Air Partition Coefficients of Volatile Methylsiloxanes and Their

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Octanol/Air Partition Coefficients of Volatile Methylsiloxanes and Their Temperature Dependence Shihe Xu* and Bruce Kropscott Health and Environmental Sciences, Dow Corning Corporation, 2200 W Salzburg Road, Midland, Michigan 48686, United States ABSTRACT: The octanol/air partition coefficient (KOA) is a key parameter used to predict the long-range transport potential (LRTP) of volatile and semivolatile organic compounds and their bioaccumulation in terrestrial biota. Despite the enormous importance of this parameter, reliable KOA values are not available for volatile methylsiloxanes (VMS). In this study, a method of syringe headspace analysis was developed to determine KOA values for six common VMS at trace levels. It was found that log KOA values of any given VMS were linearly related to the reciprocal of the environmental temperature (T) from −3 °C to 40 °C, whereas at any given T, the log KOA values of different VMS were linearly related to their molecular mass and normal boiling point temperatures (Tb). Based on those findings, empirical models were developed to predict log KOA values of methylsiloxanes using Tb (or molecular mass) and T as the only independent variables. The log KOA values for common VMS at 25 °C were in the range from 2.98 (for hexamethyldisiloxane) to 5.77 (for dodecamethylcyclohexasiloxane). Judging from the log KOA values, these VMS have little potential for long-range transport and deposition to remote surface media and for bioaccumulation in terrestrial biota.

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In addition, KOA values were key parameters to predict biomagnification behavior of POPs in terrestrial biota6,7 and humans.8 Substances with low log KOA values ( 98 % by gas chromatography. The specific activity was 395.9 mCi·g−1 for 14C-D4, 406.9 mCi·g−1 for 14CD5, 399.0 mCi·g−1 for 14C-D6, 312.0 mCi·g−1 for 14C-L2, 147.4 mCi·g−1 for 14C-L3, and 228.1 mCi·g−1 for 14C-L4. High purity (> 99 %) 1-octanol was supplied by Fluka. The test system (Figure 1) consisted of 1-octanol (1 mL to 5 mL) and air contained in a 100 mL gastight syringe with a valve

individual compounds was analyzed by high-performance liquid chromatography with a radiochemical detector (HPLC/RAM). The total radioactivity was determined by LSC (Packard Liquid Scintillation Analyzer, Tri-Carb 2500TR) using six aliquots (10 μL each). Each aliquot was mixed with a 5 mL liquid scintillation cocktail (Ultima Gold from PerkinElmer). The aliquots were taken before and after air sampling (three aliquots in each case). After a minimum 1 h equilibration, the syringe was mounted onto a syringe pump (KD Scientific Syringe Pump, model no. 780230). The syringe was connected to the cold gas trap with the syringe needle inserted through the inlet septum, while a vent needle was then inserted in the outlet to complete the air passage. The target volume of air (10 mL to 90 mL) from the syringe headspace was injected using a syringe needle and pumped through the cold gas trap with at least part of the glass coil immersed in dry ice/acetone bath (−76 °C) at 20 mL·min−1 to 30 mL·min−1. After the targeted volume of air was sampled, the valve was closed, and the syringe needle was removed from the inlet of the cold trap. A sample of 1 mL of a mixed solvent [(60 % methanol/40 % acetonitrile (v:v)] was injected into the glass coil to dissolve the trapped VMS. Half of the mixed solvent collected from the cold gas trap was analyzed by HPLC/RAM and the rest analyzed by LSC. The HPLC/RAM system consisted of Hewlett-Packard 1050 solvent conditioning module and Hewlett-Packard 1050 quaternary pump equipped with a flow scintillation analyzer (Packard Radiomatic FLO-ONE Beta Series A-500 radiochromatograph detector, model no. C515TR). For the HPLC/ RAM analysis of VMS, a reversed phase column (Agilent Eclipse XDB-C-18, 5 μm, 4.6 mm × 150 mm) with a typical solvent gradient as follows: 0 min, water 30 %, methanol 35 %, acetonitrile 35 %; 6 min, methanol 50 %, acetonitrile 50 %; 17 min, methanol 40 %, acetonitrile 60 %; 22 min, methanol 40 %, acetonitrile 60 %; 25 min, water 30 %, methanol 35 %, acetonitrile 35 %. The solvent flow rate was 1.5 mL·min−1, and the cocktail flow rate was 4.5 mL·min−1. After all the air samples were collected, the octanol solution from the 100 mL gastight syringe was transferred to a clean glass vial, and three aliquots of the solution were analyzed by LSC. The DPM from this analysis was referred to as post-LSC counts.

