Thermodynamics of bolaform electrolytes. III. Partial and apparent molal volumes in H2O and D2O

The apparent,φ _v, and partial, (bar V) ₂, molal volumes of a series of homologousbis-tetraalkylammonium bromides have been determined in H₂O and D₂O at 25°C from precision density measurements carried out with a buoyancy densimeter and a dilatometer. The Debye-Hückel theoretical limiting slope forφ _v and (bar V) ₂ as a function of the square root of molar concentration is approached for all of the salts studied. the concentration dependence ofφ _v and (bar V) ₂ is anomalously large and negative in both solvents, with the deviations being less negative in D₂O than in H₂O. The bolaform salts have larger (bar V) ₂ ^o values in H₂O than in D₂O, contrary to the observed behavior of R₄NX salts. Possible origins of the solvent isotope effects observed are discussed in terms of structural and cavity contributions to the measured volumes. A comparison of thermodynamic transfer functions (H₂O→D₂O) for Et₄N Br and the corresponding bolaform analog appropriate in the consideration of cation-cation pairing of Et₄N⁺ ions shows inconclusive evidence for the support of the cation pair theory.