J. Phys. Chem. B 2010, 114, 12589–12596
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Rationalizing the Diverse Solid-Liquid Equilibria of Binary Mixtures of Benzene and Its Fluorinated Derivatives Diana Ruivo,† Ana B. Pereiro,†,‡ Jose´ M. S. S. Esperanc¸a,*,† Jose´ N. Canongia Lopes,*,†,§ and Luis P. N. Rebelo*,† Instituto de Tecnologia Quı´mica e Biolo´gica, (www.itqb.unl.pt), UNL, AV. Repu´blica, Apartado 127, 2780-901 Oeiras, Portugal, Chemical Engineering Department, Vigo UniVersity, P.O. Box 36310, Vigo, Spain, and Centro de Quı´mica Estrutural, (cqe.ist.utl.pt), IST/UTL, 1049 001 Lisboa, Portugal ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: August 19, 2010
The solid-liquid phase behavior of benzene plus hexafluorobenzene binary mixtures is characterized by a stable congruent-melting binary solid, (C6H6 · C6F6). In this work, differential scanning calorimetry (DSC) was used to build, for the first time, the temperature-composition phase diagrams of ten other binary mixtures involving benzene and its fluorinated derivatives. Distinct types of solid-liquid equilibria were observed, namely those exhibiting the usual eutectic behavior associated with ideal or quasi-ideal solubility conditions, or, in other cases, systems with equimolar congruent-melting solids. Data have been interpreted and rationalized using a unifying framework that takes into account the molecular dipole and quadrupole moments of the two components of each binary system and the structural motifs associated with each type of crystal. Introduction Fluorinated organic compounds display unique physicochemical properties that have been used in many commercial and industrial applications. They are generally perceived as alternative substances toward the development of more environmentfriendly processes. Industrial production of these compounds has increased significantly since the early 1980s and fluorinated organics are commonly used as refrigerants, surfactants, polymers, as components of pharmaceuticals, fire retardants, lubricants, and insecticides.1 In recent years the possibility of their use as gas-carriers, including the possibility of their use as synthetic blood substitutes,2 and as solubility promoters in supercritical extraction media3 has also been explored. From a molecular point of view, the properties of fluorinated organic compounds can be rationalized in terms of the unique interactions with different molecules: fluorocarbons generally exhibit low-intensity interactions with normal organic compounds4 or water, a fact related to both (i) their “inverted” quadrupole moment;unlike hydrocarbons, the carbon backbone of a perfluorocarbon molecule carries a partially positive charge due to the high electronegativity of the fluorine atom;and (ii) to their relatively rigid structure;due to the bulkier nature of the fluor atom as compared to that of hydrogen;that inhibits the existence of different conformers, and, thus, introduces a penalty of entropic-driven factors in the mixing with other substances.5 These same underlying molecular reasons explain the very high vapor pressures exhibited by fluorocarbons as compared to their hydro-counterparts.6,7 It must also be stressed that most syntheses and uses of such molecules involve the insertion of an interacting group in the otherwise inert perfluorinated chain. This strategy promotes the “docking” of the fluorinated compound in the midst of other molecules (with the possible self-organization of the former in * To whom correspondence should be addressed:
[email protected];
[email protected]. † Instituto de Tecnologia Quı´mica e Biolo´gica. ‡ Vigo University. § Centro de Quı´mica Estrutural.
