Mesophases of Alkylammonium Salts. I. First-Order Transitions

Thermal Properties and Ionic Conductivity of Tetra-n-Butylammonium Perchlorate

The thermal parameters of the phase transitions and transport properties of tetra-n-butylammonium (TBA) perchlorate (n-C4H9)4NClO4 (TBAClO4) were investigated. TBAClO4 has a polymorphous transition at 330 K and melts at 487 K. The structure of the high-temperature (HT) phase belongs to cubic symmetry and is similar to the HT phases of TBABF4 and TBAI salts. The conductivity parameters of the low-temperature and HT phases of TBAClO4 were determined from the Arrhenius plots. The thermodynamic parameters and transport properties of TBAClO4 were compared with those of other TBA salts having isostructural HT phases. The polymorphous phase transition entropy was found to correlate with the conductivity of HT phases of TBA salts; TBAClO4 has the lowest conductivity compared to TBABF4 and TBAI salts.

From a eutectic mixture to a deep eutectic system via anion selection: Glutaric acid + tetraethylammonium halides

In pursuit of understanding structure-property relationships for the melting point depression of binary eutectic mixtures, the influence of the anion on the solid-liquid (S-L) phase behavior was explored for mixtures of glutaric acid + tetraethylammonium chloride, bromide, and iodide. A detailed experimental evaluation of the S-L phase behavior revealed that the eutectic point is shifted toward lower temperatures and higher salt contents upon decreasing the ionic radius. The salt fusion properties were experimentally inaccessible owing to thermal decomposition. The data were inter- and extrapolated using various models for the Gibbs energy of mixing fitted to the glutaric-acid rich side only, which allowed for the assessment of the eutectic point. Fitting the experimental data to a two-parameter Redlich-Kister expansion with Flory entropy, the eutectic depth could be related to the ionic radius of the anion. The anion type, and in particular its size, can therefore be viewed as an important design parameter for the liquid window of other acid and salt-based deep eutectic solvents/systems.

Study on Physicochemical and Thermal Properties of Tetrabutylammonium-Based Cation Ionic Salts Induced by Al2O3 Additive for Thermal Energy Storage Application

The physicochemical and thermal properties of tetrabutylammonium bromide (TBA-Br) and tetrabutylammonium hexafluorophosphate (TBA-PF6), and their change with the addition of Al2O3, were investigated using infrared (IR) spectroscopy and by simultaneously conducting thermal thermogravimetric (TG) analysis and differential thermal analysis (DTA) to obtain the differential scanning calorimetry (DSC) thermogram. The change in the IR data is characterized by the growth of a large peak in the range of 3500 cm−1 and the reduction of peaks below 1000 cm−1 with the additive concentration. The decomposition temperature determined from the peak in the DTG curve is nearly constant for TBA-Br, and it decreases with the addition of Al2O3 for TBA-PF6, although it does not depend on the concentration of the additives. The DTA curve of ionic salts with the addition of Al2O3 shows additional peaks, which indicates a change in the sample’s temperature at disorder or phase transitions. The variation in the melting temperature with additive concentration is similar to that of decomposition temperature. The maximum heat of fusion value was approximately 67 kJ kg−1 for the doped TBA-Br and TBA-PF6 but was achieved at a different additive concentration. This is due to the additional disorder in the system induced by the dissolution of Al2O3.

