Origin of chronoamperometric responses associated with impacts of single electrolyte droplets at a polarized liquid/liquid interface

Abstract

Chronoamperometric measurements at a constant potential are used to study the single impacts of the water-in-DCE emulsion droplets containing an electrolyte (tetrapropylammonium chloride, TPrACl) at a polarized water(W)/1,2-dichloroethane (DCE) interface. The impacts give rise to the spike-like current transients, which are characterized by the initial current step (time constant less than 2 ms) followed by a slower exponential current decay with a time constant in tens of milliseconds. It is shown that the theoretical treatment of the current spikes can be based on a modification of the bulk electrolysis model including (1) the droplet collision at the W/DCE interface, (2) a partial coalescence (fusion) of the droplet with the impacted interface, (3) a reorganization of the electric double layer leading to the propagation of the interfacial potential difference from the impacted interface to the surface of the droplet, and (4) stripping (bulk electrolysis) of TPrA+ from the droplet to the DCE phase accompanied by the chloride depletion into the bulk water phase. On assuming that the stripping of TPrA+ is the rate-determining step of the impact event, the theoretical relationships for the current transient are derived, which are confirmed by the experimental data. An analysis of the current transients yields the charge corresponding to the ionic content of the droplets, which is used to evaluate the droplet diameter. Droplet size distribution with the droplet diameter of around 8 μm is verified by an analysis of the microscopic images of the W-in-DCE emulsion, and in part also by the dynamic laser scattering measurements. It is shown that the frequency of the droplet collisions with the W/DCE interface (1–10 Hz) is controlled by migration of the negatively charged droplets towards the interface.