Znd. Eng. Chem. Res. 1991,30, 1362-1366
1362
Substituent Effect on Solubilities of Solids in Supercritical Fluids. Naphthalene Derivatives Tadashi Nakatani* Department of Chemical Engineering, Research Laboratory of Applied Biochemistry, Tanabe Seiyaku Co., Ltd., Yodogawa-ku, Osaka 532, Japan
Kazunari Ohgaki and Takashi Katayama Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
Solubilities of five naphthalene derivatives (2-methoxynaphthalene, 2-naphthol, 2-cyanonaphthalene, 2,6-dimethylnaphthalene,and 2-hydroxy-1-naphthaldehyde) in supercritical carbon dioxide, ethylene, ethane, and trifluoromethane were measured at 308.15 K. One of the interesting findings is that a disubstitution does not always show the additional depression in solubilities of each monosubstituted derivative. T h e substituent effect on solubilities of these derivatives in each fluid was elucidated by use of electronic properties of substituents. The empirical principles of solubility changes have been drawn out from the discussion about solubility depression caused by substitutions, characteristic behavior of carbon dioxide for electron-releasing substituents, and good dissolution power of trifluoromethane for electron-withdrawing substituents. Finally, the empirical rules in choosing appropriate supercritical solvents have been itemized in terms of five articles.
Introduction Studies on high-pressure phase behavior of supercritical fluids involving solids have been increased because of theoretical interest and their practical importance for high-pressure extraction in pharmaceutical and related industries. Recently, a method for predicting solubilities has been advanced to give a good prediction for nonpolar fluid mixtures such as hydrocarbons by use of the equations of state. However, the equation constants, even for pure component, must be frequently estimated for asymmetric mixtures treated in supercritical fluid extraction. The physical meaning of binary parameters is also lost during a fitting procedure for such mixtures. The method may lead to significant errors for mixtures that consist of polar and/or associated compounds. This is why a new prediction method based on molecular structures or electronic states alone is highly desirable. We have investigated solubilities of some derivatives of indole, pyrimidine, and pyrazine in supercritical carbon dioxide, ethylene, ethane, and trifluoromethane in our previous studies (Sako et al., 1989; Nakatani et al., 1989; Nakatani et al., submitted). The indole derivative used in the previous study has methyl, methoxyl, oxyl, hydroxyl, amino, formyl, or carboxyl substituents in the parent compound. A hydroxyl, amino, mercapto, chloro, or carboxyl substituent lies in each parent compound of pyrimidine and pyrazine. From the previous findings, the substituent effect on the solubilities of each derivative has been provisionally explained in terms of some articles: an addition of substituents lowers the solubility by far from that of parent compound in supercritical fluids; the solubility is extremely lowered by the hydroxyl, amino, formyl, carboxyl, and mercapto substituents that are expected to take part in the self-association such as an intermolecular hydrogen bonding; supercritical carbon dioxide shows a fairly high dissolution power for a derivative that has an electronreleasing substituent; supercritical trifluoromethane is a relatively good solvent for a derivative that has an electron-withdrawing substituent; both supercritical ethylene and ethane are very good solvents for aromatic hydrocarbons. To fill the above articles, the solubilities of five naphthalene derivatives in the four supercritical fluids have
been measured in the present study. Several investigators have reported the solubilities of naphthalene in supercritical carbon dioxide (Tsekhanskaya et al., 1964; McHugh and Paulaitis, 1980; Cheong et al., 1986; Sako et al., 19881, ethylene (Diepen and Scheffer, 1948,1953; Tsekhanskaya et al., 1964; Sako et al., 1988) ethane (Johnston et al., 1982; Schmitt and Reid, 1986; Sako et al., 1988), and trifluoromethane (Schmitt and Reid, 1986). The solubilities of 2-naphthol in supercritical carbon dioxide have been published by Schmitt and Reid (19861, Tan et al. (19871, and Dobbs et al. (1987). Also the solubility of 2-naphthol in supercritical ethane has been reported by Schmitt and Reid (1986). Kurnik et al. (1981) have measured the solubilities of 2,6-dimethylnaphthalene and 2,3-dimethynaphthalene in supercritical carbon dioxide and ethylene. No data are available for other naphthalene derivatives except for 1,4-naphthoquinone reported by Schmitt and Reid (1986). Marking a link in the chain of the studies on solubility prediction from molecular structures or electronic states alone, Stahl and co-workers (1980) presented a short expression for low extractability of self-associatedcompounds in supercritical carbon dioxide. In the similar context, Schmitt and Reid (1986) pointed out the significant rules in choosing appropriate supercritical solvents. Their conclusion was related to the dissolution power of supercritical fluids: first, carbon dioxide is a very encompassing solvent, performing especially well with polar compounds; second, ethane is the best solvent for simple aromatic hydrocarbons in carbon dioxide, ethane, trifluoromethane, and chlorotrifluoromethane; third, trifluoromethane is a poor solvent for hydrocarbons but a good solvent for those molecules containing hydrogen bonding; fourth, chlorotrifluoromethane is consistently the poorest of the four solvents.
