Ind. Eng. Chem. Res. 1997, 36, 2509-2513
2509
Correlations of Henry’s Constants of Nonpolar Solutes in Molten Polypropylene and Poly(dimethylsiloxane) and Polar Solutes in Molten Polyethylene and Poly(dimethylsiloxane) Chongli Zhong and Hirokatsu Masuoka* Department of Chemical Engineering, Faculty of Engineering, Hiroshima University, Higashi-Hiroshima 739, Japan
In this work, generalized correlations for the estimation of Henry’s constants of nonpolar solutes in molten polypropylene (PP) and poly(dimethylsiloxane) (PDMS) were proposed based on the available relationships for molten polyethylene (PE) and polyisobutylene (PIB). Furthermore, the available relationships for nonpolar solutes were modified and extended to estimate Henry’s constants of polar solutes in molten polymers, and generalized correlations for PDMS and lowdensity PE(LDPE) were developed. The proposed correlations enable one to estimate Henry’s constants of nonpolar solutes in molten PP and PDMS and those of polar solutes in molten PDMS and LDPE from the critical temperature, critical compressibility factor, and acentric factor of the solute for a wide temperature range with good accuracy, which are convenient and useful for engineering purposes. Introduction Solubilities of small molecules in molten polymers are important values in the industrial processing of polymeric materials. Though many experimental measurements have been carried out, models are still required to estimate the solubility when experimental data are not available. As a result, it is necessary and important to establish generalized correlations based on the available data. Several correlations have been proposed for the estimation of solubilities of solutes in molten polymers. For example, Durrill and Griskey (1969) related Henry’s constants of gases in molten or thermally softened polymers to the Lennard-Jones force constants or the critical temperatures of the solutes. Stern et al. (1969) proposed a linear relationship between the logarithm of Henry’s constants and (Tc/T)2 for polyethylene. Maloney and Prausnitz (1976) proposed an improved correlation for Henry’s constants in low-density polyethylene (LDPE) based on the Flory-Huggins relationship for solubility in polymers. Stiel and co-workers (Stiel and Harnish, 1976; Stiel et al., 1985) developed generalized correlations for polystyrene (PS), polyethylene (PE), and polyisobutylene (PIB) based on the approach of Stern et al. (1969), and, more recently, Chiu and Chen (1996) proposed two improved correlations for polyisobutylene. The above-mentioned correlations are applicable only to nonpolar or slightly polar solutes. Recently, Bithas et al. (1995) proposed a corresponding state-type correlation for Henry’s constants in molten polymers based on the van der Waals equation of state (EOS). Correlations for both nonpolar and polar solutes in several molten polymers were given. Zhong and Masuoka (1996), on the other hand, used the SRK EOS coupled with a modified UNIFAC model (Zhong et al., 1996) to predict Henry’s constants of polar and nonpolar solutes in molten polymers. Since the polymer-dependent generalized correlations proposed by Stiel and co-workers (Stiel and Harnish, 1976; Stiel et al., 1985) and Chiu and Chen (1996) are * Corresponding author. Telephone: +81-824-247721. Fax: +81-824-227191. E-mail:
[email protected]. S0888-5885(97)00031-6 CCC: $14.00
simple and convenient for practical use, their methods were extended to two other polymers, polypropylene (PP) and poly(dimethylsiloxane) (PDMS), in this work. Furthermore, generalized correlations for Henry’s constants of polar solutes in PDMS and LDPE were also developed. Development of Generalized Correlations for Henry’s Constants of Nonpolar Solutes in Molten PP and PDMS Previous Investigations for PS, PE, and PIB. Stiel and Harnish (1976) proposed a generalized correlation for Henry’s constants of nonpolar and slightly polar solutes in molten PS as follows:
