Absorption Spectrophotometric Study of Molecular Complexation of

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Absorption Spectrophotometric Study of Molecular Complexation of Asphaltene with Bromanil Ashish Kumar Ghosh,† Asok Kumar Mukherjee,‡ and Sanjib Bagchi*,§ Central Institute of Mining and Fuel Research, Dhanbad-828108, India, Burdwan UniVersity, Burdwan-713104, India, and IISER, Salt Lake, Kolkata, India ReceiVed September 5, 2008. ReVised Manuscript ReceiVed October 23, 2008

It has been shown by UV-vis spectrophotometric method that p-bromanil forms inclusion complex of 1:2 stoichiometry with coal-derived asphaltenes. The absorption peak for the complex has been detected by difference spectral method and has been compared with those for complexes involving chloranils. The wavelengths of the maximum absorption are in the expected order of electron affinities of the chloranils and bromanil. The formation constants (K) have been determined using the Benesi-Hildebrand equation. The magnitudes of K are large compared to those of weak charge transfer complexes (for which K ≈ 101 to 103 dm3 mol-1) but are comparable to those of some recently reported supramolecular or inclusion complexes. The study reveals the presence of π-donor (i.e., aromatic) type of groups in asphaltene molecule. It also supports the view that bromanil molecules get included into structures provided by asphaltene molecules. Thermodynamic parameters ∆H0 sand ∆S0 for the complexation process have also been determined.

1. Introduction Asphaltenes consist of a group of complex polyaromatic compounds. They exist in crude oil as monomers and in selfaggregated1 forms in equilibrium. Different concentrations have been proposed for the point below which asphaltenes remain as monomer species, and an interesting behavior of them, which is similar to that of surfactants, has initiated several studies2-7 basically focusing on the molecular-level association. Thus, surface tension,8-11 calorimetry,12 or viscosity13 measurements suggested that, depending on the concentration and the type of solvent, asphaltenes form aggregated structures much like micelles in organic solvents. However, the interpretation of the results of surface tension experiments involving micelle formation has been subjected to criticism.14 Techniques like light absorption and fluorescence have also been widely used to * To whom correspondence should be addressed. Phone: +91-9434238073. Fax: +91-3323348092. E-mail: [email protected]. † Central Institute of Mining and Fuel Research. ‡ Burdwan University. § IISER. (1) (a) Nellensteyn, F. J.; Roodenburg, N. M. Chem.-Ztg. 1930, 545, 819. (b) Nellensteyn, F. J. Chem. Weekbl. 1931, 28, 31. (2) Rao, B. M. L.; Serrano, J. E. Fuel Sci. Technol. Int. 1987, 4, 483. (3) Maruska, H. P.; Rao, B. M. L. Fuel Sci. Technol. Int. 1987, 5, 119. (4) Rogacheva, O. V.; Gimaev, R. N.; Gudaidullin, V. Z.; Danilyan, T. D. Colloid J. USSR 1980, 42, 490. (5) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (6) Andersen, S. I.; Speight, J. G. Fuel 1993, 72, 1343. (7) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299. (8) Taylor, S. E. Fuel 1993, 72, 1338. (9) Oh, K.; Ring, T. A.; Deo, M. D. J. Colloid Interface Sci. 2004, 271, 212. (10) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes and Asphalts: DeVelopments in Petroleum Science; Yen, T. F., Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 1997; Chapter 13. (11) Rogel, E.; Leon, O.; Torres, G.; Espidel, J. Fuel 2000, 79, 1389. (12) Andersen, S. I.; Christensen, S. D. Energy Fuels 2000, 14, 38. (13) Storm, D. A.; Barresi, R. J.; DeCanio, S. J. Fuel 1991, 70, 779. (14) Friberg, S.; Mullins, O. C.; Sheu, E. Y. J. Dispersion Sci. Technol. 2005, 26, 513.

