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The influence of cations on DOPC adsorbed onto Hg/electrolyte interface electrochemical and impedance spectroscopy study Agak, John Odiko

Abstract

An electrochemical impedance spectroscopy (EIS) methodology has been developed, and combined with traditional electrochemical techniques to characterize the influence of applied potential on a dioleoyl phosphatidylcholine (DOPC) monolayer adsorbed at a Hg-0.1MKC1 interface. The DOPC monolayer was initially characterized using AC voltammetry in 0.1M.KC1, LiCl and TMAC1. The monolayer displayed a minimum low-capacitance region, 1.85 μF/cm² at -0.4 V. The potential was scanned from -0.4 V to -1.2 V and two capacitance peak 1 and 2 were observed at - 0.96 V and -1.04 V respectively. When the negative potential limit was made more negative, a third capacitance peak 3 was observed at -1.34 V. These peaks represent phase transformations due to changes in electrode coverage. At very negative potentials, the monolayer desorbed from Hg in KC1 and TMAC1 but no desorption was observed in LiCl. Addition of Ca²⁺ to the KC1 subphase, systematically changed the peak heights indicating a change in the kinetics of the phase transformation. In the presence of Ca²⁺, the desorption of the lipid was incomplete. However, addition of 1.0 mM of TMA⁺ enhanced the desorption of the lipid in the presence of Ca²⁺. In order to further characterize these layers, an EIS methodology was developed and tested on the Hg-KCl systems. The monolayer was modified by adding Ca²⁺ to the electrolyte subphase. Changes due to its interaction with the cations were characterized using the new technique. The potential was scanned from -0.4 V to the peak potentials and the impedance measured over a wide range of frequencies. The memodology allowed for the impedance measurements to be performed at a potential corresponding to the capacitance peak maximum without disrupting the monolayer organization. Impedance spectra at peak 1 indicated a phase change characterized by one time constant, peak 2 indicated a phase transformation characterized by two time constants and peak 3 showed a phase transformation characterized by three time constants. At the desorption potential value, the impedance spectra revealed the desorption of the monolayer from the Hg-KCl interface and incomplete desorption in the presence of Ca²⁺ in the subphase. The new methodology also allowed for changes in solution resistance due to the presence of adsorbed or desorbed DOPC monolayers to be observed. However, in the presence of DOPC no difference in solution resistance was observed between the adsorption and desorption potentials except when Ca²⁺ was introduced.

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