Polarizability Anisotropy Relaxation in Nanoconfinement: Molecular Simulation Study of Acetonitrile in Silica Pores

Abstract

We present the results of a molecular simulation study of polarizability anisotropy relaxation of liquid acetonitrile confined in approximately cylindrical silica pores of diameters in the range of 20-40 Å. Grand Canonical Monte Carlo simulation is used to determine the density of acetonitrile in pores in equilibrium with the bulk liquid, and canonical-ensemble molecular dynamics is then used to calculate the trajectories of the filled pores prepared in this way. We find that the pores are wetting, partially due to hydrogen bonding between acetonitrile nitrogen and pore silanol groups and that acetonitrile molecules have preferential orientations relative to the interface. The mobility of molecules in interfacial regions is considerably reduced and dependent mainly on their proximity to the interface. We include the contributions of molecular and interaction-induced polarizabilities to the collective polarizability anisotropy relaxation. We find that this relaxation includes a slowly relaxing component absent from the corresponding process in bulk acetonitrile and that the amplitude of this component increases as the pore diameter decreases. These results are in agreement with optical Kerr effect experiments on acetonitrile in silica pores in a similar diameter range. Further analysis of our data indicates that collective reorientation and predominantly translational "collision-induced" polarizability dynamics both contribute to the slowly relaxing portion of polarizability anisotropy decay. We further find that pore anisotropy plays a role, giving rise to different relaxation rates of polarizability anisotropy components with a different mix of axial and radial character and that collective reorientation contributing to polarizability anisotropy relaxation is somewhat faster at long times than single-molecule orientational relaxation.

Thermo-molecular orientation effects in fluids of dipolar dumbbells

“…A more involved simulation approach would include molecular electronic polarizability as well as collective effects, similar to what has been used with some success to describe effects of confinement on the OKE spectrum of acetonitrile [38]. Another approach would be to compute the vibrational density of states via Fourier transforms of the velocity autocorrelation functions [46] and attempt to relate these to the spectroscopic response.…”

“…At the same time, there is much debate on the proper way to model real molecular rotational correlation functions beyond the Debye approximation, for example as a sum of multiple exponentials or stretched exponentials. [36][37][38] In this work, we will simply obtain a numerically integrated correlation time,…”

“…To a rough approximation (assuming Debye type relaxation) the rotational correlation functions decay exponentially, and so it is feasible to simply extract a rotational correlation time τ n from a fit of C n (t) = exp(−t/τ n ). At the same time, there is much debate on the proper way to model real molecular rotational correlation functions beyond the Debye approximation, for example as a sum of multiple exponentials or stretched exponentials [36][37][38]. In this work, we will simply obtain a numerically integrated correlation time,…”

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 .…”

On the influence of the intermolecular potential on the wetting properties of water on silica surfaces

“…One of the standard systems for studying hydrophilic interactions with water are silica nanopores such as sols-gels, 1 mesoporous silica (MCM-41), [2][3][4][5][6][7] Vycorglasses, [8][9][10] controlled pore glasses (CPG). [11][12][13][14][15] Water confined in silica nanopores or near silica flat surfaces is a topic which has attracted considerable attention, [16][17][18][19][20][21][22] mainly because of the relevance of the water-silica interaction in understanding water transport in porous rocks, 23 nanofluidic devices, 24 heteregeneous catalysis in mesoporous materials, 15,25 and permeation through membrane channels. 26 Experimental investigations of water in silica nanopores have been carried out using NMR spectroscopy, 2,12 X-ray and neutron diffraction, 5,6,9,10,27,28 quasi-elastic neurton scattering, 3,4 Small-angle neutron scattering (SANS), 8 and optical Kerr-effect spectroscopy, 1 showing that the dynamics of water in such pores is slow in comparison to the dynamics of bulk water.…”