elucidate the fundamental properties of crude oils and asphaltenes.15-20 Absorption, emission, and excitation properties of asphaltenes in chloroform and benzene revealed abrupt change at concentration of ca. 5 mg/L.17,21 More recent investigation of asphaltene fluorescence in toluene revealed self-association at even lower concentrations of ca. 1.7 mg/L.19 A minimum value of thermo-optical diffusivity has been obtained at asphaltene concentrations of 50 mg/L.22,23 This minimum value is attributed to the asphaltene molecular association beyond 50 mg/L. Molecular weights of asphaltene has been found between 500 and 1000 g/mol using fluorescence depolarization measurements.24 Evdokimov et al. proposed the existence of various molecular aggregations in solutions of asphaltene by studying the structure of near-UV/visible spectroscopy and the concentration dependencies of absorptivities.25,26 These authors observed various aggregation stages with the first one at concentration as low as 2-5 mg/L. In this respect, asphaltene self-association appears to be fundamentally different from conventional micellization of surfactants with steplike change at critical micelle concentration (cmc). Nanoaggregate formation has been shown (15) Syunyaev, R. Z.; Safieva, R. Z.; Safin, R. R. J. Pet. Sci. Eng. 2000, 26, 31. (16) Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998. (17) Yokota, T.; Scriven, F.; Montgomery, D. S.; Strausz, O. P. Fuel 1986, 65, 1142. (18) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809. (19) Albuquerque, F. C.; Nicodem, D. E.; Rajagopal, K. Appl. Spectrosc. 2003, 57, 7. (20) Goncalves, S.; Castillo, J.; Ferna′ndez, A.; Hung, J. Fuel 2004, 83, 1823. (21) Kazakova, V. I.; Koretskij, A. F. Russ. Chem. Bull. 1970, 19, 1855. (22) Juyal, P.; Merino-Garcia, D.; Andersen, S. I. Energy Fuels 2005, 19, 1272. (23) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernandez, A.; Perez, P.; Caetano, M. Fuel 1999, 78, 997. (24) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677–25. (25) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 135. (26) Evdokimov, I. N.; Eliseev, N. Y.; Akhmetov, B. R. J. Pet. Sci. Eng. 2003, 37, 145.

10.1021/ef800746z CCC: $40.75  2009 American Chemical Society Published on Web 12/11/2008

Molecular Complexation of Asphaltene with Bromanil

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been analyzed to evaluate the formation constants of the complexes and other thermodynamic parameters, namely, ∆H0 and ∆S0 for complex formation. 2. Experimental Section

Figure 1. p-Bromanil, the acceptor used in the present study.

forpetroleumandcoalasphaltenesusingultrasonicspectroscopy,27,28 NMR diffusion studies, and conductivity studies.29 NMR data measuring translational diffusion constants clearly argues for sudden aggregation. Values of critical nanoaggregate concentration (CNAC) as obtained from these studies are in the range 150-200 mg/L. Steady-state fluorescence spectral studies also indicate that in the concentration range asphaltenes remain as nanoaggregates.30 Takanohashi et al. have shown, by molecular dynamics simulation, that asphaltenes aggregate in organic solvents even at 400 °C and, in general, the force behind aggregation is noncovalent aromatic-aromatic, electrostatic, and van der Waals interactions but no charge transfer.31 It has been proposed that the structure of an asphaltene molecule can be represented by several polycyclic aromatic systems joined by flexible aliphatic chains.32 The folded conformer of the structure can trap guest molecules. There are reports by Acevedo et al. regarding the high capacity of asphaltenes for strongly trapping all sorts of molecules by host-guest interaction. Since asphaltene molecules are known to contain fused aromatic and heteroaromatic rings, they are likely to behave as π-electron donors. Studies with high rank coal using p-chloranil as an electron acceptor have been done using Fourier transform infrared (FTIR) spectroscopy.33 A similar type of complex formation study of petroleum asphaltene has also been done recently with p-nitrophenol as the acceptor.34 Again, as asphaltene remains as monomer and also aggregates in solution, it is important to know about the state of aggregation of asphaltene that is involved in the formation of electron donor-acceptor (EDA) complex with the acceptor. Recent electron absorption spectral measurement from our laboratory indicated that in the concentration range 102-800 mg/L of asphaltene the two isomeric chloranils form an EDA complex involving the aggregated asphaltene species.35 Fluorescence studies using a lower concentration of asphaltene, however, indicates 1:1 complexation involving monomeric species.36 The purpose of this paper is to study the donor behavior of asphaltene with respect to p-bromanil (Figure 1) as an acceptor. To this end, we have monitored the absorption spectrum of asphaltene varying concentration containing a fixed concentration of p-bromanil at different temperatures. Spectral observations have (27) Andreatta, G.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728. (28) Andreatta, G.; Goncalves, C. C.; Buffin, G.; Bostrom, N.; Quintella, C. M.; Arteaga -Larios, F.; Perez, E.; Mullins, O. C. Energy Fuels 2005, 19, 1282. (29) Mullins, O. C.; Sheu, E. Y.; Hammami, A.; Marshall, A. Asphaltenes, HeaVy Oils and Petroleomics; Springer: New York, 2007. (30) Guerra, R. E.; Ladavac, K.; Andrews, A. B.; Mullins, O. C.; Sen, P. N. Fuel 2007, 86, 2016. (31) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Enegy Fuels 2003, 17, 135–30. (32) Acevedo, S.; Castro, A.; Negrin, J. G.; Fernandez, A.; Escobar, G.; Piscitelli, V. Energy Fuels 2007, 21, 2165, and references therein. (33) Maksimova, N. E.; Polyakova, I. A.; Rusyamova, N. D. Khim. TVerd. Topl. Rossiiskaya Akademiya Nauk 1995, 29, 30. (34) Gutierrez, L. I.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15, 624. (35) Ghosh, A. K. Fuel 2005, 84, 153. (36) Ghosh, A. K.; Bagchi, S. Energy Fuels 2008, 22, 1845.

