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Fluid Phase Equilibria 250 (2006) 165–172

The effect of temperature on the solubility of benzoicacid derivatives in water

Jia Qing-Zhu a,b,∗, Ma Pei-Sheng a, Zhou Huan b, Xia Shu-Qian a,Wang Qiang a, Qiao Yan a

a College of Chemical Engineering, Tianjin University, Tianjin 300072, P.R. Chinab College of Marine Science and Engineering, Tianjin University of Science and Technology, Tianjin 300457, P.R. China

Received 10 March 2006; received in revised form 17 October 2006; accepted 18 October 2006

bstract

Using a laser monitoring observation technique, solubilities of o-nitro-benzoic acid, p-hydro-benzoic acid, p-methyl-benzoic acid and m-methyl-enzoic acid in water have been measured in the temperature range 290.15–323.15 K. The experimental data are regressed with the Wilson equation

−4 −5

nd the λH equation. The experimental results show that solubilities of these compounds in the range of 10 –10 mole fraction in water, increaseignificantly with temperature. Except for o-nitro-benzoic acid, the solubility data are described adequately with the Wilson equation. The λHquation gives good agreement with all experimental data. The results indicate that the molecular structure and interactions affect the solubilitiesignificantly.

2006 Elsevier B.V. All rights reserved.

; m-M

pt[aapmim

acWds

eywords: o-Nitro-benzoic acid; p-Hydro-benzoic acid; p-Methyl-benzoic acid

. Introduction

The physical–chemical properties of organic substancesre essential for understanding their environmental behavior.mong these properties, aqueous solubility plays a prominent

ole in the prediction of the environmental fate of chemi-als. Solubility in water affects processes, such as evaporation,bsorption and bioaccumulation [1,2].

Although there have been many studies of the solubility ofrganic compounds in water, data on its temperature dependencere scare or unavailable [3]. However, such temperature depen-ence of physical–chemical properties is important to improvehe predictive capability of environmental models [4]. In fact,ifferences in temperature, seasonally diurnally, and betweenifferent regions of the world can influence the environmentalehavior of organic chemicals.

Benzoic acid derivatives, which are considered as importantriority contaminants or typical environmental pollutants, maympact on the environment in a number of ways and with great

∗ Corresponding author. Tel.: +86 22 60601461.E-mail address: jiaqingzhu88@126.com (J. Qing-Zhu).

2

2

((

378-3812/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2006.10.014

ethyl-benzoic acid; Solubility in water; Wilson equation; λH equation

otential risk to human health [5,6]. They are widely used reac-ion intermediates and may be discharged into the environment7,8] as well as are often formed as a result of degradation ofromatic compounds. For example, chlorinated benzoic acidsre the most common metabolites following the degradation ofoly-chlorinated biphenyls [9]. In order to evaluate the environ-ental contamination performance of benzoic acid derivatives

n soil and groundwater, basic physical–chemical parametersust be known.The purpose of this work is to report solubilities of benzoic

cid derivatives in water at several temperatures and to test theapability of selected equilibrium models to describe these data.e have chosen the Wilson [10] and λH models [11,12] to pro-

uce rapid and easy methods to provide acceptable values ofolubility as a function of temperature.

. Experimental

.1. Materials

The benzoic acid derivatives tested include: benzoic acid>99%), o-nitro-benzoic acid (>99%), p-hydro-benzoic acid>99%), p-methyl-benzoic acid (>99%) and m-methyl-benzoic

166 J. Qing-Zhu et al. / Fluid Phase Equilibria 250 (2006) 165–172

Table 1The melting temperatures (Tm) and the enthalpies of fusion (�mH) of the benzoicacid and its derivatives

Compounds Tm (◦C) �mH (kJ mol−1)

Benzoic acid 122.35 18.02o-Nitro-benzoic acid 145.80 27.99p-Hydrogen-benzoic acid 215.55 30.90pm

aoicd

2

pmse

dwrtd

wnmbtuh

Ficw

Fd

(k

tmstsbrpap

meb

-Methyl-benzoic acid 183.55 28.40-Methyl-benzoic acid 109.85 15.73

cid (>99%). The melting temperatures (Tm) and the enthalpiesf fusion (�mH) of the benzoic acid derivatives [13] are listedn Table 1. These compounds were purchased from commer-ial sources, and used without any further purification. Doublyistilled water was used.

