly, the dimension of KS will depend upon the dimension used to measure toluene solubility, with the two common units being mg toluene/l solution, and mg toluene/kg solution. At 25°C in distilled water, these values (i.e. C0) are essentially identical, since rwater =0.997 g/cm3 at 25°C [19], i.e. a concentration of x mg/l = 0.003x mg/kg (ignoring the effect of the dissolved toluene on solution density). However, in concentrated salt solutions, there is a significant difference in the numerical values of mg/l and mg/kg, again depending upon solution density. For example, a concentration of y mg/l toluene (i.e. Csalt) in a 1

635

molal BaCl2 solution, is the same as a concentration of 0.85y mg/kg. Hence, KS (C0 and Csalt as mg/kg) = KS (C0 and Csalt as mg/l) − 0.07, for BaCl2. Toluene solubility has been measured in this study as mg/l, as have most previous studies of toluene solubility in salt solutions. A number of studies [9,15,20] have reported solubilities as mg/ kg. However, all of these studies sampled experimental solutions volumetrically, so it is assumed that these concentrations should have been reported as mg/l. Hence, no correction has been applied to these studies. Salt solutions were prepared in this study as molal solutions, and values of KS in units of kg/mol are reported in Table 2. Values of KS (kg/mol) are converted to values of KS (l/mol) using compiled solution densities [18], and are reported in Tables 2 and 3. A solution density for CsBr is not available [18], so a solution density for CsCl was used instead.

4. Results

4.1. The solubility of toluene in distilled water The solubility of toluene in distilled water was determined in this study to be 562.9 9 9.6 mg/l. A comparison with values determined in other studies investigating the effect of salts on toluene solubility, and the various experimental methods used, is presented in Table 1. Table 1 demonstrates that a wide range of values for toluene solubility have been measured, ranging from 487 [21] to 588 ppm [22], with the value determined by this study falling within this range. A more exhaustive compilation of experimental studies that have measured the solubility of toluene in distilled water is available [23], with over 70 measurements performed since the first determination [24]. Quoted solubilities show a total range of 265 to 1581 mg/l, although the majority of values (the 10th to 90th percentile) lie in the range of 470 to 670 mg/l. While the value determined by this study comfortably lies within the range determined by previous studies, it is clear that there is no accurate, consensus value for the solubility of toluene in distilled water.

S.R. Poulson et al. / Talanta 48 (1999) 633–641

636

Table 1 Comparison of toluene solubility in distilled water, and experimental methods of studies investigating toluene solubility in salt solutionsa Study

Toluene solubility in distilled water

Experimental method

This study Desnoyers and Ichhaporia (1969) [22] Brown and Wasik (1974) [26] Mackay and Shiu (1975) [28] Sada et al. (1975) [21] Sutton and Calder (1975) [20] Price (1976) [9] Rossi and Thomas (1981) [15] Sanemasa et al. (1984) [11] Keeley et al. (1988) [27]

562.9 9 9.6 mg/l 588 mg/l

Equilibration with liquid toluene, aqueous concn. by UV spectrophotometry Equilibration with liquid toluene, aqueous concn. by refractive index

5669 11 mg/kg

Vapor-aqueous phase experiments, with GC analysis, extrapolated liquid solubility calculations Vapor-aqueous phase experiments, with GC analysis, extrapolated liquid solubility calculations Volumetric titration using an organic dye as an end-point indicator Equilibration with liquid toluene via vapor phase, aqueous concn. by hexane extraction, evaporation/concentration, then GC Equilibration with liquid toluene, aqueous concn. by GC Equilibration with liquid toluene, aqueous concentration by solid phase extraction, then GC Equilibration with reduced toluene vapor pressure, aqueous concentration by chloroform extraction then UV spectrophotometry, followed by extrapolation Equilibration with liquid toluene, aqueous concn. by GC of equilibrated vapor

a b

519.59 9.6 mg/l 487.3b mg/l 534.89 4.9 mg/kg 544.09 15.0 mg/kg 506.79 6.1 mg/kg 521 mg/l 5809 3 mg/l

Majority of experimental studies conducted at, or close to, 25°C. Converted from ml toluene/l, using rtoluene = 0.865.

