Thermodynamics of Dilution and the Hofmeister Series in Aqueous Solutions of Aliphatic Ionenes with Halide Counterions

Enthalpies of dilution of aqueous solutions of aliphatic x,y-ionene fluorides and iodides, with x,y being equal to 3,3-, 4,5-, 6,6-, and 6,9-, were measured in the concentration range from 0.1 mol.kg(-1) to 0.0016 mol.kg(-1). The results were combined with previously obtained data for ionene chlorides and bromides to discuss the ion-specific and charge density effects in these solutions. The comparison with theoretical results, based on Manning's limiting law and cylindrical cell model founded on the solution of Poisson-Boltzmann's equation, is presented. These theories imply the enthalpy of dilution to be exothermic. Of all the solutions studied here, only those with fluoride ions as counterions agree with theoretical predictions. Solutions of 3,3-, 4,5-, and 6,6-ionene with bromide, chloride, and iodide counterions yield endothermic effect upon dilution. For the most hydrophobic 6,9-ionene, heat is released when adding water in the low concentration regime. Using the previously measured data for 3,3-, 4,5-, 6,6-, and 6,9-ionene bromides and chlorides, we calculated the Gibbs free energy and the entropy changes upon dilution. The non-Coulomb contributions to the Gibbs free energy, enthalpy, and entropy of dilution were estimated.