.2. Apparatus and procedure

The apparatus, shown in Fig. 1, has been described in detailreviously [14], so only a brief presentation is made here. Theain elements are an equilibrium vessel, an electromagnetic

tirrer, a thermostatic controller, water bath, and laser monitoringquipment.

The experimental procedure measured the temperature ofissolution of a known, fixed-concentration solution containedithin a well-stirred vessel. In the procedure, heating is at a slow

ate with thorough mixing, so the measured benzoic acid deriva-ive concentration is the equilibrium solubility at the measuredissolution temperature.

Pre-weighed amounts of benzoic acid derivatives and waterere placed into a vessel. The vessel was then stoppered, con-ected to a circulating water bath, and stirring was started. Theixture was initially equilibrated at a temperature significantly

elow the dissolution temperature for at least 1 h. The tempera-

ure of the mixture was then slowly increased in stepwise fashionntil the temperature at which all the benzoic acid derivativesad dissolved was reached. Near the dissolution temperature

ig. 1. Equipment for measurement of solubility of benzoic acid derivativesn water. 1: thermometer; 2: thermometer; 3: whisk machine; 4: thermostaticontroller; 5: electromagnetic stirrer; 6: whisk rotor; 7: equilibrium vessel; 8:ater bath; 9: laser; 10: laser receiver; 11: recorder.

rasuw±sl

cwiTua

3

3

s

ig. 2. Experimental solubility of benzoic acid in water compared with literarualata.

more than 1 ◦C below) the temperature increase was typicallyept at 0.2 ◦C/20 min or slower.

The required temperature was maintained by circulatinghermostatically controlled water through the jacket. The laser

onitoring equipment was used to observe and monitor the dis-olution condition of the solution. A steady laser beam passeshrough an aperture and adjustable lens and then through theolvent–solute mixture. If there are solids in the path of theeam, it will be scattered and the transmitted intensity will beeduced. The intensity of transmitted laser is recorded by a com-uter in terms of photovoltage. The corresponding temperaturet given composition is determined as the one at which the solidhase just disappears.

During the experimental procedure, errors in the final resultsay arise in three ways: impurity of the compounds, weighing

rror and temperature error. The weighing error can be neglectedy mass preparation using an analytical balance with an accu-acy of ±0.00001 g. The purity of the chemicals tested is >99%nd impurity appears to be a homolog that does not affect theolubility data significantly. The temperature error is greatestncertainty in the final results. Therefore, the thermometer thatas used in the experiments was calibrated with an accuracy of0.05 ◦C and the rate of temperature increase was controlled

trictly at less than 0.2 ◦C/20 min or even slower near the disso-ution temperature.

The experimental setup and its accuracy were validated byomparing the experimental solubility data of benzoic acid inater with those in literature [15]. As shown in Fig. 2, the solubil-

ties obtained in this work are in good agreement with literature.he deviation of the measured solubilities from literature val-es was <1%. In this work, the solubilities determined had anverage relative deviation (AAD) of <5%.

