Ilya S. Balakhontsev

Organic light-emitting diode (OLED)-based devices continue to grow rapidly in popularity. This work presents a comprehensive thermodynamic study of four nitrogen-containing organic semiconductors: 1,3-bis(9H-carbazol-9-yl)benzene (mCP), 1,3,5-tri(9H-carbazol-9-yl)benzene (TCB), 1,3,5-tris(diphenylamino)benzene (TDAB), and 1,3,5-tris[(3-methylphenyl)phenylamino]benzene (m-MTDAB). A self-consistent set of phase-change thermodynamic parameters in a wide temperature range was obtained using several independent experimental and computational techniques. Vapor pressure measurements above the liquid and crystalline phases of the compounds under study were carried out using the thermogravimetry–fast scanning calorimetry method. Based on the temperature dependence of the measured vapor pressures, vaporization and sublimation enthalpies were derived. Differential scanning calorimetry was employed to determine the heat capacities of the condensed phases and the melting parameters of the studied compounds, as well as to investigate the polymorphism of TCB. Solution calorimetry was used to determine the fusion enthalpies of the compounds at 298.15 K. The obtained values were additionally compared with the literature data and calculated estimates. The results of this study may be used to predict properties for compounds with similar molecular structures.

The efficient tools for characterization of the amorphous state are key to deepening our understanding of the glass transition phenomenon, as well as controlling the conditions of glass production, storage, and exploitation. The thermodynamic stability of glasses is commonly determined by differential scanning calorimetry (DSC) by measuring the heat capacity in the devitrification region. In this way, the fictive temperature, showing the deviation between the glass and equilibrium liquid, is obtained. Fundamental limitations of such a procedure are the need to extrapolate the temperature dependences of the glass and liquid state heat capacities and possible interference of crystallization. In this work, we demonstrated that solution calorimetry can be an efficient instrument to determine the relative thermodynamic stability of glasses directly at 298.15 K. Melt-quenched and vapor-deposited glasses of 1,3,5-tris-(α-naphthyl)benzene (TNB) were obtained and studied by DSC and solution calorimetry. The difference between the solution enthalpies in benzene of the melt-vitrified and vapor-deposited glasses was consistent with the data on fictive temperatures obtained by DSC. The enthalpies of the amorphous thin films slowly deposited onto a cold substrate were substantially lower than those of the vitrified melts, agreeing with the recent studies of ultrastable glasses. The results of the study highlight the perspectives of solution calorimetry as a tool for studying the thermodynamic state of glasses that crystallize quickly on heating, including ultrastable ones, and its promising insights into the Kauzmann paradox.

1,3,5-Tris-(α-naphthyl)benzene is an organic non-electrolyte with notable stability of an amorphous phase. Its glassy and supercooled liquid states were previously studied by spectroscopic and calorimetric methods. Despite the continuing interest in its amorphous state and, particularly, vapor-deposited glasses, the thermodynamic parameters of the vaporization of 1,3,5-tris-(α-naphthyl)benzene have not been obtained yet. Likewise, the reliable evaluation of the thermodynamic parameters of fusion below the melting point, required to establish the thermodynamic state of its glass, is still an unsolved problem. In this work, the heat capacities of crystalline and liquid phases, the temperature dependence of the saturated vapor pressures, fusion and vaporization enthalpies were determined using differential and fast scanning calorimetry and were verified using the estimates based on solution calorimetry. The structural features of 1,3,5-tris-(α-naphthyl)benzene are discussed based on the computations performed and the data on the molecular refractivity. The consistency between the values obtained by independent techniques was demonstrated.

1,3,5-Tris-(α-naphthyl)benzene is an organic non-electrolyte with notable stability of an amorphous phase. Its glassy and supercooled liquid states were previously studied by spectroscopic and calorimetric methods. However, the thermodynamic parameters of phase transitions available from the literature are inconsistent. In this work, the heat capacities of crystalline and liquid phases, the temperature dependence of the saturated vapor pressures, fusion and vaporization enthalpies were determined using differential and fast scanning calorimetry and verified using the estimates based on solution calorimetry. The structural features of 1,3,5-tris-(α-naphthyl)benzene are discussed based on the computations performed and the data on the molecular refractivity. The consistency between the values obtained by independent techniques was demonstrated. The deviations in the previously published data were attributed to possible polymorphism.