Figure 1. Illustration of the syringe system for liquid (octanol)−gas equilibration and the cold trap for gas sampling in KOA measurements. The syringe was mounted to a syringe pump (not shown).

(Hamilton Samplelock Syringes from Fisher Scientific), mounted in a syringe pump. The cold trap was a glass tubing coil capped on both ends with Teflon-lined rubber septa and immersed in a dry ice/acetone bath. The syringe pump and cold trap were used to sample airborne VMS in the syringe after equilibration as described below. Equilibration. In this study, the distribution of the test compounds (14C-labeled D4, D5, D6, L2, L3, and L4) between the two phases, 1-octanol and air, were determined at a wide range of VMS concentrations in 1-octanol (subppm to tens of thousands ppm). The VMS/1-octanol solutions were prepared to contain either a single VMS compound in 1-octanol solution, a mixed linear VMS/1-octanol, or mixed cyclic VMS/1-octanol solutions. About 5 mL of VMS/1-octanol solution was transferred to the syringe for equilibration after three aliquots of the solution were sampled for total radioactivity analysis by liquid scintillation counting (LSC). The disintegrations per minute (DPM) obtained were referred to as the initial-LSC counts. The equilibration was performed at four different temperatures: −5 ± 2 °C, 7 ± 2 °C, 23 ± 2 °C (room temperature), and 40 ± 2 °C. For room temperature experiments, the VMS/ 1-octanol solution containing syringe was placed in the hood in the laboratory. For nonroom temperature experiments, the temperature-controlled environments were two walk-in cold rooms set at −5 °C and 7 °C, respectively, and a 40 °C oven (TempCon Oven, Baxter, model no. N8G20-1A). In addition, for low-temperature experiments (−5 °C and 7 °C), the syringe plunger tip was wrapped with Teflon tape to compensate for unbalanced shrinking of the Teflon tipped plunger and the glass syringe barrel to ensure the tight seal during equilibration. Finally, a foam sleeve was used to insulate the syringe to minimize the temperature change during air sampling in all nonroom temperature experiments. Gas and 1-Octanol Sample Collection and Analyses. For the 1-octanol solution, the radioactivity arising from

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RESULTS AND DISCUSSION Efficiency of Cold Gas Trap. The current syringe method for KOA determination is a new method designed specifically for volatile compounds like VMS. The effectiveness of the cold gas trap for VMS is a critical in this method. In this study, 14C-L2 and 14C-L4 were used to check trapping efficiency, where L2 and L4 represented the VMS of the lowest and highest boiling point temperatures, respectively. Briefly, 1-octanol solutions containing 14C-L2 or 14C-L4 were loaded into a gastight syringe. After equilibrium at room temperature, the syringe was then mounted onto a syringe pump. A total of 30 mL of air was then pumped at flow rates from 5 mL·min−1 to 50 mL·min−1 through a cold gas trap with an activated carbon tube connected to the outlet (Table 1). The fraction recovered from the glass coil of the trap accounted for more than 98 % of the total of radioactivity measured (in glass coil plus wash plus on charcoal tube) at the gas flow rates from 5 mL·min−1 to 30 mL·min−1, and very little break-through as captured by the charcoal tubes was detected (Table 1). Equilibrium Time. Since KOA is an equilibrium constant, it is critical to ensure that the contact time of the air with the 1137

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free) was drawn in (under-saturation) or after part of the air in the syringe was replaced with air at a higher 14 C-D 5 concentration relative to the equilibrium state (oversaturation). These samples were then analyzed by LSC and HPLC. The analytical results are presented as the distribution coefficient or kOA (Figure 2) defined below:

