Study of Weak Molecular Interactions through Thermodynamic Mixing

J. Phys. Chem. B 2006, 110, 17683-17690

17683

Study of Weak Molecular Interactions through Thermodynamic Mixing Properties Beatriz Giner, Santiago Martıń, He´ ctor Artigas, Marıá C. Lo´ pez, and Carlos Lafuente* Departamento de Quı´mica Orga´ nica-Quı´mica Fı´sica, Facultad de Ciencias, UniVersidad de Zaragoza, 50009 Zaragoza, Spain ReceiVed: April 27, 2006; In Final Form: July 11, 2006

The electron donor-acceptor abilities of some cyclic ethers (tetrahydropyran or tetrahydrofuran), benzene, and halobenzenes (fluorobenzene or chlorobenzene) and the molecular interactions between these compounds have been investigated through a wide set of thermodynamic mixing properties of their mixtures. The mixing properties have been derived from experimental measurements of density, speed of sound, refractive index, surface tension, heat of mixing, and vapor-liquid equilibrium at the temperature of 298.15 K

Introduction The electron-donor ability of some cyclic ethers is well known.1,2 It has been pointed out that, in mixtures containing cyclic ethers and aromatic hydrocarbons, the interactions established between them involve a charge-transfer mechanism among the lone pair of electrons on the oxygen atom of the ether and the aromatic ring that acts like a weak electronacceptor.3,4 On the other hand, it must be also taken into account that when a cyclic ether is mixed with an haloaromatic compound, the oxygen atom can interact directly with the halogen atom. In addition, the presence of substituents in the aromatic hydrocarbon should modulate its electron-acceptor ability,5 and in the same way the proximity of the aromatic ring to the halogen atom also affects the acceptor ability of the latter. One of the purposes of this work is to determine the extent of the modification in the acceptor character of both the aromatic ring and the halogen atom and on the other hand to systematize the differences in electron donor ability of the cyclic ethers involved here. In this work, a comprehensive thermodynamic study of mixtures formed by a cyclic ether (tetrahydropyran or tetrahydrofuran) and an aromatic compound (benzene, fluorobenzene, or chlorobenzene) has been performed. These kinds of mixtures have attracted the attention of some scientists before,3,4,6-12 although these studies were focused mainly on the interpretation of volumetric or calorimetric measurements of mixtures involving tetrahydrofuran or tetrahydropyran with benzene An entire study requires the complete knowledge of a set of properties of the studied mixtures involving both bulk and surface properties, and it is only then when a minute description of the processes occurred during the mixture can be picked up. The methodology for the thermodynamic characterization is based on measurements of density, speed of sound, refractive index, surface tension, heat of mixing, and vapor-liquid equilibrium together with the derivation of the corresponding excess or deviation properties. The proper interpretation of the results is considered one of the most extended and reliable ways of obtaining information about the structure and interactions of mixed solvents.13-15 In this work, a detailed analysis of all these * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +34-976-762295. Fax: +34-976-761202.

properties has been performed, and it has provided excellent pieces of knowledge about our study target. Experimental Section The compounds used were: tetrahydrofuran (>99.5%), benzene (>99.9%), and fluorobenzene (>99%) obtained from Aldrich, chlorobenzene (>99.5%) provided by Fluka, and tetrahydropyran (>99%) provided by Acros. The purity of the chemicals was checked by comparing the measured densities and refractive indices with those reported in the literature.16-20 No further purification was considered necessary. Densities, F, of the pure compounds and their mixtures were determined with an Anton Paar DMA-58 vibrating tube densimeter automatically thermostated within (0.01 K. The accuracy of the densimeter after proper calibration with deionized doubly distilled water and dry air is (10-5 g‚cm-3, and the precision of the density measurements was (5 × 10-6 g‚cm-3. Speeds of sound, u, were obtained with an Anton Paar DSA48 vibrating tube densimeter and sound analyzer. The temperature was automatically kept constant within (0.01 K. The precision of the speed of sound measurements is (0.1 m‚s-1. The accuracy of the speed of sound is (1 m‚s-1. Calibration of the apparatus was carried out with air and deionized doubledistilled water. The corresponding refractive indices at 589.3 nm sodium D wavelength were measured using a high-precision automatic refractometer Abbemat-HP Dr. Kernchen whose temperature was internally controlled within (0.01 K. The apparatus was calibrated with deionized double-distilled water. The reproducibility of the measurements is (1 × 10-6 and the corresponding accuracy is (2 × 10-5. Heats of mixing were determined using a Thermometric 2277 thermal activity monitor with a combination measuring cylinder (LKB 2277-204) running under constant flow conditions,21 total flow rate ) 5 × 10-3 mL‚s-1. During the experiments, the temperature was kept constant within (2 × 10-4 K. Two Shimadzu LC-10ADVP HPLC pumps were used to drive the liquids. The pumps were calibrated for each liquid before the calorimetric measurements. The uncertainty in the mole fractions of the binary mixtures, calculated from the uncertainty in the flow delivered by the pumps, is (0.001. The calibration of the calorimeter was achieved with reference to the very accurate HE of the mixture n-hexane with cyclohexane.22 The accuracy

