Apparent molar heat capacities and volumes of electrolytes and ions in acetonitrile-water mixtures

Abstract

Apparent molar volumes Vφ and heat capacities Cp,φ of NaCl, KCl, KNO3, AgNO3, KI, NaBPh4 and Ph4PCl have been measured in acetonitrile (AN)-water mixtures up to xAN=0.25 by flow densitometry and flow microcalorimetry. Limited data have also been obtained for NaF, LiCl and KBr up to x AN =0.15. Single ion volumes and heat capacities of transfer were obtained using the assumption ΔtXφ(PH4P+) = ΔtXφ(BPh4-) where X=V or C p and ΔtXφ is the change in Xφ for a species on transfer from H2O to AN-H2O mixtures. Volumes and heat capacities for simple salts show relatively little dependence on solvent composition. However, ΔtXφ for simple ions show more pronounced variations, exhibiting at least one extremum. These extrema are similar to but much less pronounced than those derived previously for ions in t-butanol-water mixtures. Surprisingly little correlation is found between the present data and other thermodynamic transfer functions. This is attributed to the predominance of ion-solvent over solvent-solvent interactions in AN-H2O solutions. ΔtVφ and ΔtCp,φ for the silver ion differ markedly from those of the alkali metal ions as a result of the well-known specific interaction between Ag+ and AN