Table 1. Recovery of VMS by a Cold Trap as Influenced by Gas Flow Ratesa VMS 14

14

C-L2

C-L4

flow rate (mL·min−1)

% VMS recovered in solvent

% VMS recovered in trap wash

% VMS recovered in charcoal

5 10 20 30 50 5 10 20 30 50

99.89 99.94 99.94 99.96 99.98 98.15 99.34 99.07 99.40 95.12

0.01 0.03 0.01 0.01 0.00 0.57 0.28 0.16 0.00 2.25

0.10 0.03 0.05 0.03 0.02 1.28 0.39 0.76 0.60 2.64

k OA = [VMS]O /[VMS]A = [DPM]O /[DPM]A

(1)

where [VMS]O and [VMS]A were concentrations of any given VMS in 1-octanol and air phase, respectively; [DPM]O and [DPM]A were radioactivity measured in DPM per mL of the given VMS in 1-octanol solution and air, respectively. As shown in Figure 2, the equilibrium time needed for log kOA to approach a constant value (i.e., the shadowed band) is very short regardless if the equilibrium was approached from undersaturation or oversaturation. For example, when a large volume (90 mL) of fresh air was pulled into the syringe, the calculated initial kOA should be around 6.4 (Point A) based on the amount of D5 in the residual air before replenishment. The actual kOA from the first sample collected 9 min later (Point B) already approached the equilibrium value. Similarly, if 50 mL of air over 1-octanol containing 500 ppm 14C-labeled D5 was drawn into a syringe to mix with 40 mL air already in equilibrium with 5 mL of 100 ppm 14C-labeled D5 (Point C), the kOA obtained 9 min later was only slightly below the equilibrium position (Point E). However, with the added D5, the initial kOA should be around 4.44 (Point D). This was verified once more with another spike (Point G). It is obvious that the equilibrium between the air and 1-octanol phase was reached in less than 15 min from either directions. The short

a 30 cm3 of gas contained a known amount of 14C-VMS was pumped through a cold trap in dry ice bath, where one charcoal tube was inserted in the outlet of each cold trap to collect any 14C-labeled L2 or L4 that broke through the trap. A sample of 1 mL of mixed solvent (60 % methanol/40 % acetonitrile) was used to dissolve the trapped VMS, and 2 mL of acetone was used to wash the trap after all the mixed solvent was removed (% recovery all based on the known radioactivity in the 30 cm3 gas).

octanol phase is sufficiently long so that equilibrium between the air and the 1-octanol solution has been reached before the air and 1-octanol are sampled for analysis. To determine the minimum equilibration time, the air above the 14C-D5/1octanol solution was sampled at predetermined time intervals ranging from minutes to hours either after fresh air (14C-D5

Figure 2. Measured concentration ratios, kOA (kOA = concentration in 1-octanol/concentration in air), of D5 as a function of equilibration time after intentional disturbance of the equilibrium: immediately (Point A) and 10 min (Point B) after dilution of air by the addition of VMS-free air; before (Points C and F), immediately (Points D and G), or 9 min (Point E) after addition of D5 vapor. Arrows indicate the directions of log kOA value changes during the initial re-equilibriation. 138

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Table 2. Average log KOA Values and Their Standard Deviations (STDEV) for VMS Measured at Different Concentrations of VMS in 1-Octanol Solution and Equilibrium Temperatures (Temp) (Room Temperature Values Are in Bold)

equilibrium time is understandable, considering the small amount of D5 in the air compartment relative to that in the 1octanol phase. KOA as Influenced by VMS Concentrations in 1Octanol. In the determination of KOA values of VMS, two major factors need to be considered: VMS−VMS interaction and the modification of the matrix (1-octanol) by VMS as their concentration increases. To determine how VMS concentrations in octanol may affect the log KOA values, KOA values of 14 C-D4 and 14C-D5 were measured in a series of 1-octanol solutions containing an equal concentration of 14C-D4, but different concentrations of isotopically unmodified D4 ranging from 0 to 0.29 g mL−1 (mole fraction of total D4 in 1-octanol: 7.9·10−8 to 0.18). However, no significant change in log KOA values was detectable until the total D4 concentration increased beyond 105 μg·mL−1 (mole fraction of D4 in 1-octanol equals to 0.056) (Figure 3). Above this concentration, log KOA slightly

compound D4

D5

D6

L2

Figure 3. Measured log KOA values of D4 and D5 as a function of their concentrations in 1-octanol solution.

