Dinàmica orientacional i estructura local en liquids moleculars sobrerefredats
- Autores: Jordi Ortiz de Urbina Viadé
- Directores de la Tesis: Gemma Sesé i Castel (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2021
- Idioma: español
- Materias:
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- Resumen
The aim of this thesis is to study, by means of molecular dynamics simulations, the behaviour of two molecular models at different temperatures, approaching the glass transition. One of them mimics methanol molecules, whereas the other differs from the former only in the ability of forming hydrogen bonds. The analysis carried out is focused on the reorientational dynamics, the dielectric response and various orientational properties that characterize the local structure.
All time correlation functions show a two-stage relaxation, separated by a plateau at the lowest temperatures. Translational relaxation times are always larger than reorientational ones when removing hydrogen bonds. A coupling between reorientation and translation has been found: higher mobility in translation is associated with larger reorientation angles.
Rotational dynamics around a principal inertial axis has also been investigated in the non-associated system. Rotational diffusion coefficients can be fit to a mode coupling law with a critical temperature which is considerably lower than that of translation. That means that reorientation is still active at temperatures characterized by a hindered translational dynamics. Results show that the Stokes-Einstein relation breaks at a higher temperature than the Stokes-Einstein-Debye relation does.
Hydrogen bonds strongly restrict the orientation of molecular dipole moments of neighbouring molecules, so that both Kirkwood factor and dielectric permittivity decrease as the ability to establish them is suppressed. At room temperature, and in the system without hydrogen bonds, the most important contribution to the total dipole correlation function is the autocorrelation of molecular dipoles. In addition, relaxation times associated with the autocorrelation and the cross-correlations are similar, and the Debye’s model of dielectric relaxation reproduces reasonably well the behaviour of the system. In methanol, correlation between orientations of different molecules is much more relevant, and the Davidson-Cole model is more appropriate. At lower temperatures, the behaviour of both systems is best represented with the Davidson-Cole model. The longitudinal and transverse components of the dipole density for the smallest wave vectors compatible with the size of the system, have allowed to recover the value of the dielectric constant.
The anisotropy of the rotational dynamics in methanol has also been analyzed and rotation around the three main molecular axes of inertia has been studied. It has been found that rotational spectra contain higher frequency contributions than those detected for the translational spectrum. Its dependence has been analyzed for different hydrogen bonded states. As the number of hydrogen bonds established by a molecule increases, the relevant peaks shift towards larger frequencies. Upon cooling, no significant changes in the frequencies involved are encountered, but rather in the relevance of the peaks. Several rotational relaxation models have been tested, which predict analytical relationships between angular momentum relaxation times and reorientational times. The Langevin model satisfactorily reproduces the behaviour of the liquid without hydrogen bonds, even in the supercooled state.
Finally, the local structure of both systems has been investigated. The Voronoi polyhedra have been calculated and their topological and metric properties have been studied. This analysis has allowed to quantify the increase in local order in both systems upon cooling: a decrease in the number of different types of polyhedra and an increase in the frequency of the most frequent polyhedra have been encountered.
