ARTICLE pubs.acs.org/est
Effects of Chain Length, Chlorination Degree, and Structure on the Octanol-Water Partition Coefficients of Polychlorinated n-Alkanes Bettina Hilger ,†,‡ Hermann Fromme ,‡ Wolfgang V€olkel ,‡ and Mehmet Coelhan *,† † ‡
Technische Universit€at M€unchen, Research Center Weihenstephan for Brewing and Food Quality, Freising, Germany Bavarian Health and Food Safety Authority, Munich, Germany
bS Supporting Information ABSTRACT: Log octanol-water partition coefficients (log Kow) of 40 synthesized polychlorinated n-alkanes (PCAs) with different chlorination degrees were determined using reversed-phase high performance liquid chromatography (RP-HPLC). In addition, log Kow values of a technical mixture namely Cereclor 63L as well as 15 individual in house synthesized C10, C11, and C12 chloroalkanes with known chlorine positions were estimated. Based on these results, the effects of chain length, chlorination degree, and structure were explored. The estimated log Kow values ranged from 4.10 (polychlorinated n-decanes with 50.2% chlorine content) to 11.34 (polychlorinated n-octacosanes with 54.8% chlorine content) for PCAs and from 3.82 (1,2,5,6,9,10-hexachlorodecane) to 7.75 (1,1,1,3,9,11,11,11-octachlorododecane) for the individual chloroalkanes studied. The results showed that log Kow value was influenced linearly at a given chlorine content by chain length, while a polynominal effect was observed in dependence on the chlorination degree of an alkane chain. Chlorine substitution pattern influenced markedly the log Kow value of chloroalkanes.
’ INTRODUCTION Polychlorinated n-alkanes (PCAs) also known as chlorinated paraffins (CPs) are a class of chemicals that have been mainly used as additives in high pressure lubricants for metal-working or flame retardants and plasticizers in polymers (mostly polyvinylchloride). Further applications as additives in rubber, sealants, paints, or as flame retardant in textiles have been reported.1-3 They are produced by free radical chlorination of different composed n-alkane fractions between C10 and C30 under forceful conditions such as UV-light or temperature.1-3 The chlorination degree varies between 30% and 70% by weight depending on the intended application. Because of different n-alkane feedstocks as well as different chlorination degrees, over 200 commercial products with different properties have been available. With regard to the chain length, CPs are classified into short- (SCCPs: C10-C13), medium- (MCCPs: C14-C17), and long chain chlorinated-paraffins (LCCPs: C18-C30). Since the chlorination reaction shows low positional selectivity, complex mixtures consisting of several thousands of homologues and isomers are formed.2 Chlorinated paraffins have been produced since 1930s. The production volume of SCCPs for Europe, Canada, and USA was estimated between 7.5 and 11.3 kt/y in 2007, while the production of CPs in China increased to about 600 kt/y in 2007.1 SCCPs are considered persistent and show a high bioaccumulation potential as they have been found in biota, sediments and air.1-3 SCCPs are classified as a possible carcinogen to humans and several steps toward risk reduction have been taken.4 In spite of the large production quantities, very limited information is available on the physical-chemical properties of PCAs and technical mixtures.1-3 The physical-chemical properties of a compound are of importance regarding bioaccumulation, metabolism, distribution, and transport in diverse environmental compartments. The log octanol-water partition coefficient r 2011 American Chemical Society
