Anal. Chem. 1996, 68, 3516-3519
Solubility Parameters and Solubilities of Metal Dithiocarbamates in Supercritical Carbon Dioxide C. M. Wai* and Shaofen Wang
Department of Chemistry, University of Idaho, Moscow, Idaho 83843 Jya-Jyun Yu Department of Environmental Engineering and Science, Feng-Chia University, Taichung, Taiwan, ROC
The solubilities of Cu, Hg, and Zn complexes with seven different dithiocarbamate ligands in supercritical fluid CO2 at 60 °C and two pressures (100 and 230 atm) are reported. In each metal chelate system, the solubility of the metal-dithiocarbamate complex shows a strong correlation with the solubility parameters of the ligands, calculated using a group contribution method. Dithiocarbamate ligands with smaller solubility parameter values form metal complexes with higher solubilities in supercritical CO2. The solubility parameter value may provide a general guideline for selecting effective ligands for metal extraction in supercritical CO2. Extraction of metal ions from solid and liquid samples using supercritical carbon dioxide as a solvent has been the subject of several recent reports.1-6 This supercritical fluid extraction (SFE) technique involves conversion of charged metal species into neutral metal chelates using an organic ligand dissolved in the fluid phase. The efficiency of this in situ chelation-SFE method for metal extraction depends largely on the chemical nature of the ligand introduced and the metal chelates formed. A suitable ligand must react rapidly with the metal ions in the fluid phase to form metal chelates that can be readily transported by supercritical carbon dioxide. The solubility of metal chelates in supercritical CO2 appears to be an important factor in determining the efficiency of metal extraction by this technique. One ligand system which has been extensively studied for SFE of metal species in the literature is the derivatives of dithiocarbamic acid of the general form R2NCS2X, where R is an alkyl group and X is a cation, which can be an alkali metal ion, an ammonium ion, or an alkylammonium ion.3-7 Dithiocarbamate derivatives are effective extractants for preconcentration of trace elements from aqueous solutions by solvent extraction. A widely used dithiocarbamate reagent for this purpose is sodium diethyldithiocarbamate (NaDDC), which is able to extract over 30 metal species from aqueous solutions into organic solvents.8 The solubilities of metal-DDC chelates in supercritical carbon dioxide are generally low, typically in the (1) Hawthone, S. B. Anal. Chem. 1990, 62, 633A. (2) Fahmy, T. M.; Paulaitis, M. E.; Johnson, D. M.; McNally, M. E. P. Anal. Chem. 1993, 65, 1462. (3) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1992, 64, 2875. (4) Wai, C. M.; Lin, Y.; Brauer, R. D.; Wang, S.; Beckert, W. F. Talanta 1993, 40, 1325. (5) Laintz, K. E.; Yu, J. J.; Wai, C. M. Anal. Chem. 1992, 64, 311. (6) Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. J. Supercrit. Fluids 1991, 4, 194. (7) Wang, J.; Marshall, W. D. Anal. Chem. 1994, 66, 1658.
3516 Analytical Chemistry, Vol. 68, No. 19, October 1, 1996
order of 10-5-10-6 mol/L in the temperature range 40-60 °C and the pressure range 100-250 atm.4 Wai and co-workers demonstrated that fluorination of DDC can greatly enhance the solubilities of the resulting metal chelates in supercritical CO2 and thus improve the SFE efficiency for metal ions.3-6 Wang and Marshall later also showed that increasing the chain length of the R group, e.g., by substituting the two ethyl groups in DDC with two butyl groups, can also increase the solubility of the resulting metal chelates in supercritical CO2.7 The solubility of a metal chelate in supercritical CO2 is obviously related to its molecular structure. For rational design and selection of effective ligands for SFE of metal species, it is necessary to understand the relationship between ligand structure and solubility of metal chelates in supercritical fluids. We have recently measured the solubilities of a number of metal-dithiocarbamate chelates in supercritical CO2 and correlated the experimental data with the Hildebrand solubility parameters of the ligands calculated using a group contribution method. The solubility parameter appears to provide a simple method for predicting the solubility of metal chelates in supercritical fluids. EXPERIMENTAL SECTION Sodium diethyldithiocarbamate (NaDDC) and ammonium pyrrolidinedithiocarbamate (APDC) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Lithium bis(trifluoroethyl)dithiocarbamate (LiFDDC) was synthesized according to a procedure outlined elsewhere.9 