Chirality dependent inverse-melting and re-entrant gelation in α-cyclodextrin/1-phenylethylamine mixtures

Solutions of cyclohexakis-(1→4)-α-d-glucopyranosyl, α-cyclodextrin, αCD, in R-(+)-1-phenylethylamine, αCD/R-PEA, and S-(−)-1-phenylethylamine, αCD/S-PEA, display abnormal phase transitions that strongly depend on supramolecular diastereomeric interactions. While αCD/R-PEA mixtures show one sol–gel inverse-melting phase transition, αCD/S-PEA mixtures show temperature dependent gel–sol–gel re-entrant behavior. NMR, Raman spectroscopy, microscopy and X-ray scattering measurements reveal that hydrogen bond weakening in solution, as well as changes in crystal composition are responsible for entropy increase and gel formation upon heating.

Solutions of α-cyclodextrin in chiral 1-phenylethylamine display abnormal phase transitions. Depending on supramolecular diastereomeric interactions, inverse-melting and re-entrant gels are formed.

Complexation reactions in pyridine and 2,6-dimethylpyridine-water system: The quantum-chemical description and the path to liquid phase separation

Phase separation in substituted pyridines in water is usually described as an interplay between temperature-driven breakage of hydrogen bonds and the associating interaction of the van der Waals force. In previous quantum-chemical studies, the strength of hydrogen bonding between one water and one pyridine molecules (the 1:1 complex) was assigned a pivotal role. It was accepted that the disassembly of the 1:1 complex at a critical temperature leads to phase separation and formation of the miscibility gap. Yet, for over two decades, notable empirical data and theoretical arguments were presented against that view, thus revealing the need in a revised quantum-mechanical description. In the present study, pyridine-water and 2,6-dimethylpyridine-water systems at different complexation stages are calculated using high level Kohn-Sham theory. The hydrophobic-hydrophilic properties are accounted for by the polarizable continuum solvation model. Inclusion of solvation in free energy of formation calculations reveals that 1:1 complexes are abundant in the organically rich solvents but higher level oligomers (i.e., 2:1 dimers with two pyridines and one water molecule) are the only feasible stable products in the more polar media. At the critical temperature, the dissolution of the external hydrogen bonds between the 2:1 dimer and the surrounding water molecules induces the demixing process. The 1:1 complex acts as a precursor in the formation of the dimers but is not directly involved in the demixing mechanism. The existence of the miscibility gap in one pyridine-water system and the lack of it in another is explained by the ability of the former to maintain stable dimerization. Free energy of formation of several reaction paths producing the 2:1 dimers is calculated and critically analyzed.

High Partial Compression of 1-Butyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Diluted in 2,6-Dimethylpyridine+Water Solvent

Phase properties, molar volumes, isentropic compressions and isobaric thermal expansions of the system 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C4mim][Tf2N]+2,6-dimethylpyridine+ water were studied and interpreted in terms of molecular interactions. The solvation of [C4mim][Tf2N] diluted in the binary solvent consists most probably in the accommodation of the ions between pyridine rings of the hydrogen-bonded hydrates of 2,6-dimethylpyridine (C7H9N...H–OH) n . Cations and anions are located in the neighbouring voids forming ionic pairs. That process is dependent on the electric permittivity of the solvent, The higher is the permittivity, i.e. the lower is the concentration of 2,6-dimethylpyridine, the larger are limiting partial compression and volumes of [C4mim][Tf2N] due to weakened Coulomb forces that act between ions. That leads to gradual decay of the ion pairs. The effect on compression is particularly pronounced, while that on volume is small, but evident. Solvation of ions causes that the limiting partial expansion of [C4mim][Tf2N] is equal to zero or at least close to that value. With increasing concentration of the ionic liquid, the solvation shells undergo disruption that leads to the phase separation.

Hydration modes of an amphiphilic molecule: NMR, FTIR, and theoretical study of the interactions in the water-lutidine system

Using (1)H and (13)C NMR spectra and relaxations, PFG NMR diffusion measurements, FTIR spectra, and quantum-chemical structure predictions and optimizations on the MP2/6-31G(d) level, we have studied interactions between water (W) and lutidine (2,6-dimethylpyridine, L) in a wide range of ratios. At low W content up to 35%, W was found to bind to L by an O-H***N hydrogen bond and form transient L-W aggregates containing two to four L molecules in cooperation with two to three other W molecules. At higher W content, these aggregates are gradually cleaved to single L molecules enwrapped by a hydration shell anchored in an O-H***N hydrogen bond. At all compositions of the mixture, the various hydrate forms are in fast mutual exchange with a correlation time on the order of 1 x 10(-5) s.