10.1021/jp062583q CCC: $33.50 © 2006 American Chemical Society Published on Web 08/12/2006

17684 J. Phys. Chem. B, Vol. 110, No. 35, 2006

Giner et al.

TABLE 1: Properties of Pure Compounds at 298.15 K property Fexptl/g‚cm3 Flit/g‚cm3 nDexptl nDlit V/cm3‚mol-1 u/m‚s-1 κS/TPa-1 R/kK-1 CP/J‚mol-1 po/kPa B/cm3‚mol-1 σ/mN‚m-1 a

tetrahydrofuran

tetrahydropyran

benzene

fluorobenzene

chlorobenzene

0.88209 0.88197a 1.404642 1.40468e 81.746 1277.8 694.4 1.243 124.05 21.610 -1170 27.04

0.87915 0.87916b 1.418723 1.41862c 97.973 1270.1 705.3 1.156 149.8 9.56 -1165 27.30

0.87354 0.87360c 1.497740 1.49792c 89.421 1299.2 678.3 1.213 135.76 12.690 -1501 28.57

1.01879 1.01872d 1.462902

1.10091 1.10099d 1.521758 1.52185e 102.242 1267.9 565.0 0.991 151.04 1.590 -1573 33.00

94.332 1166.0 722.0 1.136 146.36 10.562 -1772 27.43

Reference 16. b Reference 17. c Reference 18. d Reference 19. e Reference 20

in the determination of the heats of mixing could be expected to be (1%. More details of procedure and calibration can be found in a previous paper.23 The vapor-liquid equilibrium was studied using an all-glass dynamic recirculating type still that was equipped with a Cottrell pump.24 It is a commercial unit (Labodest model) built in Germany by Fischer. The equilibrium temperature were measured to an accuracy of (0.01 K by means of a thermometer (model F25 with a PT100 probe) from Automatic Systems Laboratories, and the pressure in the still was measured with a Digiquartz 735-215A-102 pressure transducer from Paroscientific equipped with a Digiquartz 735 display unit. The accuracy of the pressure measurements was (0.01% of reading. Compositions of both phases vapor and liquid were determined by a densimetric analysis using an Anton Paar DMA-58 vibrating tube densimeter, the error in the determination of liquid and vapor mole fractions was estimated to be (0.0002. The proper operation of the different devices was periodically checked and rearranged if necessary. The surface tensions, σ, were determined using a drop volume tensiometer Lauda TVT-2.25 The temperature was kept constant within (0.01 K by means of an external Lauda E-200 thermostat. Details of the experimental procedure can be found in a previous paper.26 The accuracy of the surface tension measurement was (0.5% of the final value of surface tension, and the corresponding reproducibility was (0.03 mN‚m-1. The pure compound properties at 298.15 K are collected in Table 1. Mixtures were prepared by mass using a Sartorius semi-micro balance with a precision of (10-5 g. The possible error in the mole fractions was estimated to be less than 10-4.

where the correct expression27 for κid S was obtained according to Benson and Kiyohara28 is:

κid S )

[

∑i φi κS,i +

Cp,i

(

VE ) x1

) (

)

M1 M 1 M2 M 2 + x2 F F1 F F2

(1)

Isentropic and excess isentropic compressibilities were estimated from densities, F, and speeds of sound, u, by using the following equations:

1 Fu2

(2)

κES ) κS - κid S

(3)

