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

How differences in the molecular structure of monohydroxy alcohols affect the tendency to crystallization

The tendency to crystallize was studied in the selected monohydroxy alcohols: 1-chloro-2-methyl-2-propanol, 1-chloro-2-propanol, 3-chloro-1-propanol, and 8-chloro-1-octanol. Performed calorimetric measurements have proved that the differences in structures of tested alcohols influence the tendency to crystallization. At a sufficiently fast heating rate, no crystallization was observed in the case of 1-chloro-2-propanol and 3-chloro-1-propanol, contrary to other two alcohols. The obtained results suggest that elongation of the alkyl chain or adding a methyl group to the hydrocarbon backbone increases the susceptibility to crystallization.

Laser-induced reprecipitation of pyrene at 77 K and its dynamics as studied by spectroscopic techniques

Reprecipitation of pyrene (Py) in the glassy solution of methylcyclohexane and isopentane at 77 K was observed by the repetitive irradiation of nanosecond (ns) laser pulses at 355 nm. The dynamics and mechanism of this reprecipitation were investigated by means of time-resolved fluorescence and absorption spectroscopies. Although only the fluorescence of the Py monomer was observed before the ns laser irradiation, fluorescence of the excimer was observed during the initial one-shot laser irradiation. From the time-resolved fluorescence measurements, it was revealed that the appearance of the excimer was due to the transient melting of the glassy medium by the local temperature increase around Py, which was induced by the iterative reabsorption of the laser light by the S1 state. The time period of melting for allowing the translational diffusion of Py was limited in the time region ≤ ca. 10 ns. With an increase in laser exposure, the fluorescence intensity of the excimer increased concomitantly with the appearance and increase of the amount of Py dimer, which was also confirmed by steady-state absorption spectroscopy. Time-resolved fluorescence spectrum recorded by only the one-shot laser exposure did not show dimer emission. This suggested that the formation of the dimer was through the excimer produced by transient melting; its dissociation into monomers was prohibited in the highly viscous environment. Upon further increase in laser exposure (several 1000 shots), solidified Py was observed due to crystal formation/aggregation with the dimers as the nucleation species.

Glass formability in medium-sized molecular systems/pharmaceuticals. I. Thermodynamics vs. kinetics

Scrutinizing critical thermodynamic and kinetic factors for glass formation and the glass stability of materials would benefit the screening of the glass formers for the industry of glassy materials. The present work aims at elucidating the factors that contribute to the glass formation by investigating medium-sized molecules of pharmaceuticals. Glass transition related thermodynamics and kinetics are performed on the pharmaceuticals using calorimetric, dielectric, and viscosity measurements. The characteristic thermodynamic and kinetic parameters of glass transition are found to reproduce the relations established for small-molecule glass formers. The systematic comparison of the thermodynamic and kinetic contributions to glass formation reveals that the melting-point viscosity is the crucial quantity for the glass formation. Of more interest is the finding of a rough correlation between the melting-point viscosity and the entropy of fusion normalized by the number of beads of the pharmaceuticals, suggesting the thermodynamics can partly manifest its contribution to glass formation via kinetics.

How much time is needed to form a kinetically stable glass? AC calorimetric study of vapor-deposited glasses of ethylcyclohexane

Glasses of ethylcyclohexane produced by physical vapor deposition have been characterized by in situ alternating current chip nanocalorimetry. Consistent with previous work on other organic molecules, we observe that glasses of high kinetic stability are formed at substrate temperatures around 0.85 Tg, where Tg is the conventional glass transition temperature. Ethylcyclohexane is the least fragile organic glass-former for which stable glass formation has been established. The isothermal transformation of the vapor-deposited glasses into the supercooled liquid state was also measured. At seven substrate temperatures, the transformation time was measured for glasses prepared with deposition rates across a range of four orders of magnitude. At low substrate temperatures, the transformation time is strongly dependent upon deposition rate, while the dependence weakens as Tg is approached from below. These data provide an estimate for the surface equilibration time required to maximize kinetic stability at each substrate temperature. This surface equilibration time is much smaller than the bulk α-relaxation time and within two orders of magnitude of the β-relaxation time of the ordinary glass. Kinetically stable glasses are formed even for substrate temperatures below the Vogel and the Kauzmann temperatures. Surprisingly, glasses formed in the limit of slow deposition at the lowest substrate temperatures are not as kinetically stable as those formed near 0.85 Tg.

