On the prediction of evaporation rates : with special emphasis on aqueous solutions

Abstract The performance of a theoretically based model for the prediction of evaporation rates of liquid mixtures is investigated with special emphasis on aqueous solutions. The model is an extension of a previously suggested evaporation model for pure volatile organic compounds. Liquid phase nonideality is corrected for by use of activity coefficients. A comparison with experimental data on evaporation of various liquid mixtures shows that the inclusion of activity coefficients is often mandatory for occupational hygiene applications. The potential risk of exposure associated with hydrophobic compounds in minute concentrations in water is demonstrated. Activity coefficients for a wide range of mixtures 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 should be applied if available. For liquid mixtures, measured and predicted evaporation rates were in satisfactory agreement, indicating that liquid phase diffusional resistance can be neglected also in strongly non-ideal aqueous solutions. It was found, however, that the presence of water-soluble polymers in aqueous solutions suppresses the evaporation of volatile organic compounds, even at low concentrations, owing to liquid phase diffusional resistance and reduced activity. Liquid phase diffusional resistance due to polymers was found to increase with liquid viscosity and volatility of the dilute organic compound. This effect cannot be predicted by the evaporation model.