Water Dynamics in Nanoporous Silica: Ultrafast Vibrational Spectroscopy and Molecular Dynamics Simulations

Abstract

Nanoporous silica materials are important in catalysis, energy, and materials applications in which water is an essential component. System performance is intimately connected to the water dynamics occurring in the confined environment. However, the dynamics and associated structures of water in nanoporous silica have proven challenging to measure and predict. Here, confined water dynamics are examined via the ultrafast infrared spectroscopy of selenocyanate (SeCN–) dissolved in the hydrated ∼2.4 nm silica mesopores of MCM41. Polarization selective pump–probe and two-dimensional infrared measurements on the CN stretching mode of SeCN– are used to probe the effect of confinement on orientational relaxation and spectral diffusion dynamics. The dynamics of SeCN– provide information on the surrounding water hydrogen bond dynamics. The long CN stretch lifetime (∼36 ps), relative to the water hydroxyl stretch (<2 ps), significantly extends the time scales that can be accessed. Complete orientational relaxation (C 2(t), orientational correlation function) and spectral diffusion (C ω(t), frequency–frequency correlation function) dynamics are presented and compared to the simulated time correlation functions in a model silica pore of the same size. A slow decay component not present in the bulk liquid is observed in both experiments, indicating that the hydrogen bond dynamics are significantly altered by confinement. The simulations reveal a qualitative difference in the functional dependence of C 2(t;d) and C ω(t;d) on d, the distance from the interface. The former becomes exponentially faster with distance while the latter makes an abrupt transition from slower to faster dynamics midway between the surface and pore center, d ≅ 6 Å.

Amorphous Silica Slab Models with Variable Surface Roughness and Silanol Density for Use in Simulations of Dynamics and Catalysis

“…While these approaches cannot generally describe chemical reactivity (outside of the SiO 2 solid), polarization, or charge-transfer effects, they have been successfully used to describe the phase behavior of silica as well as the properties of the amorphous solid. For example, they have been used effectively in describing the interactions of silica with liquids, ,− particularly for chemical phenomena that require long simulation times. , Several force fields have been shown to display transferability between crystalline phases and even to amorphous silica . Most of these force fields share a common structure of electrostatic interactions combined with shorter-ranged potentials describing van der Waals interactions. ,− They differ primarily in the quantitative balance between these two components as well as the form of the latter as either Buckingham, ,, Lennard-Jones, ,,, or Morse , potentials.…”

Understanding the Confinement Effects and Dynamics of Methylimidazole in Nanoscale Silica Pores

“…Recently, the confinement effects on the orientational relaxation and spectral diffusion of the water in the nanopores of MCM-41 via PSPP spectroscopy and 2D infrared (IR) measurements, and MD simulations have also been reported. It is found that the dynamics of confined water shows a slower decay component which is not present in the bulk liquids . The simulated results of acetonitrile confined in the nanoscale silica pore demonstrates that the acetonitrile forms an antiparallel bilayer structure in which the acetonitrile closest to the pore surface shows an ordered behavior with the N atom pointing toward the pore wall.…”

“…The Lennard-Jones (L-J) potential was used to describe the interactions between the silica, MeIm, and MeSCN molecules. The parameters of the silica model were taken from the ClayFF model, , as shown in Table . The parameters taken from the OPLS-AA force field were employed to represent the MeIm and MeSCN molecules in which the L-J parameters for the C and N atoms on the CN groups of MeSCN were described by the ANL model .…”

“…The specific parameters of the silica include surface areas of 950 ± 70 m 2 /g, pore diameters of 2.8 ± 0.1 nm, and a total pore volume of 0.82 ± 0.05 cm 3 /g. The sample preparation for Fourier transform infrared (FTIR) spectroscopy and PSPP has been also introduced in detail in the previous work. , Here, we give a brief description. For the bulk system, the mixture with the MeSCN:MeIm of 1:10 was stirred for 30 min.…”

“…Considerable efforts have been made to probe the confined effects on the structure and dynamics of the liquids (e.g., water, acetonitrile, tert-butanol, or benzene) in the cylindrical nanoscale pores of silica from both experiments and theoretical calculations. − For example, Bourge et al studied water-filled silica with pore diameters in the range of 1–4 nm. The layering behaviors of the interfacial water structure is found, and the bulk-like water disappears in 1 nm diameter pores .…”