4.2. The solubility of toluene in salt solutions The results of the solubility experiments, including five salts that have not been studied before (KBr, KHCO3, CsBr, MgCl2, and MgSO4), are listed in Table 2, and plotted in Fig. 1 as log(C0/Csalt) versus salt concentration, after Eq. (1). Values of KS (l/mol), ranging from 0.050 (for CsBr) to 0.596 (for K2SO4), are determined by linear least-squares fit to the data in Fig. 1, and are also listed in Table 2. KS values determined by this study are compared with KS values compiled from the literature in Table 3. Comparison with previous studies is difficult, due to the limited quantity of data present in the literature, with only two previous studies of any great extent [11,21]. NaCl has been the most frequently studied (6 determinations of KS), but there are only 5 other salts that have been measured more than once (NaBr, Na2SO4, KCl, K2SO4, CaCl2, and BaCl2). Values of KS determined by this study tend to be slightly lower than previous measurements (e.g. NaCl, KCl, K2SO4, and CaCl2), but some measure-

ments are higher than previous values (e.g. BaCl2), whereas the measurement for NaBr falls between two previous determinations. In general, the trends in values of the Setschenow constants measured in this study are consistent with what might be expected, using a model where smaller, more highly charged ions will bind waters of hydration more tightly, hence reducing the availability of free water to dissolve toluene. Assuming that common ions have the same effect on KS (l/mol), the effect of Na\ K\Cs, as indicated by the bromide salts (KS decreases from 0.180 to 0.150 to 0.050, for NaBr, KBr, then CsBr), and confirmed by the chloride salts (KS decreases from 0.202 to 0.188, for NaCl and KCl). Similarly, the effect of Cl \ Br, as demonstrated by both the Na salts (KS decreases from 0.202 to 0.180, for NaCl and NaBr) and the K salts (KS decreases from 0.188 to 0.150, for KCl and KBr). However, the alkaline earth chloride salts show an inconsistent pattern, in that the effect of Mg\ Ca, as expected (KS decreases from 0.354 to 0.289, for MgCl2 and CaCl2), but the value of KS for

S.R. Poulson et al. / Talanta 48 (1999) 633–641 Table 2 Experimental results of toulene solubility in various aqueous salt solutions

637

a,b,c

Salt

Salt concentration (molal)

Toulene solubility (mg/l)

KS (kg/mol)

KS (l/mol)

Water activity coefficient for 1 molal salt solution

Distilled water NaCl

– 1.00 2.00 3.00 0.53 1.11 1.77 0.50 1.00 1.50 0.50 1.00 1.50 0.50 1.00 1.50 0.20 0.40 0.60 0.50 1.00 1.50 0.45 0.90 1.35 0.50 1.00 1.50 0.48 0.97 1.45 0.48 0.72 0.97

562.9 347.9 216.2 144.4 445.0 353.8 273.8 445.6 363.4 300.0 476.7 398.8 339.7 421.5 320.6 262.5 428.5 324.6 257.3 519.9 506.1 472.7 391.0 278.5 194.4 351.0 213.9 129.8 394.4 283.9 220.3 352.1 269.2 199.3

– 0.198

– 0.202

– 0.976

0.176

0.180

0.966

0.182

0.188

0.968

0.147

0.150

0.968

0.223

0.231

0.968

0.570

0.596

0.965

0.048

0.050

0.97

0.341

0.354

0.942

0.425

0.428

0.981

0.282

0.289

0.945

0.462

0.478

0.951

NaBr

KCl

KBr

KHCO3

K2SO4

CsBr

MgCl2

MgSO4

CaCl2

BaCl2

a

Water activity coefficients for 1 molal salt solutions are calculated using PHRQPITZ [17]. Conversion of KS from kg/mol to l/mol is discussed in text. c A value for CsBr could not be calculated as the appropriate parameters for Cs+ are not available, so the value for CsBr is taken from reference [18]. b

BaCl2 (0.478) is greater than either MgCl2 or CaCl2. Similar trends were observed for Na versus K [11,21], and for Cl versus Br [11]. However, a value of KS for CaCl2 \BaCl2 has been measured [21], in contrast to this study, but this study [21] also measured a value of KS for K2SO4 \Na2SO4, which is the reverse of what might be expected.