Tetramethylammonium salts of dipropyldithiocarbamate (P3DC), dibutyldithiocarbamate (BDC), dipentyldithiocarbamate (P5DC), and dihexyldithiocarbamate (HDC) were synthesized by the reaction of CS2 with proper dialkylamines according to a procedure described in the literature.7 The starting materials, dialkylamines (R2NH, where R ) C3H7, C4H9, C5H11, and C6H13), tetramethylammonium hydroxide, and carbon disulfide, were purchased from Aldrich. Metaldithiocarbamate complexes were prepared by mixing proper dithiocarbamate salts with excess amounts of metal ions in an aqueous solution with pH around 3-4. The resulting metal dithiocarbamate precipitate was extracted into chloroform. Purification of the metal chelates was achieved by recrystallization using a chloroform/ethanol solution (1:1 v/v) at 60 °C in a water bath. After evaporation of the chloroform, the metal chelate crystals were collected by filtration. (8) Wai, C. M. In Preconcentration Techniques for Trace Elements; Alfassi, Z. B., Wai, C. M., Eds.; CRC Press: Boca Raton, FL, 1991; Chapter 4. (9) Tavlaridis, A.; Neeb, R. Fresenius Z. Anal. Chem. 1978, 242, 135. S0003-2700(96)00276-4 CCC: $12.00
© 1996 American Chemical Society
Solubility experiments were performed using an Isco (Lincoln, NE) supercritical fluid extractor (SFX 2-10) or a laboratory-built SFE apparatus described elsewhere.4 Solubility of a metal dithiocarbamate in supercritical carbon dioxide was measured by placing a known amount of the metal chelate in a glass tube or a boat in the Isco extractor preheated to a fixed temperature. SFEgrade CO2 and SFE-grade CO2 with 5% methanol (Scott Specialty Gases, Plumsteadville, PA) were used for all SFE eperiments. The system was pressurized to a given pressure and heated statically for 30 min. The static extraction time was determined by varying the extraction time until a constant metal chelate solubility value was obtained. After that, with the inlet valve closed, the outlet valve was opened, and the fluid was depressurized and bubbled through a collection vial containing 10 mL of chloroform. The sample tube was removed from the extractor, and the system was flushed dynamically with carbon dioxide at the same temperature and pressure for 20 min. The dynamic flush step may contribute up to 10% of the total metal complex collected in the collection vials. After the dynamic extraction, no detectable amount of metal dithiocarbamates was found in the extraction cell when the system was washed with chloroform. Copper and zinc in the trapped solution were back-extracted with a 50% nitric acid solution and determined by atomic absorption spectrophotometry. Mercury in the trapped solution was determined by neutron activation analysis after the chloroform solution was evaporated to dryness. The procedures of neutron activation analysis are given elsewhere.10 The procedures of mercury extraction from plant samples are similar to those described in the literature for SFE of mercury from cellulose-based filter papers and sand samples.4 The plant sample was obtained by grinding a dry aquatic plant, coontail (Ceratophyllum demersum), to pass a U.S. standard 42 mesh sieve. The extraction condidtions are given in Table 3. RESULTS AND DISCUSSION It is known that certain thermodynamic properties of dilute solutions may be estimated from the solubility parameters of liquids. Recently, Lagalante et al. showed that the solubilities of Cu(II) and Cr(III) β-diketonates in supercritical carbon dioxide are correlated with the solubility parameters of the ligands.11 Solubility parameters of structurally complex compounds may be calculated by the group contribution method described by Fedors.12,13 In this study, the solubility parameters (δ2) of dithiocarbamate ligands (in their hydrogen form) with different alkyl substitutions were calculated using the group contribution method, according to the equation
∑∆U /∑∆V )
δ2 ) (
1/2
i
i
(1)
where ∆Ui is the molar cohesive energy and ∆Vi the molar volume of the ith group of a ligand molecule. Some typical solubility parameter values for structurally different dithiocarbamate ligands are given in Table 1. According to Table 1, an increase in chain length of the alkyl group tends to lower the solubility parameter of the ligand. The change in the solubility parameter value per carbon chain number is large when the carbon number is small (10) Tang, J.; Wai, C. M. Anal. Chem. 1986, 58, 3233. (11) Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Inorg. Chem. 1995, 34, 5781. (12) Fedors, R. F. Polym. Eng. Sci. 1974, 14 (2), 147. (13) Fedors, R. F. Polym. Eng. Sci. 1974, 14 (6), 472.
