Growth and Morphology of Liquid Phase in Frozen Aqueous NaCl Probed by Voltammetry and Simulations

Abstract

Cyclic voltammograms (CVs) of Fe(CN)₆^4- are measured using a microelectrode in frozen aqueous NaCl solutions to obtain morphological information on the liquid phase developed on the electrode surface. CVs in frozen solutions feature the radial diffusion similar to that measured in bulk solution in some cases but the linear diffusion in other cases. The former suggests the sufficient growth of the liquid phase, whereas the latter implies the diffusion paths in particular directions are hindered. Two parameters, i. e. a ratio of the maximum current to the steady-state current (R) and current amplification (r_amp ), are extracted from CVs and compared with those of simulated ones. CV simulations are carried out for four geometrical models. From the relationship between r_amp and R, the FCS developed on the electrode surface can be regarded as a thin layer developed in the direction parallel to the electrode surface or a cylinder running in the direction away from the electrode. Since solutes are concentrated in this liquid phase, highly sensitive voltammetric analysis would be possible if the growth of the FCS were successfully managed. The liquid phase morphology on the electrode, which cannot be probed by other methods, is useful information for designing such highly sensitive voltammetric analyses.

Keywords: cyclic voltammetry; digital simulations; freeze concentration; ice; liquid phase.