Environ. Sci. Technol. 1994, 28, 2345-2352
Hydrolysis of Oligodimethylsiloxane-a,u-diols and the Position of Hydrolytic Equilibrium Jay Spivack' and Steven B. Dorn General Electric Corporate Research and Development, Schenectady, New York 1230 1-0008
The hydrolysis of tetramethyldisiloxane-1,3-dioland hexamethyltrisiloxane-l,5-diolin aqueous solutions has been studied. The position of equilibrium of the system including these compounds, dimethylsilanediol, and water has been determined. Concentrations of these compounds in dilute aqueous solutions were determined by coupling HPLC to ICP analysis for Si and also by extraction into ethyl acetate followed by triethylsilylation and GC analysis. It was found that the siloxanediols hydrolyze to the equilibrium mixture at environmentally significant rates and that dimethylsilanediol dominates the equilibrium in dilute aqueous solution, even at concentrations orders of magnitude above that expected in the environment. The hydrolysis of tetramethyldisiloxane-1,3-diolin water was found to be first order in [H+l and in [phosphate buffer] by studying the rates at pH 3 and 6. The hydrolysis of a mixture of higher oligodimethylsiloxane-a,w-diolsas a suspension in water is also described. The first observation of dimethylsilanediol in an environmental sample is reported.
Introduction Domestic consumption of siliconefluids in 1991has been estimated at 155-160 million lbs (1). The bulk of this material is poly(dimethylsi1oxane) (PDMS; Figure 1; 11, 111, and IV). Linear polymers and cyclics are used in many consumer products and are released into the environment through both municipal wastewater treatment facilities and direct evaporation into the air. PDMS has very low water solubility. The water solubilities of trimethylsilyl capped oligomers from the dimer to the pentamer (11, n = 0-3) have been carefully measured recently (2). Solubilities of the linear oligomers drop from 930 ppb for the dimer to only 0.070 ppb for the linear pentamer. Since the log of the solubilities (nM) were found to decrease linearly with increasing molecular weight, the authors conclude that there is "an essential absence of any ecologically significant water solubility for the conventional higher molecular weight PDMS of commerce". Because PDMS has low aqueous solubility, the bulk of this material leaving wastewater treatment facilities is in the sludge. Most of the sludge is land-filled or incinerated; the remainder is applied to soils as fertilizer. Incineration converts PDMS to carbon dioxide, water, and silica. The relatively small amount of PDMS that leaves the treatment facilities in the water would have to be attached to suspended particulates and would be deposited in sediments downstream of the facility. The environmental fate of PDMS deposited in soils and sediments is not yet well understood. Laboratory studies of linear PDMS polymers have demonstrated both hydrolytic depolymerization and cyclization on relatively dry clays (3). More recent studies ~
* E - m a i l address:
[email protected].
0013-938X/94/0928-2345$04.50/0
0 1994 American Chemical Society
I1
I11
IV Figure 1. Dimethylsilanediol and examples of simple poly(dimethy1siloxanes) (PDMS): I , dimethylsilanediol(the monomer diol); I I , a linear end-capped PDMS; I1 I , an oligodimethylsiloxane-a,w-diol; I V , a cyclic PDMS (D4 is illustrated).
indicate that hydrolytic depolymerization of high MW end-capped PDMS polymers to low MW oligodimethylsiloxane-a,w-diols takes place readily on soils containing 2-3 % moisture ( 4 , 5 ) . This soil-catalyzed process can be so rapid that, on an EPA standard soil matrix with 2% moisture, hydrophobic high polymers are partially converted to water-soluble oligodimethylsiloxane-a,w-diols with a degree of polymerization less than 8, in just days (5). Dimethylsilanediol (the monomer diol) was found to be the major component of aqueous extracts of PDMSamended soil aged for 6 months or more (5). To understand the fate of PDMS on soils, a better knowledge of the position of hydrolytic equilibria in the oligodimethylsiloxanediolseries is required. Although the position of equilibrium between hexamethyldisiloxane and trimethylsilanol has been reported, the measurements were made in methylene chloride (6) and are not directly relevant to aqueous environmental compartments. It is interesting, however, to note that if the reported equilibrium constant was applicable to aqueous solution, it predicts that at concentrations below about 4000 ppm this equilibrium would be dominated by the silanol. We would expect the equilibrium constant in water to favor the silanol to a greater extent than this due to stabilization of the silanol by solvation. In order to determine what role the further hydrolysis of the oligodimethylsiloxane-a,w-diolsformed from PDMS on soils might play in the environmental fate of PDMS, we have studied and report here the position of aqueous hydrolytic equilibrium among dimethylsilanediol (I, the monomer diol), 1,1,3,3-tetramethyldisiloxane1,&diol (111, m = 0, the dimer diol), and 1,1,3,3,5,5-hexamethyltrisiloxane-1,5-diol (111, m = 1, the trimer diol); the rates of hydrolysis of the dimer diol under environmentally relevant conditions; and the effects of exposing a mixture Environ. Sci. Technol., Vol. 28, No. 13, 1994
