Permeation of xylene isomers through supported liquid membranes containing cyclodextrins

The permeation of p- and m-xylenes through supported liquid membranes (SLM) containing cyclodextrins (CDs) was investigated in this study. Three CDs were used in the experiments, alpha-CD, beta-CD, and hydroxypropyl-beta-CD (HP-beta-CD). The mass transfer mechanisms with and without facilitating carriers (CDs) were studied individually by applying the solution-diffusion model. The facilitated mass transfer mechanism was elucidated as partitioning of xylenes from the organic phase into an aqueous phase, formation of an inclusion complex with CDs, diffusion of the complex, and extraction into the receiving phase. The formation constants in aqueous solutions for CDs and p-xylene were 1.6-2.4 times higher than that of CDs and m-xylene. The diffusivity coefficient of the xylene-CD complex (D-m) and the equilibrium extraction constant (K-ex) associated with each CD were determined. Meanwhile. K-ex, the parameter used to describe the ability of CD to extract xylene and to form a complex, of p-xylene and CDs was almost twice as much as that for in-xylene and CDs. Thus, the addition of CD into the membrane phase for xylene separation yields a two-fold benefit in favor of p-xylene: an increase in selectivity and an enhancement of the mass transfer flux.