The simple electrochemical method for detection of 3d molecular reorientation in adsorbed layers of organic substances

Reorientation of initially flatly adsorbed molecules to slanted or fully erected positions on the electrode surface seems to be a common transition during adsorption of organic substances. However, the reorientation is not easily detected and its occurrence often questioned. In this paper we present a simple electrochemical method to detect 3d molecular reorientation at the electrode surface. The method is based on the ac voltammetric measurement of the rate of change of the capacitive current, dIc/dt with time, during the adsorption process, which is qualitatively different for adsorption with reorientation in respect of that without it. The theoretical considerations of simple, diffusion-controlled adsorption without reorientation predicts a monotonic decrease of the dIc/dt curve. This is demonstrated experimentally in the adsorption of almost isotropic 1-aminoadamantanehydrochloride. Contrary to this theoretical prediction, it is shown that, in cases where the orientational ordering and reorientation take place, the rate of change of capacitive current vs. time exhibits a plateau followed by steep decrease. Such behavior is demonstrated experimentally in adsorption of the anisotropic, rod-like Triton-X-100 molecule. This has previously been shown to undergo reorientation during the adsorption process. Qualitatively the same behavior of dIc/dt was observed for both molecules in the case of diffusion controlled adsorption as well as adsorption from stirred solution.