Heat effects measured in aqueous solutions of salts of poly(anethole sulfonic acid) upon mixing with alkali-, alkaline-earth-, and tetraalkylammonium chlorides

- Non-Coulomb mixing enthalpy of poly- and simple electrolyte solutions was studied.
- The mixing enthalpies of alkali counterions decrease with their hydration enthalpy.
- The mixing enthalpies TAA+ counterions increase with their hydration enthalpy.
- The mixing enthalpies increase with the apparent size of the counterions.
- Heat is evolved if smaller ions replace larger ones and used in the opposite case.

Experimental and theoretical results for the enthalpies of mixing, ΔmixH, for aqueous solutions of selected salts of polyanethole sulfonic acid with some low-molecular-mass salts are presented. The enthalpies of mixing for aqueous solutions of alkali (Li+, Cs+) and tetraalkylammonium (Me4N+, Bu4N+) polyanethole sulfonates (PAS−) with alkali-metal chlorides (LiCl, KCl, CsCl) and tetraalkylammonium chlorides (Me4NCl, Et4NCl, Pr4NCl, Bu4NCl) were measured with an isothermal titration calorimeter at 298.15 K. The first two polysalts (LiPAS, CsPAS) were also titrated with MgCl2 and SrCl2 solutions. Manning's theory was used to predict the electrostatic contribution to the enthalpy of mixing for polyelectrolyte and simple electrolyte solutions. The observed differences between the experimental and calculated values were ascribed to the ion-specific interactions. The cumulative heat effects obtained from the titration of the poly(anethole sulfonic) salts with the alkali chlorides, including Me4NCl, for a molar ratio r = 4.6, decreased with the hydration enthalpy, ∆hydH, of the added counterions. In contrast to this, the enthalpies of mixing for poly(anethole sulfonic) salts with tetraalkylammonium chlorides increased with the length of the alkyl chain in the added counterion. The results show a strong dependence of the enthalpy of mixing on the difference in the nature of the counterions in the mixture. Additionally, the results allow us to classify the counterions as either hydrophilic or hydrophobic based on the heat effects provoked by their introduction into a polyelectrolyte solution.