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Energy & Fuels 1996, 10, 262-263
Surface Tension of Coal Extracts in Organic Solvents Kazuaki Hayasaka, Toshimasa Takanohashi, and Masashi Iino* Institute for Chemical Reaction Science, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-77, Japan Received July 6, 1995. Revised Manuscript Received October 27, 1995 Coal extracts are known1-3 to associate through noncovalent interactions such as hydrogen bondings, aromatic-aromatic, and charge transfer interactions. High extraction yields (45-79 wt %) for some bituminous coals obtained by using a carbon disulfide-Nmethyl-2-pyrrolidinone mixed solvent indicate that a very large amount of extractable molecules exist originally in the coals.3,4 We have also reported3 that the extraction yield increases by the addition of a small amount of tetracyanoethylene (TCNE) which is one of the strongest electron acceptors. This suggests that TCNE dissociates the associates among coal extracts molecules. The association behaviors may play an important role in structure and reactivity of coals, but details on their associated structure are not well understood. In this study, surface tensions of coal extract solution in organic solvents were measured. The surface tension is widely used for analyzing colloidal properties such as formation of micelles. Sheu et al.5,6 reported that asphaltene solution from petroleum vacuum residue in pyridine behaved like surfactant solutions and showed a distinctive discontinuity point in the plot of the surface tension against the asphaltene concentration which may correspond to critical micelle concentration (cmc). It is generally accepted that, below critical micelle concentration, surfactant molecules are adsorbed to the surface of a solution for lowering the system free energy to the minimum, resulting in the decrease of its surface tension. When the surface is saturated with adsorbed surfactant molecules, that is cmc, molecules form micelles. Two Argonne Premium coals (Upper Freeport (UF), 86.2 wt % C, (daf), Illinois No. 6 (IL), 76.9 wt % C, (daf)) were used. The coals were extracted with CS2-Nmethyl-2-pyrrolidinone (NMP) mixed solvent (1:1 by volume) repeatedly at room temperature.4,7 After filtration with a membrane paper with an average pore size of 0.8 µm and evaporation of supernatant, the extract was fractionated with acetone and pyridine, respectively, and acetone-soluble (AS) and pyridine solubleacetone insoluble (PS) fractions were obtained. The surface tension of AS and PS solution in NMP or pyridine at 25 °C was measured using an automatic surface tensiometer (Model CBVP-A3, Kyowa Interface Science Co.) by the Wilhelmy plate method. The surface (1) Sanokawa, Y.; Takanohashi, T.; Iino, M. Fuel 1990, 69, 1577. (2) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100. (3) Liu, H.-T.; Ishizuka, T.; Takanohashi, T. Energy Fuels 1993, 7, 1108. (4) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639. (5) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; De Canio, S. J. Fuel 1992, 71, 299. (6) Sheu, E. Y.; Storm, D. A. Fuel 1994, 73, 1368. (7) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 452.
0887-0624/96/2510-0262$12.00/0
Figure 1. Surface tension-concentration relationship for the UF-AS/NMP solution at 25 °C.
Figure 2. Surface tension-concentration relationship for the IL-AS/NMP solution at 25 °C.
tension decreased with time, and it was determined after an equilibrium was attained, which took several hours. Figures 1 and 2 show surface tension for UFAS/NMP and IL-AS/NMP solutions as a function of the logarithm of AS concentration, respectively. In these figures, there are two distinctive discontinuity points. The reason why the surface tension hardly changes with the concentration below the first discontinuity point (0.03 g/dL for UF-AS and 0.06 g/dL for IL-AS), respectively, is not clear at present. Above the first discontinuity point, the surface tension rapidly decreased with increasing the concentration until the second discontinuity point (0.12 and 0.25 g/dL, for UF-AS and IL-AS, respectively), and then the rate of decrease slowed. It is well-known that the decrease in surface tension is caused by a decrease in surface free energy of the solvent due to the adsorption of amphiphilic compounds to the surface of the solution. The AS fraction can be amphiphilic, since it consists of nonpolar aromatic rings and polar hydroxyl groups attached on them.8 The aromatic rings are considered to be adsorbed to the surface of the solution, leading to lowering of the surface tension of the coal solution. The second discontinuity point may correspond to the cmc above which micelle(8) Iino, M.; Takanohashi, T.; Obara, S.; Tsueta, H.; Sanokawa, Y. Fuel 1989, 68, 1588.
© 1996 American Chemical Society
Communications
Figure 3. Surface tension-concentration relationship for the UF-PS/NMP (O) and the IL-PS/NMP (0) solution at 25 °C.
like associates form. The structure of these associates is not clear, although the structure having aromatic rings in the core of the associates seems to be reasonable. The cmc value for asphaltene from a petroleum vacuum residue in pyridine was reported to be about 0.03 g/dL, much smaller than that observed in this
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study. The cmc value for UF-AS was lower than that for IL-AS, probably because UF-AS contains more aromatic and less polar constituents than IL-AS,7,8 with the result that UF-AS has more surface activity than IL-AS in polar solvents. Similar behaviors were observed for UF-AS/pyridine system, except that the cmc in pyridine (0.09 g/dL) was lower than that for NMP. For the PS fraction, which is a heavier fraction than AS, Figure 3 shows that little change in the surface tension with the concentration was observed. It was reported that the PS has not only higher molecular weight substances but also more heteroatoms than AS.8 Thus, many polar and nonpolar centers may exist in one PS molecule, and so they have much less surface activity in the polar solvents than that for AS. Effects of solvent and temperature on surface tension of coal extract solution and the structure of the micelle-like coal associates will be reported later. EF9501298