2.1. Sample Preparation. The crude anthracene oil was obtained from Barari coke plant, Dhanbad, Jharkhand, India. The crude anthracene oil was over 10 years old. The fraction >360 °C was obtained after distillation of crude anthracene oil under atmospheric pressure. The fraction >360 °C so obtained was then mixed with n-hexane (1:40 ) oil:n-hexane) and was kept for two days, and then the soluble portion was decanted out. The boiling range of the n-hexane used was 67-69.2 °C. The n-hexane insoluble part was again mixed with little n-hexane and was decanted out; this process was repeated several times to ensure the removal of n-hexane soluble oil. Now, the sticky black mass was dissolved in a minimum quantity of toluene. The mixture was warmed to reduce the volume, and then petroleum ether (40-60 °C) was added. It was then filtered through whatman 41 filter paper. The residue was washed with n-hexane several times once again so that the hexane soluble oil, if any, was removed. The residue (asphaltene) was dried and kept in a desiccator. It is reported that the molecular weights of asphaltene have been found between 500 and 1000 g/mol using fluorescence depolarization measurements.24 However, the reported molecular weight of asphaltene varies considerably depending upon the method and conditions of measurement. For this experiment, molecular weight of 1000 has been assumed for the sample asphaltene. 2.2. Solvent Preparation and Spectroscopic Measurement. p-Bromanil (ACROS) was purified by sublimation before use. Carbon tetrachloride were purified and dried by standard procedures.37-39 The stock solutions of asphaltene were prepared by sonication for 2 hours. All spectral measurements were carried out on a Shimadzu UV 1601 PC model spectrophotometer fitted with a peltier controlled thermobath. Absorption spectra of mixtures containing a fixed concentration of p-bromanil (1.024 mM) and varying concentrations of asphaltene (2.6-7.8 × 10-5) in CCl4 medium against the solvent as reference were recorded at various temperatures.

3. Results and Discussion Figure 2 shows the absorption spectra of asphaltene and p-bromanil in CCl4 medium at concentrations 13 × 10-5 mol dm-3 and 1.204 × 10-3 mol dm-3, respectively, against the solvent as reference. The absorption spectrum of asphaltene so obtained is similar to the absorption spectra for toluene solutions of Tatarstan solid asphaltene and of crude oil with asphaltene concentration close to 66 mg dm-3.34 A low “hump” has been reported for Tatarstan solid asphaltene spectrum at λ ∼ 440 nm. A similar hump has also been observed in the present study (in CCl4) at λ ∼ 375 nm. The shift in the hump position is attributable to change in solvent from toluene to CCl4. Representative absorption spectra of mixtures containing asphaltene and p-bromanil at 293 K are shown in Figure 3. No new absorption peak was evident in the spectra of asphaltene-bromanil mixtures. However, it was observed that the total absorbance of each mixture was higher than the sum of the absorbances of the components throughout the spectral range studied. This difference was scanned over the entire range of wavelengths under study and was plotted against wavelength using the Origin (37) Riddick, J. A.; Bunger, W. B.; Sukano, T. K. Organic SolVents Properties and Methods of Purification Techniques in Organic Chemistry, 4th ed.; Willey Interscience: New York, 1986; Vol. II. (38) Scneider, H. Solute-SolVent Interactions; Coetzee, J. F., Ritchie, C. D. S., Eds.; Marcell Dekker: New York, 1969; Vol. I. (39) Vogel, A. I. A Text Book of Practical Organic Chemistry; Long Mans: New York, 1978.

