ON THE PREDICTION OF EVAPORATION RATES - WITH SPECIAL EMPHASIS ON AQUEOUS-SOLUTIONS

The performance of a theoretically based model for the prediction of e vaporation rates of liquid mixtures is investigated with special empha sis on aqueous solutions. The model is an extension of a previously su ggested evaporation model for pure volatile organic compounds. Liquid phase non-ideality is corrected for by use of activity coefficients. A comparison with experimental data on evaporation of various liquid mi xtures shows that the inclusion of activity coefficients is often mand atory for occupational hygiene applications. The potential risk of exp osure associated with hydrophobic compounds in minute concentrations i n water is demonstrated. Activity coefficients for a wide range of mix tures can be estimated accurately using the group-contribution method UNIFAC. For certain strongly non-ideal aqueous mixtures UNIFAC is less accurate and experimental infinite dilution activity coefficients sho uld be applied if available. For liquid mixtures, measured and predict ed evaporation rates were in satisfactory agreement, indicating that l iquid phase diffusional resistance can be neglected also in strongly n on-ideal aqueous solutions. It was found, however, that the presence o f water-soluble polymers in aqueous solutions suppresses the evaporati on of volatile organic compounds, even at low concentrations, owing to liquid phase diffusional resistance and reduced activity. Liquid phas e diffusional resistance due to polymers was found to increase with li quid viscosity and volatility of the dilute organic compound. This eff ect cannot be predicted by the evaporation model.