Environmental chemistry of organosiloxanes

An unprecedented gas-phase condensation reaction mediated by a bissilylated chloronium ion intermediate

Gas phase Cl₂Si(OH)⁺ ions undergo an unusual condensation reaction with SiCl₄ to yield Cl₃SiOSiCl₂⁺ ions that by progressive hydrolysis or solvolysis give rise to a large variety of siloxane type moieties.

Decamethylcyclopentasiloxane (D5) environmental sources, fate, transport, and routes of exposure

The environmental sources, fate, transport, and routes of exposure of decamethylcyclopentasiloxane (D5; CAS no. 541-02-6) are reviewed in the present study, with the objective of contributing to effective risk evaluation and assessment of this and related substances. The present review, which is part of a series of studies discussing aspects of an effective risk evaluation and assessment, was prompted in part by the findings of a Board of Review undertaken to comment on a decision by Environment Canada made in 2008 to subject D5 to regulation as a toxic substance. The present review focuses on the early stages of the assessment process and how information on D5's physical-chemical properties, uses, and fate in the environment can be integrated to give a quantitative description of fate and exposure that is consistent with available monitoring data. Emphasis is placed on long-range atmospheric transport and fate in water bodies receiving effluents from wastewater treatment plants (along with associated sediments) and soils receiving biosolids. The resulting exposure estimates form the basis for assessments of the resulting risk presented in other studies in this series. Recommendations are made for developing an improved process by which D5 and related substances can be evaluated effectively for risk to humans and the environment.

Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study

The ubiquitous presence of cyclic volatile methylsiloxanes (cVMS) in the global atmosphere has recently raised environmental concern. In order to assess the persistence and long-range transport potential of cVMS, their second-order rate constants (k) for reactions with hydroxyl radical ((•)OH) in the gas phase are needed. We experimentally and theoretically investigated the kinetics and mechanism of (•)OH oxidation of a series of cVMS, hexamethylcyclotrisiloxane (D3), octamethycyclotetrasiloxane (D4), and decamethycyclopentasiloxane (D5). Experimentally, we measured k values for D3, D4, and D5 with (•)OH in a gas-phase reaction chamber. The Arrhenius activation energies for these reactions in the temperature range from 313 to 353 K were small (-2.92 to 0.79 kcal·mol(-1)), indicating a weak temperature dependence. We also calculated the thermodynamic and kinetic behaviors for reactions at the M06-2X/6-311++G**//M06-2X/6-31+G** level of theory over a wider temperature range of 238-358 K that encompasses temperatures in the troposphere. The calculated Arrhenius activation energies range from -2.71 to -1.64 kcal·mol(-1), also exhibiting weak temperature dependence. The measured k values were approximately an order of magnitude higher than the theoretical values but have the same trend with increasing size of the siloxane ring. The calculated energy barriers for H-atom abstraction at different positions were similar, which provides theoretical support for extrapolating k for other cyclic siloxanes from the number of abstractable hydrogens.

Bioconcentration and Aquatic Toxicity of Superhydrophobic Chemicals: A Modeling Case Study of Cyclic Volatile Methyl Siloxanes

Many chemicals in commerce are classified as "superhydrophobic", having log octanol-water partition coefficients (log KOW) approaching or exceeding 7. Examples include long-chain alkanes, halogenated aromatics, and cyclic volatile methylsiloxanes (cVMS). We show that superhydrophobic chemicals present unique assessment challenges because of their sparing solubility in water and difficulties in empirical determinations of bioconcentration factors (BCFs) and aquatic toxicity. Using cVMS as an example, BCFs are considerably lower than expected due to biotransformation. Reviewed aquatic toxicity test data for cVMS in a range of aquatic organisms show little or no toxic effects up to solubility limits in water and sediment. Explanations for this apparent lack of toxicity of cVMS, and by extension to other superhydrophobic chemicals, are explored using a conventional one-compartment uptake model to simulate bioconcentration and toxicity tests using an assumed baseline narcotic critical body residue (CBR) and a range of organism sizes. Because of the low aqueous concentrations, equilibration times are very long and BCFs are sensitive to even very slow rates of biotransformation. Most organisms fail to achieve the assumed CBR during feasible test durations even at the solubility limit. Regulatory evaluation of superhydrophobic substances requires specially designed test protocols addressing biotransformation and dietary uptake.

Effects of 'inactive' ingredients on bees

Honey bees are sensitive to widespread co-formulants used in agrochemicals, and evaluation of the role of these 'inerts or inactives' in pollinator decline is only in its formative stages. Lack of disclosure of formulation ingredients in major products and lack of adequate methods for their analysis constrain the assessment of total chemical load and agrochemical exposures on bees. Most studies to document pesticide effects on honey bees are performed without the formulation or other relevant spray adjuvant components used to environmentally apply the toxicant. Formulations are generally more toxic than active ingredients, particularly fungicides, by up to 26,000-fold based on published literature. Some 'inactive' candidates for future risk assessment for pollinators include the organosilicone surfactants and the co-solvent N-methyl-2-pyrrolidone.