Unexpected concentration dependence of the mass accommodation coefficient of water on aqueous triethylene glycol droplets.

The mass accommodation coefficient α M of water on aqueous triethylene glycol droplets was determined for water mole fractions in the range x mol = 0.1-0.93 and temperatures between 21 and 26 °C from modulated Mie scattering measurement on single optically-trapped droplets in combination with a kinetic multilayer model. α M reaches minimum values around 0.005 at a critical water concentration of x mol = 0.38, and increases with decreasing water content to a value of ≈0.1 for almost pure triethylene glycol droplets, essentially independent of the temperature. Above x mol = 0.38, α M first increases with increasing water content and then stabilises at a value of ≈0.1 at the lowest temperatures, while at the highest temperature its value remains around 0.005. We analysed the unexpected concentration and temperature dependence with a previously proposed two-step model for mass accommodation which provides concentration and temperature-dependent activation enthalpies and entropies. We suggest that the unexpected minimum in α M at intermediate water concentrations might arise from a more or less saturated hydrogen-bond network that forms at the droplet surface.