Experimental Section Materials. Carbon dioxide (purity 99.9+ mol %), ethylene (purity 99.5+ mol %), ethane (purity 99.0+ mol %), and trifluoromethane (purity 99.99+ mol %) were all obtained from Sumitomo Seika Chemicals Co. Ltd. The model solutes examined in the present study are listed in Table I. 2-Naphthol was obtained from Kata-
0888-5885191/2630-1362$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, NO. 6,1991 1363 Table I. Physical Properties of the Solutes
MW compound
gb"
T,, K
158.20
346-348
formula
structure
2-methoxynaphthalene 2-naphthol
CllHlOO
m o C H 3
2-cyanonaphthalene 2,6-dimethylnaphthalene
C11HlN
a"" mcN
ClOHBO
ClzHlz H3C
2-hydroxy-1naphthaldehyde
CllHB02
144.17
395-396
153.18
335-337
156.23
382-384
WCH' Ho
OH
172.18
A
15
0
Q
51
355-358
yama Chemical Ind. Co., 2-cyanonaphthalene was from Tokyo Kasei Kogyo, and 2-methoxynaphthalene, 2,6-dimethylnaphthalene, and 2-hydroxy-1-naphthaldehydewere obtained from Aldrich Chemical Co. The minimum purity of 2-cyanonaphthalene and 2-hydroxy-1-naphthaldehyde was 98 mol %, and all other derivatives had minimum purities of 99 mol %. All materials were used without further purification. Apparatus and Procedure. The experimental apparatus consists of an equilibration cell coupled with a supercritical fluid chromatograph. A solid of interest was charged into the stainless steel equilibrium cell (ca.12 cm3) which was installed in an air bath regulated at 308.15 f 0.1 K. Supercritical solvents were introduced to the equilibrium cell by an HPLC-type plunger pump. The cell pressure was varied from 6 to 21 MPa with an accuracy of 10.065 MPa. After equilibrium was reached, a small amount of sample was transferred to the SFC line by switching a six-port valve. The mixed mobile phase of 1.1-4.3 wt % methanol was passed through an octadecylsilane (ODS) column, and then it was released from a back-pressure regulator that kept the pressure constant a t 22.70 f 0.12 MPa. Solid solubilities were calculated from peak areas of UV analysis. The detectable limit was about lo4 mol %. A more detailed description of the experimental equipment and operating procedure can be found in a previous paper (Nakatani et al., 1989).