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Ghosh et al. Table 1. Absorbance Data for Mixtures Containing Various Concentrations of Asphaltene and p-Bromanil (at 1.024 × 10-3 mol dm-3) at Different Temperatures in CCl4 Medium Against the Solvent as Reference absorbance at 445 nm

Figure 2. Absorption spectra of (a) asphaltene (13 × 10-5 mol dm-3) and (b) p-bromanil (1.204 × 10-3 mol dm-3) against the solvent CCl4 as reference.

105[asphaltene]mol dm-3

293 K

298 K

303 K

308 K

313 K

2.6 3.9 5.2 5.9 6.5 7.2 7.8

0.033 0.063 0.065 0.098 0.080 0.099 0.096

0.028 0.057 0.058 0.084 0.071 0.081 0. 081

0.029 0.057 0.060 0.085 0.073 0.076 0.081

0.029 0.056 0.061 0.084 0.072 0.077 0.079

0.030 0.056 0.064 0.083 0.073 0.078 0.078

from the Coulombic attraction between the positively charged donor and negatively charged acceptor ions in the excited state. With the same donor and the same solvent, the band should, according to eq 1, appear at longer wavelength for an acceptor with higher EAv. We have compared the value of λCT as obtained in the present case with the results we obtained in our earlier work involving chloranils. Literature survey shows that the order of vertical electron affinity, EAv, is o-chloranil (3.4 eV) > p-chloranil (2.4 eV) ≈ p-bromanil (2.3 eV).41-43 The observed order of λCT, o-chloranil complex > p-chloranil complex ≈ p-bromanil complex, is thus consistent with eq 1. The formation constant for the complexation process can be obtained by a proper analysis of the absorbance data. To eliminate the absorbance due to the uncomplexed components, the observed absorbance data were corrected by subtracting the absorbance due to the latter as follows. If a 1:1 complex is formed according to the equilibrium A + D h AD

(2)

The observed absorbance (d) for a particular wavelength (with a cell of 1 cm path length) is d ) εA([A]0 - c) + εD([D]0 - c) + εCc )dA0 + dD0 + (εC - εA - εD)c Figure 3. Absorption spectra at 293 K of mixtures containing p-bromanil (1.204 × 10-3 mol dm-3) and asphaltene at concentrations (a) 2.6 × 10-5 mol dm-3, (b) 3.9 × 10-5 mol dm-3, (c) 5.2 × 10-5 mol dm-3, and (d) 7.8 × 10-5 mol dm-3 against solvent CCl4 as reference. Inset: CT spectra of four experimental sets (a-d) calculated by difference method.

6.0 software. Several such difference spectra for the asphaltenebromanil systems are shown in the inset of Figure 3. The sharp change around 346 nm is possibly due to a typical jump of the spectrophotometer reading when the lamp changes from UV to visible. However, a broad band in the range 400-440 nm appears in all the difference spectra. This band is presumably due to the presence of a weak complex formed by the interaction of asphaltene and p-bromanil. That the nature of interaction is mainly of electron donor acceptor (EDA) involving p-bromanil as electron acceptor and asphaltene as donor is supported by Mulliken’s equation.40 According to this equation, the maximum transition energy, hν, is given by the following expression. hν ) IDv - EAv - ∆

(1)

where IDv is the vertical ionization potential of the donor, EAv is the vertical electron affinity of the acceptor, and ∆ is an energy term coming from van der Waals interaction between the donor and acceptor molecules in the ground state, solvation, and mainly (40) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811.