κS )

( - T(

∑i xiVi)

∑i φiRi)2

∑i

(

(4) xiCp,i)

where φi is the volume fraction of component i in the mixture referred to the unmixed state, xi the corresponding mole fraction, T the absolute temperature, and κS,i, Vi, Ri, and Cp,i are the isentropic compressibility, the molar volume, the thermal expansion coefficient, and the molar heat capacity of the pure component i, respectively. All of these properties of the pure substances are listed in Table 1. Our VE results for the mixtures tetrahydrofuran or tetrahydropyran with benzene are in excellent agreement with those of Meyer7 and Andrews,3 respectively. Excess molar volumes and excess isentropic compressibilities are graphically represented in Figures 1 and 2, respectively. Refractive index deviations, ∆nD, were calculated following the suggestions of Fialkov and Fenerly29 and Fialkov30 using volume fractions:

∆nD ) nD - φ1nD,1 - φ2nD,2

(5)

where nD is the refractive index of the mixture and nD,i is the refractive index of component i. Refractive index deviations are represented in Figure 3. Excess molar enthalpies, HE, were estimated by means of the equation:

Results and Discussion Excess molar volumes, VE, were calculated from the density of the mixture, F, densities, Fi, and molar masses, Mi, of the pure compounds, and the corresponding molar fractions, xi, by means of the equation:

]

TViRi2

HE )

Q˙ n˘ 1 + n˘ 2

(6)

where Q˙ is the measured compensating heating power, which corresponds to the heat of mixing per second and n˘ i is the molar flux of component i. Our HE values for the mixtures tetrahydrofuran or tetrahydropyran with benzene are in excellent agreement with those of Inglese11 and Andrews,4 respectively. However, our results for tetrahydrofuran with chlorobenzene are placed between those of Ruı´z,12 which are lower in absolute value than ours, and those of Mahl,8 which are higher in absolute value. The excess molar enthalpies are represented in Figure 4. The pressure-composition diagrams (P - x1 - y1) are shown in Figures 5 and 6. It can be pointed out that only the mixture tetrahydropyran with fluorobenzene shows an azeotrope (Paz ) 9.2 kPa at x1,az ) y1,az ) 0.684).

Weak Molecular Interactions

J. Phys. Chem. B, Vol. 110, No. 35, 2006 17685

Figure 1. Excess molar volume, VE, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).

Figure 3. Refractive index deviation, ∆nD, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).

Figure 2. Excess isentropic compressibility, κES , at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).

Figure 4. Excess molar enthalpy, HE, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).

The activity coefficients of the components in the liquid phase were calculated from the following equations:

where xi and yi are the liquid and vapor phase compositions, P is the total pressure, poi are the vapor pressures of the pure compounds, Bii are the second virial coefficients that were estimated from PRSV-EoS,31,32 Bij is the cross second virial coefficient calculated using a suitable mixing rule, Voi are the molar volumes of the saturated liquids, R is the gas constant, and T is the temperature. The thermodynamic consistency of the experimental results

γi ) where

yiP xipoi

[

exp

]

(Bii - Voi ) (P - poi ) + (1 - yi)2 Pδij RT

(7)


Giner et al.

GE ) RT(x1 ln γ1 + x2 ln γ2)

(9)

From excess molar enthalpies and Gibbs functions, the excess molar entropies, SE, can be estimated using the thermodynamic relation:

SE )

HE - GE T

(10)

Our GE results for the mixture tetrahydrofuran with benzene are in good agreement with those of Deshpande.6 Excess molar Gibbs functions, GE, and excess molar entropies, SE, are graphically represented in Figures 7 and 8, respectively. Surface tension deviations, ∆σ, were calculated from surface tension measurements according to the following equation:35-37

∆σ ) σ - x1σ1 - x2σ2

Figure 5. Pressure-composition diagram (P - x1 - y1) at 298.15 K for tetrahydrofuran (1) + aromatic compounds (2): benzene (0, 9), fluorobenzene (O, b), and chlorobenzene (4, 2).

