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Analysis of organosilicone surfactants and their degradation products

Abstract
In this work the analysis of the organosilicone surfactant Silwet L-77 and components thereof is described. The commercial product Silwet L-77 was demonstrated to comprise of ∼70% M₂D-C₃-O-(EO)ₙ-CH₃ (1),* ∼5% M₂D-C₃-O- (EO)ₙ-H (2) and ∼25% polar constituents. M₂D-C₃-O-(EO)ₙ-CH₃ oligomers were obtained by reversed-phase chromatography of Silwet L-77, and by synthetic procedures. Pure oligomers (n = 3, 6 and 9) were synthesised by reaction of the corresponding allyl-capped oligoethoxylate monomethyl ether M₂Dᴴ over a Pt catalyst. The allyl-capped ethoxylate monomethyl ethers were synthesised by reaction of allyl chloride with the corresponding ethoxylate monomethyl ethers (n = 3, 6 and 9). The longer chained oligoethylene glycols (n = 6 and 9) were prepared by etherification of smaller oligomers. Preparation of the organosilicone compounds M₂D-C₃-O-EO-COCH₃ and M₂D-C₃-OH were also investigated. Atmospheric pressure ionisation mass spectrometry (API/MS) is demonstrated to be a valid and informative method for the characterisation and quantitation of the organosilicone surfactants. Consistently reproducible quantitative results were achieved using an online HPLC separation system. Atmospheric pressure chemical ionisation (APcI) was used to enable high sample throughput, and the resulting HPLC/APcI/MS data obtained were improved both in reproducibility and sensitivity over conventional HPLC methods. The uptake mechanisms of agrochemical formulations and the role that surfactants play in this process are not well understood. The quantitative API/MS method developed and the use of synthesised Silwet L-77 components (1) enabled investigation of Silwet L-77 uptake into plant foliage (Chenopodium album). The influence of Silwet L-77 and constituents thereof on herbicide uptake was addressed, and dosage, spread, surfactant structure, solvation and solubility were implicated as important for uptake enhancement. Aspects of Silwet L-77 degradation were also addressed, for which API/MS methods were shown to be valid and informative. The well-established instability of Silwet L-77 to acid and alkaline conditions was characterised in more detail, with products, mechanisms and relative rates presented. Certain limitations of the API/MS method (i.e. product overlap, analyte suppression) required the use of additional methods to supplement the results obtained (LC/MS, GC/MS and NMR). The range of products formed in the degradation of Silwet L-77 were numerous as a result of the ability of the siloxane moiety to rearrange into a variety of lengths and structures, and compounded by the large number of components comprising the Silwet L-77 formulation. Unequivocal assignment of degradation product structure was thus complicated. Mass recovery following degradation was reduced in all experiments indicating the loss of volatile siloxane compounds. This was confirmed by headspace analysis. An increase in overall water-solubility for the degradation products over the parent surfactant was also observed. Rates and products appeared to be dependent on the conditions used, and were especially influenced by the solution pH. The use of fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) enabled the tentative assignment of four major products in the water-soluble fraction of degraded Silwet L-77. These were CH₃O(EO)ₙH (20), [1-SiCH₃+H] (21), cyclic tetramer [28; R¹, R², R³, R⁴ = C₃-O-(EO)ₙ-CH₃] and linear dimer [24; R¹, R² = TMS; R³, R⁴ = (EO)ₙ-CH₃]. Analysis of the degradation of purified M₂D-C₃-O-(EO)ₙ-Me (ave. n ∼7.5) samples by electrospray mass spectrometry (ESI/MS), FTICR/MS and HPLC/ESI/MS indicated CH₃O(EO)ₙH (20), [1-SiCH₃+H] (21) and [1- 2SiCH₃+2H] (22) as common degradation products. The linear dimer 24, [R¹ = H, R² = TMS; R³, R⁴ = (EO)ₙ-CH₃] was also indicated. The results also demonstrated that the HO(EO)ₙH, M₂D-C₃-O-(EO)n-H (2) and (Mᴿ)₂D-O-(EO)ₙ-CH₂CH= CH₂ (9, 10, 11; R = H or CH₃) compounds observed in the Silwet L-77 degradation mixture were synthetic by-products rather than degradation products. The degradation of single M₂D-C₃-O-(EO)ₙ-R oligomers confirmed the CH₃O(EO)ₙH (20), [1-SiCH₃+H] (21) and [1-2SiCH₃+2H] (22) as degradation products, and also indicated a range of linear and cyclic products both with and without the EO chains and terminal TMS groups intact. In general, the more silylated analogues of a structural series partitioned preferentially into the heptane-soluble fraction, and higher EO content derivatives were more commonly observed in the water-soluble fraction. Typically the products formed followed that for expected silanol stabilities, with longer EO chain products more stable as low condensation polymers and silanols, and short EO chains more commonly observed in cyclic products. The GC/MS analysis of the heptane-soluble fraction of acid-degraded Silwet L-77 enabled the tentative assignment of the cyclic trimer, 27 [R¹, R², R³ = -C₃- OH] product. ¹H and ¹³C NMR data also indicated this structure. ¹H NMR of the water-soluble fraction showed proton integration values indicative of the [1- 2SiCH₃+2H] (22) structure. After 2 years, the terminal Mᴿ groups were no longer detectable and extensive condensation had occurred, as determined by ²⁹Si NMR. The results demonstrated the rapid degradation of Silwet L-77 and derivatives thereof under extremes of pH and the highly complex nature of the products which are formed. API/MS was a valid and informative method for the study of the products formed, especially the high resolution FTICR/MS technique. Investigation into the stability of Silwet L-77 solutions on typical soil substrates indicated that there is little chance of surfactant persistence in aqueous soil environments. Reduced recovery was considered to be a result of degradation and/or strong sorption processes. Losses were most significant on substrates exhibiting extreme supernatant pH values (montmorillonite, halloysite). Reduced recoveries were also higher with higher clay content. Studies on clays indicated that supernatant pH, potential for intercalation and surface charges are important factors in the recovery process. In the case of the montmorillonite and illite clays, recoveries may be more significantly affected by sorption, as strong surfactant substrate interactions were observed immediately following application. The results obtained support previous observations that Silwet L-77 is relatively benign in the natural environment. Primary degradation is rapid and ultimate degradation to naturally occurring compounds i.e. CO₂, H₂O and Si(OH)₄, can be predicted according to the structures of the observed intermediates. However covalent bonding of degradation products to substrates was observed which may result in some environmental accumulation. In land applications this could cause aggregation of silicate minerals and/or affect the sorption capacity of the soil. An increase in sorption capacity could have positive or detrimental effects. The concentrations required for biotoxicity for Silwet L-77 and degradation products thereof are in large excess of any likely input by agricultural practices. Phytotoxicity was also low and this appeared to be a result of the dilution effects caused by high spreading. *M₂D- [Si(CH₃)₃-O]₂-Si(CH₃)- ; C₃ = (CH₂)₃; EO = CG₂CH₂O
Type
Thesis
Type of thesis
Series
Citation
Bonnington, L. S. (2000). Analysis of organosilicone surfactants and their degradation products (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14481
Date
2000
Publisher
The University of Waikato
Rights
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