2345
containing higher oligodimethylsiloxane-cr,w-diolsto water. We also report what we believe to be the first observation of dimethylsilanediol in an environmental sample. Experimental Section
Analytical Instruments. NMR spectra were taken on a 500-MHz Omega from GE Instruments. Mass spectra were taken on a JEOL SX-102. Chemical ionization was done with a reagent gas consisting of 5% ammonia in methane with the source temperature at 230 "C. The ammonia used was a 1:l mixture of 14NH3:15NH3,thus labeling the M.NH,+ ions which appear as M + 18/M + 19 doublets (7). GC-FID analysis was done on a Hewlett Packard 5890 with autoinjector using a 30 m x 0.25 mm capillary column with a 0.25-pm film of DB1 from J&W Scientific. The samples were injected at an oven temperature of 50 "C, and after 5 min, the temperature was raised at 10 "C/min to 280 "C. This temperature was held for as long as 40 min when analyzing triethylsilylated higher oligomers. The injector was at 290 "C, and the detector was at 310 "C. Materials. Dimethylsilanediol, 1,1,3,3-tetramethyldisiloxane-1,3-diol,and 1,1,3,3,5,5-hexamethyltrisiloxane1,5-diolwere prepared by hydrolysis of the dichlorides in the presence of a stoichiometric amount of triethylamine, as previously reported (8),and were generously provided by John Carpenter and James Cella. 2-(Trimethylsily1)ethanol was obtained from Aldrich Chemical Co., Milwaukee, WI. Mininert Teflon valve closures for serum bottles were obtained from Dynatech Precision Sampling Corp., Baton Rouge, LA. Observation of Hydrolytic Equilibration by Proton NMR. Monomer Diol, at 1.0 mg/mL (10.8 mM), and/or dimer diol, at 0.9 mg/mL (5.4 mM), was (were) dissolved in D20 with and without the addition of 1 pL/mL tertbutanol serving as the internal standard. Solutions including 1 pL/mL DzSO4 or 52 mM neutral phosphate buffer were also prepared. A 20X buffer concentrate was made by dissolving 11.3 g of KzHPO4 (0.65 m) and 5.4 g of NaH2P0qH20(0.39 m) in 100 g of DzO. The measured pH of the buffered solutions in DzO was 7.15. Equilibrium Measurements by ExtractiodDerivatization. In order to extract the siloxanediols from aqueous solution into ethyl acetate, it is first necessary to bring the solution to near pH 7. If this is not done and the solution is far from neutral, the condensation of the siloxanediols to higher oligomer diols in the ethyl acetate layer can be fast enough to affect the measured equilibrium constant. Therefore, when the siloxanediols were brought to equilibrium in 0.5 pL/mL aqueous sulfuric acid, 10 mL of this solution was neutralized by the addition of 1 mL of 1 M pH 7.2 phosphate buffer. The resulting solution was then extracted with 2 mL of ethyl acetate followed by derivatization of 200 pL of the ethyl acetate extract with 1 mL of triethylsilyl chloride-tri-n-butylamine-tetrahydrofuran (4:6:5 by volume). The tetrahydrofuran was added to clarify the derivatized mixtures. The resulting derivatized extracts were analyzed by GC-FID. Over time, the amine salt caused some degradation of the stationary phase in the GC column and gave rise to small background peaks for the derivatives of the siloxanediols. We were able to correct for such background peaks by running blanks (derivatized extracts of buffer in water) frequently. 2346
Environ. Sci. Technol., Vol. 28, No. 13, 1994
Table 1. Ions Observed in Chemical Ionization Mass Spectra of Triethylsilylated Oligodimethylsiloxane-a,w-diolsUsing *4NH3/16NH3in Reagent Gas
MW of oligo- Et&i mer deiiv 1
2 3 4 5 6 7 8 9 10
320 394 468 542 616 690 764 838 912 986
observed ions (relative abundances) rM+ - CoH,l* M+M.NHdta MH+ NH; CZH6 I-
3381339 925) 4121413 (100) 486/487 (100) 560/561 (100) 634/635 (100) 708/709 (100) 782/783 (100) 8561857 (100) 930/931 (96)b 100411005(85)c
321 (100) 395 (47) 469 (15) 543 (5) 617 (2) 691 (2) 765 (1) 839 (3) 913 (5)
3081309 (18) 3821383 (11) 456/457 (4) 5301531 (2) 604/605 (2) 678/679 (1) 7521753 (2) 826/827 (2) 9001901 (5)
ndd
nd
291 (23) 365 (23) 439 (18) 513 (10) 587 (4) 661 (4) 735 (2) 809 (3) 883 (5) 957 (1)
Relative abundance given for higher mass ion. Ion at;932 (100). Ion at 1004 has higher abundance (87). Not detected a