ln(1/Kp) ) -2.338 + 2.706(Tc/T)2
(1)
where T and Tc are system temperature and solute critical temperature, respectively. Kp is Henry’s constant in atm‚g of polymer/cm3 (273.2 K, 1 atm), which is defined as
P1 ) KpV01
(2)
where P1 is the partial pressure of the solute, in atm, and V01 is the solubility of the solute in molten polymer, cm3 (273.2 K, 1 atm)/g of polymer. Equation 1 is a one-parameter corresponding statetype correlation. Later, two-parameter corresponding state correlations were proposed by Stiel et al. (1985) for Henry’s constants of nonpolar or slightly polar solutes in molten PE and PIB:
For PE ln(1/Kp) ) -1.561 + (2.057 + 1.438ω)(Tc/T)2 (3) For PIB ln(1/Kp) ) -1.347 + (1.790 + 1.568ω)(Tc/T)2 (4) where ω is the acentric factor of the solute. The results © 1997 American Chemical Society
2510 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Calculated Results of 1/Kp for Polypropylene AAD,a
% no. of data points eq 6 eq 7 ref
solute
temp. range (K)
propylene propane isobutane butane isopentane pentane hexane 2,2-dimethylbutane heptane octane nonane decane dodecane cyclopentane cyclohexane benzene toluene m-xylene ethylbenzene mesitylene
448.2-498.2 448.2-498.2 448.2-498.2 448.2-498.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2 448.2-523.2
3 3 3 3 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4
2.0 6.9 4.2 3.3 7.8 5.3 3.9 3.2 4.2 2.4 4.2 4.8 6.1 3.3 6.8 3.9 4.0 3.4 5.5 2.8 6.1 2.0 6.7 3.0 9.0 4.0 11.8 6.0 12.4 7.6 5.9 10.9 5.0 5.7 4.3 3.0 5.1 4.7 7.6 5.9
overall error a
AAD ) 1984.
cal ∑i|1/Kp,i
6.2 -
cal 1/Kp,i |
×
cal Kp,i /N
b b b b b b b b b b b b b b b b b b b b
4.6
× 100. Ohzono et al., b
of Stiel et al. (1985) show that eqs 3 and 4 work well for the reproduction of Henry’s constants in molten PE and PIB. More recently, Chiu and Chen (1996) proposed two generalized correlations for nonpolar solutes in molten PIB, and one of them is as follows:
ln(1/Kp) ) -3.80 - 11.8ω + (4.48 + 9.67ω)(Tc/T) (5) The results of Chiu and Chen (1996) show that eq 5 works better than eq 4 for PIB. In this work, we made an attempt to develop similar correlations for other polymers and polar solutes to provide additional useful correlations for engineering use. To do this, experimental data from various sources were collected and modified to obtain values of Henry’s constants in terms of 1/Kp. Detailed calculation methods were given elsewhere (Stiel et al., 1985). Generalized Correlation for Molten Polypropylene. Polypropylene has a structure similar to those of PE and PIB; therefore, it is worthy of testing whether a similar generalized correlation is workable for it. Gas chromatographic measurements of Henry’s constants in PP were made by Ohzono et al. (1984) for 20 nonpolar solutes from 448.2 to 523.2 K, which were used as a database for PP in this work. First, we tested the suitability of the relationship of Stiel et al. (1985) to PP. After investigation, the following expression was proposed for Henry’s constants in PP in this work:
ln(1/Kp) ) -1.517 + (2.105 + 1.209ω)(Tc/T)2 (6) The three constants in eq 6 were obtained by fitting the data of Ohzono et al. (1984) as shown in Table 1, where the absolute average deviations (AADs) of Henry’s constants were also listed. The database (Ohzono et al., 1984) used contains 20 nonpolar solutes with a temperature range of about 75 K. The overall AAD of Henry’s constants is 6.2%, which shows that eq 6 does a good job for the reproduction of Henry’s constants of nonpolar solutes in molten PP. It is interesting to find that the three constants for PP are very close to those