(11)

where σ is the surface tension of the mixture and σi is the surface tension of the component i. In Figure 9, surface tension deviations are graphically represented. The surface of a mixture is enriched usually in the component of lower surface tension (Gibbs adsorption). Quantitative information about this enrichment is given by the surface mole fractions, xσi . The composition of the surface mixture, that is, the surface mole fraction of component i, is defined by:

xσi ) Γσi /

∑iΓσi

(12)

where Γσi is the surface excess concentration of component i, which are the real populations in the surface phase. This concentration is related with Γji, the relative surface excess concentration of component i with respect component j, by means of the classical relationship:

Γji ) Γσi - (xi/xj)Γσj ) -(∂σ/∂µi)T

(13)

Finally, the magnitudes Γσi and xσi can be evaluated by taking into account that, in the surface phase, composed by a monomolecular layer, the surface excess concentrations are related through the equation:

∑iAσi Γσi ) 1

(14)

where Aσi is the partial molar area of component i, which can be calculated38 from his molar volume, Voi , as [NA(Voi )2]1/3. A representation of xσ1 - x1, that is, excess surface concentration of cyclic ether is shown in Figures 10 and 11. The values of each excess or deviation property, Q, were correlated with a Redlich-Kister polynomial equation, Ai are adjustable parameters, and y is the mole fraction of the mixture except for refractive index deviations; in this case, y is the volume fraction. Figure 6. Pressure-composition diagram (P - x1 - y1) at 298.15 K for tetrahydropyran (1) + aromatic compounds (2): benzene (0, 9), fluorobenzene (O, b), and chlorobenzene (4, 2).

δij ) 2Bij - Bii - Bjj

(8)

was satisfactorily checked using the van Ness method,33 described by Fredenslund et al.34 Excess molar Gibbs functions, GE, were calculated from activity coefficients using the equation:

r

Q ) y1y2

Ap(y1 - y2)p ∑ p)0

(15)

The values of the parameters Ai, together with the standard deviations s(Q) are given in Table 2. Analyzing the results obtained, we can notice that all the excess functions properties studied (VE, κES , HE, GE, and SE) present negative values. In absolute value, properties related to the mixtures containing tetrahydrofuran are larger than the ones that contain tetrahydropyran and the sequence followed by the


Figure 7. Excess molar Gibbs function, GE, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).


Figure 9. Surface tension deviation, ∆σ, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (9), fluorobenzene (b), and chlorobenzene (2).

Figure 8. Excess molar entropy, SE, at 298.15 K for cyclic ether (1) + aromatic compounds (2): benzene (dotted line), fluorobenzene (dashed line), and chlorobenzene (solid line).

absolute YE values is: cyclic ether + benzene < cyclic ether + fluorobenzene < cyclic ether + chlorobenzene. With respect to refractive index deviations, according Nakata and Sakurai,39 the sign of refractive index deviations is opposite to that of excess volume if the behavior of refractive index is not too nonlinear between refractive indices of pure compounds. In our mixtures, this rule is truly fulfilled in all the cases. Regarding surface tension deviations, they are positive for all the mixtures, except for tetrahydropyran with chlorobenzene, over the whole composition range and the sequence followed by the ∆σ values being the same as we have mentioned above.

Figure 10. Excess surface concentration of tetrahydrofuran, xσ1 - x1, at 298.15 K: benzene (dotted line), fluorobenzene (dashed line), and chlorobenzene (solid line).

In the case of the mixture tetrahydropyran + chlorobenzene, values of the surface tension deviations are lower than those for the mixture containing fluorobenzene, being even negative at a low mole fraction of ether. Excess surface concentrations of cyclic ether present positive values for all the mixtures except for the mixtures containing fluorobenzene at low and medium mole fractions of ether. Excess surface concentration values for the mixtures containing


Giner et al. TABLE 2: Parameters, Ai, of Eq 15 and Standard Deviations, s(Q) function

Figure 11. Excess surface concentration of tetrahydropyran, - x1, at 298.15 K: benzene (dotted line), fluorobenzene (dashed line), and chlorobenzene (solid line).