Table 1. Solubility Parameters of Dithiocarbamate Ligands in H-Form Calculated from the Group Contribution Method ligandb
δ2 (cal1/2/cm3/2)
PDC DDC P3DC BDC P5DC HDC FDDC
11.43 10.39 10.04 9.81 9.64 9.50 8.75
a Data from refs 12 and 13. The solubility parameter of CO at 60 2 °C and 230 atm is about 6.6. b PDC, pyrrolidinedithiocarbamate; DDC, diethyldithiocarbamate; P3DC, dipropyldithiocarbamate; BDC, dibutyldithiocarbamate; P5DC, dipentyldithiocarbamate; HDC, dihexyldithiocarbamate; FDDC, bis(trifluoroethyl)dithiocarbamate.
Figure 1. Changes in δ2 per CH2 unit with respect to carbon number of the n-alkyl substitution groups in dithiocarbamate ligands.
and gradually levels off when the carbon number becomes greater than approximately 10. As shown in Figure 1, from dimethyldithiocarbamic acid (C1) to dipentyldithiocarbamic acid (C5), the lowering in the solubility parameter value per carbon chain number is 0.57, 0.35, 0.23, and 0.17 cal1/2/cm3/2, respectively. The solubility parameter value of the ligand decreases from 10.96 cal1/2/cm3/2 for dimethyldithiocarbamic acid to 9.64 cal1/2/cm3/2 for dipentyldithiocarbamic acid. Further increase in chain length results in much less reduction of the solubility parameter. For example, from C7 to C8, the solubility parameter value decreases only by 0.08 units from 9.40 cal1/2/cm3/2 for diheptyldithiocarbamic acid to 9.32 cal1/2/cm3/2 for dioctyldithiocarbamic acid. This is due to the fact that the percent increase in molar volume per CH2 unit is large relative to that of cohesive energy when the carbon chain number is small. As the carbon chain number becomes large, the difference in ∆U and ∆V increment per CH2 unit becomes less significant. Consequently, the decrease in the solubility parameter value calculated by the group contribution method tends to level off when R becomes large. Substitution of six fluorine atoms for hydrogen at the terminal of the ethyl groups in DDC lowers the solubility parameter value drastically to 8.75 cal1/2/cm3/2 for FDDC. This solubility parameter value is lower than that of a C20-substituted dithiocarbamate ligand according to the results shown in Figure 1. Fluorine substitution is far more effective than alkyl chain substitution in reducing the solubility parameter of the ligand. Ring substitution appears to increase the solubility parameter of the ligand. When the two Analytical Chemistry, Vol. 68, No. 19, October 1, 1996
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Table 2. Solubilities of Metal-Dithiocarbamate Complexes in Supercritical CO2 at 60 °C and Two Different Pressuresa pressure ligandb
100 atm
230 atm
mp (°C)c
PDC DDC P3DC BDC P5DC HDC FDDC
Copper Dithiocarbamates (4.1 ( 0.5) × 10-7 (4.0 ( 0.2) × 10-6 (1.4 ( 0.4) × 10-6 (1.1 ( 0.3) × 10-5 (6.3 ( 0.2) × 10-6 (1.2 ( 0.4) × 10-4 (1.3 ( 0.5) × 10-5 (7.2 ( 0.2) × 10-4 (9.0 ( 0.3) × 10-5 (1.8 ( 0.4) × 10-3 (2.1 ( 0.3) × 10-4 (2.8 ( 0.3) × 10-3 (9.1 ( 0.2) × 10-4 (4.0 ( 0.4) × 10-3
260 dec 189-191 101-102 73-75 63-65 44-46 144-146
PDC DDC P3DC BDC P5DC HDC FDDC
Mercury Dithiocarbamates (3.5 ( 0.3) × 10-7 (3.4 ( 0.2) × 10-6 (6.8 ( 0.4) × 10-6 (5.3 ( 0.2) × 10-5 (1.2 ( 0.5) × 10-5 (2.3 ( 0.1) × 10-4 (5.6 ( 0.2) × 10-5 (5.6 ( 0.2) × 10-4 (1.0 ( 0.1) × 10-4 (2.0 ( 0.3) × 10-3 (1.6 ( 0.3) × 10-4 (3.8 ( 0.4) × 10-3 (3.0 ( 0.3) × 10-3 (1.4 ( 0.5) × 10-2
240 dec 125-127 82-84 71-73 67-69 59-63 103-105
PDC DDC P3DC BDC P5DC HDC FDDC
Zinc Dithiocarbamates (3.2 ( 0.3) × 10-7 (9.0 ( 0.3) × 10-6 (1.1 ( 0.2) × 10-6 (2.4 ( 0.2) × 10-5 (7.9 ( 0.5) × 10-6 (1.5 ( 0.2) × 10-4 (8.2 ( 0.4) × 10-5 (6.9 ( 0.5) × 10-4 (1.6 ( 0.2) × 10-4 (3.2 ( 0.4) × 10-3 (3.2 ( 0.3) × 10-4 (5.8 ( 0.3) × 10-3 (9.5 ( 0.4) × 10-4 (9.0 ( 0.5) × 10-3
264 dec 178-180 107-109 104-106 76-78 65-69 112-114
a Standard deviations were calculated from three replicate experiments. b See Table 1, footnote b for abbreviations. c dec, decomposition.
