Surfactants effects on mass transfer during drop-formation and drop falling stages

Three types of surfactants were used to study their effects on the inhibition of mass transfer in liquid-liquid extraction. A single-drop extraction apparatus was used to obtain the extraction percentage of acetic acid from dispersed CCl₄ droplets to the aqueous phase. By using various lengths of extraction column, the extraction amounts were estimated during the drop-formation and drop-falling stages. Effects of these surfactants on the mass transfer during various stages were related to the adsorption behavior of these surfactants. The results of dynamic and equilibrium tensions show that the transport of Triton X-100 is a diffusion-controlled mechanism, which shows a fast adsorption kinetic at elevated concentrations. By comparing the effectiveness of the three surfactants in decreasing interfacial tension and in mass-transfer inhibition, the mass-transfer inhibition during the drop-formation and drop-falling stages can be related, respectively, to the properties of dynamic and equilibrium interfacial tensions. For surfactant concentrations below CMC, a surfactant that is most effective in decreasing the interfacial tension always exhibits the highest effectiveness in mass-transfer inhibition. When the surfactant concentration is small (about 10^-3 mM), the mass-transfer inhibition of SDS and DTMAC is significant in the drop-formation stage, but not in the drop-falling stage. As the concentration is increased, the effectiveness of Triton X-100 increases more quickly than the others in both the formation and falling stages. The mass-transfer rate increased slightly when the Triton X-100 concentration is elevated near CMC, due to the surface remobilization caused by the fast adsorption-desorption rate of Triton X-100 at elevated concentrations. The mass-transfer inhibition of surfactant also was caused mainly by the hydrodynamic effect in the drop-formation stage, but from the obstruction of the barrier layer in the drop-falling stage.