The colloidal behaviour of dyes

Abstract

The effect of u.v. and visible light on a polarizable electrode in transparent aqueous and ethanolic solutions of various compounds was followed by the d.c. polarographic technique of mean and instantaneous currents. Whereas a cathodic photocurrent appears in every solution, an anodic photocurrent was found only in solutions of a certain type of organic compound. The cathodic photocurrent in aqueous solutions of indifferent electrolytes increases on dilution; with it a marked effect of the cation charge is observed. A particularly high cathodic photocurrent appears in solutions of some irreversible depolarizers or of scavengers of hydrated electrons. A linear relation was found to hold between the energy of the red limit of the photocurrent and the threshold potential of the electrode. Whereas the anodic photocurrents are of the same magnitude in aqueous and ethanolic solutions, the cathodic photocurrents in water are more than one order of magnitude higher than in ethanol. For the interpretation of the photocurrent the existence is assumed of a surface complex between the electrode and an adsorbed component of the solution held by charge-transfer forces. It is suggested that in the anodic photocurrent the organic compound as electron donor undergoes an electrolytic oxidation in the excited state of the complex. The cathodic photocurrent in aqueous solutions is explained by a transfer of the electron either directly to a reducible acceptor or indirectly through a common acceptor-water molecule to a hydrated electron and an electron scavenger in the solution. In ethanolic solutions when no charge-transfer interaction with the electrode can take place the photocurrent is ascribed to the emission of electrons.