1388-2481/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII S1388-2481 ( 99 ) 00025-9 Electrochemistry Communications 1 (1999) 139–144 Effect of self-assembled surfactant structures on ion transport across the liquidNliquid interface Jorg Strutwolf a , Jose A. Manzanares b, *, David E. Williams a ¨ ´ a Department of Chemistry, University College London, 20 Gordon Street, London WC1 0AJ, UK b Department of Thermodynamics, University of Valencia, E-46100 Burjassot, Spain Received 8 March 1999; received in revised form 26 March 1999; accepted 26 March 1999 Abstract In this paper, the effect of a coadsorbed polyanion–cationic surfactant system on the transport of tetraethylammonium ion across the waterN1,2-dichloroethane interface is studied. It is shown that the change in double-layer structure due to the presence of adsorbed or coadsorbed surfactant can explain the experimental observations, thus concluding that no other effects on ion transfer (e.g., steric hindrance) are relevant under these experimental conditions. The implications of these results are discussed. q 1999 Elsevier Science S.A. All rights reserved. Keywords: ITIES; Ion transfer; Self-assembled monolayers; Surfactants; Double-layer effects 1. Introduction Phospholipid monolayers adsorbed at the interface between two immiscible electrolyte solutions (ITIES) have been widely used to investigate the behaviour of biological membranes [1], especially to study charge transfer processes [2–4]. In fact, this system models only half of a biological membrane, and it has the inconvenience that the organic solvent would tend to disrupt the packing of the adsorbed layer. However, it has the advantage that the interfacial poten- tial can be controlled which opens the possibility of modi- fying the rate or stereochemistry of organic reactions. Early work [1] showed some inhibition of ion transfer across the waterN1,2-dichloroethane (DCE) interface modi- fied with a coadsorbed lecithin–cholesterol layer, the effect being maximal when the lecithin:cholesterol ratio was 2:1, the same ratio as found in natural membranes [5]. The result was interpreted by consideration of the energetics of pore formation in a closed-packed monolayer. Kakiuchi et al. have demonstrated that the rate of ion transfer across phosphati- dylserine [6] and phosphatidylcholine [7] can be enhanced or reduced depending on the state (liquid-expanded or liquid- condensed) of the monolayer. More recent work [8] has demonstrated an apparent contradiction: enhancement of ion transport rate as a consequence of lipid adsorption at the * Corresponding author. Tel.: q34-963-983-119; fax: q34-963-983-385; e-mail: manzanar@uv.es ITIES. The interpretation is that the effect simply represents the change in double-layer potential induced by a charged adsorbent [9]. There was no detectable effect of the adsorbed layer upon transport other than this. Therefore, an open ques- tion remains: whether other systems can be formed at ITIES which influence reactions at the interface in a similar way, i.e. by the electrical structure of the layer, rather than by simply steric blocking. In a recent paper [10] Paul and Corn reported investiga- tions of electrostatically induced biopolymer (poly-L-glu- tamic acid, pGlu n , ns240) surfactant coadsorption at the waterNDCE interface by second-harmonic generation (SHG). Paul and Corn used trans-4-[4-(dibutylam- ino)styryl]-1-methylpyridinium cation as an SHG active sur- factant. They showed that coadsorption of the biopolymer and surfactant resulted in a layer with greatly decreased molecular area for the surfactant (from 100 A ˚ 2 in the absence to 30 A ˚ 2 in the presence of the biopolymer). They proposed a multi-point electrostatic coadsorption between the poly- anion and the cationic surfactant since the effect was not observed for monomeric Glu but was increased for a chain length of ns8 and further increased for ns240. In the following, a cyclic voltammetry (CV) study of ion transfer across the surfactant–biopolymer layer at the waterNDCE interface is presented and the observed reduced transfer is theoretically explained on the basis of double-layer effects.