Mechanism of rate enhancement in the extraction of Zn(II) by di(2-ethylhexyl)phosphoric acid in the presence of sodium lauryl sulphate micelles

The effect of a sodium lauryl sulphate (NaLS) micellar phase on the rate of Zn(II) extraction from a sulphate medium by a 0.01 M solution of di(2-ethyl-hexyl)phosphoric acid (DEHPA) in dodecane was investigated in a constant area stirred cell. The concentration of NaLS was varied between 5.1×10−5 and 7.7×10−3 M. Below the critical micellar concentration (CMC) the presence of the surfactant decreased the extraction flux, whereas substantial flux enhancements were observed above the CMC reaching an enhancement ratio of 10 at a concentration of 4.6×10−3 M. Solubilization of DEHPA in the Zn-free micellar aqueous phase at different concentrations of NaLS was investigated using Fourier transform infra-red spectroscopy. Solubility increased with NaLS concentration, reaching a maximum at the highest NaLS used, rendering a calculated concentration of DEHPA in the aqueous phase of 0.8×10−3 M. As DEHPA is practically insoluble in water it was concluded that DEHPA molecules were incorporated in the micellar interface, thus producing a substantial enlargement of the reaction site for the interfacial Zn complexation reaction, together with a reaction rate enhancement due to micellar catalysis. It is proposed that the main mechanism of flux enhancement is the additional production of Zn complex at the micellar interface, coupled with a high rate of complex transfer to the organic phase due to micellar interaction with the system interface. The effect of the negative electrical charge at both the interface and the micellar interface is also discussed.