A differential scanning calorimetric study of phase transitions in polysubstitutedn-alkylammonium halides

Abstract

A DSC investigation of several n-alkylammonium salts is reported. The results of this investigation can be used to help decide which of several crystalline forms should be studied by X-ray diffraction for the best correlation between the liquid and solid states.The n-alkylammonium halide compounds are currently under intensive investigation for a number of reasons. For example, the quaternary salts are excellent cationic detergents when dissolved in hydrocarbons; their concentrated solutions are known as liquid membranes (and are believed to control a large number of biological processes, with their bioactivity being a function of the structure of the solute); and they are pharmacologically active substances [1]. The secondary salts are widely used in solvent extraction processes in general and in the extraction of fused salts in particular [2].During the course of these investigations, attempts have been made to correlate the properties of these materials in solution and in the molten state with the structure in the solid state [2,3]. However, the existence of several solid-solid transitions in these compounds have been previously established [4][5][6][7]. It would seem that tJ~e crystal and molecular structure of the solid phase existing closest to the melting points (Phase l) should be used as the basis for any comparisons between the solid and molten or solution phases. However, if the structural differences between Phase I and the room-temperature phase are minor, then a reasonable and more practical approach would be to base the comparison on the room-temperature phase.As a first step in making this decision, the temperatures of the phase transitions and several thermodynamic parameters should be obtained. This information will be helpful in selecting proper temperature ranges for the crystal structure analyses and in predicting the degree of disorder to be expected in the high-temperature phases.Enthalpies and entropies of transition and of fusion, as well as the transition and fusion temperatures, have been reported for a number of substituted n-alkyl-