. Results and discussion

.1. Experimental results

Solubilities of benzoic acid derivatives in water were mea-ured and are listed in Tables 2–5, where T is the temperature

J. Qing-Zhu et al. / Fluid Phase Equilibria 250 (2006) 165–172 167

Table 2The solubility data of o-nitro-benzoic acid and regression results with Wilson equation and λH equation

T (◦C) xexp × 10−4 Wilson equation, (xexp − xcal) × 100/xexp λH equation, (xexp − xcal) × 100/xexp ln �1∞ HE (J mol−1)

19.12 5.560 15.3 −2.2 4.20 1468.50020.32 5.861 14.6 −0.8 4.19 1547.99921.43 6.131 13.7 0.0 4.18 1619.31123.89 6.831 12.3 2.7 4.15 1804.19424.36 6.812 10.1 1.0 4.15 1799.17625.13 7.290 12.6 5.2 4.14 1925.42528.34 7.70 3.5 0.5 4.11 2033.71429.32 7.80 0.2 −2.5 4.10 2060.12529.84 7.98 0.0 −0.5 4.10 2107.66730.85 8.39 0.1 1.2 4.08 2215.95531.92 8.910 1.0 3.8 4.07 2353.29734.33 9.491 −4.0 2.7 4.05 2506.7535.43 9.642 −7.5 0.9 4.04 2546.63236.39 9.721 −11.4 −1.2 4.03 2567.49737.42 10.454 −8.7 2.8 4.02 2761.09638.42 10.473 −13.4 0.0 4.01 2766.11539.53 10.79 −15.7 −0.3 4.00 2849.8440.39 11.12 −16.7 0.0 3.99 2936.99941.28 11.34 −19.0 −0.7 3.98 2995.105

The xexp stands for the experimental solubility (in mole fractions of the solute). The xcal stands for the calculated solubility. The ln γ1∞ is the logarithm infinitedilution activity coefficients. The HE indicates the molar mixing enthalpy of solution.

Table 3The solubility data of p-hydro-benzoic acid and regression results with Wilson equation and λH equation

T (◦C) χexp × 10−4 Wilson equation, (χexp − χcal) × 100/χexp λH equation, (χexp − χcal) × 100/χexp ln γ1∞ HE (J mol−1)

18.92 4.561 3.9 0.0 2.62 644.822320.32 5.012 6.3 2.8 2.61 708.583525.62 5.931 −2.0 −4.4 2.58 838.509327.47 6.430 −2.6 −4.6 2.57 909.056630.17 7.310 −2.3 3.6 2.56 1033.46931.03 7.554 −2.9 −4.1 2.55 1067.96531.82 7.992 −0.8 −1.8 2.55 1129.88832.33 8.331 0.9 0.0 2.54 1177.81533.19 9.184 6.5 5.9 2.54 1298.4134.13 9.542 6.2 5.7 2.54 1349.02335.62 9.944 3.8 3.5 2.53 1405.85736.92 10.13 0.0 0.0 2.53 1432.15337.85 10.54 0.0 0.0 2.52 1490.11838.74 10.93 −0.2 0.0 2.51 1545.25539.19 11.22 0.4 0.6 2.51 1586.25440.23 11.48 −1.3 −1.4 2.51 1623.012

Table 4The solubility data of p-methyl-benzoic acid and regression results with Wilson equation and λH equation

T (◦C) xexp × 10−5 Wilson equation, (xexp − xcal) × 100/xexp λH equation, (xexp − xcal) × 100/xexp ln γ1∞ HE (J mol−1)

30.85 4.548 9.0 4.7 6.35 297.820831.95 4.759 7.6 3.7 6.33 311.637933.15 4.865 3.6 0 6.31 318.579235.00 5.050 −2.8 −5.8 6.28 330.693736.00 5.275 −3.8 6.4 6.26 345.427636.70 5.513 −3.1 −5.4 6.25 361.012737.20 5.843 0.0 −1.9 6.24 382.622441.70 8.275 0.0 10.6 6.17 541.879244.70 9.068 10.3 6.0 6.12 593.80847.20 9.743 4.6 1.8 6.08 638.009650.10 10.10 −0.5 −8.1 6.03 661.3874

168 J. Qing-Zhu et al. / Fluid Phase Equilibria 250 (2006) 165–172

Table 5The solubility data of m-methyl-benzoic acid and regression results with Wilson equation and λH equation

