Thermal effects in polyelectrolyte ion exchange reactions: the Jahn–Teller effect

Abstract

Microcalorimetric determinations were made of the thermal effects accompanying the exchange of the divalent cations of Mn, Fe, Co, Ni, Cu, and Zn in dilute aqueous perchlorate solutions with Mg2+ ion initially bound in a crosslinked polyelectrolyte gel. The calorimetric measurements, together with microchemical equilibrium distribution determinations with the same cations, were employed to estimate the standard state Gibbs energy, enthalpy, and entropy changes associated with their ion exchange reactions.All the transition elements were selectively absorbed relative to Mg2+ with the sequence being: Mn < Fe < Co < Ni > Cu > Zn, showing divalent Ni(II) cation to possess the greatest affinity for the polyelectrolyte phase. The sequence of standard state reaction enthalpy change, ΔH0, revealed that Ni2+ also was the most exothermic of all 3d transition element cations. However, the ΔH0 values became more negative in the order: Mn < Fe < Co < Ni > Cu < Zn, showing that Cu2+ ion behaved anomalously in being less negative than expected. This behavior (and that in the ΔS0 values also) suggested that a destabilization of the octahedrally coordinated Cu2+ in aqueous solutions must occur, possibly because of Jahn–Teller effects on its d9 electronic configuration when the cation is placed in an environment of cubic symmetry. Because of a thermodynamic compensation of the magnitudes of ΔH0 and ΔS0, the position of Cu2+ in the ΔG0 value sequence was regular.