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Jan 19, 2012 - the simultaneous determination of KAW, KOW, and KOA of organic compounds ... KOW values from −0.4 to 8.9, and log KOA values up to 7...
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Method for Simultaneous Determination of Partition Coefficients for Cyclic Volatile Methylsiloxanes and Dimethylsilanediol Shihe Xu* and Bruce Kropscott Dow Corning Corporation, Health and Environmental Sciences, Auburn, Michigan 48686, United States ABSTRACT: Cyclic volatile methyl siloxanes (cVMS) such as octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6) may enter the environment through industrial activities and the use of various consumer products. Reliable air/water (KAW), 1-octanol/water (KOW), and octanol/air partition coefficients (KOA) for those compounds and their common degradation product, dimethylsilanediol, are critical for accurate prediction of the environmental fate, distribution, and transport of these materials. Challenges have been encountered in determining these properties for cVMS and their degradation products mainly due to the extremely low water solubility of the organosiloxanes, low volatility of their degradation products, and reactivity of those compounds in the water/1-octanol system that can lead to inconsistent and inaccurate partition coefficients. A novel direct method is presented for the simultaneous determination of KAW, KOW, and KOA of organic compounds and was applied to these organosilicon compounds. It was tested in a range of log KAW values from −6.8 to 3.1, log KOW values from −0.4 to 8.9, and log KOA values up to 7. The advantages of the new direct method include the improved accuracy, a shortened measurement time, simultaneous measurement of three partition coefficients of multiple compounds, self-consistency among resultant partition coefficients, and a wide range of applicability including materials that may be slowly reactive in the water/1-octanol system.

E

moist soil into the atmosphere. Allen et al15 have estimated that approximately 90% of some VMS materials used in personal care products are volatilized during use with the remaining materials being discharged to water treatment systems. In the wastewater treatment processes, the majority of cVMS will partition to air and sludge with only a small fraction (1−2%) being released to surface waters.20 Aqueous cVMS are not stable and tend to hydrolyze,21 while their hydrolysis products (silanols) may undergo condensation reactions in the 1-octanol/water system during measurements of partition coefficients. In addition, for highly hydrophobic organic compounds such as cVMS, the concentration in water (CW) is limited by their low water solubility,19 while that in the 1-octanol phase (CO) could be 6 to 9 orders of magnitude higher.22 For highly hydrophilic silanols such as dimethylsilanediol (DMSD) or methylsilanetriol, however, preferential distribution in water with very little partitioning to air is expected. Accurate determination of KAW and KOW of such compounds is, therefore, extremely challenging. Several studies conducted on cVMS materials have reported values for log KOW, KOA, or KAW.22−27 However, study design and analytical aspects of some of those studies may have affected the accuracy and precision of those values. In most multimedia environmental fate models, KAW and KOW are required inputs while KOA is calculated as the ratio of KOW to KAW values on the basis of the assumption that the

quilibrium partitioning of organic contaminants in different environmental media is important in determinations of chemical fate and transport of organic contaminants in the natural environment.1−4 The principal parameters describing the partition properties are the air/water partition coefficient (KAW), the air/octanol partition coefficient (KOA), and the octanol/water partition coefficient (KOW), where the octanol is a surrogate of environmental lipids and natural organic matter. Because of the importance of the fundamental partition properties in environmental chemistry, they have been studied extensively. Many critical reviews of the methods for measuring partition coefficients and compilations of the measured values for common environmental contaminants can be found in the literature;2,3,5−14 however, a direct method that can simultaneously determine all three partition coefficients for “difficult” contaminants such as organosiloxanes and their degradation products is still not available. Organosiloxanes such as methylsiloxanes are a group of silicones in either linear (L) or cyclic (D) form. High hydrophobicity and volatility, low surface tension, and high thermal stability are some of their unique chemical properties leading to their wide industrial and commercial applications.15,16 Estimates of continental consumption/production vary but have been estimated to be close to 200 000 t for decamethylcyclopentasiloxane (D5) in western Europe alone.17 As a result of the production and use patterns of methylsiloxanes, environmental scrutiny has increased significantly worldwide. The cyclic volatile methylsiloxanes (cVMS) have high vapor pressures18 and low water solubilities19 and thus very high KAW values. These compounds readily evaporate from water and © 2012 American Chemical Society

