INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
Liquid Methanol Confined within Functionalized Silica Nanopores. 2. Solvation Dynamics of Coumarin 153
JAVIER RODRIGUEZ, M. D. ELOLA Y D. LARIA
JOURNAL OF PHYSICAL CHEMISTRY B
American Chemical Society
Año: 2011 vol. 115 p. 12859 - 12859
Equilibrium and dynamical characteristics pertainingto the solvation of the fluorescent probe coumarin 153 inliquid methanol confined within cylindrical silica pores areinvestigated using molecular dynamics techniques. Three kindsof pores are examined: (i) Soft hydrophobic cavities, in whichwallsolvent interactions were exclusively of the Lennard-Jones type; (ii) Hydrophilic cavities, in which unsaturatedoxygen sites at the wall were transformed into hydroxyl groups;(iii) Rugged pores, in which 60% of the polar groups weretransformed into bulkier and mobile trimethylsilyl moieties.Equilibrium solvation structures in the three pores differ considerably: In hydrophobic environments, the solute remains adsorbedto the pore wall, with its molecular plane mostly parallel to the interface. Upon hydroxylation, the solid interface becomespreferentially coated by methanol, leading to a bistable solvation state of the probe, with alternation of ?wall-like? and ?bulk-like?events. An increment in the interface roughness promotes a solvation structure characterized by the embedding of the probe within awall domain surrounded by trimethylsilyl groups. In hydrophobic environments, the relevant dynamical modes of the probe can becast in terms of in-the-wall rotations, whereas in hydrophilic pores, out-of-the-wall evolutions are also present. The embedding of theprobe at wall domains in more rugged pores, leads to restrained angular motions, with maximum amplitudes of the order of 20degrees.Results of early stages of the solvation response of the environment following a vertical excitation of the probe are also presented.During the initial 30 ps, we found no evidence of modifications in the spatial localizations of the probe. The overall responses are found to be between 2 and 4.5 times slower than the one observed in the bulk, being the fastest relaxation the one associated to rugged pores whereas the slowest one corresponds to hydrophilic cavities. These features are rationalized in terms of thecomposition of the first solvation shells and the local dynamical inhomogeneities prevailing within the different regions of the pores.