Results and Discussion We introduce the two relative solubilities: the first is the mole fraction ratio of a derivative to the parent compound in a fixed solvent at the same temperature and pressure, (y/yP,,,J; the second is (y/y(c2b1)which is defined at the same temperature and pressure for a fixed derivative. The former can clearly show the substituent effect on solubilities of derivatives, and the latter can directly present the magnitude of dissolution power of each supercritical fluid for a fixed derivative. For the sake of convenience, the both are explained in terms of logarithms: the first relative solubility is In (y/ym,,J and the second relative solubility is In (y/ y(c The reference pressure is arbitrarily fixed at 15 hf& because the first relative solubilities are supposed to be constant for almost all systems in the pressure range 15-20 MPa Thus the mole fraction of derivatives at 15 MPa was obtained from an ordinary interpolation method. Solubilities of 2-Methoxynaphthalene. Figure 1 shows the solubilities of 2-methoxynaphthalene in supercritical carbon dioxide, ethylene, ethane, and trifluoromethane a t 308.15 K. In the previous paper (Nakatani et al., 19891, we pointed out the relationship between the solubility behavior and
0
0.005
0.010
Yl
Figure 1. Solubilities of 2-methoxynaphthalene in the four supercritical fluids at 308.15 K.
the Hammett constant. The methoxyl group has a negative Hammett constant, -0.27 (Hansch and Leo, 1979). The negative constant means that the s-electron density of the skeleton becomes higher by electron release from the substituent. Therefore, carbon dioxide is expected to be a very good solvent for 2-methoxynaphthalene. The second relative solubilities of 2-methoxynaphthalene, In ( y / ~ ( ~in)carbon , dioxide, ethylene, and trifluoromethane are 0.30, -0.56, and -0.06, respectively. Considering the second relative solubilities of unsubstituted naphthalene (-0.61 for carbon dioxide, -0.24 for ethylene, and -1.35 for trifluoromethane), two things should be noted about these findings: first, the dissolution powers of ethylene and ethane are relatively lowered; second, those of carbon dioxide and trifluoromethane are fairly strong; especially, carbon dioxide is the best solvent for this solute. The first relative solubilities of this compound, In (y/ yC,&), in carbon dioxide, ethylene, ethane, and trifluoromethane are -0.92, -2.15, -1.83, and -0.52, respectively. Namely, the degree of decrease in solubilities caused by the substitution is largest in ethane and smallest in trifluoromethane. This behavior was also found in indole derivatives; that is, the first relative solubilities of methoxyindole in carbon dioxide, ethylene, ethane, and trifluoromethane are -1.16, -1.36, -1.50, and -0.58, respectively. The intermolecular attractive force can be explained in terms of the sum of dipole-dipole, dipole-induced dipole, and dispersion interactions except for associated compounds : dipole-dipole interaction: Uij(dip) = -2(pipj)2NA/3kTfl dipole-induced dipole interaction:
(1)
+ a,p?)NA/fl
(2)
Uij(ind) = -(ai$ dispersion interaction:
Uij(disp) = -31iIjaiaj/2(Ii
+ Ij)fl
(3)
where CL,I, and a are dipole moment, ionization energy, and polarizability, respectively. These values are obtained from a molecular orbital calculation (MNDO) presented by Dewar and Thiel (1977). Also NA, r, and k are the Avogadro constant, intermolecular distance, and Boltzmann constant, respectively. Therefore, the total attractive energies of like and unlike molecules can be calculated once the intermolecular distance is given. Unfortunately, we have no information about the potential minimum point for each system. So the coefficients of r4 term alone, being
1364 Ind. Eng. Chem. Res., Vol. 30,No. 6, 1991
-
20
A
0
0
-
15
2
=.
'
0
e
,
e"
cp
-
e e e
0
0
O A 0
10-
P
15
0
O O
P 0
c02
.
O
0
0
C Z H ~CzHs e CHFI ' A
5-
coz
A C2H4 0 CZH6
e
CHh
'
I 0
0.0004
0.0008
0
0.004
0.008
y1
y1 Figure 2. Solubilities of 2-naphthol in the four supercritical fluids at 308.15 K.