Received: November 7, 2011 Accepted: January 19, 2012 Published: January 19, 2012 1948

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Figure 1. Illustration of two double syringe systems for three phase equilibrium: the upper one was used for volatile and semivolatile compounds such as cVMS, and the lower one used for polar compounds with low volatility such as DMSD.

effectively reduce or remove much of the error normally associated with measurements of extreme partition coefficients, ensure self-consistency of the resultant partition coefficients, and reduce the time and cost for analysis.

three partition coefficients for a given compound at any given temperature are consistent with each other:1 log KOW = l og KAW + log KOA

(1)

■

In reality, the KOW, KAW, and KOA values measured separately for a compound rarely satisfy the above relationship. Instead, log KOW = log KAW + log KOA + δ

EXPERIMENTAL SECTION Chemicals. Two types of organosilicon compounds were selected to test the applicability of the new method including three cVMS and a representative silanol. Three cVMS, radiolabeled 14C-octamethylcyclotetrasiloxane (14C-D4), 14Cdecamethylcyclopentasiloxane (14C-D5), and 14C-dodecamethylcyclohexasiloxane (14C-D6), and the representative silanol, 14 C-dimethylsilanediol (14C-DMSD), were all synthesized by Dow Corning Corporation (Midland, MI 48686). The specific activity was 393 mCi g−1 for 14C-D4, 383 mCi g−1 for 14C-D5, 248 mCi g−1 for 14C-D6, and 522.4 mCi g−1 for 14C-DMSD. The radiochemical purity was 98.1% for 14C-D4, 99.3% for 14CD5, 99.2% for 14C-D6, and 98% for 14C-DMSD. 1-Octanol and 1-heptanol were reagent grade from Fluka (purity >99.5% and >99%, respectively). All other chemicals were ACS reagent grade from Sigma-Aldrich. Purified Milli-Q water was used throughout. Equilibration. The custom-made apparatus for sample equilibrium (Figure 1) consists of two, 100 mL airtight syringes

(2)

where δ is the discrepancy, which can be greater than one log unit.28 This discrepancy is partially attributed to the measurement errors and partially to the mutual solubility of water and 1-octanol. Namely, the water phase in KOW measurement (octanol-saturated) is different from that in the KAW measurement (pure water). Similarly, 1-octanol in the KOA measurement contains very little water while it is water saturated in the KOW measurement. Although this may be true, the effect of mutual solubility on the measurements of environmental partition coefficients, especially for organosiloxanes, have not been quantified independently. The objective of this study was to develop a method for simultaneously determining three partition coefficients (KAW, KOW, and KOA) applicable for nonionic organic contaminants with a wide range of partitioning properties and reactivity (especially in the 1-octanol/water system). This method should 1949

