18-Crown-6–Sodium Cholate Complex: Thermochemistry, Structure, and Stability

Abstract

118-crown-6, one of the most relevant crown ethers, and sodium cholate, steroidal surfactant 2 classified as natural bile salt, are components of novel, synthesized coordination complex; 18-crown-3 6-sodium cholate (18C6·NaCh). Like crown ethers, bile salts act as building blocks in supramolecular 4 chemistry in order to design new functionalized materials with a desired structure and properties. In 5 order to obtain thermal behavior of this 1:1 coordination complex, thermogravimetry and differential 6 thermal analysis were used, as well as microscopic observations and differential scanning calorimetry. 7Temperature dependent infrared spectroscopy (IR) gave a detailed view into phase transitions. The 8 structures during thermal treatment were observed with powder X-ray diffraction, and molecular 9 models of the phases are made. 10Hard, glassy, colorless compound 18C6·NaCh goes through crystalline -crystalline 11 polymorphic phase transitions at higher temperatures. The room temperature phase is indexed to a 12 triclinic lattice, while in the high temperature phases molecules take randomly one of the two different 13 configurations in the unit cell, resulting in the 2-fold symmetry. The formation of cholesteric liquid 14 crystalline phase occurs simultaneously with partial decomposition, followed by the isotropisation 15 with simultaneous and complete decomposition at much higher temperature, as obtained by IR. The 16 results provide valuable information about the relationship between molecular structure, thermal 17 properties, and stability of the complex, indicating the importance of an appropriate choice of cation, 18 amphiphilic, and crown ether unit in order to synthesize compounds with desired behavior.

In Situ Monitoring of a Quasi Solid-State Diffusion-Driven Cocrystallization in the Diphenylamine–Benzophenone System

“…For this purpose, variable-temperature transmission IR spectra were recorded in the 23–45 °C range with Δ t = 1 °C steps, as indicated in the phase diagram of Figure . Transmission IR spectroscopy enables a straightforward detection of phase transitions by monitoring of the baseline absorbance, , while the evolution of ν­(CO) and ν­(NH) features reflects changes in intermolecular interactions leading to melting and recrystallization.…”

Infrared spectroscopic monitoring of solid-state processes

“…For this reason, it is somewhat surprising that this method has not come into wider use, and the literature in which it has been applied is limited to a very narrow circle of authors. [187][188][189][190][191][192][193][194][195] Beyond its original use to detect temperature-induced solid-solid phase transitions, the method found use in monitoring isothermal crystallization of the cocrystalline phase from the melt. 194 Additionally, the temperature-dependant baseline measurements, together with changes in spectral features of a series of picrate surfactants indicate a phase transition, not evident from differential scanning calorimetry (DSC) or differential thermal analysis (DTA) measurements, thus indicating an adiabatic phase transition, which causes change in the optical but not thermal properties of the sample.…”

Structure and stability of supramolecular crown ether complexes

“…At the same time, the entropy penalty for binding guest molecules is relatively small compared to their open chain analogs which is caused by the preorganization, also known as the macrocyclic effect . This makes crown ethers, and in particular the well‐known 18‐crown‐6 (18c6), attractive for host–guest chemistry and still today they are widely investigated systems …”

“…[2] This makes crown ethers, and in particular the well-known 18crown-6 (18c6), attractive for host-guest chemistry and still today they are widely investigated systems. [3][4][5][6] Among other systems, alkali and alkaline earth metals as well as ammonium ions are in the focus of experimental and theoretical investigations [7][8][9][10] as these host-guest complexes have a great impact in biological and industrial processes. [11][12][13][14] Both the synthesis and the experimental determination of binding energies are highly demanding-usually involving multistep syntheses and a sophisticated set of analytical methods to evaluate the structures of the host-guest complex and its thermodynamics which has often even to be repeated in an iterative manner to accomplish a receptor with the desired binding properties.…”