The analysis was standardized by extracting a series of freshly prepared aqueous solutions of monomer diol, dimer diol, and trimer diol as well as blanks and derivatizing them all in the same manner. Under neutral conditions, very little hydrolysis or oligomerization takes place during the extraction or derivatization. Typically, derivative peaks corresponding to only a few ppm of dimer diol and trimer diol were observed when a 1000 ppm monomer diol standard was analyzed. No derivative of the monomer diol was detected in dimer diol and trimer diol standards. The ethyl acetate used contained the linear end-capped octamer of PDMS (MDcM, octadecamethyloctasiloxane) as the internal standard. The monomer diol is not readily extracted (partition coefficient for ethyl acetate-water is 0.22) but gave a highly reproducible and linear response as a function of aqueous concentration. Because there is a background peak from the derivatizing agent very near the peak due to the monomer diol derivative (the two are not base-line resolvable),we found that our standardization for the monomer diol was statistically better when we used GC peak height ratios than when we attempted to use peak area ratios. Over the range from 1.22 to 40.32 mM aqueous monomer diol, the slope of the standard curve was determined to f 4 % within 95 % confidence limits. The use of triethylsilylation allowed measurement of monomer diol in the same derivatizing reagent as the dimer diol and trimer diol. The statistics of the dimer diol and trimer diol standardization curves (usingpeak area ratios) were even better than those for the monomer diol with the slopes determined to f 2 % . The identities of the derivatized siloxanediols were confirmed by GC-MS. When utilizing electron ionization, the spectra were dominated by a strong M+ - CzH5 peak. The monomer diol derivative, ( C ~ H E , ) ~ S ~ O S ~ ( C H ~ ) ~ O S ~ (CzH5)3, (MW 320) gave a strong peak at 291 (100) and a much weaker signal at 305 (4, M+ - CH3). The dimer diol derivative (MW 394) gave ions at 365 (98) and 379 (6) and the trimer diol (MW 468) at 439 (100) and 453 (11). When utilizing chemical ionization with 14NH3/15NH3in the reagent gas, molecular ions complexed with ammonia (M.NH4+) were obtained as well as MH+, [M+ CzHJ.NH3, and M+ - CzH5. These observed ions for the first 10 oligomer diol derivatives are listed in Table 1. Trimethylsilylationwith BSTFA failed for the monomer diol because the product, octamethyltrisiloxane, could not be separated from the many contaminants in the deriva-
Table 2, Measured pH, Added NaCl, and Final Calculated Ionic Strengths for Solutions Used in Kinetic Studies of Dimer Diol Hydrolysis pHO
total POa (M)
added NaCl (M)
ionic strength (M)
2.95 2.88 2.94 2.97 5.92 5.85 5.88
0.2 0.15 0.1 0.05 0.2 0.15 0.1
0.042 0.087 0.131 0.176 0 0.055 0.110
0.216 0.209 0.218 0.220 0.220 0.218 0.219
a
Measured at final equilibrium.
tizing agent. BSTFA was excellent for derivatization of all other siloxanediolswith tetrakis(trimethylsi1oxy)silane (MdQ) used as internal standard. Hexamethyldisilazane did not give reliable results. HPLC-ICP Analysis of Dimer Diol Hydrolysis. Dimer diol (100 ppm) was dissolved in pH 3 and 6 phosphate buffers at buffer concentrations ranging from 0.050 to 0.20 M. The solutions all contained, as the internal standard, 73 ppm of 2-(trimethylsilyl)ethanol, which was confirmed to be stable under the conditions of the hydrolysis by GC analysis. The pH of these solutions was found to change very little over the course of the reaction. The ionic strength was calculated for each solution using published pK,’s for phosphoric acid (9). Sufficient 1 M NaCl was added to each solution to bring the total ionic strength up to 0.22 M, equal to that of the highest ionic strength buffer (0.20 M phosphate, pH 6). Measured final pH, added NaC1, and calculated final ionic strengths for these solutions are listed in Table 2. The solutions were all prepared in polypropylene vials and placed in the HPLC autoinjector in polypropylene vials. This was necessary because we observed that the background rate of hydrolysis of the dimer diol varied greatly in different glass vessels and concluded that catalysis by glass surfaces could not be ignored. Each vial was used for a single injection. Samples were analyzed by HPLC-ICP (10) using an Instruments SA JY24S inductively coupled argon plasma optical emission spectrometer coupled to a HewlettPackard (HP) 1090M high-performance liquid chromatograph. The ICP torch assembly position and gas supply flows were adjusted to provide optimum sensitivity for silicon at 251.611-nm emission wavelength. This optimization was made, in particular, with the goal of producing a plasma stable over several days of continuous operation. Typical parameters were as follows: forward rf power, 1250 W; observation height, 15 mm above load coil; nebulizer gas pressure, 2.4 bar; plasma gas flow, 19 L min-l; sheath gas flow, 0.4 L min-l; auxiliary gas flow, 0.6 L min-1. The ICP employed the standard Scott double-pass glass spray chamber supplied with the instrument and a Meinhard SB-50-A1low dead volume pneumatic nebulizer. Fluid coupling of the instruments was through 1/16-in. 0.d. X 0.005-in. i.d. stainless steel and 1/16-in. 0.d. Teflonsheathed 0.l-mm i.d. aluminum-clad fused silica capillaries (Polysil SGE, Austin TX). Additional components included a Rheodyne Model 7030 flow switching valve used to divert column effluent to waste and direct flow from a peristaltic pump to the nebulizer. This was used to rapidly maximize and verify emission wavelength maxima.