Figure 3. Solubilities of 2-cyanonaphthalene in the four supercritical fluids at 308.15 K.
expressed in nms J/mol, are compared with each other. The coefficients in the total potential of like molecules of carbon dioxide, ethylene, ethane, and trifluoromethane are -6.5,-13.4,-18.4,and -30.8 nme J/mol, respectively. For the trifluoromethane molecule, the coefficients of dispersion, dipole-dipole and dipole-induced dipole terms are -6.0,-23.4,and -1.4,respectively. Without a consideration of molecular distance, a comparison of the coefficients between unlike molecules cannot give strict information about the affinity of solvents. However, we can get broad information about the magnitude of dissolution power for a fixed solute that does not have a hydrogen bonding. Solubilities of 2-Naphthol. Figure 2 shows the experimental result on solubilities of 2-naphthol. The solubility data obtained in this study for the naphthol-carbon dioxide and -ethane systems are in good agreement with the literature values. The largest decrease of solubilities was observed in the five derivatives because of the selfassociation of naphthol. According to the Hammett law, the electron-releasing power of the hydroxyl group is stronger than that of the methoxyl group. The second relative solubilitiesof naphthol in carbon dioxide, ethylene, and trifluoromethane are 0.83,0.42,and 0.33,respectively. Carbon dioxide seems to be the best solvent of the four fluids, and ethylene seems to be better than trifluoromethane; however, the first relative solubilities in carbon dioxide, ethylene, ethane, and trifluoromethane are -3.41, -4.19,-4.85,and -3.17,respectively. From these values, we can conclude that ethylene and ethane show a large solubility depression; on the other hand, trifluoromethane shows the smallest depression of the four solvents. Although the self-association of naphthol is essential in these systems as mentioned the above, the dipole-dipole interaction between trifluoromethane and naphthol molecules still play an important role in increasing the solubility. The dipole moment for naphthol is 4.43 X J112m3/2 according to the MNDO calculation. Solubilities of 2-Cyanonaphthalene. Figure 3 shows the solubilities of cyanonaphthalene in the four supercritical fluids. The cyano group has a positive Hammett constant (+0.66),withdrawing electrons from naphthalene ring. Generally speaking, a molecule having a substituent with a positive Hammett constant also has a considerably large dipole moment. According to the MNDO calculation, cyanonaphthalene has a fairly large dipole moment of 11.5 X lo-% J112m312. The coefficient of a s u m of dipole-dipole and dipole-induced dipole interactions between cyanonaphthalene molecules is -194 nms J/mol, while the coefficient of total potential is -409 nms J/mol. The first relative solubilities are -1.56 in carbon dioxide, -2.48 in ethylene, -2.70 in ethane, and -0.14 in trifluoro-
methane. The second relative solubilities are 0.53,-0.02, and 1.21 for carbon dioxide, ethylene, and trifluoromethane, respectively. The most characteristic behavior for this system is explained in terms of the high polarity of cyanonaphthalene; the second relative solubility in trifluoromethane is the largest of the four solvents. The main reason is that the coefficient of dipole-dipole and dipole-induced dipole interactions between cyanonaphthalene and trifluoromethane molecules is exceedingly large, -69 nms J/mol, while those of other three mixtures are, at the most, -3 nms J/mol. The polarizability of carbon dioxide is 0.0027 nm3, and those of ethylene and ethane are 0.0043and 0.0045 nm3,respectively. Then, the coefficient of dipole-induced dipole interaction between carbon dioxide and cyanonaphthalene is -2.1 nm6 J/mol only. Carbon dioxide becomes a somewhat poor solvent for this polar compound. Also the indole derivative having a positive Hammett constant