On the behaviour of molecules of the quinoline group at the water—mercury interface Part II. The kinetics of the isoquinoline transition in the presence of an “inert” electrolyte

Abstract

The reorientation transition, which involves two distinct monolayers of isoquinoline molecules, has been investigated by using the single-potential step and the double-potential step methods under various experimental conditions. The cathodic transients correspond to a liquid → solid transition. Their morphology and half-times have been measured at various initial and final potentials, and for several isoquinoline concentrations close to the saturation value. At small overvoltages, the reorientation process is determined by heterogeneous (“instantaneous”) nucleation, while progressive nucleation and growth prevail at higher potentials. Recourse to the double-potential step method affords an efficient way of assessing separately the influence of the overvoltage on the rate constants for nucleation and growth. Determination of the critical nucleus size and the activation energy for each of the two processes has been based on an adaptation of the Brandes theory developed for two-dimensional crystallization from a supersaturated vapour phase. The kinetics are markedly dependent on the initial state, which can be easily controlled by the potential and the bulk concentration. When the potential is stepped from the region where there is superadsorption, progressive nucleation is particularly fast, while starting from a partially depleted layer gives much slower transients with extensive tailing due to diffusional hindrance. The anodic transients show consonant characteristics which indicate that the initial film can be considered as a two-dimensional solid, and that boundary defects are acting as nucleation centres, during the solid → liquid transition triggered by the potential step.