The ICP was operated in a “diagnostics” mode, monitoring only the signal for Si, with the monochromator entrance and exit slits adjusted to obtain a fairly broad peak. Wavelength stability was satisfactory over a period of at least two weeks without readjustment. Amplified photomultiplier signal was taken from a BNC output test connector on the ICP data interface board, which was connected to the H P data system via an HP Model 35900 interface. Chromatography was performed using a YMC polymer Cle column (number MPC18-112) with a cartridge precolumn of the same material. In these experiments, the column was maintained at 40 “C with a flow of 0.2 mL min-l acetonitrile-water (35:65 v/v). Injections of 25 pL were made for standards and samples. Hydrolysis of Higher Oligomer Diols. A mixture of oligodimethylsiloxane-a,o-diolsranging from the dimer diol to the pentadecamer diol was obtained from GE Silicones, Waterford, NY. A Si analysis was performed by sodium peroxide fusion in a Parr bomb, dissolution in 35 % perchloric acid, dehydration, filtration, ignition, and weighing and gave 36.33 f 0.21% Si. This is very near the calculated % Si for the hexamer diol (36.40%), which is the dominant oligomer by GC analysis. This mixture was vigorously stirred at a concentration of 10 mg/mL with deionized water. After 1 h or so, the silicone phase had broken up into very small droplets, producing a light milky suspension. This was continuously stirred for months. Samples were occasionallytaken and analyzed by GC after extraction and derivatization as described above for equilibrated monomer diol solutions. In order to study acid-catalyzed hydrolysis, 30-mg samples of the mixed oligodimethylsiloxanediolswere placed in Mininert sealed serum bottles along with 10 mL of 0.5 pL/mL aqueous sulfuric acid and shaken. At each time point a bottle was sacrificed and analyzed by GC after extraction and derivatization. At the time point, 1 mL of 1 M pH 7.2 phosphate buffer was added to a bottle to slow both hydrolysis and condensation reactions, and the bottle was placed in a -20 “C freezer. When it was time to analyze the bottles, they were thawed, 2 mL of ethyl acetate containing an internal standard (MDsM for triethylsilylation or M4Q for trimethylsilylation) was added, the layers were shaken for 10 min, and 200 pL of the upper layer was derivatized and analyzed by GC. Results and Discussion When monomer diol, dimer diol, or trimer diol was dissolved in water at concentrations in the range of 10003000 ppm, a slow convergence to an equilibrium position dominated by the monomer diol was observed. At lower concentrations, the equilibrium concentration of the trimer diol was below measurable levels (1ppm). Equilibration of the monomer diol and dimer diol took several days in glass NMR tubes and was observed by lH NMR in DzO. NMR Analysis of Aqueous Equilibrium. Both monomer diol and dimer diol produced singlets in ‘HNMR spectra in DzO. These signals were separated by A6 = 0.005 with the dimer diol downfield of the monomer diol. In spectra in which the solvent peak due to HDO was set at 6 4.8, monomer diol absorbs at 6 0.167 and dimer diol absorbs at 6 0.173. When tert-butanol was used as an internalstandard, the monomer diol signal was 1.086ppm upfield and the dimer diol was 1.081 ppm upfield of the Environ. Sci. Technol., Vol. 28, No. 13, 1994
2347
FH3
Kl
FH,
I I I
I
1
Flgure 3. Hydrolysis reactions for which the equilibrium constants K, and K2 are defined.
Analysis of Aqueous Equilibrium by GC Analysis of Derivatized Extracts. A second method of analysis I
I
I
,
, .
,
,
,
I
,
I
I B
1
-
'024
'
'o.>i
'
'ob0
r
II
0'18 0'16 PPm
i
oi4
0112
Lo ' i
Flgure 2. 'H NMR spectrum of a mixture of about 0.5 mg/mL monomer diol and 0.5 mg/mL dimer diol in D20taken within hours of preparation (A) and after 12 days (B). The peak at 6 0.167 is due to the monomer diol and that at 6 0.173 is due to the dimer diol. Reference is HDO set at 6 4.8.
