Specific counter-ion and co-ion effects revealed in mixing of aqueous solutions of 3,3 and 6,6-ionenes with solutions of low molecular weight salts.

Enthalpies of mixing of aliphatic 3,3 and 6,6-ionene fluorides with low molecular weight salts (sodium formate, acetate, nitrate, chlorate(v), and thiocyanate), all dissolved in water, were determined. In addition, to complement our previous study (Lukšičet al., Phys. Chem. Chem. Phys., 2012, 14, 2024), new measurements were performed where aqueous solutions of 3,3 and 6,6-ionene bromides were mixed with solutions of sodium fluoride, chloride, bromide, and iodide. Electrostatic theory, based on Manning's limiting law or the Poisson-Boltzmann equation, predicted the enthalpy of mixing to be endothermic in all the cases, while experiments showed that this is not always true. When an aqueous solution of 3,3-ionene fluoride was mixed with a solution of sodium fluoride (or formate and acetate) in water, the effect was indeed endothermic. For all other salts, i.e. sodium chlorate, nitrate, and thiocyanate, heat was released upon mixing. The situation was similar for 6,6-ionene fluoride solutions with an exception of mixing with sodium chlorate, where the effect was endothermic. The enthalpy of mixing was strongly correlated with the enthalpy of hydration of the counterion of the low molecular weight salt. A lyotropic series, similar to that of Hofmeister, was obtained. To examine also the effect of co-ions, ionene bromides were titrated with tetramethyl-, tetraethyl-, or tetrapropylammonium bromides. The enthalpy was exothermic for all mixtures while, somewhat unexpectedly, the co-ion specific effect was quite strong.