micelles or similar structures) or the formation of emulsions between fluorinated and organic or aqueous domains. These blends pose very interesting challenges from both the theoretical and the applied chemistry points of view. As an example of the former aspect, one can hope to gain some insight about the interactions between the fluorinated molecules. Solid-liquid equilibria (SLE) of mixtures of fluorinated compounds can also provide important information concerning their potential use in new industrial applications or as a form to emphasize more fundamental aspects underlying their interactions at a molecular level. The binary mixture of benzene and hexafluorobenzene is characterized by the presence of an equimolar congruent-melting molecular complex of the two components.8 Mixtures of hexafluorobenzene with different aromatic hydrocarbons, namely toluene, p-xylene, and mesitylene, also show congruent-melting molecular complexes, while mixtures with nonaromatic hydrocarbons, such as n-hexane or cyclohexane, exhibit eutectic and peritectic behavior.9 Finally, mixtures of hexafluorobenzene and aromatic heterocyclic compounds, namely thiophen, pyridine, and furan, have shown congruent-melting complexes in the first mixture and peritectic behavior in the other two.10 In this work, we studied the solid-liquid phase behavior of 11 different binary mixtures involving benzene and/or some of its fluorinated derivatives, and analyzed the results in the light of the dipolar and quadropolar moments of these fluorinesubstituted derivatives of benzene. The components of the mixtures included benzene, fluorobenzene, the two positional isomers of difluorobenzene (1,2-C6H4F2 and 1,4-C6H4F2), 1,3,5trifluorobenzene, 1,2,4,5-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene. The phase diagrams for the different mixtures were built with data obtained by differential scanning calorimetry (DSC). Experimental Section Materials. Benzene was purchased from Panreac, with 99.5% purity. All fluorinated compounds were purchased from Alfa Aesar: fluorobenzene, 1,4-difluorobenzene, 1,2,4,5-tetrafluo-
10.1021/jp105639p 2010 American Chemical Society Published on Web 09/14/2010
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TABLE 1: Experimental SLE Temperature-Composition Data for the Six Binary Systems with Eutectic (first five) and Peritectic Behavior (last one) x2
T/K
x2
T/K
x2
0.000 0.102
277.1 242.3
0.102 0.304
270.8 242.6
0.000 0.092
265.8 236.4
0.092 0.296
256.1 237.3
0.000 0.100 0.100
275.1 244.9 271.3
0.312 0.312
245.6 259.7
0.000 0.049 0.049
229.2 221.1 227.5
0.092 0.092 0.195
221.3 225.6 221.6
0.000 0.100 0.100
224.4 208.4 221.1
0.298 0.298
209.2 213.3
0.403 0.503
0.000 0.099 0.099 0.201 0.201
277.1 229.7 269.9 230.0 261.8
0.295 0.295 0.384 0.384 0.497
230.1 252.8 229.9 247.3 230.1
T/K
x2
T/K
x2
T/K
x2
T/K
benzene (1) + 1,3,5-trifluorobenzene (2) 0.304 258.4 0.677 242.7 0.494 242.7 0.677 248.7
0.881 0.881
242.4 254.8
1.000
265.8
1,3,5-trifluorobenzene (1) + hexafluorobenzene (2) 0.296 245.3 0.690 236.8 0.501 237.6 0.690 259.7
0.902 0.902
235.8 271.5
1.000
276.8
1,2,4,5-tetrafluorobenzene (1) + hexafluorobenzene (2) 0.497 245.5 0.594 245.6 0.705 0.497 249.5 0.705 245.5 0.904
255.0 245.1
0.904 1.000
270.6 276.8
0.697 0.697 0.902
221.1 260.6 220.2
0.902 1.000
271.3 275.1
217.2 208.0
0.699 0.902
230.2 206.7
0.902 1.000
242.5 247.2
benzene (1) + 1,4-difluorobenzene (2) 0.497 237.3 0.759 229.3 0.609 229.9 0.759 235.5 0.646 231.1 0.797 229.4 0.689 229.8 0.797 235.5 0.689 233.7 0.797 238.4
0.843 0.843 0.843 0.912 0.912
229.4 235.5 240.2 229.5 234.3
0.912 0.961 0.961 1.000
243.1 234.4 245.5 247.2
fluorobenzene (1) + 1,2,4,5-tetrafluorobenzene (2) 0.318 221.3 0.396 238.4 0.318 235.7 0.497 221.3 0.396 221.3 0.497 247.6 1,2-difluorobenzene (1) + 1,4-difluorobenzene (2) 209.0 208.4
robenzene, and hexafluorobenzene with purity levels better than 99%; 1,2-difluorobenzene, 1,3,5-trifluorobenzene, and pentafluorobenzene with purity levels better than 98%. All compounds were dried for several weeks over freshly activated type-3 Å molecular sieves supplied by Aldrich. Methods. All solid-liquid phase transitions were determined with a Q-200 TA Instruments differential scanning calorimeter (DSC) working in the 183-293 K range. Condensation in the DSC furnace was prevented by using dry nitrogen as a purge gas with a flow rate of 50 mL/min. The calibration of the instrument was regularly confirmed with use of the melting point temperature of indium. All mixtures were prepared by weighing each component into a capped vial with use of an Ohaus analytical high-precision balance with a repeatability of 0.02 mg. Each mixture (ca. 1 mL) was then stirred by using a vortex (10 s) and a magnetic stirrer (30 min) in order to obtain a homogeneous liquid. Finally, samples of each mixture, typically 3 to 15 mg, were transferred to the aluminum DSC pan, which was then hermetically sealed in order to prevent vaporization. The weight of the pan was also recorded after the DSC measurements in order to check that no sample loss had occurred. After different trials it was found the sample masses around 3 mg yielded more reproducible results. During each DSC run, the sample was first cooled to 20-30 K below its expected melting temperature at a rate of 0.083 deg · s-1 and kept at that temperature for 30 min. The sample was then heated to a temperature of 293 K (300 K in the case of the benzene plus hexafluorobenzene mixtures), at a heating rate of either 0.050 (preliminary runs) or 0.017 deg · s-1 (final measurements). The uncertainty in the melting point temperature obtained by calculation of the standard deviation of several consecutive measurements for the same sample is better than (1 K.