(log Kow) is a measure of the hydrophobicity of a substance since its distribution between the two phases, octanol (hydrophobic) and water (hydrophilic), is determined.5 As log Kow is often correlated with water solubility,6 bioconcentration factor (BCF),7 and soil absorption coefficient (Koc),8 it has been considered a key parameter in environmental fate modeling.9 Different methods are available for the determination of log Kow. Direct methods like shake-flask or slow-stir are laborious and accuracy may be a problem.8,10 Empirical methods using substance or fragment constants for log Kow calculation do not consider molecule conformation and are hardly applicable for complex molecules.8,10 Reversed-phase high performance liquid chromatography (RP-HPLC) is an indirect method for log Kow determination which uses the correlation between the log Kow value and retention time of the compounds. This method is rapid, simple, and no quantification is needed. Sensitivity and reproducibility of the HPLC method are good.8,10 Since the accuracy depends on the known log Kow values of calibration compounds, these must be selected with care, but only a few accurate values are needed to cover a broad band of log Kow values.11 Determination of the log Kow using RP-HPLC is an accepted method.12 Few studies were reported on the log Kow of CPs using slow-stir13 and reversed phase thin layer chromatography (RP-TLC)14 as well as estimating the log Kow of four PCAs by RP-HPLC.15 In the present study, we report on the log Kow values of 40 PCAs with varying chlorination degrees and different alkane chains between C10 and C28, 15 individual chlorodecanes, undecanes, and dodecanes with defined chlorine positions as Received: September 10, 2010 Accepted: January 25, 2011 Revised: December 14, 2010 Published: March 01, 2011 2842
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Table 1. Estimated Log Kow Values and Standard Deviations of Various PCAs at the Top of Peak and As Range Log Kow peak compound
Log Kow peak
top
range
CP10:44.8
4.95 ( 0.01
4.11-6.56
CP10:50.2
4.92 ( 0.01
CP10:55.0
compound
top
range
CP13:55.0
5.76 ( 0.06
5.06-7.42
CP13:60.0
5.95 ( 0.01
5.03-7.44
4.10-6.74
CP13:65.2
6.18 ( 0.01
5.35-7.78
4.97 ( 0.01
4.11-6.72
CP13:70.0
6.74 ( 0.02
5.65-8.48
CP10:60.1
5.22 ( 0.02
4.19-6.85
CP10-13:45.0
5.65 ( 0.01
4.40-7.19
CP10:65.0
5.42 ( 0.01
4.23-7.19
CP10-13:56.0
5.43 ( 0.02
4.31-7.19
CP10:69.8
5.74 ( 0.06
4.84-7.36
CP10-13:63.5
5.61 ( 0.03
4.36-7.61
CP10:75.6 CP11:45.5
6.67 ( 0.01 5.29 ( 0.03
5.73-8.67 4.37-7.32
CP10-13:65.0 Cereclor 63L
5.48 ( 0.01 5.41 ( 0.01
4.61-7.68 4.01-7.62
SCCPs
CP11:50.2
5.31 ( 0.02
4.39-7.17
CP10þ13:45.1
5.07 ( 0.00
4.18-7.37
CP11:55.2
5.22 ( 0.01
4.40-7.12
CP10þ13:59.6
4.99 ( 0.01
4.14-7.31
CP11:60.5
5.43 ( 0.01
4.47-7.43
CP10þ13:64.9
5.44 ( 0.01
4.14-7.62
CP11:65.3
5.68 ( 0.04
4.52-7.57
MCCPs
CP11:70.5
6.19 ( 0.01
5.05-7.75
CP14:47.0
6.30 ( 0.01
5.56-7.71
CP12:45.3
5.65 ( 0.03
4.73-7.16
CP15:50.4
6.65 ( 0.02
5.84-7.81
CP12:50.2 CP12:55.0
5.66 ( 0.05 5.62 ( 0.02
4.69-7.17 4.68-7.12
CP16:61.0 CP14-17:46.7
6.81 ( 0.02 6.67 ( 0.03
5.78-8.38 5.57-7.90
CP12:59.8
5.76 ( 0.05
4.70-7.29
LCCPs
CP12:65.1
5.97 ( 0.03
4.88-7.71
CP18:57.7
7.33 ( 0.05
6.58-8.60
CP12:70.0
6.40 ( 0.02
5.26-7.96
CP22:52.2
8.57 ( 0.04
7.55-9.52
CP13:44.9
5.99 ( 0.02
5.03-7.41
CP24:56.2
8.90 ( 0.04
8.26-10.42
CP13:50.2
5.98 ( 0.03
5.06-7.43
CP28:54.8
10.10 ( 0.09
9.08-11.34
well as a SCCP technical mixture Cereclor 63L. Experiments were performed with the aim to investigate the influence of chain length, chlorination degree, and the structure on the log Kow values of PCAs. The HPLC method was selected since various mixtures with expected high log Kow values had to be examined and the quantification of the PCAs especially with high alkane chains has been problematic because of their complexity. To our knowledge, this work is the first comprehensive study on the log Kow values of PCAs and individual chloro n-alkanes with known chlorine positions.