(3)

Here, [A]0 and [D]0 are the initial concentrations of A and D, respectively, in the mixture of A and D before complexation, c is the concentration of the complex, d is the absorbance of the mixture at some suitable wavelength in the perturbed absorption bands of p-bromanil measured against the solvent as reference, and dA0 and dD0 are the absorbances of A and D in solution with the same molar concentrations as in the mixture and at the wavelength of measurement. The corrected absorbance of the complex, d′, is given by d ′ ) d-dA0 - dD0 ) ε′c

(4)

The quantity ε′ ) εC - εA - εD means an effective molar absorptivity, εC is the molar absorptivity of the complex, and εA and εD are those of A and D, respectively, at the wavelength of measurement (445 nm). The absorbance data for mixtures of asphaltene + p-bromanil at five different temperatures are given in Table 1. The formation constant (K) can be found using the BenesiHildebrand equation.44 For a 1:1 complex, we have, for the condition [D]0 . [A]0, the following equation. (41) (42) (43) (44)

Peover, M. E. J. Chem. Soc. 1962, 4540. Briegleb, G. Angew. Chem. 1964, 76, 326. Briegleb, G. Angew. Chem., Int. Ed. 1964, 3, 617. Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.

Molecular Complexation of Asphaltene with Bromanil

Energy & Fuels, Vol. 23, 2009 395

[A]0[D]0 [D]0 1 ) + (5) d′ ε′ Kε′ When the present experimental data were plotted according to the Benesi-Hildebrand equation for a 1:1 stochiometry of the complex, a very wide scatter with a low correlation coefficient was observed indicating that the composition of the complex is not 1:1. The fact that the asphaltene concentration used in the experiment is close to the critical nanoaggregate concentration (CNAC) in the solvent45 prompted us to explore the possibility of formation of a complex of 1:2 stoichiometry as represented by the following equations. 2D h D2:K2

(6)

A + 2D h AD2:K ) K2K1

(8)

The expression for d′ remains the same and only ε′ becomes (εC - εA -2εD). The Benesi-Hildebrand equation, that is, eq 5, in this case gets modified to eq 9. [A]0[D]02 [D]02 1 ) + d′ ε′ Kε′

(9)

A good linear correlation has been obtained with the present experimental data as shown in Figure 4. The regression equations are as follows. For 293 K [A]0[D]02⁄d′ ) (10.500 ( 1.248) × 10-4[D]02 +

and A + D2:K1 The overall process can, however, be represented by

(2.590 ( 0.481) × 10-12 R2 ) 0.94 (10)

(7) For 298 K

Figure 4. Benesi-Hildebrand plot for complex of asphaltene with p-bromanil in CCl4 medium (λ ) 346 nm) at (a) 293 K, (b) 298 K, (c) 303 K, (d) 308 K, and (e) 313 K.

396 Energy & Fuels, Vol. 23, 2009

Ghosh et al.

Table 2. Formation Constants (K) and Thermodynamic Parameters for Asphaltene-p-Bromanil Complex in CCl4 Medium temperature (K)

formation constant K (dm6 mol-2)

∆Hf 0 (kJ mol-1)

∆Sf 0 (JK-1 mol-1)

293 298 303 308 313

(5.3 ( 0.1) × 108 (6.2 ( 0.1) × 108 (7.0 ( 0.1) × 108 (7.4 ( 0.1) × 108 (8.0 ( 0.1) × 108

15.7 ( 1.5

220.8 ( 0.2

[A]0[D]02 ⁄ d′ ) (11.300 ( 1.088) × 10-4[D]02 + (2.462 ( 0.419) × 10-12 R2 ) 0.96 (11) For 303 K [A]0[D]02 ⁄ d′ ) (11.600 ( 0.814) × 10-4[D]02 + (2.401 ( 0.314) × 10-12 R2 ) 0.98 (12) For 308 K [A]0[D]02 ⁄ d′ ) (12.100 ( 0.477) × 10-4[D]02 +

Figure 5. ln K vs 1/T plot for complex of asphaltene with p-bromanil in CCl4 medium.