0.503 0.699
Results and Discussion The experimental DSC data (melting point temperatures were taken as the onset of temperature of the melting peak for the pure compounds and the temperature of the minimum of the peak for the mixtures) were used to build the corresponding temperature-composition phase diagrams representing the solid-liquid equilibria, SLE, for each studied system. The phase diagrams show two different types of SLE behavior. They are grouped in Tables 1 and 2 and represented in Figures 1 and 2. Five binary systems exhibit the usual eutectic behavior associated with an ideal or quasi-ideal mutual solubility of the two components in the presence of solids that are always the pure components of the mixture. These five binary systems include (benzene plus 1,3,5-trifluorobenzene), (1,3,5-trifluorobenzene plus hexafluorobenzene), (1,2,4,5- tetrafluorobenzene plus hexafluorobenzene), (fluorobenzene plus 1,2,4,5-tetrafluorobenzene), and (1,2-difluorobenzene plus 1,4-difluorobenzene) mixtures. They are represented in Figure 1a-e. In cases where the enthalpies of fusion could be obtained for the pure compounds,11–14 the ideal solubility behavior (based on the corresponding coligative property) is also presented as a dashed line. On the other hand, the data for the (benzene plus 1,4-difluorobenzene) mixture show the possible existence of an addition compound that melts incongruently. However, the peritectic SLE behavior depicted in the corresponding phase diagram (Figure 1f) led us to group it with the other “eutectic” systems included in Table 1 and presented in Figure 1. Five other systems;namely (benzene plus hexafluorobenzene), (1,4-difluorobenzene plus 1,2,4,5-tetrafluorobenzene), (benzene plus 1,2,4,5-tetrafluorobenzene), (1,4-difluorobenzene plus hexafluorobenzene), and (fluorobenzene plus pentafluorobenzene) mixtures;revealed the existence of an equimolar congruent-melting binary solid. The corresponding SLE data were grouped in Table 2 and are presented in Figure 2a-e. In some cases the congruent melting temperature of the equimolar
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TABLE 2: Experimental SLE Temperature-Composition Data for the Five Binary Systems with an Equimolar Congruent-Melting Compound x2
T/K
x2
T/K
0.000 0.081 0.081 0.143
277.1 269.9 273.5 269.6
0.286 0.286 0.299 0.299
269.6 289.1 269.5 289.4
0.000 0.051 0.051 0.096 0.096
247.2 237.6 245.6 237.8 242.9
0.000 0.049 0.049 0.101 0.101
x2
T/K
x2
T/K
x2
T/K
x2
T/K
(2) 267.1 289.9 267.6
0.698 0.828 0.828
286.5 267.7 275.4
0.899 0.899 1.000
267.5 270.6 276.8
0.151 0.151 0.204 0.252 0.252
1,4-difluorobenzene (1) + 1,2,4,5-tetrafluorobenzene (2) 238.4 0.301 238.0 0.437 248.4 0.611 240.6 0.301 244.7 0.500 249.3 0.653 238.6 0.412 237.7 0.510 249.8 0.653 238.5 0.412 247.7 0.531 249.1 0.738 242.1 0.437 237.9 0.567 249.4 0.738
249.4 249.1 257.4 249.2 262.4
0.887 0.887 0.927 0.927 1.000
248.5 270.3 248.1 272.2 275.1
277.1 264.4 274.5 264.0 269.8
0.192 0.296 0.296 0.380 0.380
264.8 264.2 268.4 264.2 271.0
benzene (1) + 1,2,4,5-tetrafluorobenzene (2) 0.456 271.9 0.534 265.0 0.502 264.4 0.534 271.2 0.502 271.9 0.566 265.3 0.513 264.4 0.566 271.4 0.513 272.5 0.591 265.5
0.591 0.702 0.702 0.791 0.898
271.1 265.4 268.1 265.4 265.1
0.898 0.956 0.956 1.000
271.1 264.7 272.8 275.1
0.000 0.019 0.019 0.050 0.050
247.2 242.8 247.4 243.8 246.3
0.092 0.202 0.202 0.296
243.9 243.9 259.1 243.6
1,4-difluorobenzene (1) + hexafluorobenzene (2) 0.296 265.5 0.541 261.0 0.388 243.3 0.541 270.4 0.388 268.9 0.593 261.7 0.500 270.7 0.593 269.6
0.701 0.701 0.792 0.906
262.1 266.3 262.3 261.8
0.906 0.931 0.931 1.000
270.1 262.1 271.2 276.8
0.000 0.101 0.101 0.199
229.2 219.0 225.6 219.2
0.303 0.303 0.391 0.391
219.5 224.1 218.8 226.4
fluorobenzene (1) + pentafluorobenzene (2) 0.498 228.4 0.627 216.2 0.545 228.3 0.627 226.4 0.581 215.8 0.672 216.2 0.581 227.4 0.672 225.7
0.731 0.731 0.809
216.4 221.6 216.6
