Heat of dilution and of solution data are fitted to the form of equation corresponding to that used successfully for activity and osmotic coefficients over a wide range of concentrations. The resulting parameters give the change with temperature of the activity and osmotic coefficients. Results are reported for 84 electrolytes of 1-1, 2-1, 3-1, and 2-2 valence types.

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Thermodynamics of electrolytes. x. enthalpy and the effect of temperature on the activity coefficients.

To investigate the stability of ionic pairs in aqueous solution, polarizable continuum-model (PCM) ab initio molecular orbital calculations were carried out. Ion pairs formed from three organic ions, methylacetate, methylammonium, and guanidinium, were treated. The methylacetate ion pair forms a stable complex when the two methyl groups are in contact. When the charged groups of methylacetate and methylammonium are in contact, this pair does not form a stable complex in aqueous solution. When the methyl group of methylacetate is in contact with that of methylammonium, the ionic groups are far apart, and a stable complex is not formed, even though the complex involves a strong attractive electrostatic interaction. The guanidinium pair forms a stable complex in aqueous solution at all conformations investigated in this work even though a strong repulsive electrostatic interaction is present in this complex.

Stability of Like and Oppositely Charged Organic Ion Pairs in Aqueous Solution

We have measured the 1H nuclear magnetic relaxation rates and the self-diffusion coefficients of (CH3)4N+ ions in aqueous solution of (CH3)4NBr as a function of salt concentration and in aqueous mixed electrolyte solution of x 4m (CH3)4NBr + (1 - x) 4mMBr, (M = Li, K, Cs) as a function of the mole fraction x of (CH3)4N+. By measuring also the 2H relaxation rate of the solvent D2O, we could separate the (CH3)4N+ proton relaxation rates into the intra- and intermolecular contribution. This allowed the determination of the so-called association parameter A, which is connected with the integral over the pair correlation function. The results for the cation-cation A-parameter show that in the pure (CH3)4NBr solution no or only weak (CH3)4N+ -(CH3)4N+ attraction is present, whereas in the mixed electrolyte solutions containing KBr or CsBr a strong association of the hydrophobic cations occurs. With M = Li, this attractive interaction is also present, however less pronounced. The sequence of the effectiveness of the alkali ions is the same as in the so-called Hofmeister series for salting-out effects on biological macromolecules.

Cation-Cation Association of Tetramethylammonium Ions in Aqueous Mixed Electrolyte Solutions

The system NaH2PO4−NaClO4 has been studied from 0.15 to 7.0 mole-kg−1 at 25°C by the isopiestic method. The excess free energy of mixing is large and positive. The results confirm the formation of (H2PO4) 2 −2 dimers with a stoichiometric association constantKa=0.25±0.1 kg-mole−1 at 25°C.

Wen-Yang Wen

Papers

Free energy changes on mixing solutions of alkali halides and symmetrical tetraalkylammonium halides