Potential-mediated interaction between dextran sulfate and negatively charged phospholipids films at air/water and liquid/liquid interfaces

C.I. CÁMARA; L.M. YUDI

ELECTROCHIMICA ACTA

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2013 vol. 113 p. 644 - 652

0013-4686

The effect of dextran sulfate (DS) on distearoyl phosphatidyl glycerol (DSPG) and distearoyl phospha-tidic acid (DSPA) films formed in the presence of Ca2Cl, LiCl or KCl as aqueous electrolytes, was analyzedby cyclic voltammetry, surface pressure-area and surface potential-area isotherms and Brewster anglemicroscopy. Experiments of cyclic voltammetry at the water/1,2-dichloroethane interface showed aninterfacial adsorption/desorption process in the presence of DSPG, or DSPA, DS and LiCl. It is suggested thatthis process could correspond to the formation of an interfacial complex DS?Li+?DSPG or DS?Li+?DSPA atpositive potentials with respect to the potential of zero charge (pzc) and its desorption from the interfacetowards the aqueous phase, by complex dissociation, at potentials below pzc. The voltammetric analy-sis of tetraethylammonium (TEA+) cation transfer, from the aqueous to the organic phase, evidenced ablocking effect of DSPG and DSPA films on that process, whose magnitude depends on both the natureof the phospholipids and the cation forming the aqueous electrolyte. The structure and permeability ofthose films were altered by the presence of DS, by formation of the complex DS?cation?phospholipid,which produces an expansion of the monolayer, whose extent depends on the cation present in water,and the polar head group of the phospholipid. Electrochemical experiments were completed with com-pression isotherms for DSPG and DSPA monolayers both in the absence and in the presence of DS. In thiscase, as opposed to the voltammetric results, a negligible effect of DS was observed in the isotherms.This apparent contradiction could be explained considering the effect of polarization, which enables theaccumulation of cations at the interface, favouring the formation of the DS?cation?phospholipid com-plex which is responsible for the increase in the film permeability. Similarly, the thickness values forphospholipid films obtained from the Brewster angle microscopy experiments were the same either inthe absence or in the presence of DS at any lateral pressure value, which is a direct evidence of lack ofinteraction between DS and DSPA or DSPG molecules at open circuit. Therefore, it was concluded that theformation of the complex DS?cation?phosholipid, responsible for the increase in permeability observedin voltammetric experiments, must be attributed to the polarization of the interface. The application ofhigh positive potentials at the interface promotes cations accumulation, which neutralize the negativecharge in phosphate groups and in DS and allows their interaction.