L3

increased with the D4 concentration, suggesting that VMS− VMS interaction and/or modification of solubility parameter of 1-octanol phase may reduce the activity coefficient of D4 in the solution. Similar results were observed using 14C-D5. The data shown in Figure 3 demonstrate that VMS-VMS interactions and/or 1-octanol phase modification had very little effect on KOA values as long as the total concentration of a single VMS in the 1-octanol phase is lower than 105 ppm. Listed in Table 2 are all of the log KOA values measured at different temperatures and at VMS concentrations less than the upper limits defined above. As mentioned in the Introduction section, KOA values and their temperature dependence measured using normal 1-octanol are not available for any methylsiloxanes. However, room temperature log KOA values were measured using water-saturated 1-octanol in a water/1octanol/air three-phase equilibrium system.11 These log KOA values for wet 1-octanol are different by −0.11 log units to 0.04 log units from those for dry octanol at the same temperatures based on equations in Table 3 of the current study, still within the range of the standard deviation. KOA and Equilibrium Temperatures. Two major factors have predominant effects on those values: the equilibrium temperature and the type of VMS. As shown in Figure 4, log KOA values for each VMS were linearly related to the reciprocal of equilibrium temperature (T) regardless of type of VMS:

L4

no. temp./°C samples −3.6 −3.6 5.3 23.0 24.1 39.7 40.0 −5.0 −3.6 5.3 23.0 24.1 39.7 40.0 −3.6 5.3 23.6 24.1 39.7 −5.0 −3.6 5.6 7.0 23.0 23.6 40.0 40.2 −5.0 −3.6 5.6 7.0 23.0 23.6 40.0 40.2 −5.0 −3.7 5.6 7.0 23.0 23.6 40.0 40.2

8 16 14 37 19 18 21 8 16 14 14 18 19 8 16 14 3 17 18 17 16 15 10 41 16 20 17 17 15 16 10 38 16 20 17 17 16 16 10 36 15 20 18

log K OA = A + B /T

concentration, in 1-octanol/ (μg·mL−1) 11.9−29.8 2.8−8.8 0.2−1.0 0.3−2.8 0.5−5.5 0.7−9.7 10.5 9.9 38.3−98.5 10.8−32.2 0.6−57.6 0.9−4.7 1.5−11.0 5.3−11.6 200.6−446.0 29.2−77.9 82455 5.7−12.0 6.2−28.9 7.9−48.7 0.2−3.1 0.2−0.9 7.2 0.9−8.9 0.03−0.3 0.7−7.6 0.05−0.2 12.3−50.1 2.3−15.6 0.5−3.9 12.2 1.3−12.3 0.8 1.2−11.4 0.2−4.0 23.1−183.4 8.3−34.3 8.5−9.9 21.7 2.4−20.3 3.3−3.5 21.4−22.3 0.9−8.5

log KOA STDEV 5.23 5.08 4.79 4.43 4.22 3.92 4.02 5.71 5.93 5.63 5.00 4.96 4.57 4.54 6.85 6.49 5.86 5.75 5.31 3.51 3.43 3.27 3.30 3.03 2.95 2.84 2.71 4.47 4.46 4.19 4.27 3.86 3.68 3.58 3.37 5.53 5.47 5.21 5.23 4.76 4.55 4.41 4.14

0.05 0.05 0.04 0.11 0.03 0.06 0.12 0.09 0.04 0.05 0.09 0.04 0.05 0.06 0.05 0.08 0.02 0.07 0.10 0.08 0.07 0.05 0.03 0.06 0.10 0.10 0.08 0.09 0.04 0.06 0.02 0.04 0.03 0.05 0.07 0.11 0.07 0.07 0.07 0.06 0.02 0.09 0.03