T (◦C) xexp × 10−5 Wilson equation, (χexp − χcal) × 100/χexp λH equation, (χexp − χcal) × 100/χexp ln γ1∞ HE (J mol−1)

23.80 2.961 −3.3 6.7 9.58 74.0047725.30 3.199 −5.2 −1.7 9.53 79.9513927.40 4.217 0.0 4.7 9.46 105.389428.40 4.468 −1.0 0.4 9.43 111.666229.10 4.997 1.1 4.3 9.41 124.880530.30 5.394 0.2 0.0 9.37 134.791131.00 5.724 0.1 −1.2 9.35 143.049932.30 6.544 0.8 −0.8 9.31 163.531532.90 7.112 1.7 1.6 9.29 177.736233.65 7.297 0.3 −3.3 9.27 182.36135.15 8.249 −0.3 −5.9 9.22 206.145236.05 9.002 0.2 −5.4 9.20 224.974136.45 9.914 1.1 0.0 9.19 247.766636.55 10.443 2.1 4.1 9.18 260.979437.25 11.567 2.7 7.4 9.16 289.056238.45 11.805 0.0 1.8 9.13 295.001840.52 14.514 −0.8 0.7 9.07 362.71364 2.2 9.02 430.42174 1.0 8.97 502.4194

(fucw1

itpadaTnsi

Fl

2.02 17.224 −1.43.85 20.105 −3.2

◦C), and xexp stands for the experimental solubility (in moleractions of the solute). The experiments show that the sol-bilities of these compounds increase with temperature. Theomponents are sparingly soluble in water (<0.01 mol/L),ith solubilities that are very low, typically of the order of0−4–10−5 mol/mol.

The solubilities from different researchers are comparedn Figs. 3–5. Benzoic acid has been studied several times inhe literature [15,16]. The solubilities of nitro-benzoic acid,-hydro-benzoic acid and p-methyl-benzoic acid in water asfunction of temperature T or at a few temperatures were

etermined by Apelblat and Manzurola [15,17,18] and Li etl. [19]. The solubilities in water as a function of temperature

of o-nitro-benzoic acid and m-methyl-benzoic acid haveot been published before. As can be seen in Figs. 3–5, theolubility data of this work and the measurements publishedn literature are in reasonably good agreement, in particularly

ig. 3. Experimental solubility of p-hydro-benzoic acid in water compared withiteratural data.

Fl

fbbl

Fw

ig. 4. Experimental solubility of o-nitro-benzoic acid in water compared withiterarual data.

or p-methyl-benzoic acid. To some extent, the differencesetween this work and the work of Apelblat and Manzurola cane caused by the different experimental approach. In case ofow solubilities, in particular, the extremely low solubilities of

ig. 5. Experimental solubility of p-methyl-benzoic acid in water comparedith literatural data.

se Eq

bosba

3

aowd(

l

wmgtdr

isw

3

cspwdz(m

x

wa

3

l

l

ot

Λ

efib

tsod

etiot(

3

s[ifi

l

TT

S

oppm

A

J. Qing-Zhu et al. / Fluid Pha

enzoic acid derivatives, analysis and especially the separationf the solid dispersed in the liquid phase are difficult andensitive to deviations. As a consequence, slight deviationsetween different solubility experiments can be consideredcceptable.

.2. Models for solubility calculations

In general, solid–liquid equilibria (SLE) can be described byn equation containing pure solute properties such as enthalpyf fusion, melting temperature and so on. If, as is the case in thisork, a solid–solid transition does not occur, the equation forescription of SLE can be simplified to the form shown as Eq.1):

n γixi = −�mHi

R

(1

T− 1

Tmi

)(1)

here �mHi is the enthalpy of fusion of the solute i, Tmi theelting temperature, T the absolute temperature, R the universal

as constant, xi indicates the real mole fraction and γ i stands forhe activity coefficient. Eq. (1) was used with the experimentalata to determine parameters in an activity coefficient model byegression.

