Effect of surfactants on liquid side mass transfer coefficients: A new insight

Specific experiments are proposed to investigate the effect of surfactants on liquid side mass transfer coefficients. They are based on the determination of the liquid side mass transfer coefficient k(L) at a free gas-liquid interface, under controlled temperature and hydrodynamic conditions. Firstly, the methodology is validated in water at various rotation speeds and temperatures. In a second time, it is applied in aqueous and pure solutions of anionic surfactants: a decrease of kL with an increase of surfactant concentrations is then observed until leveling off when the CMC is reached. Deduced from experimental results. the equivalent diffusion coefficients describe an identical behavior. These results demonstrate that the lowest kL are directly linked to the presence Of surfactants at the gas-liquid interface which makes the diffusion coefficients of oxygen be reduced. At last, a comparison is performed with the data of Painmanakul et al. and Sardeing et al. [P. Painmanakul, K. Loubiere, G. Hebrard, M. Mietton-Peuchot, M. Roustan. Effect of surfactants on liquid-side mass transfer coefficients, Chemical Engineering Science 60 (2005) 6480-6491; R. Sardeing, R Painmanakul, G. Hebrard, Effect of surfactants on liquid-side mass transfer coefficients in gas-liquid systems: a first step to modelling, Chemical Engineering Science 61 (2006) 6249-6260] obtained from a chain of bubbles having diameters above to 3.5 mm. A quasi-linear relation between the kL issued from both hydrodynamic configurations is revealed in the whole range of surfactant concentrations. Such findings would prove that, in both cases, the impact of surfactants on liquid side mass transfer coefficient is correlated with the changes in the diffusion coefficients of oxygen. (C) 2008 Elsevier B.V. All rights reserved.