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determination of the optimal time for extract concentration, extraction efficiency, and total radioactivity recovery. A result of those tests will be discussed later. A 100 μL aliquot of the concentrated organic phase was analyzed by RP HPLC/RAM, and the rest was mixed with 10 mL of Ultimate Gold and analyzed by LSC. The residual water from each tube was decanted into a scintillation vial containing 12 mL of Ultima Gold XR. The tube was washed with 2 mL of acetone twice which was also added to the last scintillation vial. After mixing well, these samples were analyzed by LSC for a check of total radioactitivy recovery. Air. Air samples for cVMS analysis were obtained from the air/octanol (right) syringe via air sampling port before and after a set of water samples were taken. Briefly, 10 mL of air were pushed through a cryogenic cold trap connected to the air sampling port and immersed in dry ice/acetone bath. After 1 mL of mixed solvent [methanol/acetonitrile = 60:40 (v/v)] was injected into the cold trap, the cold trap was removed from the dry ice/acetone bath and equilibrated in the hood to bring the temperature close to room temperature (∼5 min) to completely dissolve cVMS in the trap. An analytical syringe was then used to collect the solvent from the cold trap. A portion of the collected solvent was injected for analysis by RP HPLC/RAM, and a small portion was analyzed by LSC for total radioactivity confirmation. The analytical procedure was the same for airborne DMSD, except a larger air volume (e.g., 70 mL) was sampled and a stainless steel coil was used for trapping airborne DMSD. Water was the solvent used to dissolve the condensates in the cold trap. The interception efficiency by the cold trap was above 98% even for D4, the most volatile cVMS, with little breakthrough.22 Octanol. The octanol was sampled through an orifice of the air sampling port using a microsyringe. The concentrations of cVMS and DMSD in 1-octanol were usually relatively high, and therefore, the octanol was injected directly into RP HPLC/ RAM for cVMS analysis and normal phase HPLC/RAM (NP HPLC/RAM) for DMSD analysis with or without dilution. HPLC/RAM Analyses. The reversed phase HPLC/RAM system consisted of a HP 1100 system equipped with a Flow Scintillation Analyzer (PerkinElmer Radiomatic 610TR) and a C18 column (Agilent Eclipse XDB-C18 5 μm, 4.6 × 150 mm, S/N: USKH016928). The linear mobile phase gradient was: 100% water at time zero; 1% water plus 99% acetonitrile at 5.5 min; 100% acetonitrile at 11 min; 100% acetonitrile at 15 min; 100% water at 16 and 20 min. The flow rate was 1.7 mL min−1 for the mobile phase and 5.1 mL min−1 for the scintillation cocktail (PerkinElmer, Ultima-Flo M). Normal phase HPLC/RAM analysis was essentially the same as RP HPLC/RAM analysis, except a normal phase column (Prevail Silica 5 μm, 4.6 × 150 mm, S/N: 609120434) and the following linear solvent gradient were used: 94% hexane and 6% tetrahydrofuran (THF) at zero to 3 min; 92% hexane plus 8% THF at 4 min; 80% hexane plus 20% THF at 13 min; 65% hexane plus 25% THF at 21 min; 94% hexane plus 6% THF at 25 min. LSC Analysis. For aqueous samples, the solution was mixed with an appropriate amount of Ultimate Gold XR (PerkinElmer 77-060802) to obtain a transparent mixture and analyzed by LSC (Packard Tri-Carb 2500TR, S/N: 401574). For nonaqueous samples, 10 mL of Ultimate Gold scintillation cocktail was used for each sample.

(Hamilton borosilicate glass syringe with a Teflon-faced plunger and rotary Teflon valve) with an air sampling port in one syringe and a water sampling port on the other. During the experiment involving compounds with relatively high vapor pressure such as cVMS, the right (octanol/air) syringe contained 14C-labled cVMS in 1-octanol (0.2−1.0 mL) saturated with water (or on top of a water phase) and a gas phase (Figure 1, upper graph). The left (water/air) syringe contained 1-octanol-saturated water (60−80 mL) and an air phase (20−40 cm3). A magnetic stirrer was placed in the water phase and was rotating at the lowest speed during the equilibration period. The air phase in both syringes was connected through an open middle valve during the equilibration, but the valve was closed during the air and water sampling. For compounds with low vapor pressure such as DMSD, three phase equilibrium was established using the right syringe compartment with the middle valve closed (Figure 1, lower graph). A small volume (0.2−0.5 mL) of 1-octanol was placed on top of the aqueous phase for direct octanol/water contact to speed-up attainment of octanol/water equilibration without any stirring. The temperature control during the equilibration was achieved in incubators (Percival I-30BLL) and a walk-in environmental chamber. The temperatures inside the incubator or environmental chamber were measured during sampling using a digital thermometer. Sampling and Analyses. Water. The concentration of DMSD in water was reasonably high in the three phase equilibrium system. For 14C-DMSD analysis, after a predetermined equilibrium time, three aliquots of 1−50 μL of water solution with 1−100 times dilution were directly injected into a high performance liquid chromatograph equipped with a radiometric detector and a reversed phase (C18) column (RP HPLC/RAM) described in more detail later. The concentrations of cVMS in water were extremely low in the three phase equilibrium system. Liquid−liquid extraction of water samples followed by extract concentration was conducted for accurate cVMS analysis. More specifically, two aliquots of 1octanol saturated water (∼25 mL) in the water/air syringe compartment were collected via the water sampling port and transferred into two, 50 mL glass centrifuge tubes containing 0.1 to 0.2 mL of 1-heptanol. The exact amount of water collected in each tube was determined gravimetrically. After the tube was capped, 5 mL of 30% (v/v) MgSO4 was added into each centrifuge tube. The tubes were vortexed for 5 min and then centrifuged at 2400 rpm for 7 min. 1-Heptanol on the top of the aqueous phase was transferred into a scintillation vial using a syringe. Dichloromethane (0.8 mL) was added into each tube for a second extraction. After the tubes were capped, they were vortexed for 5 min and again centrifuged at 2400 rpm for 7 min. Without disturbing dichloromethane on the bottom, the water (4 mL each) from each tube was transferred into scintillation vials each containing 12 mL of Ultima Gold XR cocktail (PerkinElmer, Lot No. 79-070401) using a transfer pipet. After mixing well, these samples were analyzed by liquid scintillation counting (LSC). The dichloromethane on the bottom of each centrifuge tube was pooled with the organic (heptanol) phase collected previously. The pooled organic phase was concentrated by exposing the open vial to a stream of air for about 3−4 min. Several tests were performed to validate this extraction method using 1-octanol-saturated water spiked with known amounts of cVMS standards. They included 1950