tert-butanol signal. At 500 MHz these signals are clearly distinguished, although not resolved to the base line (Figure 2). When monomer diol, dimer diol, or both were dissolved at a concentration of 10.8 mM Si in D20, with or without the addition of either 1pL/mL D2S04 or 52 mM neutral phosphate buffer, the spectra changed over time to one dominated by the upfield signal (monomer diol) with a definite peak also at the downfield position of the dimer diol (Figure 2). The total area under these two peaks, relative to the internalstandard, did not change with time. Thus, no appreciable formation of any other product, in particular water-insolublepolymer, apparently takes place. This equilibration was complete in less than 1h if catalyzed by 1pL/mL of sulfuric acid, took several days in D20, and was even slower with the addition of neutral buffer. Attempts to determine the equilibrium constant in D2O by measuring the separate areas of the two singlets were hampered by the inability to resolve them completely. The measurements we did make indicated that the equilibrium constant for hydrolysis of the dimer diol, K1, as defined in Figure 3, was in the range of 3.2 X 10-3-6.6 X Although this indicates that equilibrium will favor the monomer diol in dilute solutions in H20, it does not tell us to what extent: the equilibrium constants in H20 will not, in general, be the same as in D2O (11, 12). 2348
Environ. Sci. Technol., Vol. 28, No. 13, 1994
used was extraction of neutralized solutions with ethyl acetate followed by triethylsilylation. Monomer diol could be quantified down to about 50 ppm, while dimer diol and trimer diol were easily measured down to 1 ppm. To attain equilibrium rapidly, a solution of a siloxanediol in water was acidified with 0.5 pL/mL sulfuric acid. This was allowed to stand for at least 1 day. At the time of analysis, the solution was neutralized with 100 mM phosphate buffer and extracted, and the extract was derivatized. At initial Si concentrations above about 20 mM (1800ppm monomer diol), small amounts of the cyclic D4 (IV) were formed. This is a total aqueous silanol concentration far higher than any expected in environmental waters. At lower concentrations only monomer diol, dimer diol, and trimer diol were detected. The data that were obtained by this technique are in Table 3. The reactions for which the equilibrium constants are defined are shown in Figure 3. Thus K, =
[monomer diol] [dimer diol] X [H20]
and K2 =
[monomer diol] X [dimer diol] [trimer diol] X [H201
It is of interest to note that the two equilibrium constants are nearly equal and that, therefore, the ratios [monomer diol]/ [dimer diol] and [dimer diol]/ [trimer diol] are nearly equal in any particular aqueous solution. These ratios vary from about 18:l in the most concentrated solution studied here to about 38:l in the most dilute. The data here demonstrate that at a concentration of total siloxanols of 2600 ppm or below, in water, the equilibrium mixture is dominated by monomer diol. In fact, the equilibrium constants alone would predict that dimethylsilanediol would predominate in water solution at much higher concentrations than this. The factor limiting the maximum concentration at which dimethylsilanediol will predominate in equilibrium mixtures with water is the poor water solubility of the higher oligomer diols. If one starts with too high a concentration of dimethylsilanediol in water, then the low water solubility of the higher oligomer diols leads to their separation as a second phase, which would eventually come into equilibrium with the aqueous phase. Since the water concentration in the oil phase would be low the equilibrium concentration of dimethylsilanediol in that phase would be low. Hence, at higher initial concentrations of dimethylsilanediol, one cannot predict the concentration of dimethylsilanediol in equilibrated aqueous mixtures without further study.
Table 3. Initial and Final Concentrations of Siloxanediol Oligomers Equilibrated i n Ob wL/mL Aqueaus Sulfuric Acid and Equilibrium Constants initial conditions oligomer diol mM monomer diol monomer diol dimer diol dimer diol trimer dial
monomer diol (mM)
28.8 14.4 12.0 6.02 6.94
final conditions dimer-diol (mM) trimer-diol (mM)
25.4 13.8 21.7 11.8 19.0
1.38 0.41 1.07 0.29 0.75
0.0775 0.0140 0.0540 0.0086 0.0317
DIa
9% recb
K1( x l W
K2(xlW)
0.212 nda
101 101 101 103 99
8.4 8.4 7.9 0.7 8.7
8.1
0.068
nd nd
7.3
7.7 7.2 8.1
average 101 0.4 SD 1 0.3 D, probably not a t equilibrium, see text. Percent of initial Si accounted for in prcducis. a nd = not detected.
7.7 0.4
6.6
c
E 6 -
-c
5.8 5.6
A
7
0
1000
2000
3000 4000 minutes
5000
6000
0
. ~~
-,-~-~---~~---~-+~-
0
0.05
! A0.20M PO4 00.15M PO4 0 O.lOM P 0 4 ! -~ Flgure 4. Kinetics of hydrolysis of tetramethyldisiioxanc1,~ioiat pH 6 in phosphate buffers at various concentrations. Approximately 600 pM dimer diol was dissolved in buffer containin0 sufficient NaCl to maintain ionic strength near 0.22 M in all cases. Analysis was by
HPLC-ICP wlth 2-(trimethylsilyl)noi
as ihe internal standard.
However, from an environmental point of view, the behavior of the oligodimethyl-siloxanediolsystem at and below the concentrations described in this study is important. Measurements of PDMS levels in sludgeamended soils have recently been made, and most were found to be in the range of 0.6-24 ppm, with one at 318 ppm (13). Thus as soils go through periodic dry cycles in which catalysisof the hydrolysis of PDMS is most efficient, dimethylsilanediol and higher oligomers will be formed. During periodic wet cycles, there will he a tendency to dissolve the lower oligomers in water and drive the hydrolysis toward dimethylsilanediol. The factors determiningwhether and when equilibrium is attained will he the presence of water and the kinetics of hydrolysis. Kinetics of Dimer Diol Hydrolysis. The kinetics of hydrolysis of the dimer diol were studied by HPLC-ICP at p H s near 3 and 6 with a constant ionic strength of 0.22 M. The datanear pH6 atphosphate buffer concentrations of 0.20,0.15,and 0.10 Mare shown in Figure 4 as plots of lnfdimer diol] vs time. It is clear that, at each buffer concentration,the hydrolysis of dimer diol is pseudo-firstorder in dimer diol. There is a linear dependence of the pseudo-first-order rate constant on buffer concentration as shown in Figure 5. Thus, the reaction is first order in total phosphate as well as in dimer diol. We find near pH 6, with total phosphate, [PO41 (in M), and the pseudofirst-order rate constant (in min-') the following:
,
~
0.1 [P041, M
0.15
0.2
Flgure 5. Dependence of ihe pseudo-lkst-arder rate constants for the hydrolysis of tetrameihyldisiioxanc1,~ioi at pH 6 on ihe buffer concentration. Rate constams are ihe slopes of the regression lines in Figure 4.