A new polymorph of anhydrous β-cyclodextrin (polymorph III) was obtained and characterized for the first time using powder X-ray diffraction, infrared spectroscopy, and thermal analysis. The solution enthalpy and time of dissolution in water were determined using solution calorimetry for this polymorph and compared with those of the dried commercial form of β-cyclodextrin (polymorph I), its amorphous form, and 2-hydroxypropyl-β-cyclodextrin. The specific heat capacities of polymorphs I and III were determined using differential scanning calorimetry across a wide range of temperatures, providing enthalpy and Gibbs energy values for the polymorphic transition at 298 K. The affinities of polymorph III and 2-hydroxypropyl-β-cyclodextrin for water were characterized by determining their hydration isotherms, which provided values of hydration Gibbs energy. Being energy-rich, the new-found polymorph of β-cyclodextrin has a significantly higher dissolution rate and an increased affinity for water compared with the dried commercial form of β-cyclodextrin. These properties render the new polymorph promising in industrial applications for guest inclusion in aqueous solutions and pastes, and may be a desirable alternative for water-soluble β-cyclodextrin derivatives.

A new polymorph of anhydrous β-cyclodextrin (polymorph III) was obtained and characterized for the first time using powder X-ray diffraction, infrared spectroscopy and thermal analysis. Solution enthalpy and time of dissolution in water were determined by solution calorimetry for this polymorph and compared with those of dried commercial form of β-cyclodextrin (polymorph I), its amorphous form and 2-hydroxypropyl-β-cyclodextrin. Specific heat capacity of polymorphs I and III was determined by differential scanning calorimetry in the wide range of temperatures, which gives enthalpy and Gibbs energy of polymorphic transition at 298 K. Affinity of polymorph III and 2-hydroxypropyl-β-cyclodextrin for water was characterized by determination of their hydration isotherms, which gives the values of hydration Gibbs energy. Being energy-rich, the found new polymorph of β-cyclodextrin has a significantly higher dissolution rate and an in-creased affinity for water compared with the dried commercial form of β-cyclodextrin. Such properties make the new polymorph promising in industrial applications for guest inclusion in aqueous solutions and pastes and may be a desirable alternative for water-soluble β-cyclodextrin derivatives.

In this work, we present a comprehensive study of the thermodynamic properties of 3-and 4-ethoxyacetanilides. The heat capacities in crystalline, liquid, and supercooled liquid states from 80 to 475 K were obtained using adiabatic, differential scanning (DSC), and fast scanning (FSC) calorimetries. The fusion enthalpies at T_m were combined from DSC measurement results and the literature data. The fusion enthalpies at 298.15 K were evaluated in two independent ways: adjusted according to Kirchhoff’s law of thermochemistry, and using Hess’ law. For the latter approach, the enthalpies of the solution in DMF in crystalline and supercooled liquid states were derived. The values obtained by the two methods are consistent with each other. The standard thermodynamic functions (entropy, enthalpy, and Gibbs energy) between 80 and 470 K were calculated.

Evaluating the temperature dependence of the fusion enthalpy is no trivial task, as any compound melts at a unique temperature. At the same time, knowledge of the fusion enthalpies under some common conditions, particularly at the reference temperature of 298.15 K, would substantially facilitate the comparative analysis and development of the predictive schemes. In this work, we continue our investigations of the temperature dependence of the fusion enthalpy of organic non-electrolytes using solution calorimetry. As an object of study, n-octadecanophenone, an arylaliphatic compound was chosen. The solvent appropriate for evaluating the fusion enthalpy at 298.15 K from the solution enthalpy of crystal was selected: p-xylene. The heat capacity and fusion enthalpy at the melting temperature were measured by differential scanning calorimetry to derive the fusion enthalpy at 298.15 K from the Kirchhoff’s law of Thermochemistry. An agreement between the independently determined values was demonstrated. This particular result opens a perspective for further studies of the fusion thermochemistry of arylaliphatic compounds at 298.15 K by solution calorimetry.