Adsorption and ion-pairing interactions of phospholipids in the system of two immiscible electrolyte solutions. Part II: The formation and behaviour of a lecithin layer at the water/1,2-dichloroethane interface in the presence of multivalent anions in the aqueous phase

Abstract

The present paper investigates voltammetrically the properties of the layers formed, whereas Part I of this series studied the electrochemical formation of l-α-lecithin (dipalmitoyl, DPPC) adsorbed layers at the water/1,2-dichloroethane interfaces. These are primarily determined by ion pairing between the positively charged layer and the anions present on the aqueous side of the interface. Cerium(IV) sulfate has been used as the source of the model anion and a five-step mechanism proposed for the formation and behaviour of DPPC layers. For the sake of comparison, the same experiments have been carried out with trimethyloctadecylammonium cation (TODA+) which have helped in elucidation of certain parts of the mechanism. In the absence of anions of higher charges, both DPPC and TODA+ form monolayers at the interface and the process involves simple free diffusion transport of the layer components through the bulk of the solution. In their presence, both DPPC and TODA+ form multilayers, the transport of the components is slower and is given by lateral diffusion along the layer surface and diffusion through the layer. The passage of ions across the interface is virtually not hindered by the monolayers, unless the ions are very large. However, DPPC and TODA+ multilayers strongly slow down or even prevent passage of larger ions (trimetylammonium (TMA+), PF6- and K+ ions assisted by a crown compound were used in the study).