(2.388 ( 0.171) × 10-12 R2 ) 0.99 (13) For 313 K [A]0[D]02 ⁄ d′ ) (12.600 ( 0.112) × 10-4[D]02 + (2.396 ( 0.430) × 10-12 R2 ) 0.96 (14) The formation constant (K) is obtained by dividing the slope by the corresponding intercept of each plot. For the complex, K was calculated at various temperatures, and the results are given in Table 2. Calculations, taking λ ) 400 nm and λ ) 420 nm, at 303 K gave almost identical values of K which confirms the existence of formation equilibrium of the complex in the systems. Although no charge transfer was found from the molecular dynamics study on asphaltene aggregates by Takanohashi et al.,31 in this theoretical study only asphaltene (and no other adduct) was investigated. However, the present experimental study points to the existence of an intermolecular complex involving asphaltene (donor) and bromanil (acceptor). This does not contradict the earlier theoretical finding31 on asphaltene aggregation; instead, the high values of K support the aggregation model. The presently found magnitudes of K (∼108) are large compared to those of weak charge transfer46 complexes (101 to 103) but are comparable to those of some recently reported47-51 supramolecular or inclusion complexes. As can be seen from eq 8, the observed value of K is given by K ) K2K1. The high value of K obtained in the present study can thus be explained according to eqs 6-8. Acevedo et al. have recently reported the high capacity of asphaltene to trap molecules by host-guest interaction.32 Our present study also indicates that a molecule of p-bromanil is included between two asphaltene donors. Figure 5 shows the variation of formation constant (K) with temperature. Thermodynamically, ln K would vary linearly with 1/T according to the following equation. (45) Ghosh, A. K.; Srivastava, S. K.; Bagchi, S. Fuel 2007, 66, 2528. (46) Foster, R. Organic Charge Transfer Complexes; Academic Press: London, 1969. (47) Atwood, J. L.; Koutsoantonis, G. A.; Raston, C. L. Nature 1994, 368, 229. (48) Hatano, T.; Ikeda, A.; Akiyama, T.; Yamada, S.; Sano, M.; KaneKiyo, Y.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2000, 2, 909–912. (49) Mizyed, S.; Georghiou, P. E.; Ashram, M. J. Chem. Soc., Perkin Trans. 2000, 2, 277. (50) Bhattacharya, S.; Nayak, S. K.; Chattopadhyay, S.; Banerjee, M.; Mukherjee, A. K. J. Phys. Chem. A 2002, 106, 6710. (51) Bhattacharya, S.; Nayak, S. K.; Chattopadhyay, S.; Banerjee, M.; Mukherjee, A. K. J. Phys. Chem. B 2003, 107, 11830.

ln K )

-∆Hf0 ∆Sf0 + RT R

(15)

where ∆Hf0 and ∆Sf0 are enthalpy and entropy of formation of the complex. Values of ∆Hf0 and ∆Sf0, as calculated from the slope and intercept of the plots in Figure 5, have been listed in Table 2. The high positive value of entropy of formation needs special mention. The positive value indicates that during complex formation the acceptor molecule (p-bromanil) gets included in the cavity of the structure provided by asphaltene molecules and many solvent molecules are released from the cavity thereby increasing the entropy of the system. The high value of ∆Sf0 can be understood by the formation of 1:2 complex. Thus, ∆Sf0 per mole of asphaltene comes as 110.4 JK-1 (26.4 cal K-1), which is a reasonable value. The positive value of ∆Hf0 is not clearly understood. This only means that the extent of complexation increases with an increase in temperature (eq 12). However, an opposite trend is usually observed for molecular complexes. Thus, the complex formed between asphaltene and p-bromanil is not a normal EDA complex, and the process is entropy driven. 4. Conclusion It has been established from absorption spectrophotometric evidence that p-bromanil forms 1:2 inclusion complexes with coal-derived asphaltene. The absorption peak correseponding to the complex has been detected by difference spectral method. The wavelengths of the absorption maximum of the complex parallel the electron affinity of bromanil as expected from Mulliken’s theory. The formation constants of the complexes support the view that in CCl4 solution the bromanil molecules get trapped by structures provided by asphaltene dimers. The positive sign of ∆Hf0 indicates unusual molecular complexes of asphaltene. The positive entropy of formation indicates that during complex formation the acceptor molecule gets included in the cavity of the aggregate and many solvent molecules are released from the cavity thereby increasing the entropy of the system. Acknowledgment. A.K.G. thanks Director, CIMFR, India, for support. A.K.M. thanks UGC, India, for financial support (DSA, Chemistry, BU, Phase III). EF800746Z