Heat capacity changes and standard partial molal heat capacities of aqueous solutions of some tetra-alkyl ammonium halides at 30°C and the effect on water structure

Abstract

ΔC°_p values of tetra-alkylammonium halide salts Me₄NBr, Me₄NI, Et₄NBr, Et₄NI, Pr₄ NBr, Pr₄NI and Bu₄NBr in aqueous solutions at 30°C have been determined by measuring integral heats of solution at 25 and 35°C ΔC°_p values which vary from –16.5 to 179 cal deg.^–1 mole^–1 are in the increasing order of Me₄NI < Me₄NBr < Et₄NBr ≃ Et₄NI < Pr₄NBr ≃ Pr₄NI < Bu₄NBr. Results are discussed in terms of the influence of tetra-alkylammonium halide salts on the water structure. Me₄NBr and Me₄NI break water structure, while all other tetra-alkylammonium bromide and iodide salts enhance or promote water structure. The influence of R₄N⁺ salts in promoting water structure increases greatly with increase in the size of the alkyl group R. The influence of anions (bromide and iodide) does not appear to be significant in modifying the structure of water. Results from this work support the concept of “cation-cation interactions” in explaining the nature of interaction of tetra-alkylammonium halide salts with water.