Adsorption behavior of surfactants and mass transfer in single-drop extraction

Three surfactants-SDS, Triton X-100, and DTMAC-were used to study effects of surfactants on the mass transfer in liquid-liquid extraction. Single-drop extraction apparatus was used to obtain the mass-transfer coefficient of acetic acid from dispersed CCl4 droplets to the continuous water phase. The excess mass-transfer resistance was related to the adsorption behavior of surfactants on the interface. All of these surfactants significantly reduced the mass-transfer rate. Mass-transfer coefficient K-R, decreases rapidly with increasing surfactant concentration and reaches a minimum at concentrations less than 0.04 mM. The effectiveness of the surfactants in decreasing the mass-transfer rate increases in the order: DTMAC < SDS = Triton X-100. This effectiveness has no direct relation with the ionic types of surfactants, but is affected by the adsorption rate and adsorbed surface concentration on the interface. A slight increase in the mass-transfer coefficient was found at higher concentrations of Triton X-100 due to its higher adsorption kinetics. The excess mass-transfer resistance exerted by the surfactant mainly resulted from the obstruction of the adsorbed barrier layer. The contribution of the inhibition of interfacial flow to the mass-transfer resistance was considered to be minor.