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Table 1. Efficiency of cVMS Extraction from Water by Heptanol/Dichloromethane Extraction and Total Recovery of the Water Extraction/Concentration Steps

a

extr. vol. (mL)

spiked DPMa

DPM extracted as cVMSb

DPM extracted as othersb

DPM remaining in waterc

extraction efficiency (%)

recovery (%)

D4

25.01 25.14 25.52 25.1

6168 6201 6293 6191

3641 4517 4281 3906

1077 239 829 683

600 459 235 220

D5

25 25 25 25

5207 5207 4086 4086

4584 4717 4148 4095

0 0 0 0

168 44 66 26

D6

50 50

4542 4542

4147 4400

112 0

167 157

88.7 91.2 95.6 95.4 92.7 98.3 101.0 100.3 101.1 100.2 99.3 100.4 99.9

86.2 84.1 84.9 77.7 85.1 91.3 91.4 103.1 100.9 96.7 97.4 100.3 98.9

run no.

cVMS

1 2 3 4 average 5 6 7 8 average 9 10 average

The purity of cVMS in spiking solution is 98.1% for D4, 99.3% for D5, and 99.2% for D6. bDetermined by HPLC/RAM. cDetermined by LSC.

■

Equilibration of cVMS. One issue in determination of partition coefficients involving a 1-octanol saturated water phase is the formation of microemulsions in water. This is especially true when surface active compounds or compounds with low cohesive and surface energy like cVMS are involved. For this group of compounds, three phase equilibrium was achieved without a direct contact of 1-octanol with water, which may slow the equilibration among the three phases. In order to determine the time needed to establish equilibrium between the air and water phases in such a system, water and air were sampled at various times ranging from 1 to 94 h. At each sampling time, the ratio of D4 radioactivity in water to that in air (kAW) was determined and plotted in Figure 2A. Shown in Figure 2A was also the same ratio using total radioactivity to replace D4 radioactivity for comparison. Although the log kAW based on total radioactivity continued to decrease after 20 h, the log kAW based on D4 radioactivity remained relatively constant past 20 h, indicating that equilibrium or steady state was reached after 20 h. Notice that kAW was defined as the air-to-water concentration ratio for D4 in the above discussion. This is a quantity different from that of KAW which is equal to kAW when the partition equilibrium between the air and water is reached. The same applies to kOW and kOA. In determining the equilibrium time for this three phase system, several physicochemical processes need to be considered. They are octanol-to-air partitioning of D4, gas phase diffusion in the headspaces of the connected double syringes, adsorption of D4 on the wall of the syringe system, air-to-water partition, and finally hydrolysis of D4 in water. Of those processes, 1-octanol/air partition of D4 in the syringe should be very fast according to a previous study which shows the octanol/air equilibrium of cVMS in a similar syringe system can be reached in less than 10 min.22 The air/water partitioning was accelerated by slow stirring of the water which reduces the time for cVMS distribution in the water phase. Adsorption of cVMS on the wall of the syringe could contribute to the loss of some cVMS. The significance of adsorption of cVMS on the wall of the syringe was not quantified in this study. The assumption was made that adsorption equilibrium could be reached relatively fast and thus should not have any effects on the measurement of concentration ratios.