tetrahydrofuran and tetrahydropyran are very similar. For the mixtures containing benzene and fluorobenzene, values are quite small, the excess surface concentration of ether for the mixtures formed by chlorobenzene being the largest ones. Excess functions are defined as the difference between an actual value of a function and that corresponding to an ideal mixture at the same pressure, temperature, and composition. A description of the real behavior of the liquids when they are mixed can be obtained from the analysis of these excess properties. Nonideality of a mixture can be attributed on one hand to structural effects: interstitial accommodation, changes in free volume, and differences in shape and size of the mixed components and, on the other hand, to the energetical effects, that is, molecular interactions that can be weakened or destroyed or established during the mixing process. These last interactions are mainly hydrogen bonding and charge-transfer complex formation, classified as short- and medium-range forces. As a result of the existence of these molecular interactions, complexes or clusters can be created. Similar considerations can be made in the case of deviation from linearity properties. Liquids of not too different molecular size, if interactions between unlike molecules are absent, usually mix with an increase in both volume and isentropic compressibility,40 that is, positive VE and κES values and negative ∆nD values. Examining minutely the excess molar volumes, excess isentropic compressibility, and refractive index deviations obtained, we can conclude that neither structural effects such as changes in free volume nor interstitial accommodation nor the weakening of the dipole-dipole interactions in the pure compounds during the mixing process can explain on their own the observed behavior, and consequently, molecular interactions between the unlike molecules must play an important role. Furthermore, excess molar enthalpies, excess molar Gibbs functions, and excess molar entropies present negative values, and this behavior is usual when polar but nonassociating compounds are mixed and solvation can take place,41 the thermodynamic behavior being dominated by enthalpic (energetic) effects.

A1

A2

A3

s(Q) 0.003 0.1 0.000016 0.01 2 1

Tetrahydrofuran + Fluorobenzene at 298.15 K -0.023 0.112 VE/cm3‚mol-1 -1.735 -0.220 -70.4 -13.2 -8.4 -6.5 κES /TPa1∆nD 0.010492 -0.000056 -0.000511 -0.00528 1.13 1.34 1.84 ∆σ/mN‚m-1 2.25 HE/J‚mol-1 -2883 -343 260 89 -109 -81 81 GE/J‚mol-1 -919

0.004 0.2 0.000012 0.01 6 3

Tetrahydrofuran + Chlorobenzene at 298.15 K -0.112 -0.161 VE/cm3‚mol-1 -2.132 -0.335 -87.5 -30.7 -9.7 -7.6 κES /TPa1∆nD 0.012876 0.000194 0.000981 -0.000695 1.15 1.26 -0.97 ∆σ/mN‚m-1 3.09 HE/J‚mol-1 -3078 -451 451 354 -21 206 GE/J‚mol-1 -1078 29

0.003 0.1 0.000044 0.01 6 3

Tetrahydropyran + Benzene at 298.15 K -0.662 -0.032 -0.039 -0.067 -25.4 5.2 -5.8 -1.8 0.001768 0.000010 0.000278 -0.000042 0.71 -0.07 0.37 -0.33 -1014 -79 94 -25 -352 -27 5 45

0.001 0.1 0.000004 0.01 2 1

Tetrahydropyran + Fluorobenzene at 298.15 K 0.242 0.373 VE/cm3‚mol-1 -0.997 -0.292 -40.1 -4.5 6.8 8.7 κES /TPa-1 ∆nD 0.005639 0.001436 -0.000222 -0.001110 -0.04 0.07 -0.59 ∆σ/mN‚m-1 1.16 242 139 HE/J‚mol-1 -2208 -342 GE/J‚mol-1 -723 -79 -56 74

0.003 0.1 0.000008 0.01 4 2

Tetrahydropyran + Chlorobenzene at 298.15 K -0.219 0.424 VE/cm3‚mol-1 -1.280 -0.370 -69.4 -18.1 3.9 6.8 κES /TPa-1 ∆nD 0.011548 0.001880 -0.000222 -0.002239 0.00 -0.66 1.34 ∆σ/mN‚m-1 0.95 448 179 HE/J‚mol-1 -2484 -334 GE/J‚mol-1 -856 132 -221 133

0.003 0.1 0.000042 0.01 4 3

VE/cm3‚mol-1 κES /TPa1∆nD ∆σ/mN‚m-1 HE/J‚mol-1 GE/J‚mol-1

xσ1

A0

Tetrahydrofuran + Benzene at 298.15 K -1.008 -0.134 0.121 0.058 -55.0 -2.8 -12.3 -5.6 0.004568 0.000073 -0.000164 0.001437 1.44 0.32 0.66 0.59 -1417 -92 6 81 -468 -44 28

