Crystal structures and thermodynamic properties on phase change materials n-CnH2n+1NH3·H2PO4(s) (n=8, 9 and 10)

Crystal structures of three long-chain n-alkylammonium dihydrogen phosphates n-CnH2n+1NH3·H2PO4(s) (n=8, 9 and 10) (abbreviated as C8P, C9P and C10P), which may be used as the phase change materials in storage and utilization of solar energy, were determined in layered structures by X-ray crystallography, and the same hydrogen bonding network is observed for these long carbon chain compounds. Low-temperature heat capacities of the compounds were measured by a precision adiabatic calorimeter over the temperature range from 78 to 380 K. Two solid–solid phase transitions were observed for C8P and C9P, but only one solid–solid phase transition was observed for C10P. The temperatures, molar enthalpies and entropies of the phase transitions were determined based on the analysis of heat capacity curves. Experimental heat capacities for each compound were fitted to two polynomial equations. The smoothed heat capacities and thermodynamic functions ( H T ο − H 298.15 ο ) and ( S T ο − S 298.15 ο ) of the three compounds were calculated with an interval of 10 K in terms of the polynomial equations.