0.910 0.910 1.000
216.3 220.5 223.9
benzene (1) + hexafluorobenzene 0.332 269.8 0.651 0.332 292.0 0.651 0.508 296.1 0.698
binary solid is higher than that of either one or both pure components of the mixture. In the case of Figure 2b the existence of a congruent melting binary crystal can be inferred by the shift in the eutectic temperatures near the equimolar composition. The starting point to rationalize the two different types of SLE found for these systems is the seminal work of Williams15 that related the molecular electric quadrupole moment with solid-state architecture. The author used this very simple concept to explain both the SLE phase diagram of the (benzene plus hexafluorobenzene) mixture, including its congruent-melting equimolar binary solid, and also the distinct patterns found in the two pure solids and the (C6H6 · C6F6) crystal. We will extend Williams’ analysis to the other members of the fluorinated benzene family by borrowing some ideas from our previous work that correlated the molecular dipole and quadrupole moments of all 13 members of the family with their solubility in an ionic liquid solvent.16 According to Williams, the characteristic herringbone structures of both pure benzene and hexafluorobenzene crystals are a consequence of the packing of the molecules according to their molecular quadrupole moments (Figure 3b and also Figure 2 of ref 16). This means that in both cases the crystals will exhibit aromatic planes oriented in directions perpendicular to each other (Figure 3a). Conversely, in a C6H6 · C6F6 binary crystal17 where molecules with different (symmetrical) molecular quadrupole moments coexist, the molecules can pack together forming a layered structure that maximizes their quadrupole interactions (Figure 4b and Figure 2 of ref 16) and where all aromatic planes can face the same direction and interact via π-π interactions (Figure 4a). These facts imply the enhanced stability of the binary crystal and its appearance as a congruent
melting solid in the SLE phase diagram of benzene plus hexafluorobenzene mixtures, as originally found by Patrick and Prosser.8 How can these ideas;the relation between multipolar interactions and structure at a molecular level and its consequences in terms of SLE behavior;be transferred to the other systems under discussion? Figure 5 shows benzene and its 12 fluorinated derivatives arranged as a function of their molecular electric dipole moments and their aromatic electric quadrupole moments.16 The first row includes benzene, hexafluorobenzene, and three other fluorinated benzene molecules that due to their symmetry have null molecular electrical dipole moments. Eight out of the eleven binary systems studied in this work are composed of molecules belonging exclusively to this group. The second row contains five fluorinated benzene derivatives with permanent molecular electrical dipole moments around 5.6 × 10-30 C · m, corresponding to the dipole moment of one hydrogen and one fluorine atom substituted at opposite vertices of the benzene ring. Only two binary mixtures with at least one component belonging to this group were considered: the (fluorobenzene plus pentafluorobenzene) system and the (fluorobenzene plus 1,2,4,5-tetrafluorobenzene) system. Finally, the last two rows represent benzene fluorinated derivatives with electrical dipole moments larger than 9.5 tme · 10-30 C · m. Only the mixture (1,2-difluorobenzene plus 1,4-difluorobenzene) has one component belonging to this group. Figure 6 shows a matrix representation of the SLE results obtained for the different systems, ordered by the molecular electrical dipole moment of the components of the mixtures. We will consider first only the eight systems involving components with null dipole moment represented in the upper left triangle of Figure 6. Four of these systems;including the already discussed (benzene plus hexafluorobenzene) system;
