Chemical models of the hydrophobic interaction: Apparent molal volumes and heat capacities of three symmetrical bolaform electrolytes and their homologous monomers in aqueous solution at 25°C

Abstract

The densities and volumetric specific heats of aqueous solutions of Bu₃NHBr, Pent₃NHCl, and three diazonium salts, HN−Oct₃−NHBr₂, HN−Dec₃−NHCl₂, and Bu₃N−Oct−NBu₃Br₂, have been measured at 25°C. From these data, the apparent molal volumes φ _v and apparent molal heat capacities φ _c have been calculated and are reported here. In the series of compounds chosen, the diazonium (higher homologs) can be regarded as dimers of the alkyl-substituted ammonium ions (lower homologs), and these systems are examined as chemical models for the hydrophobic interaction. With the three homologous pairs studied here, the chemical model predicts that the strong interaction (limitingly, chemical binding) of two hydrocarbon chains in water leads to a major decrease in both φ _v and φ _c of the interacting solutes, ca.−22 cm³-mole⁻¹ and −200 J-^oK⁻¹-mole⁻¹. These predictions constitute limiting behavior — useful, but not sufficient, to explain the observed concentration dependence of φ _v and φ _c in aqueous solutions of the lower homologs Bu₃NHBr, Pent₃NHCl, and Bu₄NBr. An explanation for the concentration dependence of φ _c is suggested with reference to ultrasonic relaxation data.