Surfactant mixtures at the oil-water interface

We report the structural study of mixed monolayers of partially deuterated N,N'-di-hexadecyl-(d(33))-4,13-diaza-18-crown-6 ether (d-ACE16) and palmitic acid (PA) at the oil-water interface, in order to understand the mechanism of metal ion transport through Permeation Liquid Membrane (PLM) devices. The composition of the mixed monolayers remains constant with increasing spread amount and the saturation of the interface is achieved at a relatively low spread amount. The excess PA material is accommodated in the oil phase, playing an important role in equilibrating the interfacial concentration of ACE-16. The presence of PA increases the surface concentration of ACE-16 at low spread amount and facilitates its dissolution into the oil phase at the high spread amount. The result suggests a dynamic exchange between the bulk phase and the interface ensuring a continuous turnover which reflects their relevance in PLM devices. The conclusions regarding the role of a fatty acid in regulating the surface concentration of the alkylated azacrown ether and its dominant role in the bulk transport of metal ions through the membrane are consistent with the results of macroscopic studies reported earlier. (C) 2013 Elsevier Inc. All rights reserved.