for PE (eq 3), which may be due to the similar structure of the two polymers. We further tested whether the relationship proposed by Chiu and Chen is workable for PP. As a result, the following expression was obtained by fitting the same data:
ln(1/Kp) ) -2.316 - 8.885ω + (3.290 + 8.545ω)(Tc/T) (7) The results calculated with eq 7 were listed in Table 1. The overall AAD of Henry’s constants for 20 nonpolar solutes is only 4.6%, which shows that it works very well and gives better accuracy than with eq 6. Generalized Correlations for Molten Poly(dimethylsiloxane). PDMS is an industrially important polymer, and we want to test whether it is possible to develop a similar correlation for it. Gas chromatographic studies of solubility in PDMS for a range of temperatures were made by many researchers (Summers et al., 1972; Galin, 1977; Chien et al., 1981; Ward et al., 1981; Roth and Novak, 1986; etc.), which provide a reliable database for developing a generalized correlation for PDMS. A total of 27 nonpolar solutes was collected. On the basis of the analysis of these data, we found that the results were unsatisfactory when identical constants were used for both aromatic and nonaromatic hydrocarbons. As a result, the parameters were fitted for them separately, and two generalized correlations similar to that of Stiel et al. (1985) for PE and PIB were obtained as follows:
For nonaromatic hydrocarbons: ln(1/Kp) ) -1.195 + (1.887 + 1.261ω)(Tc/T)2 (8) For aromatic hydrocarbons: ln(1/Kp) ) -1.268 + (1.749 + 1.270ω)(Tc/T)2 (9) The constants in eq 8 were obtained by fitting the data of 23 nonaromatic hydrocarbons as shown in Table 2, where the AADs of Henry’s constants were also listed. Considering the wide range of temperatures for some solutes and the relatively large uncertainties included in the data, the results can be thought of as being satisfactory. The constants in eq 9 were obtained by fitting Henry’s constants of aromatic hydrocarbons. The results together with the references were listed in Table 3, which shows that eq 9 works well for a wide temperature range. Furthermore, the suitability of the relationship of Chiu and Chen (1996) to PDMS was also tested, and two correlations were obtained as follows by fitting the same data.
For nonaromatic hydrocarbons ln(1/Kp) ) -3.986 - 9.228ω + (4.778 + 7.811ω)(Tc/T) (10) For aromatic hydrocarbons ln(1/Kp) ) -3.152 - 13.954ω + (4.025 + 10.758ω)(Tc/T) (11) The calculated results with eqs 10 and 11 were reported in Tables 2 and 3, respectively. The overall
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2511 Table 2. Calculated Results of 1/Kp for Poly(dimethylsiloxane) (Nonaromatic Hydrocarbons) solute
temp. range (K)
pentane 2-methylbutane 1-hexene cis-3-hexene trans-3-hexene hexane hexane hexane cyclohexane cyclohexane cyclohexane 2-methylpentane 2,2-dimethylbutane 2,3-dimethylbutane 1-heptene heptane heptane heptane 2-methylhexane 2,2-dimethylpentane 2,3-dimethylpentane 2,4-dimethylpentane 3,3-dimethylpentane octane 2-methylheptane 2,2,5-trimethylhexane 2,2,4-trimethylhexane 2,3,4-trimethylhexane decane
313.2-353.2 298.2 303.2-353.2 303.2-353.2 303.2-353.2 313.2-353.2 303.2-353.2 313.2-413.2 313.2-353.2 303.2-353.2 328.2-453.2 298.2-343.2 303.2-353.2 303.2-353.2 303.2-353.2 313.2-353.2 303.2-353.2 328.2-453.2 298.2-343.2 303.2-353.2 303.2-353.2 303.2-353.2 303.2-353.2 313.2-353.2 298.2-343.2 303.2-353.2 303.2-353.2 303.2-353.2 303.2-353.2
AAD, % no. of data points eq 8 eq 10 ref 5 1 6 6 6 5 6 7 5 6 8 4 6 6 6 5 5 8 4 6 6 6 6 5 4 6 6 6 6
7.9 7.9 10.5 12.5 4.8 5.3 7.7 7.0 12.2 11.5 16.8 2.0 11.5 10.3 1.4 1.5 3.2 13.0 4.7 5.2 3.5 6.6 4.0 1.1 7.9 5.1 3.6 2.6 3.6
6.5 3.3 8.4 10.2 4.2 2.2 5.7 1.5 11.0 4.6 16.4 4.6 8.3 7.4 3.2 2.1 1.6 3.7 5.8 3.2 2.7 4.6 3.5 4.5 7.0 5.5 3.2 3.1 4.3
7.0
5.4
overall error a
b
a b c c c a c d a c d b c c c a c d b c c c c a b c c c c
Figure 1. 1/Kp versus (Tc/T)2 for polar solutes in PDMS.
