Phase behaviour and heat capacities of DBN and DBU based protic ionic liquids – Insights into the ionic character and nanostructuration

Herein, we report the thermal behavior and high-precision heat capacity values, at T = 298.15 K, and in the range from 283 to 363 K, for several protic ionic liquids (PILs), derived from the 1:1 liquid mixtures of the organic superbases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with some carboxylic acids (propionic, butyric, hexanoic and octanoic). Glass transition, Tg, crystallization, Tc, cold-crystallization, Tcc, and melting, Tm, temperatures were determined for the PILs studied, showing marked differences with the base – Tg is lower for all DBN PILs, and no Tm, Tc, and Tcc could be obtained for the DBU PILs. The standard molar heat capacities, Cp,m0, were obtained with an uncertainty of less than ±0.3 % and their dependence on the base and on the acid’s chain length was studied in detail. The Cp,m0 of the DBU PILs were found to be significantly higher than those of the corresponding DBN PILs, which corroborates the greater ionic character of DBU PILs. The heat capacity data suggested the existence of a trend-shift with the chain length of the carboxylic acid. Similar to aprotic ionic liquids, the shift occurs around n = 5 (pentanoic acid) and suggests the existence of some degree of nanostructuration into polar and non-polar domains in PILs with larger acids. Moreover, PILs can display abnormally high excess heat capacities, resulting from the formation of an ionic mixture from neutral species. Analysis of the calculated excess heat capacities indicates that PILs tend to become less ionic as the temperature increases, which goes in accordance with the acid-base equilibrium shift.