Wojciech Marczak

Molecular clusters in aqueous solutions of pyridine and its methyl derivatives

Small-angle neutron scattering proved that molecules in aqueous solutions of pyridine, 2-methylpyridine and 2,6-dimethylpyridine form clusters. The clusters are dynamic aggregates consisting of hydrogen-bonded water-amine complexes. Strengthening of the hydrogen bonds between water and amine molecules due to the methyl groups in the ortho position in the pyridine ring makes the structures more stable, as was evidenced by relatively long times of the structural relaxation. The strong intermolecular forces affect the thermal expansion of the systems. No aggregates similar to those in aqueous systems are present in the methanolic ones. That points to the crucial role of water in the molecular clustering. A molecule of methanol, although capable of hydrogen bonding with the amines, cannot participate in larger structures because of the lack of protons that could form the enhanced network. Thus, even if the amine-methanol complexes occur, they are incapable of further association. It was shown that the co-operative nature of hydrogen bonds and the propensity of water to association are the main factors that determine the properties of aqueous systems.

Internal pressure of a thermodynamically ideal mixture and the excess internal pressure

The excess internal pressure is often discussed in terms of molecular interactions in liquid mixtures. The additivity of internal pressures in the mole fraction scale is often erroneously assumed for an ideal mixture. In this paper, a thermodynamically correct formula for the excess internal pressure is suggested and compared with that based on the mole fraction additivity.

Water-induced aggregation and hydrophobic hydration in aqueous solutions of N-methylpiperidine

Liquid system N-methylpiperidine–water shows a miscibility gap with a lower critical solution temperature of 316.7 K. The phase separation is most likely due to the aggregation of N-methylpiperidine–water complexes, evident in the intensity of the small-angle neutron scattering at temperatures much lower than the LCST. Such complexes arise because the attraction forces between unlike molecules are stronger than the water–water ones, and aggregation is possible through the O–H⋯O bonds involving the hydration water molecules. The aggregates are dynamic structures with nanoseconds-order relaxation times, as it was estimated by the ultrasonic absorption experiment. While hydrophilic aggregation prevails at relatively high concentrations of the amine, the hydrophobic hydration is possible at low concentrations, likely consisting of the formation of structures resembling those in the sH clathrates observed in the solid state. The hydrophobic hydration of N-methylpiperidine is manifested in the minima of the partial volume isotherms at the amine mole fraction close to 0.01 and in the limiting partial molar compression approximately equal to zero. Essential similarity of the N-methylpiperidine–water system to aqueous solutions of pyridine and its methyl derivatives studied previously, suggests that those amines are potential clathrate hydrate promoters.

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

Abstract 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.