Thermodynamics, structure, and dynamics in room temperature ionic liquids: the case of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF6]).

A detailed investigation of the phase diagram of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]) is presented on the basis of a wide set of experimental data accessing thermodynamic, structural, and dynamical properties of this important room temperature ionic liquid (RTIL). The combination of quasi adiabatic, continuous calorimetry, wide angle neutron and X-ray diffraction, and quasi elastic neutron scattering allows the exploration of many novel features of this material. Thermodynamic and microscopic structural information is derived on both glassy and crystalline states and compared with results that recently appeared in the literature allowing direct information to be obtained on the existence of two crystalline phases that were not previously characterized and confirming the view that RTILs show a substantial degree of order (even in their amorphous states), which resembles the crystalline order. We highlight a strong connection between structure and dynamics, showing the existence of three temperature ranges in the glassy state across which both the spatial correlation and the dynamics change. The complex crystalline polymorphism in [bmim][PF(6)] also is investigated; we compare our findings with the corresponding findings for similar RTILs. These results provide a strong experimental basis for the exploration of the features of the phase diagram of RTILs and for the further study of longer alkyl chain salts.

Thermodynamic study of alkyl-cyclohexanes in liquid, glassy, and crystalline states.

The specific heat of some alkyl-cyclohexanes in their liquid, supercooled liquid, crystalline, and (for the first time) glassy states has been measured by quasiadiabatic calorimetry. Thermodynamic properties as well as the glass forming ability have been studied as a function of systematic changes of the molecular structure. Only one stable crystalline phase is observed experimentally for ethylcyclohexane, propylcyclohexane, and butylcyclohexane. In the case of methylcyclohexane, experimental evidence is provided of a crystal-to-crystal transition at temperatures just below the melting.