RESULTS AND DISCUSSION

Sample Analysis Methods. Water Analysis for cVMS. In the three phase equilibrium, cVMS concentrations in water were expected to be low. The liquid−liquid extraction with heptanol/dichloromethane was designed to concentrate cVMS for HPLC/RAM analysis. The choice of heptanol and dichloromethane as the extracting agents was based on a series of pre-experiments with several organic solvents such as: hexane, toluene, octanol, heptanol, dichloromethane, heptane, and octane. 1-Heptanol was found to form a round droplet on top of water with the smallest contact angle and thus was much more easily collected while nonpolar organic solvents spread out on the water surface and were thus difficult to be removed from the water surface. For extract concentration, the volume of the collected 1heptanol/dichloromethane mixture was reduced by blowing down the solvent with a stream of nitrogen at room temperature as described in the method section. When such a mixture was spiked with known amont of 14C-cVMS and then subjected to the concentration process, the weight loss of the solvent mixture was rapid (data not shown). After 3 min of blowing, the majority of dichloromethane was removed. However, the measured radioactivity in the remaining 1heptanol solution was essentially identical to that initially spiked (data not shown), indicating that using a concentration step did not change the cVMS quantity in the extracts within the tested time. Extraction efficiency was defined as the radioactivity extracted from the water as the percent of the total radioactivity originally in the water sample. Recovery was the total radioactivity measured in the extract after extract concentration plus that remaining in the water as the percent of total radioactivity spiked in the original water samples before extraction. As shown in Table 1, the extraction efficiency is greater than 92% for D4, D5, and D6. The average recovery rate is 85% for D4 but greater than 98% for D5 and D6, indicating that the extraction is quantitative without significant analyte loss or contamination. The recovery of the total radioactivity decreased as the radio-purity of the spiking solution decreased (e.g., in case of D4). This may be caused by low recovery for silanols, the hydrolysis products of the cVMS. 1951

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These same trends were observed when CA/CW was determined for D5 and D6. However, D5 and D6 had much slower hydrolysis rates than that of D4, and the effects of hydrolysis on the distribution of D5 and D6 between air and water phases were much smaller than those observed for D4. Like D4, air/water equilibrium was established in the D5 and D6 systems at 20 h. Equilibration of Dimethylsilanediol (DMSD). Dimethylsilanediol, an ultimate hydrolysis product of cVMS, has a combination of partition properties completely different from that of cVMS: high estimated solubility in water and low estimated vapor pressure27 and no surface activity. For such compounds, three phase equilibrium without direct 1-octanol/ water contact, as used for the cVMS, required a long equilibrium time and the procedure described for cVMS may not be practical. To accelerate the partitioning, three phase equilibration for DMSD was conducted with a small droplet of 14C-DMSD/1octanol solution (0.4 mL) directly placed on top of water phase (10 mL) in the right syringe compartment with the middle valve closed. In Figure 3, the log kAW, log kOA, and log kOW

Figure 2. (A) Dependence of log kAW (where kAW = CA/CW) of D4 for water sampled at different equilibrium times (A), calculated on the basis of D4 radioactivity (high and low D4 conc) determined by HPLC/RAM and total radioactivity (high and low total) determined by LSC. Each data point represents the average of a duplicate. The overall average temperature was 21.6 °C. (B) A typical HPLC/RAM chromatogram for water after equilibration with 14C-D4 containing 1octanol for 20 h.