5. Discussion and conclusions Comparison of KS values measured in this study to previous determinations shows only moderate agreement. This may not be surprising in light of apparent uncertainties in various experimental and analytical methods. There does not

638

S.R. Poulson et al. / Talanta 48 (1999) 633–641

Fig. 1. Variation of toluene solubility (expressed as C0/Csalt, where C0 =toluene solubility in distilled water, and Csalt =toluene solubility in salt solution) versus salt concentration for various inorganic salts.

appear to be an accurate consensus value for the solubility of toluene in distilled water, which should be a more straightforward determination compared to the measurement of toluene solubility in salt solutions. However, comparison of just the 10 studies listed in Table 1 indicates a range of measurements for toluene solubility in distilled water from 487 to 588 ppm, and this range becomes much wider when considering the multitude of other studies which have measured toluene solubility (see earlier section). Clearly, measurement of toluene solubility in distilled water is an important factor in determining values of KS. As an example, log(C0/Csalt) versus NaCl concentration is plotted in Fig. 2, using the value of C0 measured in this study (563 mg/l), but also using values of C0 of 543 and 583 mg/l (i.e. 920 ppm) with the same values of toluene solubility in NaCl solutions listed in Table 1. Interestingly, values of R2 for the linear regressions are essentially identical in Fig. 2, indicating that the value of R2 is not very sensitive to the exact value of C0, but the value of KS varies significantly, depending on the choice of C0 (KS =0.193, 0.198, or 0.202 kg/mol, for C0 =543, 563, and 583 mg/l, respec-

tively). As discussed earlier, a consensus value for the solubility of toluene in distilled water is not available, so this remains a factor of considerable uncertainty for measurements of KS. Values of the activity of water for 1 molal salt solutions, calculated using PHRQPITZ [17], are listed in Table 2, and plotted against values of KS in Fig. 3. For a simple model of solution of a single organic compound in a variety of salt solutions, it might be expected that there would be a negative correlation between the value of KS, and the activity of water in the salt solution. However, Fig. 3 clearly demonstrates that there is no significant correlation between water activity and value of KS for toluene for the salts investigated by this study. Consideration of the number of water molecules bound in ionic hydration spheres, and hence the effect upon the effective concentration of bulk water in solution, could potentially predict values of KS. This model does indeed qualitatively predict trends in values of KS, e.g. a number of studies indicate a decrease in solvation numbers from Na to K to Cs, and also from Cl to Br [25]. As solvation numbers decrease, the amount of water tied up in hydration spheres decreases,

S.R. Poulson et al. / Talanta 48 (1999) 633–641 Table 3 Compilation of KS values for toluene in various aqueous salt solutions Salt

KS (l/mol)

Ref.

LiCl Li2SO4 NaF NaCl

0.191 0.593 0.336 0.202 0.205

Sada et al. (1975) [21] Sada et al. (1975) [21] Sanemasa et al. (1984) [11] This study Mackay and Shiu (1975) [28] Sada et al. (1975) [21] Price (1976) [9] Sanemasa et al. (1984) [11] Keeley et al. (1988) [27] This study Desnoyers and Ichhaporia (1969) [22] Sanemasa et al. (1984) [11] Sanemasa et al. (1984) [11] Sanemasa et al. (1984) [11] Sanemasa et al. (1984) [11] Sada et al. (1975) [21] Sanemasa et al. (1984) [11] This study Sada et al. (1975) [21] Sanemasa et al. (1984) [11] This study This study This study Sada et al. (1975) [21] This study Sada et al. (1975) [21] Sada et al. (1975) [21] This study This study This study Sada et al. (1975) [21] This study Sada et al. (1975) [21] Sanemasa et al. (1984) [11] Sada et al. (1975) [21] Sada et al. (1975) [21] Rossi and Thomas (1981) [15] Brown and Wasik (1974) [26] Sutton and Calder (1975) [20] Price (1976) [9]

