Effects of pore size on water dynamics in mesoporous silica

Abstract

Water confined in mesoporous silica plays a central role in its many uses ranging from gas sorption to nanoconfined chemical reactions. Here, the influence of pore diameter (2.5–5.4 nm) on water hydrogen bond (H-bond) dynamics in MCM41 and SBA15 mesoporous silicas is investigated using femtosecond infrared vibrational spectroscopy and molecular dynamics simulations on selenocyanate (SeCN−) anions dissolved in the pores. As shown recently, SeCN− spectral diffusion is a reliable probe of surrounding water H-bond structural motions. Additionally, the long CN stretch vibrational lifetime facilitates measurement of the full range of confined dynamics, which are much slower than in bulk water. The simulations shed light on quantitative details that are inaccessible from the spatially averaged observables. The dependence of SeCN− orientational relaxation and that of spectral diffusion on the distance from the silica interface are quantitatively described with an exponential decay and a smoothed step-function, respectively. The distance-dependence of both quantities is found to be independent of the diameter of the pores, and the spatial distribution of SeCN− is markedly non-uniform, reaching a maximum between the interface and the pore center. The results indicate that the commonly invoked two-state, or core–shell, model is a more appropriate description of spectral diffusion. Using these insights, we model the full time-dependence of the measured dynamics for all pore sizes and extract the “core” and “shell” dynamical correlation functions and SeCN− spatial probability distributions. The results are critically compared to those for water confined in reverse micelles.

Ultrafast Dynamics and Liquid Structure in Mesoporous Silica: Propagation of Surface Effects in a Polar Aprotic Solvent

“…In particular, liquids in mesoporous silicas (pore diameter 2–50 nm) have broad importance, e.g., hybrid electrolytes, , heterogeneous catalysis, , and carbon capture . Due to the ubiquity and importance of aqueous systems, the structure and dynamics of confined water have been studied extensively. − Simulation − and experimental − (neutron scattering, optical Kerr effect, and NMR) confinement studies have been conducted for a few simple organic liquids like acetonitrile and benzene. In general, dynamics slow with increasing confinement, but the rate of formation of an imidazolium-based ionic liquid (IL) by chemical reaction from 1-methylimidazole increased upon confinement in mesoporous silica .…”

“…Confined dynamics can be described by the two-state model when the shell and core dynamics are independent of the pore size and cleanly separated at a certain distance from the pore surface. Variations of the two-state model have been invoked in both theoretical and experimental studies of liquid dynamics in porous glass and silica materials, ,,,,, as well as in reverse micelles , and polymer electrolyte fuel cell membranes . Furthermore, orientational relaxation and spectral diffusion of SeCN – in water confined in mesoporous silicas have been successfully described with modified two-state models .…”

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.…”

Structural Characterization of Proteins Adsorbed at Nanoporous Materials

“…54 Recently, similar slow diffusivity of the confined water was examined by NMR spectroscopy (silica pore diameter, 2 to 57.7 nm) 55 and nonlinear IR spectroscopy (silica pore diameter, 2.5 to 5.4 nm). 56 These studies predicted that the water/silica interaction is a critical factor for the slow diffusivities of confined water. This prediction was supported by fact that the diffusivities of water confined inside hydrophilic mesoporous silica pores were slower than those inside hydrophobic mesoporous organosilica pore (pore diameter: 3.0 nm).…”