(2)

where A and B are temperature-independent constants. The slope B is related to the internal energy change (ΔUOA) for VMS transferred from 1-octanol to air as: ΔUOA = −2.303RB

(3)

where R is the ideal gas constant. Using the data shown in Figure 4, ΔUOA for the six VMS were found to range from −25.5 kJ·mol−1 for L2 to −57.5 kJ·mol−1 for D6 (Table 3). The absolute values of these ΔUOA are much smaller than those for 139

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Table 3. Intercept (A), Slope (B), Correlation Coefficients (r), and Number of Measured Values for the Linear Regression of log KOA Values of VMS against the Reciprocal of the Equilibrium Temperature (T): log KOA = A + B/Ta VMS L2 L3 L4 D4 D5 D6

A −1.49 −2.70 −3.33 −3.38 −3.29 −4.39

(0.30) (0.55) (0.63) (0.58) (0.61) (0.33)

B 1332.3 1927.9 2377.6 2292.8 2456.0 3000.6

(87.2) (158.1) (181.3) (168.4) (175.2) (95.6)

ΔUOA/(kJ·mol−1)

r(n)

−25.5 −36.9 −45.5 −43.9 −48.7 −57.5

0.987(8) 0.980 (8) 0.983 (8) 0.987 (7) 0.987 (7) 0.998 (5)

log KOA at 25 °C

(1.7) (3.0) (3.5) (3.2) (3.4) (1.8)

2.98 3.77 4.64 4.31 4.95 5.77

a ΔUOA, the energy of phase transfer, was calculated as −2.303RB, where R is the ideal gas constant. All the values in the parentheses in columns A, B, and ΔUOA are standard errors.

Simple linear relationships between ΔUOA and molecular mass (Figure 5c) or boiling point temperature (Figure 5d) were also observed for all the VMS. The boiling point temperature is a better predictor for ΔUOA than molecular mass. Based on the above observations, a simple model for predicting log KOA values from the boiling-point temperature (or molecular mass) and equilibrium temperature may be obtained. In the literature, volatility measured by log P, where P is the saturated vapor pressure, is related to boiling-point temperature (Tb) and equilibrium temperature (T) in the following form:13 ln P = a + b(Tb/T ) + c log(Tb/T )

(4)

where a, b, and c are constants. When log KOA values obtained at various temperatures for each compound from the current study were used in the place of “ln P” to fit the above equation, the following relationship was obtained: Figure 4. Relationship between the measured log KOA values and the reciprocal of equilibrium temperatures.

log K OA = − 9.33(± 1.46) + 11.41(± 1.63)Tb/T − 20.42(± 5.70)log(Tb/T )

r 2 = 0.985

(n = 43)

chlorinated carbons of similar molecular weights. For example, the ΔUOA values are −49.3 kJ·mol−1 to −91 kJ·mol−1 for chlorinated benzenes (CBs) and biphenyls (PCBs) with the molecular weights ranging from 147 to 395.12 Similarly, ΔUOA values < −75 kJ·mol−1 are also reported for polycyclic aromatic hydrocarbons with molecular weights in a similar range (160 to 300).13 Practically, the smaller absolute values of ΔUOA for VMS will manifest as a smaller temperature dependence for KOA and thus a smaller tendency for airborne VMS to partition out from air as temperature decreases. K OA and ΔU OA of Different VMS. At any given temperature, different VMS have different KOA values. The distribution of any given VMS between the 1-octanol and the air phases may be correlated with several molecular properties affecting the volatility of VMS and their solublization in 1octanol including molecular weights, boiling point temperatures, and saturated vapor pressures. Since the former two are widely available for most volatile organic compounds, a relationship between the log KOA and these quantities will be very useful for environmental modeling. For the six VMS at any given temperature (e.g., 25 °C), good linear correlations were obtained between log KOA values and molecular weight (Figure 5a) or boiling point temperature (Figure 5b). The good correlation between log KOA and boiling point temperature implies that volatility, not the solubility in 1-octanol, may be the major factor in determining the KOA values. This can be expected since molecular interactions (mainly dispersion) between these methylsiloxanes and the solvent (1-octanol) may be correlated with their molecular size.

(5)

This equation represented data well with the p values