NaBr

0.267 0.195a 0.242 0.201b 0.180 0.113

0.190 0.066 0.154 0.139 0.650 0.684 KCl 0.188 0.205 0.214 KBr 0.150 KHCO3 0.231 K2SO4 0.596 0.674 CsBr 0.050 NH4Cl 0.055 (NH4)2SO4 0.415 MgCl2 0.354 MgSO4 0.428 CaCl2 0.289 0.401 0.478 BaCl2 0.278 0.414 CuSO4 0.580 ZnSO4 0.517 Natural sea wa- 0.083c ter 0.154c Artificial sea water 0.149c NaSCN NaNO3 NaClO4 Na2SO4

0.131 a

c

Calculated from solubility data. Units of KS unclear-KS given in terms of ionic strength, but definition of ionic strength (i.e. in terms of molal or molar) not provided. c Value=log (C0/Cseawater), where C0 = toluene solubility in distilled water, and Cseawater = toluene solubility in seawater b

639

and the concentration of water available to dissolve toluene increases. Hence, values of KS are predicted to decrease from Na to K to Cs, and also from Cl to Br, which is observed experimentally. However, quantification of this model may be impossible due to the difficulty in assigning solvation numbers to each ion. Widely varying solvation numbers have been measured by different experimental methods [25], and solvation numbers may well be dependent upon salt concentration. Consideration of the results of this study, and a previous study [21], suggests that the effects of salts on toluene solubility are not additive, in contrast to the results for benzene [12] and naphthalene [13,14]. If individual ion effects are additive, they will also be conservative, such that if two salts have a common anion, the difference in KS values between salts of cation A versus cation B should be independent of the identity of the common anion. Similarly, in the case of salts with a common cation, the difference in KS values between salts of anion X versus anion Y should be independent of the identity of the common cation. In this study: Na-K: KS(NaCl)-KS(KCl)=0.014 "KS(NaBr)-KS(KBr) =0.030 Cl-Br: KS(NaCl)-KS(NaBr)= 0.022 " KS(KCl)-KS(KBr) = 0.038 Similarly, there are many additional examples for toluene [21], including: Na− K: KS(NaCl)− KS(KCl)=0.062 "0.5[KS(K2SO4)− KS (K2SO4)]= − 0.012 SO4 − 2Cl: KS(Na2SO4)− 2[KS(NaCl)]= 0.116 " KS([NH4]2SO4)− 2[KS(NH4Cl)]= 0.305 This contrasts with data for benzene [12], for example:

S.R. Poulson et al. / Talanta 48 (1999) 633–641

640

Fig. 2. Example of dependence of KS value upon C0 (toluene solubility in distilled water).

Fig. 3. Variation of activity of water for a 1 molal salt solution versus salting-out coefficient, KS.

Na− K: KS(NaCl)− KS(KCl) = 0.029 :KS(NaBr)− KS(KBr) =0.026 Cl−Br:

KS(NaCl)− KS(NaBr)= 0.040 : KS(KCl)−KS(KBr)= 0.047 Similarly, direct experimental measurements of the solubility of naphthalene demonstrates addi-

