Understanding the dynamic surface tension of solutions containing volatile organic compounds (VOCs)

Measurement of time-dependent surface tension, i.e. dynamic surface tension, has been found useful for monitoring rates of the removal of volatile organic compounds (VOCs) in environmental/chemical engineering applications. However, the mechanism of the dynamic surface tension of solutions containing VOCs has not been fully understood. This paper demonstrates that the conventional models based on diffusional mass transfer across the interface cannot be used to explain the measured surface tension data. The effect of the evaporation of VOC molecules at the interface must be included in the overall rate analysis. The evaporation kinetics can be modeled by Statistical Rate Theory (SRT). The combined diffusion–evaporation model can then predict the surface mass transfer and surface composition, and further explain dynamic surface tension. The new model is compared with the experiment using tetrachlorethylene (PCE), a typical VOC, dissolved in water; an agreement is found for the first 1000 s of the experiment. The model fails to predict dynamic surface tension for longer time ranges; this is because the diffusion portion of the new model is only a short time approximation and is not applicable for later stages of the experiment.