showed the lowest solubility in carbon dioxide for the four solvents (Nakatani et al., 1989). For the naphthalene derivative, carbon dioxide is not the poorest but is fairly low comparing with other systems. The change of degree between the two compounds is based on the property of each parent compound. As a matter of fact, the dipole moment of indole is given as 6.58 X lo-% J112m312by the MNDO calculation, while that of naphthalene is zero. Solubilities of 2,6-Dimethylnaphthalene.When a methyl group, having a negative Hammett constant (-0.17), is attached the naphthalene molecule, the melting temperature of the derivative becomes quite low. We could not measure the precise solubility of monomethylnaphthalene, because our experimental apparatus was designed for measuring the solubility of solids in supercritical fluids. However, it can be supposed to be very large. Then we adopted a disubstituted compound here because a disubstituted compound usually shows a solubility lower than a monosubstituted one. The first relative solubilities of dimethylnaphthalene in supercritical trifluoromethane (-1.42),carbon dioxide (-1.84),ethylene (-2.001, and ethane (-2.12)decrease in that order. The second relative solubilities in carbon dioxide, ethylene, and trifluoromethane are -0.33,-0.12,and -0.65,respectively. Thus ethane is the best solvent, while trifluoromethane is the poorest of the four solvents as shown in Figure 4. The ionization energies of unsubstituted naphthalene (0.827 X los J/mol) and dimethylnaphthalene (0.826X losJ/mol) are almost identical with each other; moreover, they both have negligible dipole moments. The polarizabilities are also not so much different, 0.017 nm3 for naphthalene and 0.020 nm3 for dimethylnaphthalene. The coefficients of dispersion inter-
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1365 o
20
n A
t
0.003
0
0.006
Yl
Figure 4. Solubilities of 2,6-dimethylnaphthalene in the four supercritical fluids at 308.15 K. I
0
I
0.002
0.004
Yl
Figure 5. Solubilities of 2-hydroxy-1-naphthaldehydein the four supercritical fluids at 308.15 K.
actions of dimethylnaphthalene with carbon dioxide, ethylene, ethane, and trifluoromethane are -39.7, -57.8, -66.7, and -37.4 nm6 J/mol, respectively. The order of coefficients corresponds to the order of the second relative solubilities. There is an interesting problem concerning the solubilities of an isomer, 2,3-dimethylnaphthalene,in supercritical carbon dioxide and ethylene (Kurnik et al., 1981). The isomer, having two methyl groups on adjacent positions in the naphthalene ring, is given similar electronic properties of 2,6-dimethylnaphthalene by the MNDO calculation. Upsetting the expectation, the solubility of this isomer is larger than that of 2,6-dimethylnaphthalene. This fact may be explained in terms of a kind of so-called ortho effect. In any case, this problem should be made clearer in the further study. Solubilities of 2-Hydroxy-1-naphthaldehyde.The solubilities of this compound are shown in Figure 5. The fmt relative solubilities in carbon dioxide, ethylene, ethane, and trifluoromethane are -1.91, -3.03, .-3.39, and -1.98, respectively. The large depression in solubilities is observed in the two hydrocarbon solvents. All of the second relative soiubilities are positive, that is, ethane is the poorest soivent: 0.87 for carbon dioxide; 0.12 for ethylene; 0.06 for ’crifluoromethane. 2-Hydroxy-1-naphthaldehydehas two substituents of different properties: the hydroxyl group is, as mentioned the above, an electron-releasingtype (a negative Hammett constant, -0.37); on the other hand, the formyl group is an electron-withdrawing type (a positive Hammett constant, +0.42). There is a thing in common between the two, viz., the self-association (intermolecular hydrogen bonding) is expected when a compound has such kind of Substituent. According to the statement of Stahl et al.