Fusion and Thermal Degradation Behavior of Symmetric Sulfur-Containing Quaternary Ammonium Bromides

Quaternary ammonium salts are widely used in consumer products and industrial processes, where their instability at elevated temperatures limits their range of applications. In this work, the thermal behavior of a new class of quaternary ammonium salts was investigated using differential scanning calorimetry. These salts contain a sulfur atom in each chain at the fourth position from the central nitrogen and are thus termed thiaquats. The temperatures at which these salts melt and thermally degrade were determined, and enthalpies and entropies of fusion were evaluated. Their melting points increase with chain lengths, in contrast to the behavior of traditional quaternary ammonium salts. Furthermore, they exhibit enthalpies and entropies of fusion significantly lower than corresponding tetraalkyl analogues. These trends provide physical insight into the molecular-level behavior of these salts, suggesting that they do not fully dissociate upon melting. The thiaquats also exhibit thermal stability to markedly higher temperatures than traditional quaternary ammonium bromides, a phenomenon that can be explained in by strong pairing between the quaternary cation and bromide anion, which inhibits possible decomposition mechanisms. This enhanced thermal stability may enable applications of these salts in processes where traditional salts are not viable, such as phase-transfer-catalyzed systems performed at elevated temperatures.

Shape-memory effect in an organosuperelastic crystal

An organoionic crystal of tetrabutyl-n-phosphonium tetraphenylborate thermally recovers its shape through superelasticity, similar to shape-memory alloys.

Shape-memory materials, i.e., polymers (SMPs: shape-memory polymers) and alloys (SMAs: shape-memory alloys), have been developed in very different ways since they are historically far apart in material type as well as physical property. In the deformation process, SMPs require only a slight stress due to the properties of organic polymer solids, and they reveal a smaller recovery force during the thermoplastic process whereas SMAs require a relatively large stress due to metallic properties, and they thermally tighten to generate a larger recovery force via destabilization of the stress-induced phase. An investigation into the unexplored area of the material adjoining both ends of SMPs and SMAs would lead toward a better understanding of shape-memory materials and extend future applications and material types. Here, we report the discovery of a shape-memory effect in an organic crystal bearing a combination of crystal transformability like in SMAs with organic components like SMPs.

Understanding the Polymorphic Behavior of Sibenadet Hydrochloride through Detailed Studies Integrating Structural and Dynamical Assessment

Three polymorphs of sibenadet hydrochloride (AR-C68397AA, Viozan) have been shown to exist at ambient temperature and have been characterized. Each undergoes a solid-state transition to a common high temperature form, which melts at approximately 220 degrees C. X-ray powder diffraction (XRPD) indicates that Polymorphs I and II share a similar layered structure not exhibited by Polymorph III. All three ambient temperature polymorphs show evidence of varying degrees of dynamic disorder of the terminal phenyl group as shown by solid-state NMR spectroscopy. Furthermore, the carbons alpha to the ether link also undergo different rates of mobility in Polymorphs I and II. This variation in the extent of dynamic disorder results in an alteration in the short-range structure resulting in distinct polymorphs. Polymorph I is the thermodynamically stable form at room temperature as indicated by solution calorimetry and assessment by thermodynamically driven solution mediated phase transition studies. The present study aims to address the types of discriminatory data required to make a clear distinction between physical forms and define, unequivocally, the presence of polymorphism.

Cubic mesophase in an unsymmetrical alkyl ammonium salt. Synthesis and structural model

N,N,N-butylethylpentylpropylammonium iodide 4 and related molecules have been selectively synthesised from commercially available aldehydes, amines and alkyl iodides using a reductive alkylation procedure. The crystalline texture of 4 obtained on cooling is optically isotropic between crossed polarisers, indicating a cubic structure. Differential scanning calorimetry (DSC, +10 K min-1) reveals a glass phase transition at -59 degrees C and a melting point at 192 degrees C. The melting entropy (23.9 J mol-1 K-1) indicates a first-order transition between a highly disordered mesophase and the isotropic liquid. Powder X-ray diffraction patterns were indexed in the cubic system (a = 14.08A; Pm3n space group). In this cell, the molecular packing with Z = 6 corresponds to a rather low compactness of 65%. Iodine and tetraalkylammonium ions occupy positions with a 4m2 site symmetry. These highly symmetrical states may be generated by stepwise rotation of the ammonium cation. The same structural model for orientationally disordered crystal (ODIC) phases can be applied to a series of tetraalkylammonium bromides and iodides.