Thermal characteristics of solid-solid phase transitions in long-chain dialkyl ammonium salts

A series of symmetrical dialkyl ammonium salts, DCnX, has been prepared and characterized with respect to temperature and enthalpy of solid-solid phase transitions, temperature of melting, thermal stability as well as the reversibility of the phase transitions. The number of carbon atoms, C-n, was varied between 8 and 18 and as anions X halides, nitrate, chlorate, perchlorate and hydrogen sulphate had been chosen. In dependence on chain length and anion type transition temperatures from 20 to 100 degrees C were observed. Mass specific solid-solid transition enthalpies reach values of 185 J g(-1), which makes this class of substances attractive for heat storage applications. The influence of anion type on the transition enthalpies is explained in terms of packing requirements, hydrogen bond network formation and contributions from hindered anion rotation. (C) 2005 Elsevier B.V. All rights reserved.