The effect of pressure on the electrical conductivity of liquid iodine, iodine chloride, iodine bromide and bromine trifluoride

Abstract

The electrical conductivities of the pure liquids iodine, iodine chloride, iodine bromide and bromine trifluoride have been measured at temperatures up to 177 °C and pressures to 100 MPa (1 kbar). In all cases the conductivity increased with pressure at constant temperature. At constant density, the conductivity always increased with temperature, but the behaviour at constant pressure was more complex; ICl and IBr showed conductivity maxima, and the other liquids had negative temperature coefficients of conductivity. These variations are attributed mainly to corresponding displacements in self-ionisation equilibria of the type AB ⇌ A⁺+ B^–(where the ions produced may be solvated by the molecular substance). Attempts to account for the measured pressure coefficients using a model based on the Born equation for the free energy of solvation were only qualitatively successful. Better agreement could be obtained if it was assumed that the charges in these liquids are delocalised over a volume containing ca. 100 molecules, as has previously been suggested by Ludwig et al. It is also likely that a part of the increase in conductivity with pressure observed for liquid iodine is due to an increase in the mobility of I^– with pressure.