Computer Simulations of Ionenes, Hydrophobic Ions with Unusual Solution Thermodynamic Properties. The Ion-Specific Effects

On the crossroads of current polyelectrolyte theory and counterion-specific effects

Aqueous solutions of ionene polyelectrolytes highlight the need for combining the scaling concepts of polyelectrolyte solutions with those of ion specificity, to encompass the wealth of phenomena taking place in these systems.

Presence of hydrophobic groups may modify the specific ion effect in aqueous polyelectrolyte solutions

Enthalpies of dilution of aqueous solutions of aliphatic 6,12- and 12,12-ionene bromides and fluorides and enthalpies of mixing with low molecular-weight salts, such as sodium fluoride and bromide, are determined. In the second part of the study, the various x,y-ionenes (x, y are numbers of methylene groups between the adjacent charges) with fluoride, bromide, and iodide counterions are mixed with aqueous sodium sulfate solution. The polyelectrolytes examined in this part of the work are 3,3-, 6,9-, 6,12-, and 12,12-ionenes. A comparison with theoretical results, based on the Poisson-Boltzmann cell model, is presented. The theory predicts for the enthalpy of dilution to be exothermic and the enthalpy of mixing endothermic, while experiments show that signs of the heat effects depend on the nature of the counterion of the added salt, as also on the hydrophobicity (numbers x, y of methylene groups) of the ionene. We show that the salts when ordered by heat effects produced by mixing of NaF and NaBr with 3,3-, 6,9-, or 6,12-ionene fluorides and bromides follow the opposite ordering than in the case when the same alkali halide salts are mixed with more hydrophobic 12,12-ionene salts. The results for the enthalpy of mixing of ionenes under study with Na2SO4 follow the same order as obtained for monovalent salts.

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.

Isothermal titration calorimetry and molecular dynamics study of ion-selectivity in mixtures of hydrophobic polyelectrolytes with sodium halides in water

Aliphatic x,y-ionenes are polyelectrolytes in which x and y denote the numbers of methylene groups separating quaternary ammonium ions. They represent useful model substances for studying hydrophobic and charge effects in aqueous solutions. We used isothermal titration calorimetry to measure the enthalpies of mixing, ΔH(mix), of 3,3- and 6,6-ionene fluorides and bromides with low molecular weight salts (NaF, NaCl, NaBr, and NaI) at 298 K in water. The signs and magnitudes of the measured enthalpies depend on the hydrophobicity of the ionene and on the nature of the added salt. For example, addition of sodium fluoride to solutions of 3,3- and 6,6-ionene fluorides produced endothermic effects, while addition of sodium bromide to 3,3-ionene bromide resulted in a strong exothermic effect. Interestingly, mixing of 6,6-ionene bromide and NaBr solutions in water gave a small exothermic heat effect. Polyelectrolyte theories, based on continuum-solvent models, predict enthalpies of mixing to be positive (endothermic) for all the solutions examined in this work. The ion-specific effect is more strongly expressed in ionene solutions with higher charge density (3,3-ionene). The most important result of this work is the finding that the enthalpy of mixing of 3,3- (and of 6,6-ionene) fluorides with sodium halides can be expressed as a linear function of the enthalpy of hydration of the halide counterions. The experimental results were complemented with an explicit water molecular dynamics simulation of solutions of oligoions modelling 3,3- and 6,6-ionenes. The computer simulation results for various nitrogen-counterion pair distribution functions were in most cases consistent with the enthalpy measurements.

Interplay of ion-specific and charge-density effects in aqueous solutions of weakly charged ionenes as revealed by electric-transport measurements

In 3,3-ionenes, one quaternary nitrogen is bonded to a chain of three methylene groups on each side, and in 6,9-ionene, it is bonded to a chain of six on one side and nine on the other. We examined how the solution properties of several ionenes changed with increased hydrophobicity of the polyion, depending on the nature of the counterion. We determined the electrical conductivities of aqueous solutions of 3,3-, 4,5-, 6,6-, and 6,9-ionene fluorides and bromides in the range of concentrations from 5 x 10(-3) to 1 x 10(-1) M and for the temperature interval 5-35 degrees C. Over these ranges, the conductivities of the ionenes were found to decrease with increasing concentration and increase with increasing temperature. The conductivity of 3,3-ionene bromide was lower than that of its fluoride analogue throughout the whole range of concentrations, whereas for the 6,9-ionenes, the trend was reversed. For 4,5- and 6,6-ionene, we observed a crossover in the concentration dependence of conductivity. The conductivity data were compared with the predictions of Manning's theory and scaling theory. Separately determined transport-number values were combined with the conductivity data to obtain the fractions of so-called "free" counterions, f. For bromide samples, f increased from 3,3- to 6,9-ionene. In the case of fluoride counterions, the fraction of free counterions was the lowest for 3,3-ionene and, within the experimental uncertainty, approximately constant for the other less charged ionenes.