VE/cm3‚mol-1 κES /TPa1∆nD ∆σ/mN‚m-1 HE/J‚mol-1 GE/J‚mol-1

As we have previously mentioned, cyclic ethers are considered electron-donor compounds. However, there are differences in its ability depending on the number of atoms of the ring1,2 the five-membered cyclic ether, tetrahydrofuran, being a better electron-donor than the six-membered ether, tetrahydropyran. Our results truly fulfill this statement; the excess or deviation properties values of mixtures containing tetrahydrofuran are larger than the ones that are formed by tetrahydropyran. There are two kinds of molecular interactions that can occur between the components of our mixtures. On one hand, n(O)-π interactions can be established3 between the cyclic ether and the aromatic ring, and on the other hand, for fluorobenzene or chlorobenzene, the oxygen atom of the ether can interact directly with the halogen atom. When a hydrogen atom of the benzene is replaced by a chlorine or fluorine atom, charge in the σ bond is transferred away from the ring to the halogen atom (inductive effect), leaving a positive charge in the ring. However, although the halogen atom is in this way a σ-bond acceptor, it is also a nπ donor in a donor-acceptor action (dative conjugation).42 As a result of these two opposite effects, there is a net transfer of negative charge from the ring to the halogenated atom as well as an enhancement of the acceptor character of the aromatic



TABLE 3: Partial Molar Excess Enthalpies of Component 1 at Infinite Dilution, H h E,∞ 1 , at 298.15 K -1 H h E,∞ 1 /J‚mol

+ benzene + fluorobenzene + chlorobenzene + cyclohexane + chlorocyclohexane

tetrahydrofuran

tetrahydropyran

-1400 -2369 -2530

-816 -1763 -1881

3127 -197

1906 -325

ring. Therefore, the interaction between the aromatic ring and the cyclic ether must also be increased when a chlorine or fluorine atom is attached to the ring. To clarify these ideas, let us make use of partial molar excess enthalpies43-45 of the mixtures studied here and the corresponding ones of some reference systems.46 In Table 3, the partial molar excess enthalpies of cyclic ether at infinite dilution, H h 1E,∞, are presented, these values can be evaluated by means of the following equation:

(

∂H ∂x1

H h E,∞ h E1 ) lim HE + (1 - x1) 1 ) lim H x1f0

x1f0

)

E

(16)

As a first aproximation, H h E,∞ for cyclic ether with benzene 1 represents the energy necessary to break up all the dipoledipole interactions in the pure cyclic ether, tetrahydrofuran, or tetrahydropyran, together with the energy involved in the O-π interaction, the value of the dipole-dipole interactions can be evaluated from the value for the system cyclic ether with cyclohexane in which there are not specific interactions. In the light of the H h E,∞ values obtained, the O-π interaction involv1 ing tetrahydrofuran is stronger than the corresponding one with tetrahydropyran. Taking into account now the H h E,∞ values for 1 the reference system cyclic ether with chlorocyclohexane, we can conclude that the O-Cl interaction is weaker than the O-π interaction. Finally, comparing the H h E,∞ 1 values for cyclic ether E,∞ with halobenzene with the H h 1 values shown by the mixtures with benzene, it is clear that there is an enhancement of the interaction between the cyclic ether and the halogenated aromatic compound. The extent of the interaction between cyclic ether and chlorobenzene is bigger than the interaction etherfluorobenzene. Because of the characteristics of chlorine and fluorine atoms, the dative conjugation effect can be considered almost equal in both cases, and consequently, the differences of the observed behavior are only attributed to the stronger inductive effect of the fluorine atom. Special attention has to be paid in order to describe surface tension deviation results. Surface tension can be considered the result of several phenomena that can take place not only in the surface of a liquid but also in the bulk. The cyclic ethers present slightly lower surface tension values than the aromatic compounds; they are more surface active and therefore are expected to be displaced to the surface, while the aromatic compounds will tend to stay in the bulk. Usually surface tension deviation values should be negative (Gibbs adsorption), and the bigger the difference between surface tension of pure components, the more negative surface tension deviations should be.47 But during the mixing process, new effects can affect the surface behavior, like changes in the structure of components, repulsion, or attraction between unlike molecules,48 these effects can make, for instance, the ethers remain in the bulk, leading to positive values of surface tension deviations. The surface behavior shows for our mixtures positive surface tension deviations, except for tetrahydropyran with chlorobenzene at low ether molar fractions;