65 6
a
H
555
B
e 5 al E45 ?2 5 4 35
3 -__. 0
50
100
150
200
250
minutes A 0 20M PO4
0 15M PO4 0 0 10M PO4 X 0 05M PO4
Flgure E. Kinetics of hydrolysis of teVamethyldisiloxanb1,~ioiat pH 3 in phosphate buffers at various concentrations. Approximately 600 pM dimer diol was dissolved in buffer containing sufficient NaCl to maintain ionic strength near 0.22 M in all cases. Analysis was by HPLC-ICP with 2-(trimethylsilyi)ethanol as the internal standard.
pseudo-first-order rate constant = (5.46X lo4) X [PO,]
+ (1.48 X lo4)
This can be interpreted as catalysis by some or all components of the phosphate system, a case of general acid catalysis, and a rate constant of 1.48 X 10-5 min-' from catalysis by 1.3 X 1 P M H+ a t the average pH of 5.88. Near pH 3,the rate constants are independent of [PO4] from 0.20 to 0.10 M and slightly lower at 0.050 M. The data are shown as Figure 6. The rate constants are 1.41 Envirm. Sci. Temnol.. Voi. 28.
NO. 13. I994 2549
~
~
D4
2
1
3
4
6
5
7
8
9 1 0 1 1 1 2 1 3
Oligomer Fyun 7. Effect on individual oligomeru,udiol concentrations when 3 mglmL of an ollgomer diol mixture was stlned wlth 0.5 &lmL aqwoua
sulfuric ac!d The bar heights represent the relative GC peak heights of ttw triethylsiiylated oligomer diols after neutralization, extraction into ethyl acetate, and derivatization at each time point Therefore,the WncBnlTBtion of the monomer diol is greatty underestimated by the bar heights (see Figure 8) X 10-*,1.45 X 10-2,l.UX lW, and 1.23 X l@Zmin-'with the average pH at 2.94. This constancycan he interpreted as meaning that catalysisby H+ dominates near pH 3, and the effect of increasing [POJ is therefore negligible. The rate constantnear pH 3 is very close to IOOOX the estimated rate constant at zero [PO41 near pH 6. Thus the reaction appears to he first order in H+ as well, and the data fit a simple rate law of the form:
rate = -
a[dimer diol] -
at ~~
(k,[PO,l
+ k,[H+I)
X
[dimer diol]
where k, = 5.5 X 10-1 M-I min-' and k~ = 12 M-' min-'. The environmentally important observations are the fnst-order dependenceon [H+landthe existence ofgeneral acid catalysis. This latter observation implies that a great many environmental catalysts of this hydrolysis may exist. Hydrolysis of Higher Oligomer Diols. Having determined that the dimer diol and trimer diol spontaneously hydrolyze to monomer diol in dilute aqueous solution and knowing that a mixture of higher a,*-siloxanediols are produced from PDMS polymer by soil catalysis (5), we decided that knowledge of the behavior of such higher oligomer diols in aqueous suspension would help us to understand theenvironmental fate of PDMS in soils. Such a study was necessary because these higher oligomer diols are much less water soluble than the trimer diol, and a mixture of these higher oligomer diols and water would produce atwo-phasesystem. This two-phase system would 2550 Envhn. Sd. Tschnol.. Vd. 28. No. 13. 1994
be much more characteristicofthe environmentalsituation than the onephase aqueoussystem described above. Since polymerizationintheoilphaseisexpected (I4),therelative importance of hydrolysis required experimental investigation. We chose a commercial mixture of oligodimethyln,o-siloxanediols with a degree of polymerization from 2 to 15 with the hexamer as the major constituent. This mixture was chosen because a wet THF extract of PDMSamended and aged soil has been reported to contain a series of oligomer diols up to the heptamer (5). We stirred the mixture with water and observed the changes in oligomer diol distribution by ethyl acetate extraction, derivatizingwith eithertriethylsilylchloride/tributylamine or BSTFA and analyzing the derivatized extracts by GC. The reaction with water was observed with and without catalysis by mineral acid. Inoneseries of experiments, ahout 30mgofthe oligomer diol mixture and 10 mL of 0.5 p L / d aqueous sulfuric acid were placed in each of several Mminert sealed serum bottles. The bottles were shaken for hours, and each was sacrificed a t a time point and analyzed by GC after extraction and derivatization. The results of such an experiment are illustrated in Figure 7. The oligomer diol mixture used is dominated hy the hexamer diol and trimer diol, with the nonamer diol representing a local maximum in the distribution as well, and there is no monomer diol initially present. As timeproceeds,theconcentrationsof alloligomerdiolsfrom the trimer diolup to the decamer dioldecriase. Thedimer diol concentration rises and then falls. The monomer diol
- - ; - . - 1520 2 0) -
15
E
i7j 10
5
0
Hours
'
!+ ronomer diol dimer diol I-; trimer diol i
1~
D4
i t
~
sum m+d+t Flgure 8. Quantitative data for the concentrations of monomer diol, dimer diol, trimer diol, D4,and the sum of monomer diol 4- dimer diol trimer diol for the same experiment as described in Figure 7. The concentrations are expressed as mole percent of total Si.
