Partitioning based determinations of water solubility for highly hydrophobic liquid chemicals

Water solubility is a fundamental parameter for the environmental risk assessment of organic chemicals. It is experimentally and analytically challenging to determine the aqueous solubility for highly hydrophobic liquid chemicals, because they are prone to the formation of micro-droplets and emulsions in the saturated water. Two partitioning based methods were therefore recently developed for saturating water while avoiding direct contact with the hydrophobic liquid test substances. The first method is based on first saturating a silicone rod with the test substance and then using the silicone rod for saturating the water. The second method is based on saturating water from the liquid phase chemical via the headspace. Equilibrium kinetics of the headspace dosing method was determined in order to explore its applicability domain. Headspace passive dosing of 5 chemicals with log Kow of 4-6 and Kaw of 0.8-200 reached 95% of equilibrium within a few hours. Headspace passive dosing was very precise and well-behaving for several of the test compounds, but challenges remained for chemicals with a log Kow >6 that are most prone to the formation of micro-droplets, emulsions and also other potential artefacts. A third passive dosing format has thus been developed and will be tested aiming for increased robustness of solubility determinations for highly hydrophobic liquid chemicals of logKow > 6. Finally, the solubility of several model compounds will be determined in pure water and five other media such as algae toxicity test medium, lake water and seawater.