ethyl groups in DDC are connected to form a C4 ring structure, the solubility parameter increases from 10.39 to 11.43 cal1/2/cm3/2 for PDC. The solubilities of Cu, Hg, and Zn complexes with different dithiocarbamate ligands at 60 °C and two pressures (100 and 230 atm) in supercritical CO2 are given in Table 2. The melting points of the metal-dithiocarbamate complexes, also given in Table 2, indicate that the melting points of the non-fluorinated C2-C6 dialkyl homologs decrease with increasing chain length. The solubility of each metal-dithiocarbamate system follows the order M(FDDC)2 > M(HDC)2 > M(P5DC)2 > M(BDC)2 > M(P3DC)2 > M(DDC)2 > M(PDC)2. In each metal-dithiocarbamate system, the fluorinated metal complex M(FDCC)2 is always more soluble in supercritical CO2 than the other non-fluorinated ones. The least soluble one is the PDC-metal complex, M(PDC)2, with a ring structure associated with the ligand. The other five alkylsubstituted dithiocarbamate-metal complexes show a linear trend of increasing solubility in supercritical CO2 with respect to increase in chain length. Linear regression analyses of the C2-C6substituted solubility data (log M versus δ2 as shown in Figure 2) reveal slopes of -2.80 ( 0.18 (R2 ) 0.98), -2.10 ( 0.15 (R2 ) 0.98), and -2.76 ( 0.16 (R2 ) 0.99) in (cal/cm3)-1/2, for the copper, mercury, and zinc dithiocarbamates, respectively. The solubility of metal dithiocarbamates in supercritical CO2 increases with pressure (or density), as reported in the literature,6 but the degree of increase appears to vary with the metal. The solubilities of the metal-dithiocarbamate complexes in supercritical CO2 show a strong correlation with the calculated solubility parameters of the ligands (Figure 2). For example, the solubility of the mercury-dithiocarbamate complexes increases logarithmically with decreasing solubility parameters of the ligands. The fluori3518 Analytical Chemistry, Vol. 68, No. 19, October 1, 1996
Figure 2. Solubilities of metal-dithiocarbamate complexes in supercritical CO2 (60 °C, 230 atm) versus the solubility parameters of ligands. Table 3. Efficiencies of Mercury Extraction from the Aquatic Plant Coontail with Methanol (5%)-Modified CO2 Containing Different Dithiocarbamate Ligands at 60 °C and 200 atma ligandb
extraction (%)
FDDC P5DC BDC P3DC DDC PDC
95 ( 2 88 ( 2 87 ( 3 86 ( 3 72 ( 2 25 ( 3
a Experimental conditions: 20 mg of plant + 40 mg of ligand + 40 µL of H2O; 20 min static followed by 20 min dynamic extraction. The SFE procedures are similar to those described in ref 4. The plant contains 100 ppm of Hg obtained from a bioaccumulation experiment. b See Table 1, footnote b for abbreviations.
