Structural analysis of hydrazinium trifluoroacetate aqueous solution by X-ray diffraction and empirical potential structure refinement modeling in the temperature range of 25 to −125 °C

Abstract

X-ray diffraction of a hydrazinium trifluoroacetate ([N2H5]+ [TFA]-) aqueous solution at 14.4 mol% of N2H5TFA is measured in a temperature range from 25 to −125 °C. The solution in a supercooled region exhibits an anomalous temperature behavior of the heat capacity without ice crystallization taken place for neat water. The X-ray interference functions of an N2H5TFA solution are subjected to empirical potential structure refinement (EPSR) modeling to obtain the site-site pair distribution functions, the coordination number distribution, the angle distribution, and the spatial density function (3D structure). The tetrahedral-like network structure as observed for neat water remains for an N2H5TFA aqueous solution, and the structure is enhanced at temperatures below −75 °C, although the solution does not freeze. The enhancement of the tetrahedral-like network structure could be responsible for the anomalous behavior of the heat capacity of the solution at a low temperature. It is consistent with the scenario of the second critical point of neat water where the transition from high-density water to low-density water is considered to take place.