Hydrolysis of cVMS in water follows first order kinetics with the actual rate depending largely on pH and temperature.21 Under the experimental conditions, the half-life of D4 in pure water is expected to be about 4 days.21 The exact half-life of D4 in 1-octanol saturated water is presently unknown. However, significant amounts of hydrolytic products were detected in the water phase after 20 h in the three phase equilibrium experiment described above (Figure 2B). The hydrolysis of D4 in water was likely responsible for the differences observed between the apparent log kAW values calculated on the basis of total radioactivity and that calculated on the basis of the D4 radioactivity shown in Figure 2A. The constant log kAW values based on D4 radioactivity observed after 20 h equilibration could be attributed to two possible causes: a steady state or a partition equilibrium. The CW at a steady state, which involves air phase controlled diffusion as an input and hydrolysis as a removal mechanism, should depend on the CA which controls the input rate. When a volume of 1-octanol was added to the D4-containing 1-octanol phase to decrease CA in the syringe compartment, however, the identical constant log kAW values were observed at equilibrium time greater than 20 h (low D4 concentration in Figure 2A). Therefore, the constant log kAW values after a 20 h equilibration were due to equilibrium and thus represented the log KAW value.

Figure 3. Concentration ratios of DMSD at 20 °C as influenced by equilibrium time in the air (A)/water (W)/1-octanol (O) three phase equilibration for two initial conditions: (1) DMSD initially contained in the water phase only (open symbols); (2) DMSD initial contained in 1-octanol phase only (solid symbols). The concentration ratios are defined as kAW (=CA/CW) for water/air partition, kOW (=CO/CW) for 1-octanol/water partition, and kOA (=CO/CA) for 1-octanol/air partition; CA, CW, and CO are DMSD concentrations in the air, water, and 1-octanol, respectively. Dashed lines represent the corresponding equilibrium partition coefficients for DMSD at 20 °C.

values for DMSD were plotted against equilibrium times for two different initial conditions (Figure 3). When the equilibration was started with 14C-DMSD-containing water and DMSD-free 1-octanol phase (under-“saturation”), the time for the system to approach equilibrium was very short as indicated by the fact that all measured partition coefficients never deviated much from the equilibrium positions (Figure 3). This was probably due to the fact that the amount of 14CDMSD actually transferred from water to other two phases was small as dictated by small octanol/water and air/water partition coefficients. When the equilibrium started with DMSD-free 1952

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water (10 mL) and 14C-DMSD-containing 1-octanol (0.4 mL), the equilibration was started from the initial over-“saturations” in 1-octanol phase (points a and b in Figure 3). It took 20 to 48 h to reach equilibrium (Points b, d, e, and f in Figure 3). DMSD is also a reactive compound and can form many different condensation products in the water/octanol system. The homocondensation products such as tetramethyldisiloxane-α, ω-diol (dimer diol), and hexamethyltrisiloxane-α, ω-diol (trimer diol), accounted for ∼2% radioactivity in original 14CDMSD containing water. As shown in Figure 4, after the water

condensation reaction in 1-octanol phase. The resultant partition coefficients under different initial conditions converged to the same equilibrium conditions as shown in Figure 3, indicating that the measured partition coefficients were, in fact, obtained from the equilibrium states, not steady states. Simultaneously Determined Partition Coefficients. On the basis of the above information, a minimum of 20 h was the required equilibrium time for all cVMS in the double syringe system while that for DMSD started with 14C-DMSD in water can be as low as 2 h. Using those equilibrium times, the average concentrations of cVMS and DMSD in the three phases and their three partition coefficients were calculated and are listed in Table 2. This method is unique in that the three compartments achieve and remain in equilibrium and much of the uncertainty associated with the fate and distribution of volatile or semivolatile compounds in the system is accounted for, which ensured the self-consistency of all three partition coefficients (e.g., δ ∼ 0 in Table 1). Other studies have determined partition coefficients for cVMS using a variety of methods without carefully checking the status of the equilibration and accuracy of cVMS quantification; subsequently, a wide range of values have been reported for the three partition coefficients, with δ ranges from −2.46 to 1.94 log units for D4 and −2.74 to 1.94 log units for D5.22 Effect of Mutual Solubility of Water and 1-Octanol on Consistency of Partition Coefficients. Strictly speaking, the self-consistency of three partition coefficients can only be achieved when all three partition media are also consistent, as exemplified in the current study. That is, all the “water” was the 1-octanol-saturated water; all “1-octanol” is the water-saturated 1-octanol (∼4% water by weight at 25 °C, from ref 3), and all the “air” contains both saturated water and 1-octanol vapors. In this aspect, the KOW in this study was measured the same way as those in the literature, while KOA and KAW were measured slightly differently. The 1-octanol used in KOA measurements in the literature is often obtained from commercial sources as a normal (nonanhydrous) solvent (∼0.01% water). How different are the KOA values measured with the commercial “non-anhydrous” 1octanol from those measured using the water-saturated 1octanol? In a separate experiment, KOA values for D4, D5, and D6 were measured using a single syringe system and untreated 1-octanol. The measured KOA values were 4.40 for D4 at 21.7 °C, 4.96 for D5 at 24.6 °C, and 5.72 at 23.6 °C,22 very similar to those measured at the same temperatures in the three phase equilibrium system in the current study (Table 1). This

