Comparing the Performance of Computational Estimation Methods for Physicochemical Properties of Dimethylsiloxanes and Selected Siloxanols

Siloxanes are used in high amounts in a wide range of applications. There is evidence that siloxanes may be persistent and bioaccumulative in the environment. Despite their high production volumes and their wide applications, only few empirical data on their physicochemical properties are available. For a selection of linear and cyclic siloxanes and siloxanols, we computed (subcooled) liquid vapor pressure, (subcooled) liquid water solubility, and the partition coefficients among air, water, and octanol with the property estimation methods, EPI Suite, SPARC, and COSMOtherm and compared the results to experimental data from the literature. The estimates obtained with the three methods generally show decent qualitative and limited quantitative agreement in the estimated trends of property data for the homologous series of linear and cyclic dimethylsiloxane oligomers. The number of measured property data available in the literature is small, but the data show considerable scatter, and their ranges exceed the differences among the three estimation methods in several cases. A strong impact of molecular configurations and conformations on substance properties could be demonstrated by means of COSMOtherm. In a case study we illustrate that these effects are large enough to greatly influence the environmental fate of the transformation products of octamethylcyclotetrasiloxane (D₄)