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Osmotic coefficients have been measured at 65" for aqueous solutions of tetra-n-butylammonium fluoride, tetramethyl-, tetra-n-propyl-, and tetra-n-butylammonium chloride, and tetramethyl-and tetra-n-propylammonium iodide. These data were combined with the available thermodynamic data at 25" to estimate relative partial molal entropies of the solvent. The thermodynamic properties of symmetric tetra-n-alkylammonium halide solutions have been interpreted in terms of the effect of increasing anion size and increasing temperature on the local structure of water. This interpretation is justified in terms of the recently published X-ray diffraction results on water and on aqueous ammonium halide and tetra-n-butylammonium fluoride solutions. The concept of "water-structure-enforced ion pairing" has been used to explain the reversal in the sequence of the magnitude of the osmotic coefficients on substituting iodide for chloride ions. It is suggested that this concept is not valid in cases involving either cations or anions which reduce the amount of local order in the solvent. However, osmotic coefficient reversals may be accounted for on the basis of the competition between increased hydrogen bonding of water as the size of the R4N+ cation increases and decreased hydrogen bonding as the size of the halide ion increases.

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the entire ionic strength fraction range. Using an isopiestic vapor pressure apparatus, we have determined osmotic coefficients for aqueous solutions containing sodium chloride and the clathrate-forming salt tetra-zi-butylammonium chloride over a wide range of total ionic strength and ionic strength fraction at 25 °C. We have used the Scatchard-Rush equations to fit the osmotic coefficient data for solutions of each pure component and the mixtures. From these equations we calculated the excess free energy of mixing and the activity coefficients of each solute component as a function of ionic strength fraction and total ionic strength. Friedman's interaction parameters, g0 and gb are calculated and the results discussed.

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