Computer simulation of an excess proton in aqueous systems
This thesis aims at studying the microscopic physical-chemical properties of an excess proton in aqueous systems. From bulk water environments to narrow hydrophobic channels constructed with a double-layered graphene slab, the local proton structure has been computed and analyzed together with a var...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2016 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/396325 |
| Acceso en línea: | http://hdl.handle.net/10803/396325 https://dx.doi.org/10.5821/dissertation-2117-96386 |
| Access Level: | acceso abierto |
| Palabra clave: | Àrees temàtiques de la UPC::Física 004 539 |
| Sumario: | This thesis aims at studying the microscopic physical-chemical properties of an excess proton in aqueous systems. From bulk water environments to narrow hydrophobic channels constructed with a double-layered graphene slab, the local proton structure has been computed and analyzed together with a variety of dynamical properties, including proton diffusion, transfer rates and spectroscopic vibrational bands. All these properties have deep relationship with the most relevant structures associated with the proton: the hydronium ion (H3O)+, the Zundel dimer (H5O2)+, and the Eigen complex (H9O4)+. The influence of temperature and confinement has been systematically evaluated in a wide variety of thermodynamic conditions, ranging from low temperature amorphous ices to high temperature sub-critical states. Classical molecular dynamics simulations have been used to model dynamics of the systems, whereas a multi-scale empirical valence bond method has been applied to model the specific quantum nature of the proton. Within this approach, a variety of diabatic bond states is defined and a semi-classical Hamiltonian is constructed. The linear combination of all diabatic states involves a different quantum weight for each state, so that proton characteristics have contributions from high to low values of each coefficient, implicitly accounting for the delocalization of the proton. We have observed that a lone proton in unconstrained water produces important changes in the local water structure, especially at low temperatures. Below 273 K, the mobility of the proton gets significantly reduced, compared to ambient conditions, although it can still be transferred along the whole range explored. The activation energy barriers show a clear Arrhenius-like dependence and range of the order 1 kJ/mol. When the system is confined inside a hydrophobic channel, the microscopic behavior of the lone proton is seriously affected: as the distance between graphene plates is reduced below 1.5 nm, the local structure of the proton shows a clear enhancement and indicates a tendency of lone quantum charge to move closer to the interfaces. At the shortest interplate distances (0.9 and 0.7 nm), the system becomes nearly planar, and two-dimensional water sheets formed by one to two layers have been observed. In such cases, both Zundel and Eigen structures are still seen (indicated by a signature 2500 1/cm frequency band), with a tendency of mixed Zundel-Eigen moieties to disappear and to be replaced by single hydronium species. The combined effect of hydrophobic plates and temperature changes has been evaluated at densities between 0.02 and 0.07 1/Å^3. As a general trend, a competition between the two effects has been observed. So, as it was indicated above, confinement has strong influence on the local structure of the proton, whereas changes in temperature mainly affect proton's dynamics. Both proton transfer and proton diffusion are activated processes with energies up to 10 kJ/mol, depending of the particular thermodynamic state of the system. |
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