© Copyright 2004 American Chemical Society
FEBRUARY 17, 2004 VOLUME 20, NUMBER 4
Letters Specific Interactions between Aromatic Electrons of Organic Compounds and Graphite Surfaces As Detected by Immersion Calorimetry E. Castillejos-Lope´z,† D. M. Nevskaia,† V. Mun˜oz,† I. Rodrı´guez-Ramos,‡ and A. Guerrero-Ruiz*,† Dpto. Quı´mica Inorga´ nica y Quı´mica Te´ cnica, Facultad de Ciencias, UNED, P° Senda del Rey 9, 28040 Madrid, Spain, and Ins. de Cata´ lisis y Petrol, CSIC, Campus Cantoblanco, 28049 Madrid, Spain Received July 24, 2003. In Final Form: December 12, 2003 The interaction of toluene and methylcyclohexane with the surface of a series of graphites differing in surface area has been studied by immersion calorimetry. Comparison of the immersion enthalpies that evolved when the solids were immersed into the distinct liquids reveals the formation of charge-transfer complexes among aromatic electrons of the organic molecules and the edges and lateral faces of the graphite surface.
Introduction Calorimetric determination of the immersion enthalpy of powdered adsorbents into various chemical compounds is widely used to characterize the adsorbents in terms of surface area and pore size distribution.1-3 The enthalpy of immersion is related to the formation of an adsorbed layer of the molecules of the liquid on the surface of the solid. When nonspecific interactions between the surface of the solid and the molecules of the liquid occur, only the surface accessibility of the immersion liquid accounts for the total value of the immersion enthalpy. The pore size distribution is mainly based on the use of chemical probes with different molecular sizes, while for surface area * Corresponding author. Phone: 34913987344. 34913986697. E-mail:
[email protected]. † UNED. ‡ CSIC.
Fax:
(1) Denoyel, R.; Fernandez-Colinas, J.; Grillet, Y.; Rouquerol, J. Langmuir 1993, 9, 515. (2) Gonzalez, M. T. G.; Sepulveda-Escribano, A.; Molina-Sabio, M.; Rodrı´guez-Reinoso, F. Langmuir 1995, 11, 2151. (3) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powders and Porous Solids. Principles, Methodology and Applications; Academic Press: London, 1999.
determination, it is generally assumed that the immersion heat per surface area unit is a constant for a given adsorbate/adsorbent system. However, the immersion calorimetry provides a direct measurement of the energy involved in the interaction of vapor molecules of the immersion liquid with the surface of the solid. The energy of interaction depends on the chemical nature of the solid surface sites and of the probe molecule, that is, specific interactions between the solid surface and the liquid increase the immersion enthalpy. This is the case, for example, of the wetting of a polar surface with a polar liquid. Comparison between enthalpies of immersion into liquids with different polarities provides a picture of the surface chemistry of the solid. Particularly for carbonaceous materials, the surface can be considered as a combination of basal planes and unsaturated sites at the edges of the graphene layers that constitute the carbon crystallites. The edge carbon atoms are frequently combined with other heteroatoms, like oxygen, forming functional groups at the surface. There is general agreement in attributing acidic and basic centers to functionalities present on carbon surfaces that induce specific interactions between the carbon surface and the
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immersion liquid. Thus, the enthalpy of immersion of carbonaceous materials into water has been correlated with the amount and nature of their surface oxygen functional groups.4,5 However, for a nonpolar liquid such as benzene, the enthalpies of immersion remained unaffected by changes in the content of oxygen functional groups of the surface.6,7 In addition, carbon surfaces also contain basic sites that are thought to be oxygen-free sites of Lewis-type located at regions rich in π electrons within basal planes. It has been demonstrated that water molecules may also interact with oxygen-free basic sites.6,7 A major question that remains to be studied is whether different nonpolar organic compounds are able to originate specific interactions when interacting with the diverse types of sites exposed on the functionality-free surface of carbons. Specifically, comparison of saturated hydrocarbons with aromatic hydrocarbons would be important because of the possibility the latter have to form donor-acceptor complexes with the π electrons of the aromatic ring.8 The formation of these donor-acceptor complexes could involve a larger immersion enthalpy than the nonspecific interaction. This work is then aimed at studying the interaction of organic molecules with and without aromatic character with the carbon surface. To avoid microporous structure, the carbonous materials were various graphite samples differing in surface area and, therefore, in surface heterogeneity (i.e., with different proportions of basal planes, edges, corners, and surface defects). Immersion calorimetry was used to reveal the interactions. Two organic compounds, one with aromatic character, toluene, and another that was saturated, methylcyclohexane, were used as probe molecules.
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Figure 1. Nitrogen adsorption isotherms at 77 K of the various graphite samples: (squares) G1, (circles) G2, (upward-pointing triangles) G3, (downward-pointing triangles) G4, (diamonds) G5, and (left-pointing triangles) G6.