According to Eq. (1), the activity coefficient γ i is a key valuen calculating the solubility, knowing Tmi and �mHi.. Out ofeveral activity coefficient models which can be selected, in thisork, the Wilson equation was chosen.

.2.1. Empirical formulaSince the aqueous solubility is the highest equilibrium con-

entration of a compound in water at certain temperature, thepecified temperature is the most important and fundamentalarameter. In this work, an empirical formula shown in Eq. (2)as used to correlate the aqueous solubilities of benzoic aciderivatives, both the presented and literature results for ben-oic acid. All regressions produced high correlation coefficientsR2 > 0.99), indicating satisfying descriptive capability of theodels obtained.

= AT 3 + BT 2 + CT + D (2)

here x is the aqueous solubility (as mole fraction of the solute)nd T stands for the dissolving temperature (◦C).

fFr

able 6he regression results of Wilson equation and λH equation

amples Wilson equation

g21 − g11 g12 − g22

-Nitro-benzoic acid 7788.55 40298.17-Hydrogen-benzoic acid 3925.87 23547.88-Methyl-benzoic acid 13526.84 28336.41-Methyl-benzoic acid 23625.47 113.859

AD∗ =∑

n

|Xexp−Xcal|Xexp

× 100n

.

uilibria 250 (2006) 165–172 169

.2.2. Wilson equationFor a binary system, the Wilson equation is equated as fol-

ows:

n γ1 = 1 − ln (χ2Λ12 + χ1)

−(

χ1

χ1 + χ2Λ12+ χ2Λ21

χ2 + χ1Λ21

)(3)

f which Λij is the Wilson equation parameter, which is a func-ion of temperature.

ij = Vj

Vi

exp

(−gji − gii

RT

)(4)

In general, the energy parameter (gji − gii) in the Wilsonquation is thought to be independent of temperature. There-ore, if the difference in molar volume between the constituentss ignored, two parameters remain in the Wilson equation for ainary system: (g21 − g11) and (g12 − g22).

Eq. (1) was combined with the Wilson equation to determinehe Wilson parameters by regression of the measured aqueousolubilities. The results are shown in Tables 2–5. The valuesf the objective function AAD and the parameters that wereetermined by regression are listed in Table 6.

To test the accuracy of the regressed parameters of the Wilsonquation, the degree of confidence was calculated according tohe F-function, the degree of confidence is calculated with thencomplete beta function. The results show that all the degreesf confidence of the Wilson parameters (g21 − g11) regressed forhe different substances are >0.9825. For the Wilson parametersg12 − g22), the degree of confidence is >0.9985.

.2.3. λH equationThe λH equation, Eq. (5), is another way to describe the

olution behavior and was suggested firstly by Buchowski et al.11,12], which is used particularly for SLE. It is thermodynam-cally correct and gives a good description of experimental dataor many systems although only two parameters, λ and H, arenvolved.

n

[1 + λ(1 − xi)

xi

]= λH

(1

T− 1

Tmi

)(5)

In Eq. (5), λ and H are two equation parameters, and xi standsor the mole fraction of a solute in a binary, saturated solution.or all the systems studied, the parameters λ and H, which wereegressed using a non-linear optimization method, showed no

λH equation

AAD* λ H (K) AAD*

9.5 0.008207 321046 1.52.5 0.02243 173764 2.45.1 0.005584 791050 4.91.9 0.03064 302472 2.7

170 J. Qing-Zhu et al. / Fluid Phase Equilibria 250 (2006) 165–172

Table 7The solubility data of benzoic acid, the ln γ1∞ and the HE

T (◦C) xexp × 10−4 ln γ1∞ HE (J mol−1)

17.20 3.889 5.78 2114.50818.15 4.011 5.77 2179.73719.86 4.272 5.74 2321.06622.82 4.623 5.69 2511.31824.00 4.951 5.67 2690.69824.23 4.972 5.67 2701.56930.01 5.948 5.58 3234.27331.33 5.988 5.56 3256.01633.57 6.801 5.53 3696.31235.29 7.071 5.50 3843.07740.01 8.285 5.43 4500.80344

stHT

3

ttst

ataaocoo

TT

T

2223333334444444

tbwt

atemaTtTvit

0.03 8.354 5.43 4538.8541.50 8.752 5.41 4756.284

ignificant variation with temperature. The regression results ofhe aqueous solubilities and the values of the parameters λ and

as well as AAD for the solubility calculations are shown inables 2–6.