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Figure 1. T-x phase diagrams for the six binary systems with eutectic and perictectic behavior: (a) benzene (1) + 1,3,5-trifluorobenzene (2); (b) 1,3,5-trifluorobenzene (1) + hexafluorobenzene (2); (c) 1,2,4,5- tetrafluorobenzene (1) + hexafluorobenzene (2); (d) fluorobenzene (1) + 1,2,4,5tetrafluorobenzene (2); (e) 1,2- difluorobenzene (1) + 1,4-difluorobenzene (2); and (f) benzene (1) + 1,4-difluorobenzene (2). All graphs are represented in the same temperature range. The solid lines are guides to the eye representing the liquidus and eutectic lines, adjusted to the experimental data and the known melting point temperatures. The dashed lines represent the ideal solubility of the two pure solids, calculated from available enthalpy and temperature of fusion values.11–14
exhibit a congruent-melting binary compound, while the other four exhibit either eutectic or (in the case of the (benzene plus 1,4-difluorobenzene)) a peritectic behavior. At first sight the (1,4-difluorobenzene plus 1,2,4,5-tetrafluorobenzene) system could be considered as an attenuated version of the (benzene plus hexafluorobenzene) mixture, with less intense but still symmetrical aromatic electric quadrupole moments (Qzz). However, a more careful analysis shows that the more intense components of the electric quadrupole moment in these two molecules are not those normal to the aromatic plane (Qzz) but the Qyy component in 1,4-difluorobenzene and the Qxx component in 1,2,4,5-tetrafluorobenzene, respectively.16 The different relative orientation between the dominant component of the molecular electric quadrupole moment and the aromatic plane has a profound impact on the crystallization of the pure components: the X-ray diffraction spectra of 1,4difluorobenzene and 1,2,4,5-tetrafluorobenzene do not show the herringbone structures typically found in benzene or hexafluorobenzene but quasilayered structures that show that the molecules are now able to maximize their electric quadrupole interactions while maintaining the aromatic planes parallel to each other and performing 90° in-plane rotations of the
molecules, Figure 7. In the case of the benzene plus hexafluorobenzene mixtures, the extra stability of the binary solid is conferred by the change in the crystal architecture, from herringbone structures in the pure components to a layered structure in the equimolar solid compound (Figures 3 and 4);in the case of (1,4-difluorobenzene plus 1,2,4,5-tetrafluorobenzene) mixtures, such advantage does not exist. Nevertheless, the system still forms a congruent-melting solid at the equimolar composition that melts at a temperature between those of the two pure components (Figure 2b). The driving force for the formation of such binary solid lies in the fact that the two dominant (and symmetrical) quadrupole moments of each component can still find a way to interact with each other while maintaining a layered structure in the binary crystal. The interactions between layers also help to stabilize the crystal via the secondary (and also symmetrical) aromatic electric quadrupole moments. The same principle applies to the formation of binary solids in the (benzene plus 1,2,4,5-tetrafluorobenzene) and (1,4-difluorobenzene plus hexafluorobenzene) mixtures (Figure 2c,d): a layered structure is still compatible with the dominant molecular electric quadrupole moments of the two
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Figure 2. T-x phase diagrams for the five binary systems with an equimolar congruent-melting compound: (a) benzene (1) + hexafluorobenzene: open triangles and squares are data from refs 8 and 10, respectively; (2); (b) 1,4-difluorobenzene (1) + 1,2,4,5-tetrafluorobenzene (2); (c) benzene (2) + 1,2,4,5-tetrafluorobenzene (1); (d) 1,4-difluorobenzene (1) + hexafluorobenzene (2); and (e) fluorobenzene (1) + pentafluorobenzene (2). Lines as in Figure 1.
