Thermodynamic analysis of the interaction of partially hydrophobic cationic polyelectrolytes with sodium halide salts in water

Abstract

Isothermal titration calorimetry was used to determine the temperature and concentration dependence of the enthalpy of mixing of 3,3-and 6,6-ionene fluorides, bromides, and iodides with low molecular weight salts (NaF, NaCl, NaBr, and NaI) in water. The magnitudes of the enthalpies, measured in the temperature range from 273 to 318 K, depended on the number of methylene groups on the ionene polyion (hydrophobicity), and on the anion of the added salt (ion-specificity). All enthalpies of mixing of 3,3-and 6,6-ionene fluorides with low molecular weight salts (NaCl, NaBr, and NaI) were negative, which is in contrast to the predictions of standard theories of polyelectrolyte solutions. This fact was interpreted in the light of the ion-water short-range interactions that are not accounted for in those theories. In contrast, the enthalpies of mixing of 3,3-and 6,6-ionene bromides and iodides with NaF were positive, being in accord with theory. Using the calorimetric data, we performed a model thermodynamic analysis of the polyelectrolyte-salt mixing process to obtain changes in the apparent standard Gibbs free energy, enthalpy, entropy, and heat capacity relative to the pure ionene fluorides in water. The results prove that halide ions replace fluoride counterions with a strength increasing in the order chloride < bromide < iodide. The process is enthalpy governed, accompanied by a positive change in the heat capacity.

Counter-ion binding and mobility in the presence of hydrophobic polyions – combining molecular dynamics simulations and NMR

“…23 temperatures and using this information obtained the excess Gibbs free energy, enthalpy, and entropy contributions for displacement of fluoride ions in the mixing process with NaBr, NaCl, and NaI salts at T = 298 K. The results for 3, 3-ionenes, shown in Table I of that paper, indicated that excess free energy function decreases in order from F − toward I − . The same holds true for the relevant enthalpy and entropy terms.…”

“…ii) Experimental 22,23 and MD simulation data concerning the strength of counter-ion binding to the ionene oligoion arrive at the following sequence for the halide ions: F − (weakest binding) < Cl − < Br − < I − (strongest binding). [26][27][28][29] The situation recently encountered with the hydrophobic 12, 12-ionenes seems to differ from the above.…”

“…17): ∆H m is mostly negative and decreases in the order F − > Cl − > Br − > I − . Similarly to the above sequence, the counter-ion trends concerning the strength of counter-ion binding to the ionene oligoion, observed in experimental 22,23 and MD simulation data, 17,[26][27][28][29] were: F − (weakest binding) < Cl − < Br − < I − (strongest binding). Very interestingly, the calorimetric (∆H m and ∆H dil ) data for 12, 12-ionenes, presented in our recent study 21 did not follow the above trends.…”

“…The same trend is observed for 6, 6-and 6, 9-ionenes, but in the latter case, the difference in ion-binding between the two ionic species is within the experimental error. In yet another study, 23 we used calorimetric data to perform a model thermodynamic analysis of the polyelectrolyte-salt mixing process. The results prove that halide ions replace F − counter-ions with a strength increasing in the order Cl − < Br − < I − .…”

“…Experimental studies have been performed for x, y equal to 3,3-, 4,5-, 6,6-, and 6,9-to 12,12-ionenes, that is for increasingly more hydrophobic polyelectrolytes. [16][17][18][19][20][21][22][23] Among physico-chemical properties we measured enthalpies of dilution and mixing, osmotic coefficients, dielectric relaxation, conductivities and transport numbers. These solutions have also been examined using NMR, X-ray and neutron scattering methods.…”