Binding of Alkali and Alkaline-Earth Cations to Monolayers of a Noncyclic Ionophore

A new type of ionophore monolayer was prepared from 3,7-dioxaazelaamide derivatives: N,N′-dimethyl-N,N′-dioctadecyl- (la), N,N′-dimethyl-N,N′-dioctadecyl- (1b), and N,N,N′,N′-tetraoctadecyl-3,7-dioxaazelaamide (1c). Their monolayer properties have been studied by the surface pressure-area isotherm measurement, fluorescence microscopy, and surface reflection spectroscopy. By use of an anionic dye, 8-hydroxy-l,3,6-pyrenetrisulfonate ion, as counterion for metal cation, the ion binding characteristics of these ionophore monolayers were determined from reflection spectrum intensities. These monolayers bind Ca²⁺ ion effectively, and their affinities to metal cations depend on their chemical structures, being in the order of 1b > 1a > 1c. These different affinities can be interpreted in terms of the flexibility of the binding sites, which are regulated by alignment of long alkyl chains in monolayers.