Structural relaxation of vapor-deposited molecular glasses and supercooled liquids

The properties of vapor-deposited molecular glasses largely depend on deposition conditions, and stable and/or dense glasses are formed with several compounds.

Thermal properties and Brillouin-scattering study of glass, crystal, and "glacial" states in n-butanol

We investigated through noncommercial calorimetry and elastoacoustic Brillouin experiments the phase diagram of n-butanol and measured the specific heat and the thermal conductivity in a wide low-temperature range for its three different states, namely, glass, crystal, and the so-called "glacial" states. The main aim of the work was to shed light on the controversial issue of these allegedly polyamorphic transitions found in some molecular glass-forming liquids, first reported to occur in triphenyl phosphite and later in n-butanol. Our experimental results show that the obtained glacial state in n-butanol is not a homogenous, amorphous state, but rather a mixture of two different coexisting phases, very likely the (frustrated) crystal phase embedded in a disordered, glassy phase.

Thermal expansion of silver iodide-silver molybdate glasses at low temperatures

Ionic glasses obtained combining silver iodide and silver molybdate are characterized by quite low values of the glass transition temperature T(g) around 320-350 K, by high values of the dc ionic conductivity even at room temperature and by a peculiar behavior of the mechanical response at ultrasonic frequencies. In fact, at temperatures well below their glass transition temperature, these glasses exhibit an intense peak of acoustic attenuation well described by two different and almost overlapping relaxational contributions. Considering also that negative thermal expansion has been reported for some molybdate crystalline compounds, we have investigated in this work the thermal expansion of two silver iodomolybdate glasses (AgI)(1-x)(Ag(2)MoO(4))(x) for x=0.25 and x=0.33 in a wide temperature range (4.2-300 K) from cryogenic temperatures up to some 20 K below T(g) using a precision capacitance dilatometer aiming to understand whether the expansivity shows some possible fingerprint corresponding to the above-mentioned mechanical response. Two different measuring methods, a quasiadiabatic and a continuous one, have been used for the thermal expansion measurements. The results are discussed in comparison with the information obtained from previous investigations based on the extended x-ray absorption fine structure (EXAFS) technique and with the behavior of other ionic glasses.

Dynamics of glass-forming liquids. XII. Dielectric study of primary and secondary relaxations in ethylcyclohexane

The dynamics of ethylcyclohexane are investigated by high resolution dielectric spectroscopy aiming to characterize the relevant relaxational features of this simple system in its fluid, supercooled liquid, and glassy states. The dielectric signature of structural relaxation is a primary loss peak with amplitude Deltaepsilon=0.01, and a secondary loss process is found in the glassy state. This beta relaxation is compared with a "slow" process revealed by ultrasonics and with previously found gamma and chi processes in similar materials containing the cyclohexyl group. The results suggest that this secondary process is an intramolecular mode rather than a Johari-Goldstein process, consistent with its persistence in the liquid state at slow relaxation times which exceed those of the alpha process. The dielectric activity of such a slow process requires that the dipole magnitude changes with the intramolecular transition, whereas a change in dipole direction only would be masked by the faster structural relaxation.

Multiple Mechanical Relaxations in Ethylcyclohexane above the Glass Transition Temperature

The mechanical response of ethylcyclohexane has been investigated at ultrasonic frequencies in a large temperature range from 300 K down to the glass transition region. The results indicate the existence of a secondary relaxation not yet reported for this system. The comparison with literature data leads to a rather complex dynamic behavior. In fact, this molecular liquid exhibits three different mechanical relaxations above the glass transition temperature: a main structural process and two additional processes, both having a possible intramolecular origin.

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.