Introduction

The interpretation of electrochemical experiments at the liquid/liquid interface relies heavily on ideas about the molecular structure and composition of the interface. This information is difficult to get from experiments and therefore a variety of models has been used. In this work we compare a lattice model, Monte Carlo simulation (MC) and molecular dynamics (MD).

Discussion

Our original area of interest was the potentiometric and voltammetric behavior of ion-selective electrodes of the ˝liquid membrane˝ type. These electrodes typically consist of a plasticized PVC membrane separating the sample and an internal electrolyte. Ion distribution near the interface in equilibrium and ion transport under nonequilibrium conditions needs to be understood to interpret the sometimes peculiar behavior of these electrodes. Impedance measurements and spectroscopic studies have revealed the importance of adsorption phenomena at the interface^1,2.

A lattice type model has been used successfully to describe many experimental anomalies observed in potentiometric measurements³. Although we are dealing here with a plasticized polymer, the liquid/liquid model proved to be satisfactory. Difficulties arose, though, when an attempt was made to describe adsorption in a realistic way, because the estimation of the quantity and orientation of adsorbed molecules requires a more realistic modeling of molecular details.

MC and MD methods had been used before with success to obtain a molecular level picture of the interface. Software to do such calculations is available on the internet. We decided to validate such software by simulating the same system: water/1,2-dichloroethane by both methods. We then compared the MC and MD results with each other and with earlier published results. We found a generally good agreement.

Subsequently we studied the adsorption of a relatively simple ion, thiocyanate, at the water/DCE interface. This ion has been used in SHG studies of ion-selective electrodes. First results with the simulation showed a pronounced orientation of the ions. This result needs, however, further confirmation.
 

1. Qingshan Ye, G. Horvai, A. Toth, I. Bertoti, M. Botreau, Tran Minh Duc: Studies of ion-selective solvent polymeric membranes by X-ray photoelectron spectroscopy and time-of-flight static secondary ion mass spectroscopy.
Anal. Chem. 70 (1998) 4241-4246

2. Qingshan Ye, A. Vincze, G. Horvai, F.A.M. Leermakers:Partial blocking of ion transport at the interface of an ion-selective liquid membrane electrode by neutral surfactants. Experiments and computer simulation
Electrochimica Acta 44 (1998) 125-132

3. A. Vincze, G. Horvai, F.A.M. Leermakers:Modeling of the electrified interface of liquid membrane ion-selective electrodes
J. Phys. Chem. B 103 (1999) 852-859