Figure 4. HPLC/RAM chromatograms for water (reversed phase) and 1-octanol (normal phase) after DMSD-free 1-octanol (0.4 mL) was equilibrated with 14C-DMSD-containing water (10 mL) at 20 °C for 20 h (blue) and 96 h (black). DMSD = dimethylsilanediol; dimer = tetramethyldisiloxane-α, ω-diol; trimer = hexamethyltrisiloxane-α, ωdiol.

phase was brought in contact with 1-octanol for 20 h, much larger fractions of trimer diol and dimer diol were found in the octanol phase together with a series of unidentified compounds. As the equilibrium time increased to 96 h, the concentrations of the unidentified compounds increased. Identification of all the compounds formed in 1-octanol is beyond the scope of the current study. However, distinction between the steady state and equilibrium states for DMSD in the three phase equilibrium may be important due to the

Table 2. Summary of the Experimental Results for Room Temperature Partition Measurements Based on Three Phase Equilibration average concentration (mg L−1) organic

a

exp. ID

D4

1 2

D5

1 2

D6 DMSD

1 1

no. of samplea 17 20 average 18 20 average 16 12

average temp. (°C) 21.6 21.7 21.7 24.9 24.3 24.6 23.6 20.1

octanol

air

water

1.49 × 104 8.00 × 103

0.741 0.417

1.53 × 10−3 8.34 × 10−4

2.43 × 104 1.75 × 104

0.324 0.175

2.43 × 10−4 1.29 × 10−4

8.25 × 104 29.0

0.114 1.0 × 10−5

1.09 × 10−4 70.0

log KAW 2.68 2.70 2.69 3.13 3.13 3.13 3.01 −6.84

± ± ± ± ± ± ± ±

0.12 0.14 0.13 0.13 0.13 0.13 0.12 0.34

log KOA 4.30 ± 0.02 4.28 ± 0.04 4.29 ± 0.03 4.87 ± 0.07 5.00 ± 0.08 4.94 ± 0.08 5.86 ± 0.14 6.40 ± 0.31

log KOW 6.98 6.98 6.98 8.00 8.13 8.07 8.87 −0.41

± ± ± ± ± ± ± ±

0.15 0.11 0.13 0.27 0.16 0.22 0.14 0.10

δb 0 0 0 0 0 0 0 0.03

Number of replicates analyzed. bδ = log KOW − log KAW − log KOA. 1953

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various phases and direct quantification of the test compounds in three phases at the equilibrium. Compared with some methods for rapid determination of KOA such as a generator column method13 or the calibrated GC retention method,30 the current method may be more demanding in analysis, which can be viewed as a limitation. One must recognize that the aforementioned methods are designed specifically for KOA determination with their own limitations and, obviously, separate experiments with different methods are needed to couple with those methods if all three partition coefficients are needed. In addition, one may also argue that the current method may be inferior to an indirect analysis of an equilibrium that avoids the need for measuring the abundance of the test materials in both phases. An example of such a method is the phase ratio variation (PRV) method used for determination gas−liquid partition coefficient such as of KAW31 or the equilibrium partitioning in closed system (EPICS) method.23 Unfortunately, both methods depend on a critical assumption that the test compound introduced into the test system only undergoes the partitioning process so that the mass balance of the test substance in the partition medium remains unaltered. They are not suitable for cVMS and DMSD due to unavoidable side reactions in the test system such as hydrolysis of cVMS in the octanol-saturated water shown in Figure 2b, condensation of DMSD in the water saturated 1-octanol demonstrated in Figure 4, and sorption of cVMS on magnetic bar in water or sorption of DMSD on the glass wall. In addition, both PRV and EPICS is not sensitive enough for compounds with extreme KAW values (e.g., too small like KAW for DMSD or too large like those for cVMS). In summary, as demonstrated with cVMS and DMSD, the custom-made double syringe apparatus is a better method for determination of KAW, KOA, and KOW for volatile, semivolatile, or low-volatility organic compounds, especially labile organic compounds with extremely low water solubility.