.3. Comparison among the solubility of substances

For completeness of discussion and in order to calculatehe infinite dilution activity coefficients (ln γ1∞) and the par-ial molar excess enthalpy of solution (HE), previously reportedolubilities of m-nitro-benzoic acid [11] were also included inhe analysis of this work.

Since the solubilities of the benzoic acid derivatives in waterre very low, the correlations (3) and (5) can be extrapolated tohe limit of infinite dilution in the solutes. The infinite dilutionctivity coefficients (ln γ1∞), calculated with Wilson equation,re shown in Tables 2–5, 7 and 8. It becomes clear that the ln γ1∞

f p-hydro-benzoic acid are much lower than those of the otheromponents studied. This can be attributed to the smaller sizef the p-hydro-benzoic acid molecules and stronger interactionsf the p-hydro-benzoic acid molecules with water. According

able 8he solubility data of m-nitro-benzoic acid, the ln γ1∞ and the HE

(◦C) xexp × 10−4 ln γ1∞ HE (J mol−1)

6.05 2.090 5.61 187.13047.45 2.350 5.59 210.40989.60 2.880 5.55 257.86391.20 2.890 5.52 258.75923.22 2.871 5.49 257.05815.50 3.527 5.45 315.79377.54 4.256 5.42 381.06558.82 4.544 5.40 406.85199.80 4.954 5.38 443.56171.00 5.081 5.36 454.93281.81 5.856 5.35 524.32322.78 6.084 5.33 544.73743.24 6.292 5.32 563.36094.13 6.501 5.31 582.07405.70 7.787 5.28 697.21747.45 8.583 5.25 768.4881

s(

l

rtwmm

TT

S

Boppmm

Fig. 6. The relationship between ln r and 1/T.

o the analysis of Tsonopoulos and Prausnitz [20] for methyl-enzoic acid, its molecules are bigger, while its interactions withater are expected to be weaker and its activity coefficients are

herefore higher than those of others.The infinite dilution activity coefficients (ln γ1∞) versus 1/T

re plotted in Fig. 6. According to Eq. (6), fitting to Eq. (7) yieldshe parameter B, which is equal to R times the partial molarxcess enthalpy at infinite dilution, and A which is the partialolar excess entropy at infinite dilution compared to the solute

s a hypothetical liquid. Positive values of A and B, shown inable 9, indicate that both enthalpic and entropic effects reduce

he solubility. This is the case for most of the compounds inable 9. Note that the m-methyl substitution has a much loweralue of A and a higher value of B than the others, exceed-ng �mH. This is an indication of the hydrophobic nature ofhe methyl substitution and the hydrophilic nature of the otherubstitutions.

∂ ln γi

∂T

)p,x

= − H̄Ei

RT 2 (6)

n r1∞ = A + B1

T(7)

Due to the different positions of –NO2 group in the benzeneing, the molecular volume of the m-nitro-benzoic acid is larger

han that of the o-nitro-benzoic acid. Thus, much more “energy”ould be needed to break the hydrogen bonds of water in the-nitro-benzoic acid solution. As a consequence, the value B of-nitro-benzoic acid is larger than that of o-nitro-benzoic acid.

