Samantha T. Hung

Stanford

“Ultrafast Dynamics and Liquid Structure in Mesoporous Silica”

Ultrafast infrared spectroscopy provides access to the picosecond dynamics of liquids confined in mesoporous silica, an orderly and well-characterized nanomaterial with applications from medicine to energy. The tunable silica framework enables the quantitative study of confinement effects on the molecular interactions and properties of liquids. With insights from molecular dynamics simulations, the effects of confinement on the ultrafast dynamics of a nonaqueous solvent were explained in terms of changes in the liquid structure and compared to the effects on the dynamics of water confined in the same silica framework.

ABSTRACT

Nanoconfined liquid dynamics are often distinct from that of the bulk, and desirable differences can be harnessed to design novel processes and materials. Liquids confined in mesoporous silica (pore diameter 2-50 nm) have applications in hybrid electrolytes, heterogeneous catalysis, and drug delivery. The dynamics of a nonaqueous solvent, 1-methylimidazole, confined in mesoporous silica (2.8, 5.4, and 8.3 nm pore diameters) were examined using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations of a dilute probe, the selenocyanate anion. Compared to water, a polar protic solvent, the polar aprotic solvent, 1-methylimidazole, experiences a much more dramatic slowdown of liquid dynamics upon confinement in mesoporous silica. The effects were quantified by modified two-state models used to fit three spatially averaged experimental observables: vibrational lifetime, orientational relaxation, and spectral diffusion. Modeling the subnanometer distance dependences of the observables with insights from simulations guided interpretation of the molecular roots of confinement effects, illustrating the importance of electrostatic effects and H-bonding interactions in the behavior of confined liquids.
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