’ MATERIALS AND METHODS Reagents and Standards. The CP mixture Cereclor 63L was kindly supplied previously by the former Imperial Chemicals Industries (ICI, Runcorn, England). Polychlorinated n-alkane mixtures with defined chain lengths and varying chlorine contents synthesized as described elsewhere16 were used in our experiments (Table 1). In the synthesized mixtures, PCAs having different carbon chain lengths are each present in equal quantities. CP10-13 means a mixture of C10 to C13 n-alkane chains, and CP10þ13 means a mixture of C10 and C13 n-alkane chains. For example, CP10:44.8 refers to a mixture of polychlorinated n-decanes (C10) with a mean chlorine content of 44.8% by weight. Solutions containing PCAs as well as Cereclor 63L were prepared by dissolving 30 mg of each substance in 1 mL of ethanol. Solutions of individual deca-, undeca-, and dodecachloroalkanes with known chlorine substitution (Table 5) were made for 20 mg/mL in ethanol and diluted to proper concentrations (for synthesis see ref 17). Benzene,9,11,12,18,20-24,26 toluene,9,11,12,18-24 p-xylene,18,22,23 biphenyl,9,11,12,18,20-24,26 p,p0 -DDD,20,21,24 p,p0 DDT,9,11,12,18-21,23-25
p,p0 -DDE,9,19-21,24-26 benzo[a]pyrene,9,11,25 hexachlorobenzene (HCB),9,11,18-21,23,24,26 and diethylhexylphthalate (DEHP)12 covering the log Kow range from 2.34 to 7.86 were used for calibration. These compounds were dissolved in ethanol or in acetone for HCB, and appropriate volumes were mixed for the calibration mixture. Octachlorodibenzodioxin was analyzed as a potential calibration compound, but it was not used for calibration due to the large differences between literature values (log Kow 7.59 up to 12.97). HPLC System. A Perkin-Elmer series 200 HPLC system (Shelton, CT, U.S.A.) consisting of autosampler, vacuum degasser, pump, column oven, and diode array detector was used. The stationary phase used was an ODS-Hypersil-column (Thermo Scientific, West Palm Beach, FL, U.S.A.) 250 4.6 mm i.d. with 3 μm particle size in combination with a 10 4 i.d. mm guard column. An isocratic mixture of methanol/water (90:10, v:v) served as the mobile phase. HPLC measurements were carried out at a temperature of 40 °C and a wavelength of 200 nm. At the beginning of the experiments, a refractive index detector was tested for detection. As it revealed, a UV detector was also suitable, the latter was preferred due to the stability of baseline and better reproducibility. The flow was 0.5 mL/min excluding polychlorinated alkanes of C24 and C28 chains where a flow of 0.8 mL/min was used to achieve sharper peaks for better detection. The injection volume was varied: 20 μL for SCCP, MCCP, individual chloroalkanes and calibration mixture, 40 μL for C18 and C22, and 60 μL for C24 and C28 to receive sufficient peak intensity. For the construction of the regression functions, only mean log Kow values of calibration compounds estimated by RP-HPLC from literature were used in order to obtain a better comparison (see the Supporting Information). 2843
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Figure 1. HPLC chromatograms of CP16:61.0, benzo[a]pyrene, and 1,1,1,3,6,7,10,12,12,12-decachlorododecane.
Table 2. Influence of the Chlorination Degree on the Log Kow Values of Short-Chain PCAs C10
a
C11 Δlog Kow
Log Kow
4.92
-0.03
∼55%
4.97
∼60% ∼65%
C12 Δlog Kow
Log Kow
5.31
0.02
0.05
5.22
5.22
0.25
5.42
0.20
∼70%
5.74
0.32
∼75%
6.67
0.93
Cl %a
Log Kow
∼45%
4.95
∼50%
b
C13 Δlog Kow
Log Kow
5.66
0.01
5.98
-0.01
-0.09
5.62
-0.04
5.76
-0.22
5.43
0.21
5.76
0.14
5.95
0.19
5.68
0.25
5.97
0.21
6.21
0.26
6.19
0.51
6.40
0.43
6.74
0.53
b
5.29
b
5.65
Δlog Kow
b
5.99
Exact chlorination degree of each alkane chain is given in Table 1. b The difference log Kow value to the subchlorination degree of the same n-alkane chain.