components and the layers are stabilized via the secondary and symmetrical aromatic electric quadrupole moments. This state of affairs also explains why (1,2,4,5-tetrafluorobenzene plus hexafluorobenzene) and (benzene plus 1,4difluorobenzene) mixtures do not form binary solids (Figure 1c,f). Albeit layer formation is still possible between the two components (while maximizing their dominant electric quadrupole moments), the aromatic electric quadrupole moments of the two components have the same sign, which destabilizes interlayer interactions. In fact, the five benzene species with null dipole moment, which are ordered by increasing aromatic electric quadrupole moment, Qzz, in the first row of Figure 5, are divided into two groups by the 1,3,5-trifluorobenzene molecule species (with an almost null Qzz value). Figure 6 shows that only mixtures formed by two species from opposite sides of the divide exhibit congruent melting binary solids in their SLE diagrams, while those with two components from the same side will exhibit eutectic (or peritectic) behavior. Moreover (and as expected), mixtures containing 1,3,5-trifluorobenzene will always exhibit eutectic behavior (Figure 1a,b).
When dipole moments are introduced due to unsymmetrical fluorination of the benzene molecule, the relation between the interactions arising from the molecular dipole and quadrupole moments and the ensuing architecture of the crystals (either as pure components or binary solids) become more complex. The (fluorobenzene plus pentafluorobenzene) system, where both components have similar molecular dipole moments (5.8 × 10-30 and 5.4 × 10-30 C · m, respectively), exhibits a congruentmelting solid (Figure 2e). As in the cases where the phase diagram is characterized by the presence of an equimolar binary compound, the relation between the dominant and secondary interactions in the solids and the corresponding crystal architectures is able to explain the SLE behavior. Pure fluorobenzene and pentafluorobenzene show herringbone-like structures that represent a compromise between head-to-tail rows of molecules that maximize the dominant dipole-dipole interactions and perpendicular aromatic planes that maximize the secondary interactions between dipoles of the same sign (Figure 8). In the case of fluorobenzene, the herringbone structure is broken by such head-to-tail rows (Figure 8a), in the case of pentafluo-
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Figure 3. Two distinct snapshots showing the crystalline structure of solid benzene, including (a) the perpendicular orientation of the aromatic planes and (b) the characteristic herringbone structure of a quadrupolar fluid (illustrated by the white lines in a fishbone arrangement). The negative aromatic electric quadrupole moments of benzene are represented in part b by a red arrow representing the concentration of negative charge along the normal to the aromatic plane and a blue circle corresponding to the depletion of negative charge around that plane. The structural data were collected from the Cambridge database.
Figure 4. Two distinct snapshots showing the crystalline structure of the equimolar binary solid of benzene and hexafluorobenzene. The structure is characterized by the (a) stacking of the aromatic molecules into columns where the (b) aromatic planes are oriented parallel to each other. In part b the negative aromatic electric quadrupole moments of benzene are represented as in Figure 3b whereas the positive aromatic electric quadrupole moment of hexafluorobenzene is represented by a blue arrow representing the concentration of positive charge along the normal to the aromatic plane and a red circle corresponding to the depletion of negative charge around the aromatic plane. The structural data were collected from the Cambridge database.
robenzene the rows are paired in order to yield a “double” herringbone structure (Figure 8b). When fluorobenzene and pentafluorobenzene are mixed, a binary crystal can be formed where head-to-tail rows can be replaced by rows of head-to-head and tail-to-tail pairs of unlike molecules (according to the dominant dipole-dipole interactions) while the aromatic quadrupole moments with opposite signs can interact in planar or quasiplanar arrangement of aromatic planes. This is illustrated in Figure 9, where a hypothetical structure for such binary crystal is represented. In fact, the (fluorobenzene plus pentafluorobenzene) system acts in a similar fashion as the (1,2-difluorobenzene plus 1,2,4,5tetrafluorobenzene) system, with the dominant Qxx-Qyy quadrupole interactions of the latter mixture being replaced by
dipole-dipole interactions in the former system, and the aromatic quadrupole interactions, Qzz, of opposing sign acting in both systems as a secondary stabilizing agent between aromatic planes. Finally, the (fluorobenzene + 1,2,4,5-tetrafluorobenzene) and (1,2-difluorobenzene + 1,4-difluorobenzene) mixtures exhibit a SLE behavior characterized by the presence of a single eutectic point (Figure 1, panels d and e, respectively). In both cases one component with a strong dipole moment is mixed with a species that has null dipole moment. The dominant dipole-dipole interactions present in pure 1,2-difluorobenzene or fluorobenzene are not found in the other two pure species or in the cross interactions that would be present in the hypothetical binary solids. Those weaker cross interactions are detrimental to the
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Figure 5. Plot of benzene and its 12 fluorinated derivatives ordered by their aromatic electric quadrupole moments Qzz (first values between parentheses represent x-axis in the plot) and molecular electric dipole moment µ (second values between parentheses represent y-axis in the plot). The relative position of the molecules in the (Qzz,µ) plane gives their correct order in terms of the two but their distances in the horizontal are not to scale. The molecules in bold were used in this work.