c
Roth and Novak, 1986. Summers et al., 1972. Chien et al., 1981. d Galin, 1977. Table 3. Calculated Results of 1/Kp for Poly(dimethylsiloxane) (Aromatic Hydrocarbons) solute
temp. range (K)
no. of data points
benzene benzene toluene toluene p-xylene ethylbenzene ethylbenzene chlorobenzene
298.2-343.2 313.2-453.2 298.2-343.2 333.2-453.2 298.2-343.2 298.2-343.2 333.2-453.2 433.2-468.2
4 8 4 7 4 4 7 3
overall error a
b
AAD, % eq 9 eq 11 3.0 3.5 3.7 5.0 3.9 4.3 4.9 8.5
4.2 3.2 3.8 4.9 3.9 4.5 3.3 6.1
4.4
4.1
ref a b a b a a b c
c
Summers et al., 1972. Galin, 1977. Ward et al., 1981.
AAD of Henry’s constants for 23 nonaromatic solutes from eq 10 is 5.4%, which shows better accuracy than eq 8. For aromatic solutes, eqs 9 and 11 give similar accuracy, and both of them are fairly good. Development of Generalized Correlations for Henry’s Constants of Polar Solutes in Molten PDMS and LDPE In previous researches (Stiel and Harnish, 1976; Stiel et al., 1985; Chiu and Chen, 1996), only nonpolar or slightly polar solutes were considered. In this work, the extension of similar correlations to polar solutes was tried. Generalized Correlation for Polar Solutes in Molten PDMS. Experimental Henry’s constants of polar solutes in PDMS are scarce, and 14 systems were collected from two literatures. The data of Munk et al. (1990) were measured by gas chromatography at only one temperature, 373.15 K, while those from Becerra et al. (1991) were measured with a wide temperature
Figure 2. 1/Kp versus (Tc/T) for polar solutes in PDMS.
range. Henry’s constants of Becerra et al. (1991) versus (Tc/T)2 and (Tc/T) were shown in Figures 1 and 2, respectively. From Figure 1, it is obvious that the logarithm of Henry’s constant changes almost linearly with (Tc/T)2. If linear functions are used to simulate the trends, they will have a common intercept, which is similar to those of nonpolar solutes. As a result, the relationships for Kp and (Tc/T)2 for nonpolar solutes should be applicable to polar solutes in PDMS. The key problem is to find an appropriate relationship between the slopes and the characteristic properties of the polar solutes. From Figure 2, it is evident that the logarithm of Henry’s constant changes also linearly with (Tc/T), and if linear functions are used to simulate the trends, they will have a common slope with different intercepts. After investigation, the following two expressions were proposed in this work:
ln(1/Kp) ) -1.706 + [-0.161 + 46.256(ω‚zc) 201.262(ω‚zc)2](Tc/T)2 (12) and
2512 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
ln(1/Kp) ) -10.913 + 94.546(ω‚zc) 410.557(ω‚zc)2 + 6.219(Tc/T) (13) where zc is the critical compressibility factor of solute. Equation 12 is a modification and extension of the relationship of Stiel et al. (1985) for nonpolar solutes, while eq 13 is a similar treatment of that of Chiu and Chen (1996). The four constants in each equation were obtained by fitting the experimental data shown in Table 4. The results calculated with eqs 12 and 13 were shown in Table 4. The two expressions give good accuracy except that for 1-octanol, and eq 12 is a little better than eq 13. It should be pointed out that the use of polynomials in (ωzc) for polar solutes is empirical. We chose zc as an additional parameter for polar solutes because it is a readily available value for many substances, and it is possible to use other parameters instead of zc. Generalized Correlation for Henry’s Constants of Polar Solutes in LDPE. Stiel et al. (1985) have proposed a generalized correlation for nonpolar and slightly polar solutes in LDPE. In this work, the data of polar solutes were collected and used to develop similar correlations for polar solutes. Liu and Prausnitz (1976) measured Henry’s constants of acetone, 2-propanol, methyl ethyl ketone, vinyl acetate, and methyl chloride in LDPE from 398.2 to 523.2 K by gas chromatography, and similar measurements for vinyl acetate in LDPE from 397.2 to 573.2 K were made by Maloney and Prausnitz (1976). Based on the analysis of their data, the following expression was proposed for LDPE in this work:
