Effects of liquid VOC concentration and salt content on partitioning equilibrium of hydrophilic VOC at air–sweat interface

Volatile organic compounds (VOCs) must initially be absorbed by sweat on the surface of skin for human VOC dermal exposure. The partitioning equilibrium at the air–sweat interface is given by , where pc is the partitioning coefficient, and is the gaseous concentration in equilibrium with the aqueous VOC concentration (CL) at a constant water temperature (Tw). A series of thermodynamic functions of are presented, as well as the values of pc, and the heat of gaseous–liquid phase transfer (ΔHtr) for tested VOCs, including iso-propanol (IPA, CL=12–120 mg L−1) and methyl ethyl ketone (MEK, CL=10–80 mg L−1) to determine the effects of liquid VOC concentration and salt contents of sweat on pc of hydrophilic VOCs. Experimental data reveal that the pc values of IPA and MEK drop as the liquid VOC concentrations increasing from 10 to 120 mg L−1. However, sodium salt content in human sweat (sodium chloride and sodium lactate) induces the effect of salt, indicating the increase in pc. Notably, neither urea nor ammonia in human sweat increase pc. Artificial sweat, consisting of sodium chloride 0.47%, urea 0.05%, ammonia 0.004% and sodium lactate 0.6%, was used to evaluate the increase in the pc values of IPA and MEK. The liquid VOC concentration effect simultaneously develops together with the salt effect on the partition at the interface of air–sweat for hydrophilic VOC solutions. The pc values of IPA for artificial sweat decrease as much as 32.5% as CL increases from 12 to 120 mg L−1 at 300 K, and those of MEK drop by as much as 70.9% as CL increases from 10 to 80 mg L−1 at 300 K. This investigation provides a basis for elucidating the assessment of human dermal exposure to hydrophilic VOCs.