Figure 7. Two distinct snapshots comparing the crystalline structures of (a) 1,4-difluorobenzene and (b) benzene. The (positive) electric quadrupole moment shown in part a is not the aromatic one (Qzz) but one oriented along the aromatic plane (Qyy). The structural data were collected from the Cambridge database.
components of the binary systems are from different sides of the dipole divide (cf. Figure 6). Conclusions
Figure 6. Matrix of the results obtained for the several mixtures studied. P stands for peritectic behavior, E for eutectic behavior, and BC for the appearance of an binary crystal. The compounds in the matrix were ordered by dipole moment, µ (cf. Figure 5), shown in the last column of the table.
formation of binary solids. In other words, in order to be able to obtain congruent-melting binary crystals it is necessary (but not sufficient) that both molecules exhibit similar interactions (both dipolar, both quadrupolar). In the case of the (fluorobenzene + 1,2,4,5-tetrafluorobenzene) and (1,2-difluorobenzene + 1,4-difluorobenzene) mixtures, the lack of a driving force to form binary solids simply stems from the fact that the
Binary mixtures of different fluorinated derivatives of benzene can show quite diverse solid-liquid equilibria behavior, as attested by their temperature-composition phase diagrams, which were built with melting temperature data obtained by differential scanning calorimetry. Nonetheless, all experimental data can be interpreted within a unifying framework by taking into account the relation between the multipolar interactions present in each type of system and the corresponding crystalline structures. Simply two factors seem to condition the formation of congruent-melting binary solids: (i) The dominant interactions must be of the same type;dipole-dipole or quadrupole-quadrupole interactions. This factor excludes the possibility of binary solid formation in systems like (fluorobenzene (dipolar) plus 1,2,4,5-tetrafluorobenzene (quadrupolar)), (1,2-difluorobenzene (dipolar) plus 1,4-difluorobenzene (quadrupolar)), (benzene (quadrupolar) plus 1,3,5-trifluorobenzene (apolar)), and (1,3,5-trifluorobenzene (apolar) plus hexafluorobenzene (quadrupolar)). (ii) The two components of the mixture must have aromatic electric quadrupole moments of opposite sign (Qzz), a fact that promotes stabilization of the binary solids via the formation of layered
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Figure 8. Two snapshots showing the crystalline structures of solid (a) fluorobenzene and (b) pentafluorobenzene. Both structures exhibit motifs based on herringbone structures (depicted by the green fishbone lines). The red arrows indicate the direction of the molecular dipole moments of the molecules. In both cases the structures are dominated by rows of molecules oriented head-to-tail. The structural data were collected from the Cambridge database.
through projects PTDC/CTM/73850/2006 and PTDC/QUIQUI/101794/2008. J.M.S.S.E. acknowledge FCT for a contract under Programa Cieˆncia 2007. References and Notes Figure 9. Schematic representation of an hypothetical congruent equimolar melting crystal of fluorobenzene and pentafluorobenzene. The posited structure shows how both the molecular dipole and aromatic quadrupole moments can be aligned while maintaining a layered architecture.
crystals, as opposed to the herringbone structures found in the solids of most of the pure components. This second factor excludes the formation of congruent-melting binary solids in systems like (benzene plus 1,4-difluorobenzene) and (1,2,4,5tetrafluorobenzene plus hexafluorobenzene). Many of the arguments just presented still lack additional supporting evidence in the form of the crystalline structures of the congruent-melting binary solids characterizing the SLE behavior of 5 of the 11 binary mixtures studied in this work;only the structure of the equimolar (benzene plus hexafluorobenzene) mixture is known. The first attempts to obtain such data proved unsuccessful. A new effort;whose results will be presented in due time;is currently under way in order to make the necessary X-ray diffraction experiments. Acknowledgment. Financial support to carry out this work was provided by Fundac¸a˜o para a Cieˆncia e Tecnologia (FCT)
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