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

Abstract

Enhancement of processes ranging from gas sorption to ion conduction in a liquid can be substantial upon nanoconfinement. Here, the dynamics of a polar aprotic solvent, 1methylimidazole (MeIm), 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 (SeCN − ) anion. The long vibrational lifetime and sensitivity of the CN stretch enabled a comprehensive investigation of the relatively slow time scales and subnanometer distance dependences of the confined dynamics. Because MeIm does not readily donate hydrogen bonds, its interactions in the hydrophilic silanol pores differ more from the bulk than those of water confined in the same mesopores, resulting in greater structural order and more dramatic slowing of dynamics. The extent of surface effects was quantified by modified two-state models used to fit three spatially averaged experimental observables: vibrational lifetime, orientational relaxation, and spectral diffusion. The length scales and the models (smoothed step, exponential decay, and simple step) describing the transitions between the distinctive shell behavior at the surface and the bulk-like behavior at the pore interior were compared to those of water. The highly nonuniform distributions of the SeCN − probe and antiparallel layering of MeIm revealed by the simulations guided the interpretation of the results and development of the analytical models. The results illustrate the importance of electrostatic effects and H-bonding interactions in the behavior of confined liquids.

Concentration Dependence of Dynamics and Structure among Hydrated Magnesium Ions: An Ultrafast Infrared Study

“…The frequency independent lifetime (Table ) decreases monotonically with increasing salt concentration, from 36.2 ps in pure D 2 O to 23.5 ps in the 1–12 solution. The vibrational lifetime measures how rapidly the excited SeCN – dissipates vibrational energy through solvent-assisted intramolecular vibrational energy redistribution from the excited CN stretch to other SeCN – modes (e.g., C-Se stretch and SeCN bend) and through intermolecular vibrational energy relaxation to solvent modes. ,, The stronger probe–solvent interactions (the interactions between SeCN – with the waters that become slightly positively charged in the presence of the Mg 2+ cations) that led to the blue shift in the CN center frequency may also accelerate the vibrational relaxation by making energy transfer to the solvent modes more efficient. − …”

“…The SeCN – probe was chosen as it was proven to be a reliable probe of ultrafast water dynamics in deuterated water (D 2 O), giving almost identical spectral diffusion time scales as a native hydroxyl probe . The long SeCN – vibrational lifetime (20–40 ps), large transition dipole, and substantial solvatochromism − in MgCl 2 solution facilitate the measurement of a broad range of time scales and the detection of diverse and changing solvent structures.…”

Long-Range Interface Effects in Room Temperature Ionic Liquids: Vibrational Lifetime Studies of Thin Films

“…A plot of the amplitude functions is shown in Figure S5. This model is similar to the modified two-state model previously used to describe the spatial dependence of orientational relaxation in water and 1-methylimidazole confined in mesoporous silica. , …”

“…Changes in vibrational lifetimes upon confinement have been observed previously in chemical systems including water in reverse micelles, , Nafion fuel cell membranes, and clay minerals; 1-methylimidazole in mesoporous silica; and metal carbonyls in Pd 6 L 4 nanocages . Because the vibrational lifetime is sensitive to the local chemical and structural environment, the thickness dependence of the lifetime data gives information about the length scale of the interface effect as well as the differences between the interfacial and bulk structures.…”

Dynamics of Acrylamide Hydrogels, Polymers, and Monomers in Water Measured with Optical Heterodyne-Detected Optical Kerr Effect Spectroscopy

“…Typically, the inertial component ranges from 0.075 to 0.175, but it is frequently close to 0.1. For the purposes of this analysis, A in = 0.1 was chosen, but as shown below, the following analysis is not very sensitive to this choice. ,− …”