+
appears within the first hour and continues to rise in concentration. The concentration of cyclic D4 rises and apparently levels off or decreases. The final concentration of D4 is attained by 6 h, which is the time point at which the tetramer-diol has disappeared. It is thus possible that D4 is primarily produced by cyclization of the tetramerdiol. The higher oligomer diols, the dodecamer-diol and tridecamer-diol, increase in concentration. Because the monomer diol does not extract well from water into the ethyl acetate phase, its relative concentration is much higher than it appears to be in Figure 7. We were able to quantify the concentrations of monomer diol, dimer diol, trimer diol, and D4 in these mixtures (Figure 8). The Si content of the oligomer diol mixture was measured, and we could therefore calculate these concentrations in units of Si mole percent. The monomer diol increases from zero to 23 % of the total Si by 119 h. The sum of monomer diol + dimer diol + trimer diol is also represented in Figure 8. Note that this remains constant through the first hour, during which most of the trimer diol hydrolyzes. Consequently, monomer diol production during the first hour is attributable to hydrolysis of trimer diol and dimer diol. Beyond that first hour, however, this sum continuously increases (becoming almost entirely monomer diol by 119 h). Thus most of the monomer diol produced must be attributed to the hydrolysis of oligomer diols higher than the trimer diol. In other experiments, the extracts were derivatized with BSTFA. The more volatile trimethylsilylated derivatives
made observation of higher oligomer diols easier. This confirmed that oligomer diols from the dodecamer diol and higher increase in concentration, with oligomer diols as high as the nonadecamer diol appearing. Also the concentration of cyclics D4, D5, and D6 increased (no higher cyclic oligomers were observed). The presence of acid catalysis is not required to observe changes similar to these. When approximately 10 mg/mL of the oligomer diol mixture was stirred with deionized water for 81 days, monomer diol was produced at the expense of higher oligomer diols. The monomer diol rose to nearly 16 mM (about 13% of all Si) by day 81. The hydrolysis of dimer diol and trimer diol alone account for 7 mM monomer diol, so the remaining 9 mM must arise from hydrolysis of higher oligomer diols. It is apparent that hydrolysis, condensation to higher oligomer diols, and cyclization compete in this mixture. Hydrolysis is very important, with monomer diol yields as high as 7 2 mol % observed (at a total oligomer diol concentration of 500 ppm shaken for 11weeks in 0.5 pL/ mL aqueous sulfuric acid). It is probable that the relative importance of these three paths will vary with the siloxanol to water ratio, pH, and availability and nature of active catalytic surfaces in environmental situations. This study was done at oligodimethylsiloxanediol concentrations much higher than expected in the environment and in a closed system. The presence of much more water, the high water solubilities of the monomer diol and the lower oligomer diols, and the presence of flowing water which would remove the water-soluble products as they are formed should all conspire to push this system toward monomer diol in periodically wet soil environments. Furthermore, since a high polymer on soil containing 2 % moisture has been observed to hydrolyze to oligomer diols similar to those studied here (51, it does not seem reasonable to expect those same oligomer diols to repolymerize, on that same soil, unless the system suffered nearly complete loss of water. Dimethylsilanediol in an Environmental Sample. The results above predict the production of dimethylsilanediol from PDMS in environmental situations where water and hydrolytic catalysts are present along with the PDMS. If the degradation rate of dimethylsilanediol in some such environments is less than its rate of production, then it may accumulate to levels high enough for us to detect. We therefore began to look for dimethylsilanediol in environmental samples. A local municipal wastewater treatment plant uses a process for biodegradation of municipal waste including aerobic and anaerobic biodegradation followed by composting of the anaerobic sludge in combination with wood chips and wood ash. Samples of the influent, effluent, aerobic sludge, anaerobic sludge (product sent to composter), fresh compost, and compost stored outdoors for about 4 months were gathered and analyzed for monomer diol, dimer diol, and trimer diol by HPLC-ICP. None of these were detected in the aqueous phases of the influent, effluent, aerobic sludge, or anaerobic sludge (detection limit 1ppm). Aqueous extracts of the composts, however, apparently contained dimethylsilanediol as evidenced by the appearance of a Si-containing peak at the expected residence time. To confirm the identity of this material, an aqueous extract of the 4-month-old compost was neutralized with 100 mM phosphate buffer and extracted five times with ethyl acetate; the extract was concentrated Environ. Sci. Technol., Vol. 28, No. 13, 1994
