Iodide islands in iodine. The electrical properties of iodides in liquid iodine

Abstract

Liquid iodine is a good solvent for potassium and tetrabutylammonium iodide and for organic molecules of low ionization potential, such as p-phenylenediamine. Their dissolution leads to a considerable conductivity, mainly (95%) carried by the iodide ions. The dependence of the equivalent (per particle) conductivity from 0 to 10 mol% can be described quantitatively by assuming chemical equilibrium between associated and dissociated ion pairs. At infinite dilution the ions are dissociated, at finite concentrations considerable association occurs, but at higher concentrations (> 1%) redissociation takes place, because all ions are then in the (Debye–Hückel) field of all other ions.

Thermoelectric measurements show that the iodide species should at low and intermediate concentrations be described as an electron delocalized over ca. 100 iodine molecules. These ‘iodide islands’ move by density fluctuations, which accounts for the very small temperature dependence of the conductivity. At concentrations < 1% the solution becomes ‘full’, and on increasing the concentration further the islands shrink in size until only four iodine molecules participate in island formation and the solubility limit is reached.