indicates that the change in water contents of the 1-octanol in this range has a very small effect on the measured KOA values for cVMS. Chen and Siepmann29 have also found that the water saturation of the 1-octanol phase has a negligible effect on the helium-to-1-octanol partitioning of nonpolar compounds such as alkanes and small but apparent (by 0.1 to 0.2 log units) effect for polar compounds such as small alcohols. The solubility of 1-octanol in water is relatively small (0.06% 1-octanol by weight at 25 °C3). The effect of the 1-octanolsaturation of the water on KAW of D4 was measured using ndodecane to replace 1-octanol in the three phase equilibration. The average log KAW value for D4 with 1-octanol-free water was 2.48 ± 0.15 for the ten measurements at 21.6 ± 0.1 °C. This value is slightly smaller than that for D4 measured with 1octanol-saturated water, but the difference is very small (e.g., close to one standard deviation of those measurements). Similarly, the effect of the 1-octanol-saturation of the water on the measured KAW value of DMSD was also studied using pure water in a single syringe system which only contained 14CDMSD dissolved in water and a headspace of ∼90 cm3. The measured log KAW value was −7.03 ± 0.28 for DMSD at 20.6 ± 0.2 °C and compared well with −6.84 ± 0.34 for DMSD in 1octanol saturated water at 20.1 °C (Table 2). The small difference is again not significant compared with their standard deviations. The above data are still very limited. The effect of mutual solubility on the measurements of KOA and KAW and, thus, the overall consistency of the three partition coefficients of any compounds should be examined for a wide range of chemicals in future research. However, if self-consistency of the measured partition coefficients is the goal of a method, effect of the mutual solubility should be removed from the measured partition coefficients by adopting consistent media such as that used in the current study. Otherwise, all the measurements for the partition coefficients will bear uncertainty not just from the measurement errors but from the mutual solubility effect as well. Advantages and Limitations of Current Method. The current method has several advantages. First, the simultaneousness and self-consistency are two major advantages. Another advantage of the method is its compatibility with multiple quantification techniques including, but not limited to, HPLC, LC/MS, GC/MS, and GC-FID. This versatility makes the method useful in a variety of laboratory situations. In addition, this method is useful in determining multiple coefficients for volatile and semivolatile compounds because it allows one to work with smaller volumes of compound than would be needed in individual measurement methods and could potentially be used to quantify partition properties of multiple compounds simultaneously. One may argue that the three phase equilibrium method ultimately depends on the ability to accurately and quantitatively measure concentrations of the analytes which may be limited by the size of the equilibrium vessel. For example, two, 100 mL airtight syringes were used to construct the vessel for the three phase equilibrium in the current study. However, this should not be viewed as a limitation of the current method. On the basis of the idea shown in Figure 1, a similar setup can be built with much larger gastight syringes or gas sample bags for an accurate quantification of compounds with lower water solubilities and/or vapor pressures. The three phase equilibrium method is a direct measurement method that requires a check of equilibrium status among

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 989-496-5961. Fax: 989-496-5956. E-mail: shihe.xu@ dowcorning.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Donald Mackay, Mr. Gary Kozerski, and two anonymous reviewers for their valuable comments on the draft of this manuscript, and Mr. Christopher Susynski for drawing the illustration shown in Figure 1.

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REFERENCES

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Analytical Chemistry

Article

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