Criteria for distinguishing heterogeneous from homogeneous conditions for aromatic diisocyanates in water†

Environmental fate and toxicity testing typically requires knowledge of the water solubility of the test substances. Determining the solubility of aromatic diisocyanates in water poses great challenges because of their hydrophobic nature and water-reactivity. The reactive dissolution process is dynamic and the establishment of a steady-state equilibrium cannot readily be observed. In preparation of experimental work, computer simulation was used to derive and evaluate criteria that enable distinguishing homogeneous (i.e., substances would be fully dissolved in water) from heterogeneous (i.e., a separate organic phase would be present) conditions. The simulation utilized available kinetic information and models representing the main physical and chemical processes taking place. It was found that the transition to heterogeneous conditions (i.e., the exceedance of the solubility limit with increasing loading) can be identified by observing either a rapid decline in ultimate yield of the diamine hydrolysis product from near-stoichiometric to much lower values, or a decrease in rate of formation of the diamine hydrolysis product relative to its ultimate yield. The latter criterion is expected to be the more powerful indicator. These criteria can be used in future work to define and interpret an experimental program for determining solubility limits for aromatic diisocyanates or other poorly-soluble, water-reactive substances.