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  • Title: Liquid methanol confined within functionalized silica nanopores. 2. Solvation dynamics of coumarin 153.
    Author: Elola MD, Rodriguez J, Laria D.
    Journal: J Phys Chem B; 2011 Nov 10; 115(44):12859-67. PubMed ID: 21932806.
    Abstract:
    Equilibrium and dynamical characteristics pertaining to the solvation of the fluorescent probe coumarin 153 in liquid methanol confined within cylindrical silica pores are investigated using molecular dynamics techniques. Three kinds of pores are examined: (i) Soft hydrophobic cavities, in which wall-solvent interactions were exclusively of the Lennard-Jones type; (ii) Hydrophilic cavities, in which unsaturated oxygen sites at the wall were transformed into hydroxyl groups; (iii) Rugged pores, in which 60% of the polar groups were transformed into bulkier and mobile trimethylsilyl moieties. Equilibrium solvation structures in the three pores differ considerably: In hydrophobic environments, the solute remains adsorbed to the pore wall, with its molecular plane mostly parallel to the interface. Upon hydroxylation, the solid interface becomes preferentially 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 a wall domain surrounded by trimethylsilyl groups. In hydrophobic environments, the relevant dynamical modes of the probe can be cast in terms of in-the-wall rotations, whereas in hydrophilic pores, out-of-the-wall evolutions are also present. The embedding of the probe at wall domains in more rugged pores, leads to restrained angular motions, with maximum amplitudes of the order of 20°. 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 the composition of the first solvation shells and the local dynamical inhomogeneities prevailing within the different regions of the pores.
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