UCLA

Surface active agents depress gas transfer at gas-liquid interfaces. They are present as measurable trace contaminants at all environmental and at most industrial interfaces. An experimental apparatus to concurrently measure dynamic surface tension and mass transfer was constructed and tested for single-bubble and multi-bubble experiments. In this work, the parameters describing time-dependent bubble surface contamination were characterized. The application of a Ward-Tordai transient model and of a Langmuir saturation model showed that for fine-bubbles in low molecular weight surfactant solutions the interfacial surfactant accumulation equilibrates before bubble detachment. This is reflected in the bubbles behaving as solid-spheres, which is shown in our dimensionless results. For a given contamination, interfaces with higher renewal rates have higher mass transfer. At higher renewal rates, the variation due to different contamination is smaller than the variation at lower renewal rates, concluding that higher interfacial flow regimes can offset contamination. Our experimental evidence shows a gas transfer reduction of 30-70% of pure water values in surfactant solutions, which confirms full-scale field measurements. Results are consistent with expectations and correct previous Frossling-like dimensionless correlations for pure water systems. Our results offer a tool for mass transfer prediction from flow regime and surfactant properties. (c) 2006 Elsevier Ltd. All rights reserved.