(1980),the solubility of this compound would be expected to become fairly low because of self-association. The large depression of solubilities is certainly observed; however, the more important thing lies in filling up the expressions for substituent effect. We have to discuss the larger solubilities of 2-hydroxy-1-naphthaldehydecompared to those of 2-naphthol in the four solvents; for example, the first relative solubilities of naphthol and hydroxynaphthaldehyde in supercritical carbon dioxide are -3.41 and -1.91, respectively. It is contradictory to the empirical rule where a further depression in solubilities is expected by an additional substitution of the formyl group. As a matter of fact, we could not detect the solubilities of thymine and uracil in our previous study (not published) because of their very low solubilities in the four solvents. Both enol forms of pyrimidine homologues are substituted by two hydroxyl groups. There are two ideas for interpreting this contradiction: first, the electron-releasingproperty of the hydroxyl group and the electron-withdrawingproperty of the formyl group cancel each other; second, the intramolecular hydrogen bonding between the two substituents may occur because the two groups substitute adjacent positions on naphthalene ring. It is not easy at this point to decide on the propriety of these ideas; however, the latter seems to be more reasonable. Because the dipole moment of this J112malz) is far from that of compound (11.56 X naphthol (4.43 X J112mal2)and because hydrogen bonding is essential for the 2-naphthol system, cancellation of the two substituents is rather difficult to be imagined. When an intramolecular hydrogen bond is formed between two substituents in a molecule, the total intermdecular force including hydrogen bonding becomes weaker than that of an ordinary hydrogen-bonded molecule such as naphthol. This is why the first relative solubility of the company having intramolecular hydrogen bonding is larger than that of each monosubstituted compound having intermolecular hydrogen bonding. Integrating the above discussions, we try to fill up the provisional rules in choosing appropriate supercritical solvents for a derivative when the solubility of the parent compound is available. The rules are divided into two parts: one is related to dissolution powers of supercritical fluids; the other is related to the substituent effect on solubilities. Rules Concerning Dissolution Powers. (a) Hydrocarbons such as ethylene and ethane are good solvents for solid hydrocarbons because of the natural affinity between the same kind of compounds. However, they become notably poor solvents by the introduction of functional groups except for the alkyl group to the parent compound. (b) Polar compounds such as trifluoromethane are consistently good solvents for all solid substances except for nonpolar hydrocarbons. The solubilities of polar but nonassociated compounds are especially high in such solvents. (c) Carbon dioxide is generally a fairly good solvent. An electron-releasing substituent makes it a good solvent, while an electron-withdrawing one relatively reduceb its dissolution power. Rules Concerning Substituent Effect on Solubilities. (d) Introduction of substituents reduces, in a broad sense, the solubility compared to that of the parent compound. In this context, the solubility of a disubstituted compound is additionally reduced from that of each monosubs tituted compound. (e) When a compound has a substituent that is expected to be hydrogen bonding, the solubility is extremely low-
1366 Ind. Eng. Chem. Res., Vol. 30,No. 6, 1991
ered, as is well-known. However, additional substitution with the same type of group does not lower the solubility of a monosubstituted compound at all times. It depends on the position substituted by the additional substituent.
Regietry No. COz, 124-38-9; HzC=CH2, 74-85-1; CH8CH8, 74-84-0; F&H, 75-46-7; 2-metho~ynaphthalene~ 93-04-9; 2naphthol, 135-19-3; 2-cyanonaphthalene, 613-46-7; 2,6-dimethylnaphthalene, 581-42-0; 2-hydroxy-l-naphthaldehyde, 708-06-5.
Conclusion The rules for choosing appropriate solvents in supercritical extraction are, to a certain extent, well arranged from a study of substituent effect on solubilities. The solvent-selectionrules for supercritical fluids are somewhat similar to those for ordinary liquid solvents as expressed by Schmitt and Reid (1986). The further study should be performed with respect to the effect of positions substituted. The findings in the 2-hydroxy-l-naphthaldehydesystems would be explained in terms of the intramolecular hydrogen bonding between the hydroxyl and formyl groups in the same ring. Moreover, 2,3-dimethylnaphthalene shows larger solubilities than 2,6-dimethylnaphthalenein supercritical carbon dioxide and ethylene. Such behavior would be caused by something like the ortho effect; it is however not clear a t this point. This overall knowledge is quite important in separations of not only derivatives but also isomers by use of supercritical fluids.
