Apparent molar heat capacities and volumes of electrolytes and ions int-butanol-water mixtures

Abstract

The apparent molar volumes Vϕ and heat capacities Cp,ϕ of NaCl, LiCl, NaF, KI, NaBPh4 and Ph4PCl have been determined in solutions of H2O containing up to 40 mass% t-butyl alcohol (TBA) by flow densitometry and flow microcalorimetry. Combination of these results with literature data allows calculation of Vϕ and Cp,ϕ for 16 ions in these mixtures using the assumption that ΔtXφ(Ph4P+) = ΔtXφ(BPh4−) where X=V or Cp and ΔtXφ is the change in Xϕ for a species on transfer from H2O to TBA-H2O mixtures. These are the first reported single ion values for Cp,ϕ in a mixed solvent. While whole electrolyte volumes and heat capacities show relatively smooth changes with solvent composition, ΔtXφ(ion) exhibit two well-developed extrema at around 10 and 25 mass% TBA. The shape of the ΔtXφ(ion) curves shows considerable uniformity among the alkali metal cations and the halide ions but the extrema become more pronounced with increasing size among the tetraalkylammonium ions. These extrema are analogous to those observed in aqueous organic mixtures of surfactants and are probably indicative of microphase transitions in these strongly interacting solvent mixtures.