Abstract

Fluids, especially liquid crystals, enclosed in porous materials exhibit a rich phase behaviour which can be exploited in both the study of the fundamental properties of the fluid itself, as well as its confinement. Nuclear magnetic resonance spectroscopy of xenon dissolved in the confined fluid is a sensitive and noninvasive method to study these systems. The purpose of this thesis is to develop and validate computational methods which can be used to interpret, as well as predict the structure and ¹²⁹Xe NMR of the fluid.

The work used a combination of classical simulations of coarse-grained molecular models with quantum-chemical parameterisation for the pairwise-additive potential energies and the ¹²⁹Xe nuclear shielding tensors. More specifically, Monte Carlo molecular simulation was used to study the structure and phase behaviour of a uniaxial liquid crystal in a cylindrical nanocavity. Next, a small number of xenon atoms was added to the simulation and the nuclear shielding of ¹²⁹Xe was computed from the simulated configurations. Finally, the factors contributing to the maximum of the chemical shift of ¹²⁹Xe in water were studied with a semianalytical cavity model, which was parameterised using a combination of molecular dynamics simulations and quantum-chemical calculations.

It was found that planar anchoring of liquid-crystal molecules at the walls of a cylindrical cavity promotes orientational order well above the isotropic-nematic phase transition temperature and the sharp isotropic-nematic transition of the bulk liquid crystal is replaced by a gradual paranematic-nematic transition. At lower temperatures the formation of translationally ordered phases is hindered by the packing of molecules at the wall.

The ¹²⁹Xe shielding computed from the simulations corresponds qualitatively to earlier experimental results for xenon dissolved in a liquid crystal enclosed in the small cavities of controlled pore glass. The gradual change of orientational order at the paranematic-nematic transition is hard to observe in the isotropic shielding, especially in very small pores, but the transition reveals itself in the anisotropic part of the shielding.

The semianalytical cavity model qualitatively reproduces the maximum of the ¹²⁹Xe chemical shift in water. The extremum is interpreted as arising from the interplay of the variation in water density and the collisions of the Xe solute with its nearest water molecule neighbours. The interpretation suggests the chemical shift maximum could be observed also in other solvents.

Osajulkaisut / Original papers

Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon / Original papers are not included in the electronic version of the dissertation.

1. Karjalainen, J., Lintuvuori, J., Telkki, V.-V., Lantto, P., & Vaara, J. (2013). Constant-pressure simulations of Gay–Berne liquid-crystalline phases in cylindrical nanocavities. Physical Chemistry Chemical Physics, 15(33), 14047. https://doi.org/10.1039/c3cp51241j

2. Karjalainen, J., Vaara, J., Straka, M., & Lantto, P. (2015). Xenon NMR of liquid crystals confined to cylindrical nanocavities: a simulation study. Physical Chemistry Chemical Physics, 17(11), 7158–7171. https://doi.org/10.1039/c4cp04868g

3. Peuravaara, P., Karjalainen, J., Zhu, J., Mareš, J., Lantto, P., & Vaara, J. (2018). Chemical shift extremum of 129Xe(aq) reveals details of hydrophobic solvation. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-25418-4

   Rinnakkaistallennettu versio / Self-archived version