Literature Cited
Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Culture, Japan. We acknowledge the technical assistance of Mr. Y. Ishizu of Tanabe Seiyaku Co., Ltd. We also thank to Drs. K.Yoshida and K. Tatsumi of Osaka University for their valuable discussions on electronic properties of naphthalene homologues and on the MNDO calculations, respectively. Nomenclature
Z = ionization energy, J/mol k = Boltzmann constant, J/K
NA = Avogadro constant, mol-' p = pressure, MPa
r = molecular distance, nm T = temperature, K U = interaction potential energy, J/mol y = mole fraction of solute in supercritical fluid, 1 a = polarizability, nm3 cc = dipole moment, J112 Subscripts i, j = each molecule
Cheong, P. L.; Zhang, D.; Ohgaki, K.; Lu, B. C.-Y. High Pressure Phase Equilibria for Binary Systems Involving a Solid Phase. Fluid Phase Equilib. 1986,29, 555-562. Dewar, M. J. S.; Thiel, W. Ground States of Molecules, 38. The MNDO Method Amroximations & Parameters. J Am. Chem. SOC.1977, 99, 4899. Diepen, G. A. M.; Scheffer, F. E. C. The Solubility of Naphthalene in Suoercritical Ethylene. J. Am. Chem. SOC.1948,70.4085-4089. Diepen,-G. A. M.; Scheffer, F. E. C. The Solubility of Naphthalene in Supercritical Ethylene 2. J. Phys. Chem. 1953, 57, 575-577. Dobbs, J. M.; Wong, J. M.; Lahiere, R. J.; Johnston, K. P. Modification of Supercritical Fluid Phase Behavior Using Polar Cosolvents. Znd. Eng. Chem. Res. 1987,26,56-65. Hansch, C.; Leo, A. Electronic Parameters. In Substituent Constants for Correlation Analysis in Chemistry and Biology; John Wiley and Sons: New York, 1979; p 3. Johnston, K. P.; Zlger, D. H.; Eckert, C. A. Solubilities of Hydrocarbon Solids in Supercritical Fluids. Ind. En#. Chem. Fundam. 1982,21, 191-197. Kurnik, R. T.; Holla, S. J.; Reid, R. C. Solubility of Solids in SuDercritical Carbon Dioxide and Ethylene. J. Chem. Ena. - Data 1981,26,47-51. McHugh, M.; Paulaitis, M. E. Solid Solubilities of Naphthalene and BiDhenvl in SuDercritical Carbon Dioxide. J. Chem. Ena. - Data 1960,2%, 326-329. Nakatani, T.; Ohgaki, K.; Katayama, T. Solubilities of Indole Derivatives in Supercritical Fluids. J. Supercn't. Fluids 1989,2, S14. Nakatani, T.; Tohdo, T.; Ohgaki, K.; Katayama, T. Solubilities of Pyrimidine- and Pyrazine-Derivatives in Supercritical Fluids. J. Chem. Eng. Data, in press. Sako, S.; Ohgaki, K.; Katayama, T. Solubilities of Naphthalene and Indole in Supercritical Fluids. J. Supercrit. Fluids 1988,1, 1-6. Sako, S.; Shibata, K.; Ohgaki, K.; Katayama, T. Solubilities of Indole, Skatole, and 5-Methoxyindole in Supercritical Fluids. J. Supercrit. Fluids 1989,2, 3-8. Schmitt, W. J.; Reid, R. C. Solubility of Monofunctional Organic Solids in Chemically Diverse Supercritical Fluids. J. Chem. Eng. Data 1986,31, 204-212. Stahl,E.; Schilz, W.; Schutz, E.; Willing, E. Extraction with Supercritical Gases; Schneider, G. M., Stahl, E., Wilke, G., Eds.; Verlag Chemie: Weinheim, 1980, pp 93-114. Tan, C. S.; Weng, J. Y. Solubility Measurements of Naphthol Isomers in Supercritical Carbon Dioxide by a Recycle Technique. Fluid Phase Equilib. 1987, 34, 37-47. Tsekhanskaya, Y. V.; Iomtev, M. B.; Mushkina, E. V. Solubility of Naphthalene in Ethylene and Carbon Dioxide under Pressure. Rum. J. Phys. Chem. 1964,38, 1173-1176.
Receiued for reuiew September 17, 1990 Accepted December 26,1990