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under vacuum, derivatized by triethylsilylation, and analyzed by GC-MS. This gave a peak at the same residence time as the derivative of authentic dimethylsilanediol with a parent ion at mlz 291 (100, M+ - CzH5) and a weaker signal at mlz 305 (5, M+ - CH3. On the basis of dry weight, the 4-month-old compost contained 46 ppm dimethylsilanediol. GPC-ICP data on THF extracts of the 4-month-old compost gave 408 ppm total PDMS. Therefore, dimethylsilanediol represents 9 mol % of the total PDMS. Fresh compost from the same plant was found to contain 338 ppm total PDMS and 22 ppm (5 mol % ) dimethylsilanediol. Although hydrolysis of such hydrophobic material as PDMS in a composting process may be surprising, we believe it reflects the fact that this hydrolysis can be catalyzed by a wide variety of materials. We have observed catalysis of siloxanediol hydrolysis by acid, phosphate buffer, and glass surfaces and of PDMS hydrolysis by acid, base, soil, and composted cattle manure. Composted cattle manure (7% water, pH 6.25) was spiked to 912 ppm (by dry weight) with 350 centistoke PDMS fluid. The manure was aged for 5 weeks at room temperature in a capped jar. A 2-g sample of the manure was extracted with 5.0 mL of water by shaking for 10 min. The aqueous extract was analyzed by HPLC-ICP, and the concentration of dimethylsilanediol in the sample was 40 ppm. Thus 9 mol 96 of the applied PDMS was hydrolyzed to dimethylsilanediol. A similar experiment with the manure moistened to 50 % water yielded no detected hydrolysis of the PDMS in 5 weeks. If composted material can catalyze this reaction at room temperature in a nearly dry state, it is somewhat less surprising to find the reaction catalyzed in a composting process where the temperature is 55-60 "C, even though the water content in the compost reactor is much higher (60% ) and the catalytic activity is thereby reduced. Conclusion
Previous work has shown that poly(dimethy1siloxanes) hydrolyze on soils to give low molecular weight oligodimethylsiloxane-a,w-diols. What we have now shown is that these oligomer diols hydrolyze to the monomer diol, that equilibrium lies far toward monomer diol at the low concentrations observed in the environment, and that hydrolysis is subject to both specific and general acid catalysis. Hydrolysis to the monomer diol has been observed by proton NMR studies of dimer diol and monomer diol in DzO, by GC analyses of derivatized extracts of equilibrated aqueous solutions and of water/ oligodimethylsiloxanediolsuspensions, and by HPLC-ICP
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Environ. Sci. Technol., Vol. 28, No. 13, 1994
analyses of dimer diol solutions in water. The position of hydrolytic equilibrium was measured by GC analyses of derivatized extracts of equilibrated aqueous solutions. The kinetics of dimer diol hydrolysis were examined at pH 3 and 6 and varying concentrations of phosphate buffer by HPLC-ICP. Our work on the environmental fate of PDMS continues, with an emphasis on the biological and chemical fate of dimethylsilanediol. Acknowledgments
We wish to thank Paul Donahue for NMR spectra, Denise Anderson for helping with HPLC-ICP analyses, Hans Grade for GC-MS analyses, John Carpenter for the work with cattle manure, and Richard Allen,James Cella, Eileen Skelly Frame, and Terry Leib for many helpful discussions. Literature Cited
(1) ChemicalEconomicsHandbook, SRI International: Menlo Park, CA, 1994. (2) Varaprath, S.; Frye, C. L.; Hamelink, J. L. X X V I I Organosilicon Symposium; Rensselaer Polytechnic Institute: Troy, NY, 1994. (3) Buch, R. R.; Ingebrigston, D. N. Environ. Sci. Technol. 1979,13,676-679. (4) Lehman, R. G.; Varaprath, S.;Frye, C. L. Enuiron. Toxicol. Chem. 1994,13 (71, 1061-1064. (5) Cella, J. A.; Carpenter, J. C.; Dorn, S. B. Enuiron. Sci. Technol., submitted for publication. (6) Wilczek, L.; Chojnowski, J. Makromol. Chem. 1983, 184, 17-90. (7) Ligon, W. V., Jr.; Grade, H. J. Am. SOC.Mass Spectrom. 1994, 5 , 596-598. ( 8 ) Cella, J. A.; Carpenter, J. C. J. Organomet. Chem.,in press. (9) Weast, R. C. CRC Handbook of Chemistry and Physics, 64th ed.; CRC: Boca Raton, FL, 1983; p D-169. (10) Dorn, S.B.; Skelly Frame, E.M. Speciation and Quantitation of Silicones in Environmental Samples: From Silanols to Polysiloxanes. Analyst, in press. (11) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd ed.; Harper & Row: New York, 1987; pp 241-244. (12) Schowen, R. L. Prog. Phys. Org. Chem. 1972, 9, 275. (13) Pilot Monitoring Program for Silicones in theEnvironrnent, Draft Report; Silicones Environmental Health and Safety Council: Washington, DC, 1994. (14) Saam, J. C.; Huebner, D. J. J. Polym. Sci.: Polym. Chem. Educ. 1982,20,3351-3368.
Received for review March 21, 1994. Revised manuscript received July 21, 1994. Accepted August 15, 1994." Abstract published in Advance ACS Abstracts,September 15, 1994. @