ln(1/Kp) ) -1.448 + [2.062 - 6.326ω + 23.831(ωzc)](Tc/T)2 (14) Equation 14 is a modification and extension of the relationship of Stiel et al. (1985) for nonpolar solutes (eq 3); the constants were obtained by fitting the experimental data as shown in Table 5, where the calculated results were also listed. Good correlations were obtained; however, the expression should be tested further when more data are available. Finally, it should be pointed out that eqs 12-14 were developed for polar solutes in particular, which may not give good predictions for nonpolar solutes. Conclusions Generalized correlations for Henry’s constants of nonpolar solutes in molten PP and PDMS were proposed in this work. With the critical temperature and acentric factor of the solute, Henry’s constants in molten PP and PDMS can be estimated satisfactorily for a wide range of temperatures. Furthermore, similar correlations for polar solutes in molten PDMS and LDPE were developed, which enable one to estimate Henry’s constants of polar solutes in them with reasonable accuracy as long as the critical temperature, critical compressibility factor, and acentric factor of the solute are known. As a result, the present work provides several useful and convenient correlations for engineering use. Nomenclature Kp ) Henry’s constants, atm‚g of polymer/cm3 (273.2 K, 1 atm)
Table 4. Calculated Results of 1/Kp for Polar Solutes in Poly(dimethylsiloxane) solute
temp. range (K)
methyl acetate ethyl acetate ethyl acetate 1-propyl acetate 1-butyl acetate acetone methyl ethyl ketone 2-pentanone 1-propanol 1-butanol 1-butanol 1-pentanol 1-octanol pyridine
373.2 363.2-473.2 373.2 373.2 373.2 373.2 373.2 363.2-473.2 373.2 363.2-473.2 373.2 373.2 363.2-473.2 363.2-473.2
AAD, % no. of data points eq 12 eq 13 ref 1 6 1 1 1 1 1 6 1 6 1 1 6 6
overall error a
15.5 3.3 3.7 2.1 1.6 8.4 15.1 4.4 17.4 4.1 2.9 10.8 48.9 5.5
18.9 1.2 7.3 3.7 10.3 7.6 16.9 5.0 21.4 3.4 4.3 14.8 52.3 4.7
12.2
12.9
a b a a a a a b a b a a b b
Munk et al., 1990. b Becerra et al., 1991.
Table 5. Calculated Results of 1/Kp for Polar Solutes in Low-Density Polyethylene
solute
temp. range (K)
no. of data points
AAD for eq 14, %
ref
2-propanol acetone methyl ethyl ketone vinyl acetate vinyl acetate methyl chloride
398.2-523.2 398.2-523.2 398.2-523.2 398.2-523.2 397.2-573.2 398.2-523.2
6 6 6 6 5 6
5.0 2.6 9.5 6.8 7.9 1.1
a a a a b a
overall error a
5.4 b
Liu and Prausnitz, 1976. Maloney and Prausnitz, 1976.
N ) number of data points P1 ) partial pressure of solute, atm T ) temperature, K Tc ) critical temperature, K V01 ) solubility of the solute in molten polymer, cm3 (273.2 K, 1 atm)/g of polymer zc ) critical compressibility factor Greek Letters ω ) acentric factor Subscripts i ) component i c ) critical value Superscripts cal. ) calculated value exp. ) experimental value
Literature Cited Becerra, M. R.; Fernandez-Sanchez, E.; Fernendez-Torres, A.; Garcia-Dominguez, J. A.; Santiuste, J. M. Evaluation of the Effect of the Cyanopropyl Radical on the Interaction of the Methylene Group with Silicone Stationary Phases. J. Chromatogr. 1991, 547, 269. Bithas, S.; Kontogeorgis, G. M.; Kalospiros, N.; Fredenslund, Aa.; Tassios, D. P. Correlation and Prediction of Henry’s Constants for Liquids and Gases in Five Industrially Important Polymers Using a CS-type Correlation Based on the van der Waals Equation of State. Comparison with Other Predictive Models. Fluid Phase Equilib. 1995, 113, 79. Chien, C. F.; Kopecni, M. M.; Laub, R.-J. Specific Retention Volumes and Retention Indexes of Selected Hydrocarbon Solutes with OV-1 and SE-30 Poly(dimethylsiloxane) Solvents. J. High Resolut. Chromatogr. Chromatogr. Commun. 1981, 4, 539.