nate ligand FDDC, which has the lowest solubility parameter value, forms the most soluble mercury complex in supercritical CO2 relative to the other six mercury-dithiocarbamate complexes studies. PDC, which has the largest solubility parameter, forms the mercury complex Hg(PDC)2 with the lowest solubility in supercritical CO2. Increase in alkyl chain length of the dithiocarbamate ligand also increases the solubility of the resulting metal chelates. However, as the chain length increases, other properties of the ligand must be considered. Long-chain dithiocarbamate ligands are difficult to synthesize. The starting material dialkylamine with carbon number greater than 10 is not commercially available. Its reaction with CS2 is less effective because of the steric hindrance caused by the bulky R groups.14 On equal molar basis, high molecular weight ligands require more materials than the low molecular weight ones. SFE experiments indicate that BDC is an effective ligand for metal extraction in supercritical CO2.7 Preliminary experiments conducted in our laboratory showed that the SFE efficiencies of P3DC or P5DC for some metals in solid samples are about the same as those observed for BDC. A typical example is shown in Table 3. It appears that a suitable alkyl substitution for dithiocarbamate ligand for metal extraction in supercritical CO2 is probably in the range of C3H7 to C5H11. Our preliminary results also indicate that the extraction (14) Hulanicki, A. Talanta 1967, 14, 1371.
efficiency of FDDC for some metals in solid samples is usually better than that of P5DC (Table 3). Small fluorinated ligands are probably more effective than large ligands with long alkyl chain substitutions for metal extraction in supercritical CO2. Lin et al. reported that hexafluoroacetylacetone (HFA) is more effective than fluorinated long-chain β-diketones such as 2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (FOD) for uranium extraction from mine wastes.15 The smaller ligands are probably easier to diffuse into the soil matrix to form metal complexes which are also easier to migrate out of the solid phase. The extraction efficiency of PDC for metal ions in supercritical CO2 is much lower relative to that of DDC. Ring-substituted ligands are probably not effective for metal extraction in supercritical CO2. The group contribution method can also be used to calculate solubility parameters of metal-dithiocarbamate complexes if the contributions of the metal ion bound to the thio groups of the ligands are known. Fedors’s table provides the group contribution values for some metals bound to carbon which are not applicable to the metal-dithiocarbamate systems. In principle, the solubility parameters of metal dithiocarbamates can be calculated from their measured solubilities in supercritical CO2 based on the Scatchard-Hildebrand equation:16
ln x2 ) ln a2 - (V2Φ12/RT)(δ2 - δ1)2
(2)
Where a2 is the activity of solute, x2 is the mole fraction of solute, V2 is the molar volume of solute, Φ1 is the volume fraction of solvent, and δ1 and δ2 are the solubility parameter values of solvent and solute, respectively. From the solubility parameters of metaldithiocarbamate complexes, we can evaluate the contributions of (15) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1994, 28, 1190. (16) Hildebrand, J. H. J. Am. Chem. Soc. 1919, 41, 1067.
metal-sulfur groups. However, because of lack of proper thermodynamic data, the solubility parameters of metal dithiocarbamates cannot be calculated at the present time. Using the group contribution approach, the solubility parameters of metaldithiocarbamate complexes would be obtained by adding the ∆U and ∆V contributions of the metal-sulfur bonds (constant) to the values of two dithiocarbamate ligands (structurally dependent) according to eq 1. Since the solubilities of metal-dithiocarbamate complexes have been shown to correlate with the solubility parameters of the ligands, we expect the former would also correlate with the solubility parameters of the metal complexes. The results presented in this paper indicate that the solubilities of metal-dithiocarbamate complexes in supercritical CO2 are correlated with the solubility parameters of the ligands in metaldithiocarbamate systems. Since the group contribution method can be used to evaluate solubility parameters of ligands of known structures, this approach provides a simple method of evaluating relative solubilities of metal chelates in supercritical CO2. The prediction may provide a general guideline for selecting effective ligands for metal extraction in supercritical CO2. Correlations between the solubility parameters of the ligands and solubilities of metal chelates in supercritical CO2 have been observed in metal β-diketonates11 and in metal dithiocarbamates. Testing the validity of this prediction method for other metal chelate systems in supercritical CO2 is currently in progress. ACKNOWLEDGMENT This work was supported by the NSFsIdaho EPSCoR Program under NSF Cooperation Agreement OSR-9350539. J.J.Y. was supported by a grant from the National Science Council, ROC (NSC 83-0208-M-035-016). Received for review March 20, 1996. Accepted July 22, 1996.X AC960276I X
Abstract published in Advance ACS Abstracts, September 1, 1996.
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