this fact confirms again the existence of interactions between the unlike molecules. In the case of the mixture containing tetrahydropyran and chlorobenzene, the lower electron-donor ability of the ether cannot surpass its tendency at low mole fractions of cyclic ether to migrate to the surface by a mechanism of adsorption. In the light of the results obtained for the excess surface concentration of cyclic ether, we can see that, for the mixtures containing benzene and especially chlorobenzene, there is a little amount of cyclic ether that migrates to the surface, that is, the migration of the cyclic ether to the surface is hindered by the interactions between the unlike molecules. On the other hand, for mixtures containing fluorobenzene in which the difference between surface tension of pure components is very small, the adsorption is only possible when the amount of ether in the mixture is large, otherwise the interactions are so strong that there is even a desorption of the cyclic ether molecules from the surface. Conclusions In this work, a comprehensive thermodynamic study of mixtures formed by a cyclic ether (tetrahydropyran or tetrahydrofuran) and an aromatic compound (benzene, fluorobenzene, or chlorobenzene) has been performed throughout the determination of a set of thermodynamic properties. By analyzing the results obtained, we have systematized the differences in electron donor ability of the cyclic ether and we have determined the extent of the modification in the acceptor electron character of both the aromatic ring and the halogen atom. Acknowledgment. We are grateful for financial assistance from Ministerio de Educacioń y Cultura and Fondos FEDER (BQU 2003-01765). Authors are also indebted to D.G.A. and Universidad de Zaragoza for financial support (INFR 423-06). B. Giner thanks the predoctoral fellowship from Ministerio de Educacioń y Cultura. References and Notes (1) Searles, S.; Tamres, M. J. Am. Chem. Soc. 1951, 73, 3704. (2) Arnett, E. M.; Wu, C. Y. J. Am. Chem. Soc. 1962, 84, 1684. (3) Andrews, A. W.; Morcon, K. W. J. Chem. Thermodyn. 1971, 3, 513. (4) Andrews, A. W.; Morcon, K. W. J. Chem. Thermodyn. 1971, 3, 519. (5) Alkorta, I.; Rozas, I.; Elguero, J. J. Org. Chem. 1997, 62, 4687. (6) Deshpande, D. D.; Oswal, S. L. J. Chem. Thermodyn. 1975, 7, 155. (7) Meyer, R.; Giusti, G.; Meyer, M.; Vincent, E. J. Thermochim. Acta 1975, 13, 379. (8) Mahl, B. S.; Kooner, Z. S.; Khurma, J. R.; J. Chem. Thermodyn. 1977, 9, 61. (9) Mahl, B. S.; Kooner, Z. S.; Khurma, J. R.; J. Chem. Eng. Data 1978, 23, 150. (10) Geier, K.; Bittrich, H. J. Z. Phys. Chem. (Leipzig) 1979, 260, 705. (11) Inglese, A; Wilhelm, E.; Grolier J.-P. E.; Kehiaian H. V. J. Chem. Thermodyn. 1981, 13, 229. (12) Ruı´z, B.; Otıń, S.; Gutierrez Losa, C. J. Chem. Thermodyn. 1984, 16, 25. (13) Coto, B.; Cabanãs, A.; Pando, C.; Menduinã, C.; Rubio R. G.; Renuncio, J. A. R. J. Chem. Soc., Faraday Trans. 1995, 91, 2779. (14) Aparicio, S.; Alcalde, R.; Garcıá, B.; Leal, J. M. Ind. Eng. Chem. Res. 2005, 44, 7575. (15) Douhe´ret, G.; Davis, M. I.; Chem. Soc. ReV. 1993, 22, 50. (16) Kiyohara, O.; D’Arcy, P. J.; Benson, G. Can. J. Chem. 1979, 57, 1006. (17) Inglese, A.; Grolier J.-P. E.; Wilhelm, E. J. Chem. Eng. Data 1983, 28, 124. (18) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic SolVents. Physical Properties and Methods of Purification, 4th ed.; Wiley-Interscience: New York, 1986. (19) Kimura, T.; Sugihara Y.; Takagi, S. Fluid Phase Equilib. 1997, 136, 323.

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