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2513 Chiu, R. M. H.; Chen, B. D. Correlation of the Henry’s Law Constants for Nonpolar Liquids in Molten Polyisobutylene. Ind. Eng. Chem. Res. 1996, 35, 4386. Durrill, P. L.; Griskey, R. G. Diffusion and Solution of Gases into Thermally Softened or Molten Polymers: Part 2. Relation of Diffusivities and Solubilities with Temperature, Pressure and Structural Characteristics. AIChE J. 1969, 15, 106. Galin, M. Gas-Chromatographic Investigation of the Thermodynamic Interactions of Poly(dimethylsiloxane) or Poly(diethylsiloxane) with Some Solvents Between 60 and 180 °C. Macromolecules 1977, 10, 1239. Gunduz, S.; Dincer, S. Solubility Behavior of Polystyrene: Thermodynamic Studies Using Gas Chromatography. Polymer 1980, 21, 1041. Liu, D. D.; Prausnitz, J. M. Solubilities of Gases and Volatile Liquids in Polyethylene and in Ethylene-Vinyl Acetate Copolymers in the Region 125-225 °C. Ind. Eng. Chem. Fundam. 1976, 15, 330. Maloney, D. P.; Prausnitz, J. M. Solubilities of Ethylene and Other Organic Solutes in Liquid, Low-density Polyethylene in the Range 124-300 °C. AIChE J. 1976, 22, 74. Munk, P.; Hattam, P.; Du, Q.; Abdel-Azim, A. A. Determination of Polymer-Solvent Interaction Coefficients by Inverse Gas Chromatography. J. Appl. Polym. Sci., Appl. Polym. Symp. 1990, 45, 289. Ohzono, M.; Iwai, Y.; Arai, Y. Mass-Fraction Henry’s Constants for Hydrocarbon Gases in Molten PP and PS. Kagaku Kogaku Ronbunshu 1984, 10, 536. Roth, M.; Novak, J. Thermodynamics of Poly(dimethylsiloxane)Alkane Systems by Gas-Liquid Chromatography. Macromolecules 1986, 19, 364. Stern, S. A.; Mullhaupt, J. T.; Gareis, P. J. The Effect of Pressure on the Permeation of Gases and Vapours Through Polyethylene.
Usefulness of the Corresponding States Principle. AIChE J. 1969, 15, 64. Stiel, L. I.; Harnish, D. F. Solubility of Gases and Liquids in Molten Polystyrene. AIChE J. 1976, 22, 117. Stiel, L. I.; Chang, D. K.; Chu, H. H.; Han, C. D. The Solubility of Gases and Volatile Liquids in Polyethylene and Polyisobutylene at Elevated Temperatures. J. Appl. Polym. Sci. 1985, 30, 1145. Summers, W. R.; Tewari, Y. B.; Schreiber, H. P. Thermodynamic Interaction in Poly(dimethylsiloxane)-Hydrocarbon Systems from Gas-Liquid Chromatography. Macromolecules 1972, 5, 12. Ward, T. C.; Sheeny, D. P.; Riffe, J. S.; McGrath, J. E. Inverse Gas Chromatography Studies of Poly(dimethylsiloxane)-Polycarbonate Copolymers and Blends. Macromolecules 1981, 14, 1791. Zhong, C.; Masuoka, H. Prediction of Henry’s Constants for Polymer-Containing Systems Using SRK Equation of State Coupled with a New Modified UNIFAC Model. Fluid Phase Equilib. 1996, 126, 1. Zhong, C.; Sato, Y.; Masuoka, H.; Chen, X. Improvement of Predictive Accuracy of the UNIFAC Model for Vapor-Liquid Equilibria of Polymer Solutions. Fluid Phase Equilib. 1996, 123, 97.
Received for review January 3, 1997 Revised manuscript received March 12, 1997 Accepted March 17, 1997X IE970031B
X Abstract published in Advance ACS Abstracts, May 1, 1997.
