Determination of Henry's Law Constant Using Diffusion in Air and Water Boundary Layers

A simple and precise method for determining the Henry’s law constant for hydrophobic organic chemicals was developed. Henry’s law constants were obtained as air–water partition coefficients in a dilute solution from the two partition coefficients between polydimethylsiloxane (PDMS) and air (KPDMSa) and between PDMS and water (KPDMSw). Measurements of the low concentrations in air or water at phase equilibrium were avoided. Instead, the KPDMSa and KPDMSw values were obtained by measuring the mass transfer rate constants in a boundary layer and relating them with KPDMSa and KPDMSw using a film diffusion model. Twenty hydrophobic chemicals (11 polycyclic aromatic hydrocarbons, 5 chlorinated benzenes, 2 phthalate esters, and 2 aromatic nitro musks) with literature values of the Henry’s law constant ranging from 10–2 to 102 Pa·m3·mol–1 were chosen to evaluate the method. The Henry’s law constants derived in this study agreed very well with the experimentally determined values from the literature. Because the proposed method provides a fast and simple method for measuring the Henry’s law constant within three days, it is a very promising method for generating the Henry’s law constants widely used in the assessment of the environmental fate of hydrophobic chemicals.