Diffraction pattern and structure of the system tetra-n-butylammonium fluoride-water at 25°C

Abstract

The diffraction pattern of a concentrated solution of tetra-n-butylammonium fluoride in water (Bu4NF·41H2O) has been measured and analyzed at 25°C. The radial distribution function for the solution is surprisingly similar to that of pure water at the same temperature. Each water molecule has ∼3.8 neighbors at an average distance of 2.80 Å, compared to ∼4.4 neighbors at 2.85 Å in pure water. Intensity and radial distribution functions have been calculated for two different models. The respective models assume modified ice-I and gas hydrate structures, surrounded by a random distance distribution, as an adequate description for the average short-range order in the solution. In both models the F- ions and the N+ atoms of the cations are part of a hydrogen-bonded network, while the butyl chains of the cations are located inside the cavities which, although of different size and shape, are typical of both structures. The ice-I model describes the x-ray data of the solution and of pure water, and its parameters are related to bulk thermodynamic properties in a physically reasonable way. The gas hydrate model is incompatible with the experimental radial distribution function of liquid water, and the parameters of this model for the solution do not explain the large enthalpy and entropy of fusion of the solid hydrate.