Experimental Section The variety of graphites used in this study were provided by Lonza, Ltd., Switzerland. The samples, denoted as Gs, were characterized by N2 adsorption at 77 K. These determinations were carried out in an automatic Micromeritics ASAP 2010 volumetric system. The BET equation was applied to the N2 isotherms to obtain the surface area values: G1 (0.4 m2/g), G2 (12 m2/g), G3(14 m2/g), G4 (66 m2/g), G5 (193 m2/g), and G6 (297 m2/g). All the samples were treated at 1173 K, for 120 min, under helium flow (100 cm3 min-1), to eliminate the residual oxygen surface groups generally exhibited on the carbon surfaces. The enthalpies of immersion of the samples into methylcyclohexane and toluene were determined at 303 K with an isothermal calorimeter of the Tian-Calvet type, Setaram C-80. The used chemicals were methylcyclohexane (puriss) and toluene (analytical reagent grade) from Fluka. Prior to the experiments, the graphite samples were in situ outgassed at 383 K for 12 h. The experimental procedure is similar to that described in ref 1, with the corrections corresponding to the energy of bulb breaking and to the energy of liquid vaporization having been considered. The immersion enthalpies were determined several times; therefore, the dispersion of determinations (deviation errors) can be estimated as lower than 5%.
Results and Discussion Figure 1 shows N2 adsorption isotherms for the graphite samples. It can be observed that all the higher surface area graphites (G4-G6) have nearly the same type of isotherm, which is characteristic of mesoporous materials. (4) Barton, S. S.; Evans, M. J. B.; MacDonald, J. A. F. Langmuir 1994, 10, 4250. (5) Lopez-Ramo´n, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMartin, F. Carbon 2000, 38, 825. (6) Lopez-Ramo´n, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMartin, F. Langmuir 2000, 16, 5967. (7) Stoeckli, F.; Lavanchy, A. Carbon 2000, 38, 475. (8) Dougherty, D. A. Science 1996, 271, 163.
Figure 2. Immersion enthalpies in toluene (triangles) and in methylcyclohexane (squares) as a function of the different graphite surface areas.
No micropores are exhibited by any of these materials. The small hysteresis loop in isotherms of samples G5 and G6 is indicating that there is restriction in the entrance of some slit-shaped mesopores. It is worthy to note that the graphite samples were submitted to high-temperature pretreatment in helium to remove functional groups from the surface. Figure 2 displays, for the different graphites, the methylcyclohexane and toluene immersion enthalpies as a function of the BET surface area values. For a given sample, the immersion enthalpy of toluene is higher than that of methylcyclohexane. Moreover, the immersion enthalpies of toluene increase with the increasing surface area of graphites in a higher extension than those of methylcyclohexane. That is, two straight diverging lines with two different slopes are observed. This finding indicates an enhancement of the immersion enthalpy of toluene when the surface area of the graphite becomes larger. Despite the different minimal molecular dimensions of benzene (0.37 nm) and methylcyclohexane (0.48 nm), this enhancement cannot be assigned to a higher accessibility of toluene to the porous structure, given that these high surface area graphites have porosities in the range of only mesopores. Therefore, it is inferred that a specific interaction between the molecules of toluene and
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the surface of the graphite is developed when its surface area increases. The origin for the relative variation of immersion enthalpies can lie in the difference in electronic structures between methylcyclohexane and toluene, the latter with delocalized electrons (aromatic character) that can form donor-acceptor complexes with the graphite surface. It is well-known that the graphite surfaces offer at least two adsorption sites, one located at the center of the carbon hexagons (basal planes) and another at the edges (unsaturated carbon atoms located in lateral planes, etc.), and their relative amounts in the surface change when the surface area of graphites is modified. Thus, the proportion in lateral faces increases with respect to the basal planes with the increasing surface area. From the immersion enthalpy measurements, it is deduced that the interaction of aromatic electrons with the edge sites is stronger than with sp2 carbon atoms in basal planes. It is hypothesized that the edges of graphite microcrystals are able to form complexes with aromatic molecules, which probably involves an electron transfer to the graphitic structure or vice versa, resulting in a enhanced energy of interaction between toluene and the graphite surface. The attractive and electrostatic interactions between the toluene aromatic ring and the π electrons of the graphitic microcrystals9 have been postulated by some authors, but (9) Menendez, J. A.; Phillips, J.; Xia, B.; Radovic, L. R. Langmuir 1996, 12, 4404.
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direct experimental evidence is scarce. The realization of these interactions is particularly important in the interpretation of immersion enthalpy determinations. Thus, variations in immersion enthalpies achieved for reference materials, which are sometimes erratic,10 could be interpreted as a consequence of specific interactions between carbon surface sites and the molecules of the immersion compound. Conclusion The careful determination of the immersion enthalpies of a series of graphite samples with varied surface areas in organic compounds with and without aromatic character (toluene and methylcyclohexane) has allowed us to detect the formation of specific surface compounds, probably with the formation of electron-transfer bonds, between the edges of the graphite layers and the aromatic molecules. The creation of these entities contributes to an enhancement of the immersion enthalpy. Acknowledgment. E.C.-L. acknowledges receipt of a predoctoral grant from the UNED. The authors thank the financial support of the Spanish Science and Technology Ministry (Project Nos. MAT2000-0043-P4-03 and MAT2002-04189-C02). LA035349P (10) Villar-Rodil, S.; Denoyel, R.; Rouquerol, J.; Matinez-Alonso, A.; Tascon, J. M. D. J. Colloid Interface Sci. 2002, 252, 169.