’ RESULTS AND DISCUSSION A disadvantage of the RP-HPLC method for log Kow determination is that it is not a direct method though even agreement with results obtained using direct methods like slow-stir or shakeflask is generally good.12,13,18,22 The reference substances should preferably have similar structures, but in our case this could not be realized because of the lack of sufficient log Kow data of compounds with a structure similar to PCAs. In a RP-HPLC study, the average error for an estimated log Kow was 0.5, and this error was similar for aliphatic and aromatic compounds although only the latter ones were used as reference compounds.19 We preferred isocratic elution HPLC since the log Kow values of our reference compounds were estimated by the same method. In estimating log Kow, the most widely used organic modifier is methanol which has a more waterlike structure and is considered not to disturb the hydrogen-bonding network of water.27 Though a minimum water content of 25% in the eluent has been recommended, the dependence of log Kow on the methanol/water ratio has been shown to be low and superimposed by other effects for the most substances.12,28 An eluent containing 85% methanol and 15% water was used for some of our reference compounds and other highly hydrophobic compounds like chlorinated dioxins and polychlorinated biphenyls by several authors.11,26 Thus, after testing 85% and 90% methanol as the modifier, we decided on the latter composition since the log Kow values were unchanged and the broadness of the peaks was reduced significantly. This improved the reproducibility of data. Standard deviations (sd) were usually between 0.01 and 0.02 log units for PCAs (Table 1) and for individual compounds (Table 5). For instance, the log Kow values of four estimations for
CP10:55.0 were 4.98, 4.98, 4.96, and 4.98, respectively, resulting in a standard deviation of 0.01 log units. Deviations from literature data cannot be given since none of the compounds has been studied before. Since the upper calibration value was 7.52 log units, compounds possessing higher log Kow values in particular (e.g., 10.10 for CP28:54.8), whose values were obtained by extrapolation, might show large deviations when direct methods or other conditions (e.g., using other reference compounds) were used. Considerable variability has been reported for p,p0 -DDT even when using the same method.29 Log Kow values were given for p,p0 -DDT between 5.13-6.51 which were estimated by RP-HPLC, 5.73-6.914 by stir-flask, and 3.986.36 by the shake flask method. Nevertheless, the comparison of the hydrophobicity of different PCAs and individuals compounds is possible since all measurements were performed under the same conditions. For discussion below, the log Kow values at the top of peaks were considered unless otherwise stated. Polychlorinated n-Alkanes. The estimated log Kow values of the analyzed mixtures and standard deviations are summarized in Table 1. HPLC chromatograms of PCAs showed in common with the GC-chromatograms a broad band due to the presence of thousands of congeners. For a comparison, HPLC chromatograms of CP16:61.0, 1,1,1,3,6,7,10,12,12,12-decachlorododecane, and calibration compound benzo[a]pyrene are displayed in Figure 1. To determine the complete log Kow range, besides the retention time of the signal top, also the retention times of the beginning and the ending of the bands were investigated. Especially for MCCP and LCCP, the end of the peaks was difficult to locate due to the flatness of signals. Influence of Chlorination Degree. The log Kow values of a single polychlorinated n-alkane chain did not vary significantly 2844
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between 45% and 55% chlorine content (Table 2). For instance, polychlorinated decanes possessed log Kow values of 4.95, 4.92, and 4.97 at chlorination degrees of 44.8%, 50.2%, and 55.2%, respectively. The differences between the log Kow values of two consecutive chlorination degrees (Δlog Kow) were -0.03 and 0.05 between CP10:44.8 and CP10:50.2 as well as between CP10:50.2 and CP10:55.0, respectively. For chlorine content from 55% to 70% (75% for C10), the Kow values did not increase linearly. Rather, the relationship between the chlorination degree and log Kow followed a polynomial correlation of second order (Figure 2). The log Kow was markedly influenced by the high levels of chlorination degree as shown in the following example. An increase in the chlorine content in polychlorinated n-decanes from 44.8% to 65.0% by 20.2% points led to a higher log Kow from 4.95 to 5.42 by only 0.47 units, while a further increment from 65.0% to 75.6% chlorine content by around 10.6% points resulted in a jump from 5.42 to 6.67 by 1.25 units. The C11, C12, and C13 PCAs studied showed a similar behavior. From the course of graphics in Figure 2 and from the higher log Kow values of unchlorinated n-alkanes19 compared to moderately chlorinated n-alkanes (i.e., ∼45-55% Cl), it may be concluded that the log Kow values of PCAs below 45% chlorine
Figure 2. Influence of chlorination degree on log Kow values of SCCPs.
content would increase with decreasing chlorination degree. The dashed trend line for the region intervening 0% chlorination and 45% chlorination in Figure 2 is purely empirical. Therefore, it may follow another function if experimental data are available. Influence of Chain Length. For a better comparison, the estimated log Kow values were pooled in terms of chlorine content yielding the following classifications:
