Transport of Alkali Halides through a Liquid Organic Membrane Containing a Ditopic Salt-Binding Receptor

A ditopic receptor is shown to have an impressive ability to recognize and extract the ion pairs of various alkali
halides into organic solution. X-ray diffraction analysis indicates that the salts are bound in the solid state as
contact ion pairs. Transport experiments, using a supported liquid membrane and high salt concentration in the
source phase, show that the ditopic receptor can transport alkali halide salts up to 10-fold faster than a monotopic
cation or anion receptor and 2-fold faster than a binary mixture of cation and anion receptors. All transport systems
exhibit the same qualitative order of ion selectivity; that is, for a constant anion, the cation selectivity order is K+
> Na+ > Li+, and for a constant cation, the anion transport selectivity order is I- > Br- > Cl-. The data suggest
that with a ditopic receptor, the polarity of the receptor−salt complex can be lowered if the salt is bound as an
associated ion pair, which leads to a faster diffusion through the membrane and a higher maximal flux.