S.R. Poulson et al. / Talanta 48 (1999) 633–641

[4] G.J. Hirasaki, Soc. Petr. Eng. Form. Eval. 6 (1991) 217. [5] H.O. Yildiz, N.R. Morrow, J. Petr. Sci. Eng. 14 (1996) 159. [6] G.Q. Tang, N.R. Morrow, Soc. Petr. Eng. Res. Eng. 12 (1997) 269. [7] R.P. Schwarzenbach, P.M. Gschwend, D.M. Imboden, Environmental Organic Chemistry, Wiley, New York, 1993, p. 681. [8] S.W. Karickhoff, D.S. Brown, T.A. Scott, Water Res. 13 (1979) 241. [9] L.C. Price, Am. Assoc. Pet. Geol. Bull. 60 (1976) 213. [10] J. Setschenow, Z. Phys. Chem. Vierter Band 1 (1889) 117. [11] I. Sanemasa, S. Arakawa, M. Araki, T. Deguchi, Bull. Chem. Soc. Japan 57 (1984) 1539. [12] W.F. McDevit, F.A. Long, J. Am. Chem. Soc. 74 (1952) 1773. [13] J.E. Gordon, R.L. Thorne, J. Phys. Chem. 71 (1967) 4390. [14] J.E. Gordon, R.L. Thorne, Geochim. Cosmochim. Acta 31 (1967) 2433. [15] S.S. Rossi, W.H. Thomas, Environ. Sci. Technol. 15 (1981) 715. [16] D.G. Shaw (Ed.), Solubility Data Series. Volume 37, Hydrocarbons with water, seawater. Part 1: Hydrocarbons C5 to C7, Pergamon Press, New York, 1989, pp. 369 – 431. [17] L.N. Plummer, D.L. Parkhurst, G.W. Fleming, S.A. Dunkle, U.S. Geological Survey; Reston; VA, Water Res. Inv. Rep. (1988) 88 – 4153. [18] I.D. Zaytsev, G.G. Aseyev (Ed.), Properties of Aqueous Solutions of Electrolytes (M.A. Lazarev, V.R. Sorochenko, Trans.), CRC Press, Boca Raton, FL, 1992, p. 1773. [19] J.A. Dean (Ed.), Lange’s Handbook of Chemistry, 13th ed., McGraw-Hill, New York 1985, p. 1792. [20] C. Sutton, J.A. Calder, J. Chem. Eng. Data 20 (1975) 320. [21] E. Sada, S. Kito, Y. Ito, J. Chem. Eng. Data 20 (1975) 373. [22] J.E. Desnoyers, F.M. Ichhaporia, Can. J. Chem. 47 (1969) 4639. [23] D. Mackay, W.Y. Shiu, K.C. Ma, Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals, Monoaromatic hydrocarbons, chlorobenzenes, and PCBs, vol. 1, Lewis Publishers, Boca Raton, FL, 1992, p. 697. [24] H. Fu¨hner, Chem. Ber. 57 (1924) 510. [25] Y. Marcus, Ion Solvation, Wiley, New York, 1985, p. 306. [26] R.L. Brown, S.P. Wasik, J. Res. Nat. Bur. Stds. (A. Phys. Chem.) 78A (1974) 453. [27] D.F. Keeley, M.A. Hoffpauir, J.R. Meriwether, J. Chem. Eng. Data 33 (1988) 87. [28] D. Mackay, W.Y. Shiu, Can. J. Chem. Eng. 53 (1975) 239.

tive behavior for binary mixtures of NaCl with either MgCl2, Na2SO4, CsBr, CaCl2, and KBr [13], and also artificial and natural sea water [14], although mixtures of NaCl with organic salts ((n-C4H9)4NBr, (CH3)4NBr, and C4H9SO3Na) were non-additive [13]. A possible explanation for the non-additive behavior observed for toluene in salt solutions compared to the additive behavior observed for benzene and naphthalene in inorganic salt solutions may be the slightly polar nature of toluene (dipole moment of 0.45 debyes: [19]) versus the non-polar nature of benzene and naphthalene. The slightly polar nature of toluene suggests that specific electrostatic interaction between toluene and ions in solution is possible, whereas similar interaction between non-polar benzene and naphthalene with ions in solution is unlikely. The exact mechanism whereby specific interaction between the organic compound and ions in solution is responsible for non-additive salt behavior is unclear, but is consistent with the observation that naphthalene exhibits non-additive behavior for mixtures of NaCl with organic salts [13]. In this case, specific interaction between naphthalene and the organic salts may be covalent, rather than electrostatic, in nature.

Acknowledgements This study has been funded by an award from NSF-Environmental Geochemistry and Biogeochemistry (grant cEAR-9631735), with additional funding from the NSF-EPSCoR Biogeochemistry cluster award to the University of Wyoming (grant cOSR-9550477). References [1]

G.P. Willhite, Waterflooding Society Petrol Engineers; Textbook Series, vol. 3, Society Petrol Engineers, Richardson, TX, 1986, p. 326. [2] T. Sale, D. Applegate, Ground Water 35 (1997) 418. [3] J.I. Gerhard, B.H. Kueper, G.R. Hecox, Ground Water 36 (1998) 283.

.

641

.