able 9he fitting results of ln γ1∞ and 1/T

amples A B R2

enzoic acid 1 1388.8 1-Nitro-benzoic acid 1 936.8 1-Hydrogen-benzoic acid 0.9991 472.45 1-Methyl-benzoic acid 0.9998 1627.1 1-Methyl-benzoic acid 0.0021 2843.3 1-Nitro-benzoic acid 0.293 1591.8 1

se Eq

Ooismtiitm

oeitsT

H

rmrvg

3

tabid[tdipf

4

tcoiid

1

2

3

4

LABgHHH

lRTTxx

GΛ

A

(

R

[

[

[[

J. Qing-Zhu et al. / Fluid Pha

n the other hand, though both –OH group and –CH3 group aren the p-position in the benzene ring, because firstly, the –OHs polar and the –CH3 is non-polar, and secondly, the –OH isimilar to that of water, much more hydrogen bonds with waterolecules would be formed in the p-hydro-benzoic acid solu-

ion. As a result, the value of B of p-hydro-benzoic acid solutions much lower than that of p-methyl-benzoic acid solution, asndicated in Table 9. Hence, it could be concluded that the struc-ural and interaction origins might affect the dissolution behavior

arkedly.In the λH equation, the parameter H is related to the enthalpy

f solution per mole of solute. The expression due to Buchowskit al. [11] is given in Eq. (8), where HE is the molar mix-ng enthalpy of the solution. Eq. (8) can be used to estimatehe values of HE in order to get a better understanding of theolution characteristics. The values of HE are also shown inables 2–5, 7 and 8:

R = �mHi + HE

xi

(8)

The values of HE are positive, which indicates the existence ofepulsive interaction between benzoic acid derivates and waterolecules. Also, if there are two polar groups in the benzene

ing, like in nitro-benzoic acid and in hydro-benzoic acid, thealues of HE are much higher than those with only one polarroup in the benzene ring.

.4. Comparison between models

From the results shown in Tables 2–6, it can be observed thathe goodness of fit for the Wilson equation and the λH equationre almost the same for p-hydro-benzoic acid and p-methyl-enzoic acid. In the case of o-nitro-benzoic acid, the λH equations much better than the Wilson equation, which is in correspon-ence with the results by U.J. Domanska and Mapeisheng et al.21,22,23]. Also, it seems that the problem with the Wilson equa-ion for the o-nitro compound is that its parameter is temperatureependent. This is not surprising since this substance would haventernal H-bonding that would decrease more rapidly with tem-erature than the physical effects. This will be investigated inuture work.

. Conclusions

Determination of the temperature dependence of solubili-ies in water is important for improvement of the predictiveapability of environmental models. Using a laser monitoringbservation technique, solubilities of benzoic acid derivativesn water as a function of temperature have been determinedn this work. From all results, some conclusions can berawn:

. The solubilities of o-nitro-benzoic acid, p-hydro-benzoicacid, p-methyl-benzoic acid and m-methyl-benzoic acid inwater are very low, usually of the order of 10−4–10−5 mol/mol.

[

[

uilibria 250 (2006) 165–172 171

. The solubilities of the compounds increase with temperature.Owing to the special molecular structure, the solubilities ofdifferent benzoic acid derivatives in water as a function oftemperature vary significantly.

. Empirical formulas are applicable to regress the aqueous sol-ubility of these compounds, and the relativity coefficients R2

were above 0.99.. The experimental solubilities are well represented by the

Wilson equation and especially the λH equation.

ist of symbolsparameter in Eq. (7)parameter in Eq. (7)

21 − g11, g12 − g22 parameters in Wilson equationparameters in λH equation (K)

E molar mixing enthalpy of solution¯ E

i partial molar excess enthalpy of solutionn γ1∞ infinite dilution activity coefficient in the solutes

Gas constant, 8.314 J mol−1 K−1

experimental temperature in Eq. (1) (K)mi fusion temperature in Eq. (1) (K)1 mole fraction of the solute in Wilson equation2 mole fraction of the solvent in Wilson equation

reek lettersij parameters in Wilson equation

cknowledgement

Thanks for the fund support of the Nation Natural ScienceNo. 20676101).

eferences

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